Janis Krumins
Klara Ramm
Oskars Purmalis
Marcis Mezulis1
Maris Klavins- 1Department of Environmental Science, University of Latvia, Riga, Latvia
- 2Warsaw University of Technology, Chamber of Economy “Polish Waterworks”, Warsaw, Poland
Eutrophication is a critical environmental challenge in the Baltic Sea, driven predominantly by excessive inputs of nitrogen and phosphorus. While diffuse agricultural runoff constitutes the largest share of nutrient pollution, point sources, particularly municipal wastewater discharges, continue to contribute significantly in specific regions. This paper investigates the potential of reclaimed water reuse as a strategic measure to mitigate nutrient loads to the Baltic Sea. Reclaiming water from wastewater and utilizing it in agriculture, industry, and urban greening shows a potential opportunity for reducing direct nutrient discharges while conserving freshwater resources and decreasing dependency on synthetic fertilizers.
1 Introduction
Discharge of wastewater, depending on its type (domestic, industrial, commercial, agricultural, and others), can be considered as a major anthropogenic impact on the environment. Wastewater typically contains organic and inorganic substances, pathogens, nutrients, heavy metals, microplastics, and emerging contaminants like derivatives of pharmaceuticals and personal care products (Eriksson et al., 2002; Huang et al., 2010).
From a regulatory and environmental perspective, wastewater is considered a source of pollution, particularly when discharged untreated, partially, or even fully treated into receiving water bodies (Kou et al., 2023; Richards et al., 2022; Sarrazin et al., 2024). Hence, wastewater management through wastewater treatment plants (WWTPs) is essential for protecting public health, aquatic ecosystems, and downstream or receiving water bodies. Proper treatment ensures the removal of key pollutants, including biological oxygen demand (BOD), pathogens, and nutrients, before the water is safely returned to the environment or reclaimed and reused for non-potable purposes (Pesqueira et al., 2020; Nishat et al., 2023; Silva, 2023).
However, nutrients in wastewater, particularly nitrogen (N) and phosphorus (P), are a major driver of eutrophication in aquatic ecosystems (Dodds and Smith, 2016; Wurtsbaugh et al., 2019; Sun et al., 2022). Excessive nutrient inputs stimulate algal and cyanobacterial blooms; subsequent biomass decay accelerates microbial oxygen consumption, leading to hypoxia or anoxia (Wurtsbaugh et al., 2019). Some cyanobacteria also produce hepatotoxic, cytotoxic, or neurotoxic compounds, posing risks to drinking water safety (Hitzfeld et al., 2000; Falconer and Humpage, 2005) and aquatic biota. Consequently, effective removal of N and P during wastewater treatment is essential.
Unfortunately, unlike conventional pollutants such as organic matter or pathogens, nutrients are not fully removed by basic wastewater treatment processes and require specialized biological or chemical technologies to achieve regulatory standards (Hasan et al., 2021; Rout et al., 2021; Derco et al., 2024). That is why, within the European Union (EU), the Urban Wastewater Treatment Directive (UWWTD) imposes an obligation of increased nutrient removal in catchments exposed to eutrophication. Many non-EU countries follow similar frameworks to guide their wastewater treatment, especially in transboundary or marine-sensitive contexts. However, these are non-binding agreements, hence provisions are often ignored when inconvenient.
Nutrient removal is further complicated by fluctuating influent concentrations, variable operational conditions, and the high energy demand of advanced N and P removal systems (Huang et al., 2020; Ahmed et al., 2021; Fallahi et al., 2021; Nguyen et al., 2022; Lee et al., 2024). Furthermore, P removal generates substantial volumes of sludge, posing additional management and cost burdens (Wei et al., 2008; Sibrell et al., 2009; Chen et al., 2024). From a governance perspective, nutrient control is also hindered by fragmented institutional responsibilities, uneven enforcement, and limited public awareness of nutrient pollution as a persistent threat (Hasan et al., 2021; Rout et al., 2021). As such, nutrient reduction remains one of the most pressing and technically demanding aspects of modern wastewater treatment, requiring both technological innovation and coordinated policy intervention to achieve sustainable water resource management and aquatic ecosystem protection aims.
Globally, large-scale water reuse applications within WWTPs, such as those implemented in Israel, Singapore, and United States (e.g., Orange County), already demonstrate the viability of reclaimed water as a key component of sustainable water and nutrient management (EPA, 2023). These nutrient-management considerations are particularly relevant in regions where hydrological constraints intensify nutrient impacts, most notably the Baltic Sea.
Nutrient management is particularly critical in enclosed or semi-enclosed marine systems, such as the Baltic Sea. The Baltic Sea is essential to the well-being of more than 85 million people; however, its fringed nature renders it acutely vulnerable to nutrient loading (Pihlainen et al., 2020; Stakėnienė et al., 2023). Owing to its restricted water exchange and long residence times, even small nutrient surpluses accumulate in the water column and sediments, intensifying bloom persistence and prolonging oxygen-depleted conditions (Saraiva et al., 2018; Raudsepp et al., 2019). These dynamics position N and P inputs as a structural threat to the Sea’s ecological functioning, regional public health, and economic stability.
Consequently, the Baltic Sea is an especially compelling case study for global and regional environmental governance, nutrient management, and wastewater policy. The removal of N and P from wastewater before discharge into freshwater systems, and ultimately the Baltic Sea, is a critical necessity for limiting eutrophication and preserving marine ecosystem integrity. Moreover, nutrient removal through processes such as valorization—i.e., converting waste-derived nutrients into usable products—can reduce environmental pressures while improving economic feasibility. P recovery from wastewater is particularly strategic, as primary P reserves are geopolitically sensitive and non-renewable. In contrast, secondary P, defined as phosphorus recovered from wastewater or other waste streams, can be considered as a renewable resource as long as agricultural and food systems continue to cycle biomass (Chowdhury et al., 2017). N is not scarce, yet its production typically is highly energy-intensive and contributes to greenhouse gas emissions, thus recovering N from wastewater in usable forms can mitigate these emissions and support circular nutrient management.
Nevertheless, regardless of the method or recovery potential, the nutrient load entering the Baltic Sea must be mitigated. The ecological vulnerability of the Baltic region, combined with persistent exceedance of maximum allowable inputs (MAIs) for nutrients, highlights the need for decisive reduction strategies. While nutrient valorization offers promising co-benefits, it cannot substitute the fundamental necessity of nutrient removal at the source (HELCOM, 2025a). Effective mitigation remains the basis of any long-term effort to stabilize the ecological condition of the Baltic Sea within safe and sustainable boundaries.
In this regard, water reclamation offers a promising long-term strategy for mitigating nutrient pollution in the Baltic Sea, particularly by reducing N and P discharges from WWTPs. By diverting treated effluent away from direct release into aquatic systems and essentially into the Baltic Sea, and instead repurposing it for varied uses, this approach can substantially lower nutrient loads that contribute to eutrophication. Furthermore, this approach aligns with the principles of circular economy by enabling the recovery and reuse of water, energy, and nutrients. Consequently, water reuse supports efforts to stabilize the ecological condition of the Baltic Sea, contributes to sustainability and resource-efficiency objectives.
Given its potential to address both environmental and resource challenges, it is imperative to further explore and promote water reclamation as a strategic component of integrated nutrient management in the Baltic Sea region. Moreover, the use of reclaimed water can also contribute to reducing the use of artificial fertilizers in agriculture, because the content of nutrients in water can reduce the amount of fertilizers necessary to be introduced into the soil.
2 The Baltic Sea
The Baltic Sea includes seven semi-enclosed sub-basins (Figure 1) that vary significantly in their hydrographic properties, catchment characteristics, and exposure to anthropogenic pressures. These sub-basins characterize ecologically and hydrologically distinct entities and form the operational basis for regional environmental governance, including nutrient load assessments and eutrophication control under the Baltic Marine Environment Protection Commission – Helsinki Commission (HELCOM) framework (Backer et al., 2010; Pyhälä, 2012; Jetoo and Tynkkynen, 2021).
Figure 1. Map of the Baltic Sea and its sub-basins with surrounding countries.
The Bothnian Bay (~36,800 km2) is an oligohaline basin with low salinity (~0.3 to 3 PSU), high freshwater inflow, and relatively low anthropogenic nutrient inputs (Rahm et al., 1995; Piiparinen et al., 2010; Neumann et al., 2020). Although pressures remain limited, the basin is sensitive to climate-driven hydrological changes and forestry-related disturbances. HELCOM’s MAIs for the Bothnian Bay (Table 1) are 57,622 t N and 2,675 t P yr.−1 (HELCOM, 2025a).
Table 1. Summary of the seven Baltic Sea sub-basins, including area, salinity range, principal nutrient pressures, maximum allowable inputs (MAIs) for nitrogen and phosphorus, and key ecological concerns.
The Bothnian Sea (~66,000 km2), is a brackish transitional basin (~3 to 5 PSU) between the oligohaline north and the more saline Baltic Proper (Bleich et al., 2011; Meier and Andersson, 2012; Rinne et al., 2018). It experiences moderate anthropogenic pressure, mainly industrial discharges and atmospheric N deposition, but retains some resilience due to its depth and circulation patterns (Njock et al., 2023). However, increased N loading from Transboundary atmospheric N inputs remain a key concern (Njock et al., 2023; Olofsson et al., 2020). HELCOM’s MAIs for the Bothnian Sea (Table 1) are 79,372 t N and 2,773 t P yr.−1 (HELCOM, 2025a).
The Gulf of Finland (~30,000 km2) is one of the most eutrophied sub-basins, characterized by low salinity (~2–6 PSU), restricted water exchange, and high nutrient inflows—particularly P—from the Neva River catchment, which drains the St. Petersburg metropolitan area (Lappalainen and Pönni, 2000; Pitkänen et al., 2008). The basin experiences recurrent cyanobacterial blooms and persistent deep-water hypoxia due to combined external loads and internal nutrient recycling (Kononen et al., 1996). HELCOM’s MAIs (Table 1) are 101,800 t N and 3,600 t P yr.−1 (HELCOM, 2025a).
The Gulf of Riga (~18,000 km2) is a shallow, semi-enclosed basin with low-moderate salinity (~2 to 6 PSU) bordered by Latvia and Estonia and limited hydrodynamic flushing, which improves nutrient retention (Savchuk, 2002). The Daugava and Lielupe rivers deliver substantial agricultural loads, and shallow depth facilitates sediment resuspension and internal P release (Stålnacke et al., 1999). These conditions sustain a high trophic state and episodic oxygen depletion (Puriņa et al., 2018). HELCOM’s MAIs (Table 1) are 88,417 t N and 2,020 t P yr.−1 (HELCOM, 2025a).
The Baltic Proper (~211,000 km2) is the largest and most nutrient-impacted sub-basin, with moderate brackish salinity (~6 to 8 PSU), strong stratification, and extensive hypoxic-anoxic zones, especially in the Gotland Deep and Bornholm Basin (Conley et al., 2009; Carstensen et al., 2014; Weidner et al., 2020; Moros et al., 2024). It receives major N and P loads from large rivers such as the Vistula, Nemunas, and Oder, and, substantial internal P loading continues to hinder recovery despite reductions in external inputs (Stålnacke et al., 1999; Pastuszak et al., 2014). HELCOM’s MAIs for this basin (Table 1) are 325,000 t N and 7,360 t P yr.−1 (HELCOM, 2025a).
The Danish Straits (~27,000 km2), comprising the Øresund, Great Belt, and Little Belt, form the main hydraulic connection between the Baltic Sea and the North Sea and are characterized by higher salinity (~8 to 20 PSU) and strong mixing (Sayin and Krauss, 1996; Stanev et al., 2018; Haid et al., 2020). Despite efficient flushing, the region receives significant nutrient inputs from densely populated and agriculturally intensive areas in Denmark and southern Sweden (Nørring and Jorgensen, 2009; Andersen et al., 2014). HELCOM’s MAIs (Table 1) are 65,998 t N and 2,601 t P yr.−1 (HELCOM, 2025a).
The Kattegat (~30,000 km2) is a transitional marine basin linking the Baltic Sea to the Skagerrak and the greater North Sea and exhibits the highest salinity in the region (~15–30 PSU) (Andersson, 1996; Bendtsen et al., 2009). Despite efficient water exchange, it remains vulnerable to eutrophication from agricultural runoff, maritime traffic, and N deposition (Håkanson and Bryhn, 2010). HELCOM’s MAIs (Table 1) are 74,000 t N and1, 687 t P yr.−1 (HELCOM, 2025a).
To maintain ecological thresholds in the Baltic Sea, HELCOM has established total MAIs of 792,209 t N and 21,716 t P yr.−1 (HELCOM, 2025a).
Several sub-basins currently exceed their MAIs. The most substantial gaps occur in the Baltic Proper, the Gulf of Finland, and the Gulf of Riga (Table 1), where N and P inputs remain above allowable thresholds, while exceedances of N loads are also noted in the Danish Straits and Kattegat. These exceedances highlight the need for more ambitious nutrient-reduction measures.
Together, these seven sub-basins show the spatial heterogeneity of nutrient pressures and ecological responses within the Baltic Sea. Their differentiation is critical for the formulation of regionally adapted nutrient management strategies and for evaluating compliance with HELCOM’s sub-basin-specific MAIs (HELCOM, 2025a). The distinct hydrophysical and biogeochemical attributes of each sub-basin necessitate an adaptive and geographically nuanced approach to eutrophication control and marine ecosystem restoration.
3 Sources of nutrient loads to the Baltic Sea
Nutrient pollution in aquatic systems originates from two main types of sources: point sources, which discharge pollutants from identifiable, confined locations such as WWTPs or industrial outfalls; and non-point sources, which refer to diffuse inputs from broad areas, primarily agricultural runoff, atmospheric deposition, and surface erosion, where the exact origin is harder to trace. Still, the agricultural runoff is the dominant and most recognized component of non-point source pollution, even though its precise origin is harder to trace spatially and temporally (Wurtsbaugh et al., 2019; Nie et al., 2018; Häder et al., 2020; Shortle et al., 2020).
3.1 Point sources of nutrient loads
Point sources, particularly municipal WWTPs, have historically been major contributors to nutrient pollution in the Baltic Sea, especially in densely populated and industrialized regions (e.g., southern Sweden, Copenhagen metropolitan area, northern Germany, Poland). These sources were especially responsible for elevated N and P during the late 20th century (Behrendt and Bachor, 1998; Kiedrzyńska et al., 2014; Andersen et al., 2017; Räike et al., 2019). However, following the implementation of EU environmental regulations, such as the UWWTD (EUR, n.d.), and substantial technological upgrades in wastewater treatment, nutrient emissions from point sources have significantly declined in recent decades (Derco et al., 2024; Farrimond and Upton, 1993; Walve et al., 2018). Inputs of N and P from direct point sources (Table 2) decreased by approximately 58 and 84%, respectively, between 1995 and 2022. In 1995, direct discharges accounted for 7.8% of total waterborne N and 15% of total P inputs; by 2022, these shares had fallen to 4.5 and 5.8%, respectively (HELCOM, 2023).
Table 2. Reductions in direct point-source N and P loads to the Baltic Sea between 1995 and 2022, and their proportional contributions to total waterborne inputs in 2022.
These reductions are evident across most sub-basins (Table 2), with the most pronounced decreases observed in the Danish Straits (−76% of total N), the Baltic Proper (−71% of total N and −87% of total P), the Gulf of Riga (−66% of total N, −88% of total P), and the Gulf of Finland (−89% of total P). Unfortunately, in sub-basins with relatively low total flow, such as the Danish Straits and the Gulf of Finland, point sources still represent a substantial proportion of total nutrient inputs—11% of total N and 4% of total P in the Danish Straits, and 8.3% of total N in the Gulf of Finland (HELCOM, 2023).
Consequently, despite the positive shift, municipal and domestic wastewater still account for a substantial share of nutrient pollution. According to recent assessments, wastewater contributes approximately 15% of anthropogenic N and 31% of anthropogenic P inputs to the Baltic Sea. Of these, municipal WWTPs are responsible for the majority, 77% of N and 64% of P, while scattered dwellings contribute 17 and 20%, and stormwater overflows add 6 and 16%, respectively (Müller-Karulis et al., 2024; Stockholm University Baltic Sea Centre, 2024).
The condition of nutrient loads among EU countries is well controlled and is continuously improving. According to the 12th Technical Assessment of UWWTD Implementation, in 2020, only Poland and Finland did not reach the more stringent wastewater treatment goal (Fribourg-Blanc et al., 2024). However, a major challenge persists at the regional level due to transboundary nutrient inflows from non-EU countries, particularly Belarus and, most notably, Russia. The current authoritarian regime, which withdrew from HELCOM in 2022, deprioritizes environmental obligations and transboundary impacts in favor of unilateral political interests.
Another challenging source of nutrients are combined sewer overflows (CSOs), due to their episodic and variable character. Yet, CSOs contribute non-negligible share of the total nutrient load (European Commission, 2022).
3.2 Diffuse sources of nutrient loads
Currently, diffuse agricultural runoff, managed forest drainage, stormwater and surface erosion have become the predominant source of nutrient inputs in many parts of the Baltic Sea basin (Räike et al., 2019; Hong et al., 2012; Wojciechowska et al., 2019). Agriculture accounts for approximately 60 to 70% of anthropogenic N and P inputs; however, these figures aggregate data across all seven sub-basins, because regional variations are substantial. For example, the Danish Straits exhibited the highest area-specific nutrient inputs in 2022 (Table 3), with 998 kg N/km2 and 32 kg P/km2, while the Bothnian Bay and Bothnian Sea recorded the lowest levels due to lower agricultural intensity and extensive forest cover within the catchment area (HELCOM, 2023). The Bothnian Bay and Bothnian Sea exhibited the lowest inputs, with approximately 175 kg N/km2 and 7.1 kg P/km2, corresponding to low anthropogenic pressures and large expanses of forested or pristine land. On average, the Baltic Sea catchment received 320 kg N/km2 and 10 kg P/km2. However, when nutrient loads are normalized to the marine receiving areas, the Gulf of Riga had the highest N input (4,200 kg N/km2), and the Gulf of Finland showed the highest P input (122 kg P/km2). Meanwhile, the lowest marine-area-specific inputs were found in the Bothnian Sea, with 506 kg N/km2 and 18 kg P/km2, further emphasizing its low anthropogenic impact.
Table 3. N and P inputs to the Baltic Sea sub-basis in 2022, including catchment-area-normalized and marine-area-normalized loads.
In 2022, riverine inputs accounted for approximately 95% of total waterborne N and P entering the Baltic Sea, reaffirming the dominance of diffuse sources in the Baltic Sea region (HELCOM, 2023). Total N inputs via waterborne pathways were estimated at 563,000 tonnes—about 9% below the 2012–2021 average—while total P inputs dropped to 17,500 tonnes, a reduction of 33% relative to the same period. These declines were closely linked to hydrological variability, as average river flow in 2022 was 8% below the long-term mean, with several basins experiencing more than 15% reductions. Flow was particularly suppressed in the Kattegat (−30%), the Baltic Proper (−19%), the Danish Straits (−14%), and the Gulf of Riga (−11%), which collectively contributed 68% of total N and 66% of total P during 2012–2021. Their reduced flows in 2022 corresponded to slightly lower shares of total nutrient loads—65 and 59%, respectively (HELCOM, 2023).
While waterborne P inputs have shown a consistent downward trend across all seven Baltic Sea sub-basins since 1995, N inputs have shown a more heterogeneous pattern, with significant increases reported in some regions. Notably, rising total N loads have been observed to the Baltic Proper, as well as from certain countries to the Gulf of Riga and Kattegat, suggesting persistent or even worsening diffuse source pressures despite overall regulatory progress. However, this pattern is complicated by interannual variability in river flow, which interacts with long-term trends in nutrient emissions. In 2022, total N inputs were 11% higher than the 2012–2021 average in the Gulf of Finland, and 6% higher in the Gulf of Riga, despite reduced flow conditions. Conversely, five basins—Kattegat (−28%), Danish Straits (−22%), Bothnian Sea (−16%), Baltic Proper (−15%), and Bothnian Bay (−6.9%)—all received below-average total N loads, largely mirroring concurrent flow reductions. For P, all seven sub-basins reported lower-than-average waterborne total P inputs in 2022, ranging from −18% in the Gulf of Finland and Bothnian Bay to −42% in Gulf of Riga. These observations indicate that while P mitigation efforts remain largely effective, N reductions are uneven, with localized increases likely driven by a combination of land use dynamics, lag effects, and hydroclimatic variability (HELCOM, 2023).
At the same time, beyond absolute reductions in load, a notable downward trend is also evident in nutrient concentrations within riverine inputs. Between 1995 and 2022, flow-weighted concentrations of total N and P in rivers decreased by 15 and 48%, respectively. These reductions were statistically significant across most sub-basins, including the Bothnian Bay, Bothnian Sea, Baltic Proper, Danish Straits, and Kattegat. The trends suggest that nutrient mitigation strategies have been broadly effective not only in reducing point source discharges but also in lowering ambient nutrient concentrations in catchment runoff (HELCOM, 2023).
Annual flow-weighted riverine nutrient concentrations in 2022 revealed contrasting dynamics for N and P across Baltic Sea sub-basins. The flow-weighted total N concentration reached 1.19 mg N/l, representing a 3.7% increase compared to the 2012–2021 average, while total P dropped to 0.036 mg P/l, marking a 21% decrease relative to the same reference period. Total N concentrations were higher than average in the Gulf of Finland (+19%), Gulf of Riga (+14%), and Baltic Proper (+6.2%), while being lower in Bothnian Sea (−14%), Danish Straits (−7.2%), and Bothnian Bay (−5.3%). In the Kattegat, the total N concentration remained close to the ten-year average despite a 30% reduction in water flow, likely due to hydrological regulation in Göta Älv catchment, which includes large lakes and artificial channels that buffer flow and nutrient loads. Flow-weighted total P concentrations declined across all sub-basins, with the steepest reductions observed in the Gulf of Riga (−35%), Baltic Proper (−24%), and Bothnian Sea (−22%) (HELCOM, 2023).
Regulation of non-point sources remains a major challenge due to the diffuse and stochastic nature of pollutant release. While point sources can be monitored and controlled, agricultural emissions depend on farm-level practices, land cover, and hydrological conditions. EU-level instruments such as the Common Agricultural Policy (CAP) and national agri-environmental measures have attempted to mitigate these pressures, but implementation gaps and inconsistent enforcement limit their effectiveness across the region (Grizzetti et al., 2021; Viitasalo and Bonsdorff, 2022). A very important source of N is livestock farming. It is estimated that livestock production is responsible for 81% of agricultural N introduced into water systems and 87% of ammonia emissions from agriculture into the atmosphere. An important regulation aimed at reducing nitrate discharges is the EU Nitrates Directive, which aims to reduce water pollution from agricultural nitrates, but has shown mixed results in achieving its objectives (European Commission, 2021). The implementation of the Nitrates Directive by the Baltic Sea Member States is not fully effective. The relevant areas are not adequately defined and the requirements set by the Member States in their action programs are not sufficiently stringent. These shortcomings largely come from insufficient administrative capacity, limited farm-level compliance monitoring, and political reluctance to impose stronger nutrient limits in regions economically dependent on intensive agriculture. Improving enforcement, expanding targeted agri-environmental incentives, and strengthening monitoring and reporting systems would substantially improve the effectiveness of existing measures. Although some improvements have been observed, significant challenges remain, in particular in reducing nitrate concentrations in groundwater and surface water. Moreover, climatic variability further complicates control efforts. In dry years, nutrients accumulate in soils and are subsequently flushed into waterways during heavy rainfall, leading to pulses of nutrient pollution (HELCOM, 2023). This asymmetry is why point-source reductions have succeeded, and why non-point sources now dominate.
4 Country contributions to nutrient loads
The Baltic Sea catchment area includes not only the immediate coastal states—Sweden, Finland, Estonia, Latvia, Lithuania, Poland, Germany, Denmark, and Russia (through Kaliningrad Oblast and the area near St. Petersburg), but also extends into several upstream countries such as Belarus, Ukraine, Slovakia, the Czechia, and Norway, whose wastewater emissions are transported to the sea via transboundary rivers, hence also contributing to nutrient loads (Capell et al., 2021; HELCOM, 2021a).
It is important to note that reporting methodologies differ across countries. As a result, direct cross-country comparisons should be interpreted with caution, as differences in data availability, monitoring intensity, and national reporting practices affect the apparent composition of nutrient sources. For example, HELCOM contracting parties follow harmonized guidelines for nutrient source apportionment, distinguishing agricultural, point, atmospheric, and natural background inputs. At the same time, non-HELCOM upstream countries do not provide fully comparable data, and some categories (e.g., atmospheric deposition, diffuse-source sub-components) are not consistently reported.
4.1 Non-HELCOM countries
Belarus is the most prominent non-coastal contributor, with substantial N and P loads entering the Baltic via the Daugava and Nemunas rivers. Its emissions stem from both agricultural runoff and insufficiently treated municipal wastewater, making it a key focus for basin-wide nutrient reduction efforts (Stakėnienė et al., 2023; Müller-Karulis et al., 2024; Capell et al., 2021; HELCOM, 2021a). In contrast, Ukraine is a minor contributor, with nutrient discharges limited to its western border region. These inputs are transported via the San and Western Bug rivers, which are tributaries of the Vistula River (HELCOM, 2021a; Hägg et al., 2013; HELCOM, 2025b). Czechia contributes a moderate share of nutrient loads through the Oder River basin, with pollutants originating in northern Czech territories and flowing through Poland and Germany before reaching the southern Baltic (Capell et al., 2021; HELCOM, 2021a; Hägg et al., 2013). Slovakia contributes only marginally, as just a small portion of its northern territory lies within the Baltic catchment; its nutrient loads are carried via Oder River tributaries across Poland (HELCOM, 2021a; Hägg et al., 2013). Finally, Norway is a very minor contributor, with only a small portion of its far northern territory draining into the Baltic Sea via the Torne and Paatsjoki rivers. These areas contribute predominantly natural background nutrient loads, with minimal anthropogenic influence (HELCOM, 2021a).
Further complicating these patterns are interannual variations in precipitation and hydrology. For example, in 2022, below-average rainfall in parts of Poland, Latvia, and Lithuania resulted in reduced runoff and river flow, hence lowering nutrient delivery to the sea. However, preceding droughts in 2020–2021 led to nutrient accumulation in soils, which was subsequently mobilized and flushed out during later wet periods (HELCOM, 2023).
Consequently, all countries within the Baltic Sea region contribute nutrient loads to the marine environment through the discharge of municipal and industrial wastewater, either directly into coastal waters or indirectly via the extensive fluvial network that drains into the sea (Stålnacke et al., 1999; Kiedrzyńska et al., 2014).
Another source of pollution in the Baltic Sea is nutrient loads from shipboard waste streams, yet their contribution is comparatively much smaller than the one from municipal and industrial wastewater (Raudsepp et al., 2019; Lappalainen et al., 2024). At the same time, nutrient contributions to the Baltic Sea vary considerably across countries, indicating differences in land use intensity, population density, agricultural practices, and the effectiveness of wastewater treatment infrastructure.
4.2 HELCOM countries
Among the most significant contributors to nutrient load to the Baltic Sea (Table 4) is Poland, whose large agricultural sector and extensive river network, including the Vistula and Oder rivers, convey considerable N and P loads into the Baltic Proper (Wojciechowska et al., 2019; Wulff et al., 2014; Pecio, 2024). For P, point sources remain the dominant contributor, accounting for 42% of total P inputs, compared to 34% from agricultural activities. In contrast, N inputs are largely driven by diffuse agricultural runoff, which represents 45% of the total, followed by 31% from point sources. Natural background contributions amount to 16% for N and 18% for P, while atmospheric deposition plays a minor role (3% N, 1% P). Transboundary nutrient loads from upstream countries such as Slovakia or Ukraine constitute only 4–5% of the total nutrient loads reaching the Baltic Sea via Poland, highlighting the importance of domestic mitigation measures (HELCOM, 2023). Although substantial investments have been made in improving wastewater treatment facilities in recent decades, the scale of diffuse pollution from fertilized cropland continues to exceed Poland’s nutrient input ceilings under HELCOM’s MAI framework (Wojciechowska et al., 2019; Pecio, 2024).
Table 4. Percentage contribution of N and P sources from HELCOM countries.
Russia also plays a notorious role in nutrient loading (Table 4), particularly in the Gulf of Finland, where the Neva River drains the highly urbanized and industrialized St. Petersburg metropolitan area (Ylöstalo et al., 2016; Knuuttila et al., 2017). In contrast to other countries in the Baltic Sea catchment, Russia reports that the majority of its riverine nutrient loads are of natural origin. According to available data, 83% of N and 65% of P inputs are attributed to background losses from soils and unmanaged landscapes. Among anthropogenic sources, diffuse pollution dominates P (21%) and N (10%), while point sources contribute 14 and 6%, respectively. Notably, Russia has not reported data on atmospheric deposition, which may understate total anthropogenic contributions (HELCOM, 2023). Russia’s contributions to P exceed HELCOM reduction targets for the Gulf of Finland, moreover, Russia is responsible for about 73% of the total P input to the Gulf of Finland, while N loads remain elevated across multiple sub-basins (Knuuttila et al., 2017; Izmaylov et al., 2022; Iho et al., 2023). While significant progress has been made in upgrading wastewater treatment in the city, including P removal technologies, legacy inputs and diffuse pollution from surrounding agricultural and forested landscapes continue to challenge water quality goals in the Baltic Sea (Solovieva, 2021).
Germany’s nutrient inputs (Table 4) are primarily directed toward the western Baltic sub-basins, including the Danish Straits and the southern Baltic Proper. These inputs are mainly from diffuse agricultural sources, with notable reductions in recent decades, but challenges remain in meeting water quality targets (Werner and Wodsak, 1994; Liu et al., 2017; Kuss et al., 2020). Agricultural runoff, despite widespread implementation of best management practices, remains a persistent source of N pollution, and P reduction targets for the Baltic Proper have not yet been fully achieved (Behrendt and Bachor, 1998; Schernewski et al., 2015; Heyl, 2023). While Germany has succeeded in reducing point-source discharges through advanced wastewater treatment, diffuse sources remain more difficult to regulate, particularly in regions of intensive livestock farming (Sarrazin et al., 2024; Lam et al., 2010; Lam et al., 2011; Yang et al., 2019).
In contrast, Sweden and Finland have achieved notable reductions in both N and P inputs through comprehensive wastewater treatment reforms and nutrient abatement in agriculture (Table 4). Nonetheless, challenges persist in meeting MAI targets for the Baltic Proper and the Gulf of Finland (Räike et al., 2019; Guterstam, 1996; Andersson et al., 2005; Aronsson et al., 2016; Hellsten et al., 2019; Kinnunen et al., 2023). Sweden continues to report exceedances of P inputs to the central basin, showing the continued influence of legacy P stored in marine sediments and internal loading processes (Walve et al., 2018; Wu et al., 2016; McCrackin et al., 2018). Finland, while largely compliant with N reduction goals, has struggled to reduce P inputs sufficiently in its eastern coastal regions (Pitkänen et al., 2008; Räike et al., 2019; Huttunen et al., 2015). In Sweden, nutrient loads to the Baltic Sea are predominantly characterized by natural background losses, which account for approximately 54% of total N inputs and 66% of total P. Among anthropogenic sources, agriculture contributes 25% of N and 14% of P, while point sources add 11% of N and 16% of P. Atmospheric deposition plays a more substantial role than in many other HELCOM countries, accounting for 10% of N and 4% of P loads. This elevated impact is primarily attributed to Sweden’s extensive inland water surface coverage, with lakes occupying nearly 10% of the national land area, thereby increasing nutrient input via deposition onto aquatic surfaces (HELCOM, 2023). Similarly, Finland also exhibits a high proportion of natural background loads, which constitute 44% of total N and 34% of total P inputs. Agricultural runoff remains significant, contributing 38% of N and 53% of P, whereas point sources account for 10% of both nutrients. Atmospheric deposition is also relevant, especially for N (8%), with a slightly lower share for P (3%). As in Sweden, this is largely explained by Finland’s high percentage of lake area, which facilitates deposition-driven nutrient accumulation (HELCOM, 2023).
The Baltic states contribute varying nutrient loads (Table 4), primarily to the Gulf of Riga, the Baltic Proper, and the Gulf of Finland (Yurkovskis et al., 1993; Laznik et al., 1999; Tamminen and Seppälä, 1999; Rahm and Danielsson, 2007). Estonia’s loads remain elevated, particularly in relation to P, with significant exceedances in all its recipient sub-basins (Pachel et al., 2012; Ausmeel et al., 2024). Latvia’s nutrient contributions are largely concentrated in the Gulf of Riga, where urban wastewater and agricultural runoff from the Daugava basin continue to drive eutrophication pressures (Laznik et al., 1999; Põder et al., 2003). Lithuania similarly exceeds P reduction targets for the Baltic Proper, despite having met some requirements for the Gulf of Riga (Iho et al., 2023; McCrackin et al., 2018; Murray et al., 2019). In Estonia, diffuse sources dominate riverine nutrient loads, with agriculture assumed to be the primary contributor. These sources account for 64% of N and 75% of P loads. Natural background losses remain significant, comprising 34% of N and 21% of P, while point sources contribute only 2 and 4%, respectively (HELCOM, 2023). In Latvia, riverine nutrient inputs to the Baltic Sea are strongly influenced by transboundary inflows, especially from Belarus, which contribute approximately 37% of total N and 46% of total P. Although these loads cannot be disaggregated by source, agricultural activities and point discharges in upstream countries are presumed important. Within Latvian territory, diffuse sources account for 53% of N and 40% of P, while point sources are minimal, comprising just 0.5% of N and 2% of P (HELCOM, 2023). Lithuania also experiences high transboundary nutrient inputs, with 25% of N and 34% of P loads entering from upstream—again, mainly Belarus. Although precise source attribution is lacking, upstream contributions are believed to mirror HELCOM countries in being dominated by agriculture and point discharges. Within Lithuania, agriculture is the main domestic source, responsible for 56% of N and 42% of P. Natural background loads contribute 12% of N and 11% of P, while point sources make up 4% of N and 13% of P. Atmospheric deposition plays a minimal role, accounting for only 2% of N inputs (HELCOM, 2023).
Denmark, although geographically small, wields a disproportionately high influence on nutrient dynamics (Table 4) in the Danish Straits and Kattegat sub-basins (Macneil, 2001; Rydberg et al., 2006; Bogalecka and Pigłowski, 2024). While it is the only country currently achieving HELCOM’s N input ceilings across all sub-basins, continued P contributions from intensive agriculture and livestock production remain a concern, particularly given the high connectivity of the Danish Straits to the wider Baltic Sea system (Kronvanga et al., 2005; Dalgaard et al., 2014; Petersen et al., 2021; Dessirier et al., 2023). In Denmark, riverine nutrient inputs to the Baltic Sea are predominantly derived from agricultural activities, particularly for N. Diffuse sources account for 74% of total N and 38% of total P loads. Natural background losses contribute an additional 19% for N and 29% for P. Point sources are a relatively minor contributor to N loads (6%), but play a substantial role in P pollution, accounting for 33% of total P inputs. The contribution of atmospheric deposition is negligible—1% for N and 0.1% for P—largely due to Denmark’s low lake surface area, which limits the deposition interface (HELCOM, 2023).
Observed patterns indicate major structural differences in national nutrient load mitigation capacity. Denmark, Sweden, and Finland have achieved the most consistent progress due to strong wastewater treatment requirements, long-standing and effective agri-environmental regulations, and efficient monitoring systems. At the same time, Poland, Estonia, Lithuania, and especially Russia continue to exceed nutrient reduction targets because diffuse agricultural emissions remain high, enforcement is uneven, and in some cases institutional and political barriers (Russia) hinder effective implementation. Sustainable Baltic Sea basin-wide progress is unlikely until region-wide governance and compliance are sustained.
5 Wastewater treatment within the Baltic Sea region
Wastewater treatment in the EU follows the standards set out in the Urban Wastewater Treatment Directive (EUR, n.d.). This directive mandates that wastewater from agglomerations above certain population thresholds must undergo primary, secondary, or, in some cases, tertiary treatment before being discharged into the environment. Compliance ensures the removal of organic matter, N and P, thereby protecting water bodies from pollution and eutrophication. Yet, across the EU, implementation levels vary significantly (Figure 2). For example, Sweden, Germany, and Denmark treat 100% of their sewage in line with the directive, while Lithuania and Estonia reach 99%, Latvia 98%, and Finland 97%. However, compliance is notably lower in Czechia (78%) and Poland (71%), indicating ongoing infrastructural and regulatory challenges in achieving full EU-wide alignment (WISE Freshwater, 2024).
Figure 2. Compliance of wastewater treatment within the Baltic Sea drainage basin with EU legislation (% of all treated wastewater in 2020).
Germany leads with 3,672 treatment plants equipped for biological nutrient removal (Figure 3), followed by Poland (1,577) and Czechia (530). In the Nordic region, Sweden operates 428 such plants, Denmark 316, and Finland 146. The Baltic states, despite smaller populations, show notable investment: Lithuania has 55 plants, Estonia 49, and Latvia 20 (WISE Freshwater, 2024).
Figure 3. WWTPs within the Baltic Sea drainage basin with biological treatment with N and P removal (number of treatment plants).
However, many countries also operate plants with only basic biological treatment (Figure 4), which do not remove N or P and may not suffice for compliance in sensitive areas. Germany has 135 such plants, Czechia 79, Poland 35, Latvia 32, Denmark 14, Lithuania 6, and Estonia 2, while Sweden and Finland operate none, showing their commitment to full nutrient removal. The presence of these more basic facilities—particularly in countries with lower overall compliance like Poland (71%) and Czechia (78%)—highlights the infrastructural divide and the ongoing need for investment in upgrading wastewater treatment systems across the EU (WISE Freshwater, 2024).
Figure 4. WWTPs within the Baltic Sea drainage basin with biological treatment without N and P removal (number of treatment plants).
More concerning, however, is the continued use of primary treatment alone, which is insufficient for most EU requirements. Poland still operates 30 such plants, and Czechia has one, whereas Germany, Sweden, Denmark, Finland, Lithuania, Estonia, and Latvia have fully phased out primary-only treatment (WISE Freshwater, 2024).
6 Reclaimed water reuse as a nutrient balance improving tool
The preceding nutrient-budget analysis shows that diffuse agricultural sources dominate basin-wide N and P inputs. At the same time, point sources remain locally important in several Baltic Sea sub-basins with low flow (e.g., Gulf of Finland, Danish Straits), while exceedances of MAIs persist in the Baltic Proper, Gulf of Riga, and Gulf of Finland. The observed patterns indicate where reclaimed water reuse and nutrient-recovery technologies can exert the greatest leverage, which is primarily in areas with high agricultural pressure and in catchments where municipal wastewater still forms a substantial share of anthropogenic P inputs. Accordingly, water reuse and nutrient recovery can modify these identified pathways and contribute to reducing overall nutrient loads to the Baltic Sea.
Reclaimed water reuse is increasingly recognized as a viable and sustainable approach for addressing the challenges of water scarcity and nutrient pollution (Bixio et al., 2006). This practice not only contributes to water conservation by supplementing conventional water sources with treated effluent, but also plays a critical role in nutrient management through the recovery and reuse of N and P (Saliu and Oladoja, 2021). These nutrients, often present in high concentrations in municipal and industrial wastewater, can be recovered using advanced treatment technologies and redirected for beneficial use, particularly in agriculture (Fito and Van Hulle, 2021). Use of reclaimed water for irrigation is established practice worldwide and it support reduction of fertilizer use as well as water consumption. By substituting synthetic fertilizers with nutrient-rich reclaimed water or derived products such as biosolids and struvite, this approach helps reduce the environmental liability associated with nutrient runoff and eutrophication in aquatic ecosystems. Moreover, integrating water reuse into agricultural and urban water cycles aligns with circular economy principles by transforming waste streams into resource flows, minimizing environmental impacts, and improving the overall sustainability of water and nutrient cycles. Intrinsically, reclaimed water reuse is increasingly endorsed in environmental management as a solution for improving resource efficiency, protecting ecosystems, and supporting long-term food and water security (Vo et al., 2014).
Reclaimed water reuse and nutrient recovery can be integrated within the general wastewater pathway, spanning from influent treatment, liquid-line processing, sludge handling, sidestream nutrient recovery, and the final division between discharge and reuse (Figure 5). Given the magnitude of nutrient loads entering Baltic Sea, failing to integrate water reuse and nutrient recovery into existing wastewater pathways leaves a substantial fraction of recoverable N and P unutilized, limiting the region’s ability to meet long-term reduction targets.
Figure 5. Reclaimed water reuse and nutrient recovery integration within the general wastewater pathway.
Several technologies have been developed for the removal and recovery of nutrients from wastewater, addressing both environmental protection goals and resource circularity (Hasan et al., 2021; Śniatała et al., 2024). The primary approaches for nutrient removal can be categorized into three main types: 1. biological processes, 2. physicochemical treatments, and 3. nature-based solutions (NBS) such as constructed wetlands and soil aquifer treatment. Among biological methods, conventional technologies like activated sludge systems, sequencing batch reactors, and membrane bioreactors remain prevalent due to their high efficiency in removing N and P through microbial activity (Abdoli et al., 2024). In recent years, microalgae-based systems have gained attention for their multifunctionality, offering simultaneous nutrient uptake, oxygen production, and CO2 sequestration, while also generating biomass that can be repurposed for bioenergy or agricultural use (Wang et al., 2017; Im et al., 2024; Ugwuanyi et al., 2024). Complementing these are advanced oxidation processes—including ozonation, ultraviolet irradiation, and photocatalysis—as well as membrane-based separation technologies like reverse osmosis and nanofiltration, which enable selective and efficient nutrient removal, especially in tertiary and quaternary treatment stages (Wang et al., 2017; Ye et al., 2020). Chemical treatments such as adsorption, precipitation, and ion exchange are also widely applied, particularly for P control and recovery (Wang et al., 2017; Ye et al., 2020).
Nature-based solutions such as constructed wetlands and soil aquifer treatment utilize biogeochemical processes occurring in engineered ecosystems to remove nutrients and other contaminants. However, their effectiveness is affected by various parameters, including hydrological regime, substrate composition, vegetation type, and wetting-drying cycles (Bôto et al., 2022; Gharbia et al., 2024), making them suitable for decentralized or low-energy treatment solutions. Beyond removal, increasing focus is placed on nutrient valorization—the recovery and transformation of nutrients, primarily N and P, into useful products that support both agricultural productivity and the circular economy. This shift is particularly crucial in light of declining phosphate reserves, water pollution, and the rising demand for sustainable fertilizer alternatives. Technologies used for nutrient recovery include ion exchange resins, bioelectrochemical systems, electrodialysis, solvent extraction, and chemical precipitation methods like struvite crystallization, as well as biological approaches such as nitrification, denitrification, microalgae cultivation, and yeast fermentation (Chen et al., 2019; Yadav et al., 2022; Amara et al., 2023; Kurniawan et al., 2025).
Reclaimed water, when subjected to appropriate treatment processes, could offer a sustainable and nutrient-rich alternative for irrigation. Beyond addressing water scarcity, its application provides a valuable source of essential nutrients—primarily N, P, and K—that are critical for crop development. However, there is a conflict of goals here. Urban Wastewater Treatment Directive requires increased nutrient removal in the Baltic Sea basin. There are, therefore, too few nutrients in the reclaimed water. On the other hand, it is in the interest of reclamation to leave these nutrients in the water to reduce the use of artificial fertilizers. Therefore, water reclamation should be combined with nutrient recovery from wastewater or sludge so as to replace most artificial fertilizers. As a result, reclaimed water can simultaneously fulfill both irrigation and fertilization functions, reducing the demand for synthetic agrochemicals. Field studies conducted in various climatic and soil conditions have demonstrated that crops irrigated with treated, nutrient-rich wastewater often perform as well as—or in some cases, better than—those receiving water and synthetic fertilizers (Vergine et al., 2017; Bixio et al., 2006). These performance outcomes are attributed to the continuous supply of bioavailable nutrients, improved soil moisture retention, and potential increases in soil organic matter content over time.
In addition to direct reuse of treated and disinfected effluent for irrigation, the recovery and recycling of concentrated nutrient products from wastewater streams further contribute to agricultural sustainability. Technologies such as chemical precipitation (e.g., struvite formation), ion exchange, and biological nutrient removal allow extraction of N and P in forms suitable for use as fertilizers or soil amendments. These recovered products can be locally applied or commercially distributed, supporting circular economy goals while reducing reliance on finite mineral fertilizers and fossil-fuel-based ammonia production (Ye et al., 2020; Bixio et al., 2006; Carpanez et al., 2024). Particularly in regions with limited access to or high costs of commercial fertilizers, the use of reclaimed water and recovered nutrients presents a cost-effective strategy for improving food security, promoting resource efficiency, and lowering the environmental footprint of agriculture. As awareness of these benefits grows, policies and guidelines are increasingly encouraging safe and regulated use of reclaimed water in agriculture, provided that public health risks are adequately managed through treatment standards and monitoring protocols.
7 Reclaimed water reuse in the Baltic Sea region
Reclaimed water reuse is gaining increasing attention in the Baltic Sea region as a strategy to tackle pressing environmental challenges, particularly nutrient pollution, water scarcity, and the broader interest of environmental sustainability. The Baltic Sea is one of the most heavily eutrophicated marine ecosystems in the world, primarily due to high levels of N and P inputs from agricultural runoff, municipal wastewater discharges, and industrial effluents (Purmalis and Klavins, 2025). These nutrient loads contribute to algal blooms, hypoxia, and long-term degradation of marine biodiversity and ecosystem services. In this context, the integration of wastewater reuse and nutrient recovery has emerged as a promising approach to mitigate anthropogenic nutrient load while advancing circular economy objectives (Haddaway et al., 2019; Johannesdottir et al., 2020; Koskiaho et al., 2020).
Ecotechnologies are being investigated across the region for their potential to reclaim valuable resources from domestic wastewater streams. Among the most prominent are anaerobic digestion, source-separation of greywater and blackwater, nutrient recovery during wastewater treatment, and microalgae cultivation systems. These approaches aim not only to reduce nutrient discharge into aquatic environments but also to recover C, N, and P for reuse as bio-based fertilizers, soil conditioners, or energy carriers. Such innovations align with the shift from linear to circular urban metabolism, where wastewater is reframed from a waste problem to a resource flow (Haddaway et al., 2019; Johannesdottir et al., 2020).
Empirical studies from river basins draining into the Baltic Sea—including in Finland, Sweden, and Poland—show the potential of these technologies (Ramm and Smol, 2024). Anaerobic digestion has been shown to improve biogas production and reduce the volume of biosolids, while source-separation systems facilitate more targeted treatment and recovery of nutrients at the household or neighborhood scale. Nutrient extraction technologies, such as struvite precipitation or ammonia stripping, can yield recoverable fertilizer products that displace synthetic alternatives. Although these interventions have demonstrated some reductions in nutrient loads—particularly when combined with conventional treatment upgrades—their nutrient removal efficiency often remains lower than that achieved by established best management practices, such as buffer strips or controlled drainage in agriculture (Koskiaho et al., 2020). Nonetheless, their co-benefits are notable: they offer decentralized treatment options, support bioeconomy goals, improve agricultural productivity through recycled inputs, and contribute to energy self-sufficiency at the local level. The technologies are not viewed solely as replacements for traditional nutrient mitigation measures, but rather as complementary solutions that add value and resilience to integrated water and nutrient management systems in the Baltic Sea region (Djodjic et al., 2025).
8 Barriers and enablers for the implementation of nutrient removal from wastewater through reuse in the Baltic Sea region
One significant challenge lies in the operation of small and often under-documented WWTPs, particularly in coastal areas where population density and its fluctuations lead to highly variable influent loads. These fluctuations make it difficult for treatment systems to maintain consistent performance and meet strict environmental discharge standards, especially during peak seasons. Despite their limited scale, these decentralized systems can contribute substantially to local nutrient pollution, particularly when they lack proper oversight or upgraded technologies (Konkol et al., 2024).
A further barrier is the lack of harmonization in wastewater management regulations, data collection practices, and treatment standards across the Baltic Sea countries. While larger plants are generally subject to EU directives such as the Urban Wastewater Treatment Directive, many small or non-urban systems fall outside the scope of these frameworks, leading to regulatory gaps. This fragmentation results in inconsistent enforcement and reporting, complicating efforts to quantify and manage regional nutrient loads. To address this, there is an urgent need for unified data collection protocols and harmonized reporting systems that enable cross-border comparability and evidence-based policymaking (Konkol et al., 2024).
In terms of nutrient reuse, current practices in the region predominantly focus on the agricultural use of biosolids while water recovery from wastewater is more concerned with water scarcity and is not perceived by stakeholders as an option to reduce eutrophication (Ramm and Smol, 2024). Moreover, the direct reuse of recovered nutrient compounds, such as struvite and ammonium salts, remains limited. Despite the availability of nutrient recovery technologies, there is a marked absence of field-scale applications and long-term studies evaluating the agronomic effectiveness, economic viability, and environmental implications of such products. This points to a critical knowledge gap that must be addressed to unlock the full potential of nutrient recycling in support of circular economy goals (Johannesdottir et al., 2020).
In addition to municipal wastewater, industrial and maritime sectors play a key role in nutrient management. Many industrial facilities along the Baltic coast still rely on outdated treatment systems, and ports often lack adequate infrastructure for handling ship-generated waste. Upgrading these systems and integrating recycling and reuse approaches can significantly reduce direct nutrient discharges into the marine environment. For example, modernizing ship waste reception facilities and promoting closed-loop systems in industrial processes are recognized as effective strategies to mitigate nutrient inputs and enhance regional sustainability (Strizhenok et al., 2019; Vaneeckhaute and Fazli, 2020).
Despite the persistence of several institutional and technical barriers, there are notable enablers that support the advancement of nutrient removal and reuse from wastewater in the Baltic Sea region. At the policy level, EU frameworks, such as the Urban Wastewater Treatment Directive (EUR, n.d.), the Circular Economy Action Plan (European Commission, 2020), and the Fertilizing Products Regulation (European Union, 2025a), are beginning to recognize and incentivize the recovery of nutrients like P in the form of struvite and other bio-based fertilizers. These regulatory developments create pathways for market integration and promote innovation in nutrient recycling.
The main legal act in the EU regulating water reuse is Regulation (EU) 2020/741 of the European Parliament and of the Council of 25 May 2020 on minimum requirements for water reuse (European Union, 2025b). This Regulation aims to guarantee that reclaimed water is safe for agricultural or other use, thereby ensuring a high level of environmental protection and human and animal health protection, promoting the circular economy, and supporting adaptation to climate change. The most important field of water reuse in the EU is agriculture, especially water reuse for irrigation. This regulation aims to encourage and facilitate water reuse.
Regulation is closely related to the EU action plan for the circular economy and Commission recommendations to take actions promoting water reclamation and reuse for plant irrigation. However, the Regulation stresses that water reuse should be appropriate and cost-efficient. Reusing reclaimed water in agriculture is a market-driven process based on the demands and needs of agriculture in member states. The Regulation stresses the significance of strictly observing health standards concerning food hygiene of agricultural products; thus, minimum levels for water quality and monitoring are established based on reports of the Joint Research Centre, also considering international standards. As a continuation to facilitate water reuse, the EU Commission is preparing the tools to support the complete application of the rules in cooperation with member states and stakeholders. Water reuse in agriculture contributes to the implementation of the circular economy as it provides possibilities to recover nutrients, at first, phosphorus, and return them to agricultural plants, reducing the EU dependence on fertilizers and reducing the impacts of agricultural production on the aquatic environment.
A risk management approach is suggested as a major tool to control water reuse. Risk management comprises the identification and management of risks in a proactive way and incorporates the concept of producing reclaimed water of a specific quality required for particular uses. Risk assessment should be based on critical elements of risk management and identify any additional water quality requirements necessary to ensure sufficient environmental, human, and animal health protection. This Regulation includes minimum requirements for reused water quality and monitoring and provisions on risk management for the safe use of reclaimed water. The minimum requirements for the safe water reuse are based on available scientific knowledge and internationally recognized water reuse standards and practices and guarantee that such water can be safely used for agricultural irrigation or elsewhere, thereby ensuring a high level of protection of the environment and human and animal health.
Minimum requirements and area of use according to Regulation are in detail elaborated for agricultural applications, including food crops that are intended for human consumption in a raw or unprocessed state, processed food crops (for example, cooked or industrially processed) as well as non-food crops (for example, pastures and forage, fiber, ornamental, seed, energy, and turf crops). Minimum requirements are determined for reclaimed water quality classes depending on the food crop use and irrigation methods.
Technological enablers also play a significant role: anaerobic digestion, membrane bioreactors, nutrient crystallization, and microalgae cultivation, provide diverse options for both centralized and decentralized systems. In parallel, funding opportunities through EU programs such as Horizon Europe, LIFE, and Interreg Baltic Sea Region (Baltic Sea Region, n.d.) have supported pilot projects, infrastructure upgrades, and regional capacity building. Another critical enabler is the increasing public and political awareness of the impacts of eutrophication, which has led to greater stakeholder engagement in circular water management practices, particularly among municipalities and agricultural sectors.
It should also be emphasized that the recast Urban Wastewater Treatment Directive in Art.15 reinforces the need to recover water from wastewater wherever possible and safe. WWTPs must therefore undergo a transformation to become centers for the recovery of nutrients and resources (Soo and Shon, 2024). The implementation of projects for recovering water from wastewater is supported by regulation (European Union, 2025b), which is implemented to varying degrees in the EU (Ramm and Smoll, 2023).
One of the prospective approaches concerning the evaluation of wastewater reuse is the life cycle assessment (LCA) of water reuse, nutrient recovery, and other impacts of the technologies. The topicality of nutrient recovery from wastewater systems is increasingly being emphasized. Several technologies exist or are under development for recycling nutrients such as nitrogen and phosphorus from wastewater to agriculture (Lam et al., 2020). For the environmental sustainability dimension, life cycle assessment can be used to assess the environmental impact potentials of wastewater-based nutrient recycling alternatives, especially nitrogen and phosphorus recycling. LCA is a framework to quantify the environmental impact potentials (benefits or burdens) of processes and products throughout their life cycle. Furthermore, the Baltic Sea region benefits from strong regional cooperation through mechanisms like HELCOM, which facilitate cross-border coordination, data harmonization, and the dissemination of best practices.
Among all the identified barriers, the most structurally limiting are (1) insufficient performance and oversight of small and decentralized WWTPs, (2) fragmented regulatory and reporting frameworks across the Baltic Sea region, and (3) the persistent dominance of diffuse agricultural emissions, which remain difficult to monitor and mitigate. At the same time, the most effective current enablers are strengthening EU-level policy framework (e.g., the recast of UWWTD, the Circular Economy Action Plan, and Regulation 2020/741) and developing technological solutions for nutrient recovery that are progressively moving from pilot scale toward wider applicability.
9 Future outlook and recommendations
Reclaimed water reuse combined with biomass and nutrients recovery from sewage sludge hold significant potential to become a central component in reducing nutrient loads to the Baltic Sea, particularly when integrated with broader circular economy and climate change adaptation strategies (Baltic Marine Environment Protection Commission, 2021; Santos et al., 2024). However, realizing this potential requires a strategic and coordinated approach across sectors and governance levels (Baltic Marine Environment Protection Commission, 2021; NurseCoast II Project, 2023). First, there is a pressing need to scale up nutrient recovery technologies—such as struvite precipitation, microalgae cultivation, and advanced anaerobic systems—especially in small and medium-sized wastewater treatment plants, which currently contribute disproportionately to nutrient emissions. Future efforts should prioritize demonstration projects and long-term field studies to evaluate the agricultural, economic, and environmental benefits of reclaimed water and recovered fertilizers under Baltic Sea conditions (Baltic Sea Region, n.d.; HELCOM, 2021b; Interreg ReNutriWater Project, 2024). Additionally, policy harmonization across the region is essential. Expanding the scope of EU directives to include smaller treatment facilities and developing regionally unified nutrient reuse guidelines would ensure consistent regulatory coverage and enhance compliance. Nutrient loading reduction efforts would increase the development of national regulations in countries around the Baltic Sea. Financial and institutional support mechanisms must also be strengthened, including targeted subsidies, green public procurement policies, and knowledge-sharing platforms that engage stakeholders from the wastewater, agriculture, and environmental protection sectors (Nordic Investment Bank, 2012). In this respect financial incentives supporting initiatives of local wastewater treatment operators could have a major positive effect.
Public awareness and risk communication remain critical for overcoming persistent concerns about the safety and acceptability of reclaimed water, especially in agriculture. Without societal support, water reuse programs cannot succeed sustainably (Kehrein et al., 2020). Studies show that rejection often comes from perceived pathogen risks and limited public knowledge on treatment quality and benefits (Saliba et al., 2018; Stenekes et al., 2006). Increasing awareness through transparent communication, demonstration sites, and evidence of safety can meaningfully shift public attitude (Nkhoma et al., 2021). Economic incentives also influence acceptance, as the nutrient content of reclaimed water can reduce fertilizer costs and motivate farmers to adopt reuse practices (Ricart and Rico, 2019). Evidence from successful cases in Israel, Singapore, and United States (EPA, 2023; Tortajada and Bindal, 2020; Ormerod and Silvia, 2017) shows that acceptance improves when communication is transparent, risk information is scientifically grounded, and reclaimed-water programs are paired with visible demonstrations, long-term safety monitoring, and clear agronomic benefits. In the Baltic Sea region, similar approaches would likely accelerate behavioral uptake and strengthen societal trust.
Several indicative targets can guide regional progress toward nutrient load reduction through wastewater reuse. By 2030, expanding nutrient-recovery technologies to at least 25–30% of small and medium-sized WWTPs in the Baltic Sea region would substantially increase circular nutrient flows. The share of reclaimed water used in agriculture could be raised to 10–15% of total treated effluent in water-scarce or nutrient-sensitive catchments, supported by harmonized regional guidelines. At the same time, achieving a 10–20% reduction in anthropogenic N and P loads relative to the 2020 baseline would require broader reuse implementation and strengthened diffuse-source mitigation. By 2040, full compliance of all WWTPs with advanced nutrient removal and systematic integration of nutrient recovery into plant operation should represent a realistic long-term objective.
10 Conclusion
Reclaimed water reuse is a substantial opportunity to effectively reduce nutrient pollution in the Baltic Sea, addressing persistent eutrophication challenges while simultaneously promoting sustainability through the circular economy and climate change adaptation. Advanced nutrient recovery technologies demonstrate significant potential to transform wastewater treatment from pollutant removal to active nutrient recovery. Furthermore, there is a need to transform WWTPs to bio-factories producing water, biomass, nutrients, energy, etc. This transition can be phased through upgrading existing wastewater treatment lines to recovery-ready configurations, integrating nutrient crystallization or biogas systems, and ultimately developing fully integrated circular resource hubs within WWTPs. At the same time, practical implementation will remain constrained by financial capacity, regulatory fragmentation, and heterogenous wastewater infrastructure, requiring gradual and context-specific deployment across the Baltic Sea region.
There is a clear need for greater harmonization of policies with respect to water resource management and wastewater treatment regulation across Baltic Sea countries. Harmonizing regulatory requirements, including extending EU directives to smaller municipalities and creating regionally unified nutrient-reuse guidelines, remains essential for closing governance gaps and ensuring consistent implementation.
Scaling nutrient-recovery solutions will depend on enabling conditions such as targeted financial incentives, stable institutional frameworks, and coordinated knowledge-sharing platforms that reduce economic and administrative barriers.
Public acceptance remains a critical determinant of uptake; communication strategies that emphasize demonstrated safety, co-benefits, and successful early case studies have proven effective in increasing consumer and stakeholder confidence.
Finally, the Baltic Sea region should continue leveraging its tradition of regional collaboration through HELCOM and other transnational frameworks to align national strategies, pool resources, and monitor impacts in a coordinated manner. Sustained coordination through HELCOM remains essential for long-term accountability and coherent nutrient-load reduction across the Baltic Sea region.
Author contributions
JK: Conceptualization, Methodology, Data curation, Validation, Investigation, Writing – review & editing, Software, Visualization, Formal analysis, Writing – original draft, Resources. KR: Validation, Writing – review & editing, Writing – original draft. OP: Resources, Validation, Project administration, Conceptualization, Methodology, Writing – review & editing, Writing – original draft, Funding acquisition. MM: Writing – original draft, Writing – review & editing, Investigation, Validation. MK: Supervision, Writing – original draft, Investigation, Conceptualization, Resources, Validation, Project administration, Writing – review & editing.
Funding
The author(s) declared that financial support was received for this work and/or its publication. This research was supported by the European Union’s Interreg Baltic Sea Region Programme 2021–2027 under Priority 2: Water-smart societies, Objective 2.1: Sustainable waters. Project No. #C016 “Closing local water circuits by recirculating nutrients and water and using them in nature (ReNutriWater)” as well as Fund for PhD study support of University of Latvia.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Generative AI statement
The author(s) declared that Generative AI was not used in the creation of this manuscript.
Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
References
Abdoli, S., Asgari Lajayer, B., Dehghanian, Z., Bagheri, N., Vafaei, A. H., Chamani, M., et al. (2024). A review of the efficiency of phosphorus removal and recovery from wastewater by physicochemical and biological processes: challenges and opportunities. Water 16:2507. doi: 10.3390/w16172507
Ahmed, S., Mofijur, M., Parisa, T., Islam, N., Kusumo, F., et al. (2021). Progress and challenges of contaminate removal from wastewater using microalgae biomass. Chemosphere 286:131656. doi: 10.1016/j.chemosphere.2021.131656
Amara, N., Chukwuemeka, E., Obiajulu, N., and Chukwuma, O. (2023). Yeast-driven valorization of agro-industrial wastewater: an overview. Environ. Monit. Assess. 195, 1252–1226. doi: 10.1007/s10661-023-11863-w,
Andersen, J., Carstensen, J., Conley, D., Dromph, K., Fleming-Lehtinen, V., et al. (2017). Long-term temporal and spatial trends in eutrophication status of the Baltic Sea. Biol. Rev. 92, 135–149. doi: 10.1111/brv.12221
Andersen, J., Fossing, H., Hansen, J., Manscher, O., Murray, C., et al. (2014). Nitrogen inputs from agriculture: towards better assessments of eutrophication status in marine waters. Ambio 43, 906–913. doi: 10.1007/s13280-014-0514-y,
Andersson, L. (1996). Trends in nutrients and oxygen concentrations in the Skagerrak-Kattegat. J. Sea Res. 35, 63–71. doi: 10.1016/s1385-1101(96)90735-2
Andersson, J., Bastviken, K., and Tonderski, K. (2005). Free water surface wetlands for wastewater treatment in Sweden: nitrogen and phosphorus removal. Water Sci. Technol. 51, 39–46. doi: 10.2166/wst.2005.0283,
Aronsson, H., Hansen, E., Thomsen, I., Liu, J., Ogaard, A., et al. (2016). The ability of cover crops to reduce nitrogen and phosphorus losses from arable land in southern Scandinavia and Finland. J. Soil Water Conserv. 71, 41–55. doi: 10.2489/jswc.71.1.41
Ausmeel, M., Liira, M., Paiste, P., Lepland, A., and Suuroja, S. (2024). Phosphorus fractions and their vertical distribution in seabed sediments of the eastern Baltic Sea. Cont. Shelf Res. 282:105340. doi: 10.1016/j.csr.2024.105340
Backer, H., Leppänen, J., Brusendorff, A., Forsius, K., Stankiewicz, M., Leppänen, J. M., et al. (2010). HELCOM Baltic Sea action plan – a regional programme of measures for the marine environment based on the ecosystem approach. Mar. Pollut. Bull. 60, 642–649. doi: 10.1016/j.marpolbul.2009.11.016,
Baltic Marine Environment Protection Commission (2021). Baltic Sea regional nutrient recycling strategy. Available online at: https://helcom.fi/wp-content/uploads/2021/10/Baltic-Sea-Regional-Nutrient-Recycling-Strategy.pdf (Accessed June 5, 2025).
Baltic Sea Region Interreg Baltic Sea region project list. Available online at: https://interreg-baltic.eu/projects/ (Accessed June 5, 2025).
Behrendt, H., and Bachor, A. (1998). Point and diffuse load of nutrients to the Baltic Sea by river basins of Northeast Germany (Mecklenburg-Vorpommern). Water Sci. Technol. 38, 147–155. doi: 10.1016/s0273-1223(98)00744-6
Bendtsen, J., Gustafsson, K., Söderkvist, J., and Hansen, J. (2009). Ventilation of bottom water in the North Sea–Baltic Sea transition zone. J. Mar. Syst. 75, 138–149. doi: 10.1016/j.jmarsys.2008.08.006
Bixio, D., Thoeye, C., De Koning, J., Joksimovic, D., Savic, D., Wintgens, T., et al. (2006). Wastewater reuse in Europe. Desalination 187, 89–101. doi: 10.1016/j.desal.2005.04.070
Bleich, S., Powilleit, M., Seifert, T., and Graf, G. (2011). β-Diversity as a measure of species turnover along the salinity gradient in the Baltic Sea, and its consistency with the Venice system. Mar. Ecol. Prog. Ser. 436, 101–118. doi: 10.3354/meps09219
Bogalecka, M., and Pigłowski, M. (2024). An attempt to assess nutrients emissions from fertilisers on eutrophication in the Baltic Sea coastal zone. Rocz. Bezp. Morsk. 18, 348–388. doi: 10.5604/01.3001.0054.8291
Bôto, M., Dias, S., Crespo, R., Mucha, A., and Almeida, C. (2022). Removing chemical and biological pollutants from swine wastewater through constructed wetlands aiming reclaimed water reuse. J. Environ. Manag. 326:116642. doi: 10.1016/j.jenvman.2022.116642,
Capell, R., Bartosova, A., Tonderski, K., Arheimer, B., Pedersen, S., Pedersen, S. M., et al. (2021). From local measures to regional impacts: modelling changes in nutrient loads to the Baltic Sea. J. Hydrol. Reg. Stud. 36:100867. doi: 10.1016/j.ejrh.2021.100867
Carpanez, T., Silva, J., Otenio, M., Amaral, M., and Moreira, V. (2024). Potential for nutrients reuse, carbon sequestration, and CO2 emissions reduction in the practice of domestic and industrial wastewater recycling into agricultural soils: a review. J. Environ. Manag. 370:122443. doi: 10.1016/j.jenvman.2024.122443,
Carstensen, J., Andersen, J., Gustafsson, B., and Conley, D. (2014). Deoxygenation of the Baltic Sea during the last century. Proc. Natl. Acad. Sci. USA 111, 5628–5633. doi: 10.1073/pnas.1323156111,
Chen, B., Li, Y., Luo, Z., Lei, M., Li, J., and Zhang, X. (2024). Enhanced phosphorus removal from biological secondary effluent using denitrifying phosphorus-removal granular sludge. J. Water Process. Eng. 57:104584. doi: 10.1016/j.jwpe.2023.104584
Chen, W., Oldfield, T., Patsios, S., and Holden, N. (2019). Hybrid life cycle assessment of agro-industrial wastewater valorisation. Water Res. 170:115275. doi: 10.1016/j.watres.2019.115275
Chowdhury, R., Moore, G., Weatherley, A., and Arora, M. (2017). Key sustainability challenges for the global phosphorus resource, their implications for global food security, and options for mitigation. J. Clean. Prod. 140, 945–963. doi: 10.1016/j.jclepro.2016.07.012
Conley, D., Björck, S., Bonsdorff, E., Carstensen, J., Destouni, G., Conley, D. J., et al. (2009). Hypoxia-related processes in the Baltic Sea. Environ. Sci. Technol. 43, 3412–3420. doi: 10.1021/es802762a,
Dalgaard, T., Hansen, B., Hasler, B., Hertel, O., Hutchings, N., Hutchings, N. J., et al. (2014). Policies for agricultural nitrogen management—trends, challenges and prospects for improved efficiency in Denmark. Environ. Res. Lett. 9:115002. doi: 10.1088/1748-9326/9/11/115002
Derco, J., Gotvajn, Ž., Guľašová, P., Kassai, A., and Šoltýsová, N. (2024). Nutrient removal and recovery from municipal wastewater. PRO 12:894. doi: 10.3390/pr12050894
Dessirier, B., Blicher-Mathiesen, G., Andersen, H., Gustafsson, B., Müller-Karulis, B., Andersen, H. E., et al. (2023). A century of nitrogen dynamics in agricultural watersheds of Denmark. Environ. Res. Lett. 18:104018. doi: 10.1088/1748-9326/acf86e
Djodjic, F., Golovko, O., Kumblad, L., Rydin, E., Sandström, S., and Widén-Nilsson, E. (2025). Usual suspects meet mission impossible: nutrient losses and effects of mitigation measures on a coastal catchment in the Baltic Sea region. Ambio 54, 1026–1042. doi: 10.1007/s13280-025-02132-w,
Dodds, W., and Smith, V. (2016). Nitrogen, phosphorus, and eutrophication in streams. Inland Waters 6, 155–164. doi: 10.5268/iw-6.2.909
EPA (2023). “United States Environmental Protection Agency” in From water stressed to water secure: Lessons from Israel’s water reuse approach. 2022 U.S. delegation summary. WRAP action 11.1.
Eriksson, E., Auffarth, K., Henze, M., and Ledin, A. (2002). Characteristics of grey wastewater. Urban Water 4, 85–104. doi: 10.1016/s1462-0758(01)00064-4
EUR Directive (EU) 2024/3019 of the European Parliament and of the council of 27 November 2024 concerning urban wastewater treatment. Available online at: http://data.europa.eu/eli/dir/2024/3019/oj (Accessed June 3, 2025).
European Commission (2020). A new circular economy action plan for a cleaner and more competitive Europe. Available online at: https://eur-lex.europa.eu/legal-content/EN/TXT/?qid=1583933814386&uri=COM:2020:98:FIN (Accessed June 5, 2025).
European Commission (2021). Report from the commission to the council and the European Parliament on the implementation of council directive 91/676/EEC concerning the protection of waters against pollution caused by nitrates from agricultural sources based on member state reports from the period 2016–2019. Available online at: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:52021DC1000 (Accessed June 20, 2025).
European Commission (2022). Proposal for a directive of the European Parliament and the council concerning urban wastewater treatment (recast). Available online at: https://environment.ec.europa.eu/system/files/2022-10/Impact%20assessment%20accompanying%20the%20proposal.pdf (Accessed June 20, 2025).
European Union Regulation (EU) 2019/1009 of the European Parliament and of the council of 5 June 2019 laying down rules on the making available on the market of EU fertilizing products and amending regulations (EC) no 1069/2009 and (EC) no 1107/2009 and repealing regulation (EC) no 2003/2003. (2025a) Available online at: https://eur-lex.europa.eu/eli/reg/2019/1009/oj/eng (Accessed June 5, 2025).
European Union Regulation (EU) 2020/741 of the European Parliament and of the council of 25 may 2020 on minimum requirements for water reuse. (2025b) Available online at: https://eur-lex.europa.eu/eli/reg/2020/741/oj/eng (Accessed June 15, 2025).
Falconer, I., and Humpage, A. (2005). Health risk assessment of cyanobacterial (blue-green algal) toxins in drinking water. Int. J. Environ. Res. Public Health 2, 43–50. doi: 10.3390/ijerph2005010043,
Fallahi, A., Rezvani, F., Asgharnejad, H., Nazloo, E., Hajinajaf, N., et al. (2021). Interactions of microalgae-bacteria consortia for nutrient removal from wastewater: a review. Chemosphere 272:129878. doi: 10.1016/j.chemosphere.2021.129878
Farrimond, M., and Upton, J. (1993). A strategy to meet the nutrient (N and P) standards of the urban wastewater directive. Water Sci. Technol. 27, 297–306. doi: 10.2166/wst.1993.0509
Fito, J., and Van Hulle, S. W. (2021). Wastewater reclamation and reuse potentials in agriculture: towards environmental sustainability. Environ. Dev. Sustain. 23, 2949–2972. doi: 10.1007/s10668-020-00732-y
Fribourg-Blanc, B., Dhuygelaere, N., Berland, J.M., and Imbert, F-X. (2024). 12th technical assesment of UWWTD implementation. Publications Office of the European Union: Luxembourg.
Gharbia, A., Zákányi, B., and Tóth, M. (2024). Impact of sand media continuous drying and rewetting cyclic on nutrients transformation performance from reclaimed wastewater effluent at soil aquifer treatment. Sci. Rep. 14:8065. doi: 10.1038/s41598-024-58787-0,
Grizzetti, B., Vigiak, O., Udias, A., Aloe, A., Zanni, M., Bouraoui, F., et al. (2021). How EU policies could reduce nutrient pollution in European inland and coastal waters. Glob. Environ. Change 69:102281. doi: 10.1016/j.gloenvcha.2021.102281,
Guterstam, B. (1996). Demonstrating ecological engineering for wastewater treatment in a Nordic climate using aquaculture principles in a greenhouse mesocosm. Ecol. Eng. 6, 73–97. doi: 10.1016/0925-8574(95)00052-6
Haddaway, N., Johannesdottir, S., Piniewski, M., and Macura, B. (2019). What ecotechnologies exist for recycling carbon and nutrients from domestic wastewater? A systematic map protocol. Environ. Evid. 8, 1–7. doi: 10.1186/s13750-018-0145-z
Häder, D., Banaszak, A., Villafañe, V., Villafañe, V., Narvarte, M., Häder, D.-P., et al. (2020). Anthropogenic pollution of aquatic ecosystems: emerging problems with global implications. Sci. Total Environ. 713:136586. doi: 10.1016/j.scitotenv.2020.136586,
Hägg, H. E., Lyon, S. W., Wällstedt, T., Mörth, C. M., Claremar, B., and Humborg, C. (2013). Future nutrient load scenarios for the Baltic Sea due to climate and lifestyle changes. Ambio 43, 337–351. doi: 10.1007/s13280-013-0416-4,
Haid, V., Stanev, E., Pein, J., Staneva, J., and Chen, W. (2020). Secondary circulation in shallow ocean straits: observations and numerical modeling of the Danish straits. Ocean Model 148:101585. doi: 10.1016/j.ocemod.2020.101585
Håkanson, L., and Bryhn, A. C. (2010). “Controlling eutrophication in the Baltic Sea and the Kattegat” in Eutrophication: Causes, consequences and control. eds. A. Ansari, S. Singh Gill, G. Lanza, and W. Rast (Dordrecht: Springer), 17–67.
Hasan, M., Altaf, M., Khan, N., Khan, A., Khan, A., Hasan, M. N., et al. (2021). Recent technologies for nutrient removal and recovery from wastewaters: a review. Chemosphere 277:130328. doi: 10.1016/j.chemosphere.2021.130328,
HELCOM (2021a). Background information on the Baltic Sea catchment area for the seventh Baltic Sea pollution load compilation (PLC-7). Monitoring assesment. Available online at: https://helcom.fi/wp-content/uploads/2021/12/Background-information-on-the-Baltic-Sea-catchment-area-for-the-Seventh-Baltic-Sea-Pollution-load-compilation-PLC-7-211202.pdf (Accessed June 3, 2025).
HELCOM (2021b). Palette of solutions for nutrient recycling in the Baltic Sea region. Available online at: https://helcom.fi/wp-content/uploads/2021/07/Palette-of-Solutions-for-Nutrient-Recycling-in-the-BSR.pdf (Accessed June 5, 2025).
HELCOM (2023). BSEFS on waterborne nitrogen and phosphorus inputs and water flow to the Baltic Sea between 1995 and 2022. Available online at: https://helcom.fi/wp-content/uploads/2024/10/BSEFS-on-Waterborne-nitrogen-and-phosphorus-inputs-and-water-flow-to-the-Baltic-Sea-1995-2022.pdf (Accessed June 3, 2025).
HELCOM. Threshold values. Maximum allowable annual inputs (MAI) of nitrogen and phosphorus to the Baltic Sea sub-basins. (2025a). Available online at: https://indicators.helcom.fi/indicator/inputs-of-nutrients/ (Accessed May 30, 2025).
HELCOM (2025b). National nutrient input ceilings. Available online at: https://helcom.fi/baltic-sea-action-plan/nutrient-reduction-scheme/national-nutrient-input-ceilings/ (Accessed June 3, 2025).
Hellsten, S., Dalgaard, T., Rankinen, K., Tørseth, K., Bakken, L., Bechmann, M., et al. (2019). Abating N in Nordic agriculture - policy, measures and way forward. J. Environ. Manag. 236, 674–686. doi: 10.1016/j.jenvman.2018.11.143,
Heyl, K. (2023). Reducing phosphorus input into the Baltic Sea—an assessment of the updated Baltic Sea action plan and its implementation through the common agricultural policy in Germany. Water 15:315. doi: 10.3390/w15020315
Hitzfeld, B., Höger, S., and Dietrich, D. (2000). Cyanobacterial toxins: removal during drinking water treatment, and human risk assessment. Environ. Health Perspect. 108, 113–122. doi: 10.1289/ehp.00108s1113,
Hong, B., Swaney, D., Mörth, C., Smedberg, E., Hägg, H., et al. (2012). Evaluating regional variation of net anthropogenic nitrogen and phosphorus inputs (NANI/NAPI), major drivers, nutrient retention pattern and management implications in the multinational areas of Baltic Sea basin. Ecol. Model. 227, 117–135. doi: 10.1016/j.ecolmodel.2011.12.002
Huang, W., Gong, B., Wang, Y., Lin, Z., He, L., Zhou, J., et al. (2020). Metagenomic analysis reveals enhanced nutrients removal from low C/N municipal wastewater in a pilot-scale modified AAO system coupling electrolysis. Water Res. 173:115530. doi: 10.1016/j.watres.2020.115530,
Huang, M., Li, Y., and Gu, G. (2010). Chemical composition of organic matters in domestic wastewater. Desalination 262, 36–42. doi: 10.1016/j.desal.2010.05.037
Huttunen, I., Lehtonen, H., Huttunen, M., Piirainen, V., Korppoo, M., Veijalainen, N., et al. (2015). Effects of climate change and agricultural adaptation on nutrient loading from Finnish catchments to the Baltic Sea. Sci. Total Environ. 529, 168–181. doi: 10.1016/j.scitotenv.2015.05.055,
Iho, A., Valve, H., Ekholm, P., Uusitalo, R., Lehtoranta, J., Soinne, H., et al. (2023). Efficient protection of the Baltic Sea needs a revision of phosphorus metric. Ambio 52, 1389–1399. doi: 10.1007/s13280-023-01851-2,
Im, H., Nguyen, H., Jeong, D., and Jang, A. (2024). Wastewater treatment optimization utilizing polyvinyl alcohol cryogel immobilized microalgae for nutrient removal. Chemosphere 366:143426. doi: 10.1016/j.chemosphere.2024.143426,
Interreg ReNutriWater Project (2024). Methods and challenges: reclaiming water from wastewater. Available online at: https://interreg-baltic.eu/project-posts/renutriwater/methods-and-challenges-how-to-reclaim-water-from-wastewater/ (Accessed June 5, 2025).
Izmaylov, A., Popov, V., Briukhanov, A., Kondratyev, S., Oblomkova, N., and Grevtsov, O. (2022). Quantification of nitrogen and phosphorus inputs from farming activities into the water bodies in the Leningrad and Kaliningrad regions. Environ. Monit. Assess. 194:508. doi: 10.1007/s10661-022-10155-z,
Jetoo, S., and Tynkkynen, N. (2021). Institutional change and the implementation of the ecosystem approach: a case study of HELCOM and the Baltic Sea action plan (BSAP). Environments 8:83. doi: 10.3390/environments8080083
Johannesdottir, S., Macura, B., McConville, J., Lorick, D., Haddaway, N., et al. (2020). What evidence exists on ecotechnologies for recycling carbon and nutrients from domestic wastewater? A systematic map. Environ. Evid. 9, 1–14. doi: 10.1186/s13750-020-00207-7
Kehrein, P., van Loosdrecht, M., Osseweijer, P., Garfí, M., Dewulf, J., and Posada, J. (2020). A critical review of resource recovery from municipal wastewater treatment plants – market supply potentials, technologies and bottlenecks. Environ. Sci. Water Res. Technol. 6, 877–910. doi: 10.1039/c9ew00905a
Kiedrzyńska, E., Józwik, A., Kiedrzyński, M., and Zalewski, M. (2014). Hierarchy of factors exerting an impact on nutrient load of the Baltic Sea and sustainable management of its drainage basin. Mar. Poll. Bull. 88, 162–173. doi: 10.1016/j.marpolbul.2014.09.010,
Kinnunen, J., Rossi, P., Herrmann, I., Ronkanen, A., and Heiderscheidt, E. (2023). Factors affecting effluent quality in on-site wastewater treatment systems in the cold climates of Finland and Sweden. J. Clean. Prod. 404:136756. doi: 10.1016/j.jclepro.2023.136756
Knuuttila, S., Raike, A., Ekholm, P., and Kondratyev, S. (2017). Nutrient inputs into the Gulf of Finland: trends and water protection targets. J. Mar. Syst. 171, 54–64. doi: 10.1016/j.jmarsys.2016.09.008
Konkol, I., Kuligowski, K., Szafranowicz, P., Vorne, V., Reinikainen, A., Effelsberg, N., et al. (2024). Review of the seasonal wastewater challenges in Baltic coastal tourist areas: insights from the NURSECOAST-II project. Sustainability 16:9890. doi: 10.3390/su16229890
Kononen, K., Kuparinen, J., Mäkelä, K., Laanemets, J., Pavelson, J., and Nõmmann, S. (1996). Initiation of cyanobacterial blooms in a frontal region at the entrance to the Gulf of Finland, Baltic Sea. Limnol. Oceanogr. 41, 98–112. doi: 10.4319/lo.1996.41.1.0098
Koskiaho, J., Okruszko, T., Piniewski, M., Marcinkowski, P., Tattari, S., Johannesdottir, S., et al. (2020). Carbon and nutrient recycling ecotechnologies in three Baltic Sea river basins – the effectiveness in nutrient load reduction. Ecohydrol. Hydrobiol. 20, 313–322. doi: 10.1016/j.ecohyd.2020.06.001
Kou, Y., Yang, B., Jiang, J., Sun, H., Zhang, R., Li, Z., et al. (2023). Characteristics of dissolved organic matter in point-source wastewaters at a petrochemical plant: molecular constituents and contributions to the influent of wastewater treatment plant. Environ. Res. 238:117157. doi: 10.1016/j.envres.2023.117157,
Kronvanga, B., Jeppesena, E., Conleyb, D., Søndergaarda, M., Larsena, S., et al. (2005). Nutrient pressures and ecological responses to nutrient loading reductions in Danish streams, lakes and coastal waters. J. Hydrol. 304, 274–288. doi: 10.1016/j.jhydrol.2004.07.035
Kurniawan, S., Ahmad, A., Imron, M., Abdullah, S., Othman, A., Kurniawan, S. B., et al. (2025). Achieving a biocircular economy in the aquaculture sector through waste valorization. Toxics 13:131. doi: 10.3390/toxics13020131,
Kuss, J., Nausch, G., Engelke, C., Von Weber, M., Lutterbeck, H., et al. (2020). Changes of nutrient concentrations in the western Baltic Sea in the transition between inner coastal waters and the central basins: time series from 1995 to 2016 with source analysis. Front. Earth Sci. 8:00106. doi: 10.3389/feart.2020.00106
Lam, Q., Schmalz, B., and Fohrer, N. (2010). Modelling point and diffuse source pollution of nitrate in a rural lowland catchment using the SWAT model. Agric. Water Manag. 97, 317–325. doi: 10.1016/j.agwat.2009.10.004
Lam, Q., Schmalz, B., and Fohrer, N. (2011). The impact of agricultural best management practices on water quality in a north German lowland catchment. Environ. Monit. Assess. 183, 351–379. doi: 10.1007/s10661-011-1926-9,
Lam, K. L., Zlatanović, L., and van der Hoek, J. P. (2020). Life cycle assessment of nutrient recycling from wastewater: a critical review. Water Res. 173:115519. doi: 10.1016/j.watres.2020.115519,
Lappalainen, A., and Pönni, J. (2000). Eutrophication and recreational fishing on the Finnish coast of the Gulf of Finland: a mail survey. Fish. Manag. Ecol. 7, 323–335. doi: 10.1046/j.1365-2400.2000.007004323.x
Lappalainen, S., Tapaninen, U., and Kotta, J. (2024). Nitrogen and phosphorus discharges from cargo ships’ black and grey waters—a case study of a Baltic Sea port. Oceans 5, 560–570. doi: 10.3390/oceans5030032
Laznik, M., Stålnacke, P., Grimvall, A., and Wittgren, H. (1999). Riverine input of nutrients to the Gulf of Riga - temporal and spatial variation. J. Mar. Syst. 23, 11–25. doi: 10.1016/s0924-7963(99)00048-2
Lee, J., Antonini, G., Al-Omari, A., Muller, C., Mathew, J., Bell, K., et al. (2024). Electrochemical methods for nutrient removal in wastewater: a review of advanced electrode materials, processes, and applications. Sustainability 16:9764. doi: 10.3390/su16229764
Liu, Y., Meier, H., and Eilola, K. (2017). Nutrient transports in the Baltic Sea – results from a 30-year physical–biogeochemical reanalysis. Biogeosciences 14, 2113–2131. doi: 10.5194/BG-14-2113-2017
Macneil, J. S. (2001). Off Denmark, a drawn-out war against hypoxia. Science 291:969. doi: 10.1126/science.291.5506.969,
McCrackin, M., Gustafsson, B., Hong, B., Howarth, R., Humborg, C., McCrackin, M. L., et al. (2018). Opportunities to reduce nutrient inputs to the Baltic Sea by improving manure use efficiency in agriculture. Reg. Environ. Chang. 18, 1843–1854. doi: 10.1007/s10113-018-1308-8
McCrackin, M., Muller-Karulis, B., Gustafsson, B., Howarth, R., Humborg, C., et al. (2018). A century of legacy phosphorus dynamics in a large Drainage Basin. Glob. Biogeochem. Cycles 32, 1107–1122. doi: 10.1029/2018gb005914
Meier, H., and Andersson, H. (2012). ECOSUPPORT: a pilot study on decision support for Baltic Sea environmental management. Ambio 41, 529–533. doi: 10.1007/s13280-012-0317-y,
Moros, M., Kotilainen, A., Snowball, I., Neumann, T., Perner, K., Kotilainen, A. T., et al. (2024). Giant saltwater inflow in AD 1951 triggered Baltic Sea hypoxia. Boreas 53, 125–138. doi: 10.1111/bor.12643
Müller-Karulis, B., McCrackin, M. L., Dessirier, B., Gustafsson, B. G., and Humborg, C. (2024). Legacy nutrients in the Baltic Sea drainage basin: how past practices affect eutrophication management. J. Environ. Manag. 370:122478. doi: 10.1016/j.jenvman.2024.122478,
Murray, C., Müller-Karulis, B., Carstensen, J., Conley, D., Gustafsson, B., et al. (2019). Past, present and future eutrophication status of the Baltic Sea. Front. Mar. Sci. 6:00002. doi: 10.3389/fmars.2019.00002
Neumann, T., Siegel, H., Moros, M., Gerth, M., Kniebusch, M., and Heydebreck, D. (2020). Ventilation of the northern Baltic Sea. Ocean Sci. 16, 767–780. doi: 10.5194/os-16-767-2020
Nguyen, L., Aditya, L., Vu, H., Johir, A., Bennar, L., Nguyen, L. N., et al. (2022). Nutrient removal by algae-based wastewater treatment. Curr. Pollut. Rep. 8, 369–383. doi: 10.1007/s40726-022-00230-x
Nie, J., Feng, H., Witherell, B., Alebus, M., Mahajan, M., Witherell, B. B., et al. (2018). Causes, assessment, and treatment of nutrient (N and P) pollution in rivers, estuaries, and coastal waters. Curr. Pollut. Rep. 4, 154–161. doi: 10.1007/s40726-018-0083-y
Nishat, A., Yusuf, M., Qadir, A., Ezaier, Y., Vambol, V., Ijaz Khan, M., et al. (2023). Wastewater treatment: a short assessment on available techniques. Alex. Eng. J. 76, 505–516. doi: 10.1016/j.aej.2023.06.054
Njock, P., Zhou, A., Yin, Z., and Shen, S. (2023). Integrated risk assessment approach for eutrophication in coastal waters: case of Baltic Sea. J. Clean. Prod. 387:135673. doi: 10.1016/j.jclepro.2022.135673
Nkhoma, P. R., Alsharif, K., Ananga, E., Eduful, M., and Acheampong, M. (2021). Recycled water reuse: what factors affect public acceptance? Environ. Conserv. 48, 278–286. doi: 10.1017/S037689292100031x
Nordic Investment Bank (2012). Sanitized sewage offers relief for Baltic Sea. Nordic investment Bank report. Available online at: https://www.nib.int/articles/sanitised-sewage-offers-relief-for-baltic-sea (Accessed June 5, 2025).
Nørring, N., and Jorgensen, E. (2009). Eutrophication and agriculture in Denmark: 20 years of experience and prospects for the future. Hydrobiologia 629, 65–70. doi: 10.1007/s10750-009-9772-2
NurseCoast II Project (2023). Protecting the Baltic demands unified data. Available online at: https://interreg-baltic.eu/project/nursecoast-ii/ (Accessed June 5, 2025).
Olofsson, M., Klawonn, I., and Karlson, B. (2020). Nitrogen fixation estimates for the Baltic Sea indicate high rates for the previously overlooked Bothnian Sea. Ambio 50, 203–214. doi: 10.1007/s13280-020-01331-x,
Ormerod, K. J., and Silvia, L. (2017). Newspaper coverage of potable water recycling at Orange County Water District’s groundwater replenishment system, 2000–2016. Water 9:984. doi: 10.3390/w9120984
Pachel, K., Klõga, M., and Iital, A. (2012). Scenarios for reduction of nutrient load from point sources in Estonia. Hydrol. Res. 43, 374–382. doi: 10.2166/nh.2012.136
Pastuszak, M., Kowalkowski, T., Kopiński, J., Stalenga, J., and Panasiuk, D. (2014). Impact of forecasted changes in polish economy (2015 and 2020) on nutrient emission into the river basins. Sci. Total Environ. 493, 32–43. doi: 10.1016/j.scitotenv.2014.05.124,
Pecio, A. (2024). The share of Poland in the actual pollution status of Baltic Sea waters with nitrates in the light of HELCOM PLC research. Curr. Agron. 53, 96–115. doi: 10.2478/cag-2024-0010
Pesqueira, J., Pereira, M., and Silva, A. (2020). Environmental impact assessment of advanced urban wastewater treatment technologies for the removal of priority substances and contaminants of emerging concern: a review. J. Clean. Prod. 261:121078. doi: 10.1016/j.jclepro.2020.121078
Petersen, R., Blicher-Mathiesen, G., Rolighed, J., Andersen, H., and Kronvang, B. (2021). Three decades of regulation of agricultural nitrogen losses: experiences from the Danish agricultural monitoring program. Sci. Total Environ. 787:147619. doi: 10.1016/j.scitotenv.2021.147619,
Pihlainen, S., Zandersen, M., Hyytiäinen, K., Andersen, H., Bartošová, A., et al. (2020). Impacts of changing society and climate on nutrient loading to the Baltic Sea. Sci. Total Environ. 731:138935. doi: 10.1016/j.scitotenv.2020.138935
Piiparinen, J., Kuosa, H., and Rintala, J. (2010). Winter-time ecology in the Bothnian Bay, Baltic Sea: nutrients and algae in fast ice. Polar Biol. 33, 1445–1461. doi: 10.1007/s00300-010-0771-6
Pitkänen, H., Lehtoranta, J., and Peltonen, H. (2008). “The Gulf of Finland” in Ecology of Baltic coastal waters. Ecological studies. ed. U. Schiewer (Heidelberg: Springer), 285–308.
Põder, T., Maestrini, S., Balode, M., Lips, U., Béchemin, C., Maestrini, S. Y., et al. (2003). The role of inorganic and organic nutrients on the development of phytoplankton along a transect from the Daugava River mouth to the open Baltic, in spring and summer 1999. ICES J. Mar. Sci. 60, 827–835. doi: 10.1016/S1054-3139(03)00069-9
Puriņa, I., Labucis, A., Bārda, I., Jurgensone, I., and Aigars, J. (2018). Primary productivity in the Gulf of Riga (Baltic Sea) in relation to phytoplankton species and nutrient variability. Oceanologia 60, 544–552. doi: 10.1016/j.oceano.2018.04.005
Purmalis, O., and Klavins, M. (2025). Reclaimed wastewater reuse topicality in Baltic Sea area: perspectives and risks. IOP Conf. Ser. Earth Environ. Sci. 1474:012009. doi: 10.1088/1755-1315/1474/1/012009
Pyhälä, M. (2012). “HELCOM Baltic Sea action plan: an ecosystem approach to the Management of Human Activities” in Climate impacts on the Baltic Sea: From science to policy. eds. K. Brander, B. MacKenzie, and B. Omstedt (Heidelberg: Springer), 45–69.
Rahm, L., and Danielsson, Å. (2007). Spatial heterogeneity of nutrients in the Baltic proper, Baltic Sea. Estuar. Coast. Shelf Sci. 73, 268–278. doi: 10.1016/j.ecss.2007.01.009
Rahm, L., Håkansson, B., Larsson, P., Fogelqvist, E., Bremle, G., et al. (1995). Nutrient and persistent pollutant deposition on the Bothnian Bay ice and snow fields. Water Air Soil Pollut. 84, 187–201. doi: 10.1007/bf00479597
Räike, A., Taskinen, A., and Knuuttila, S. (2019). Nutrient export from Finnish rivers into the Baltic Sea has not decreased despite water protection measures. Ambio 49, 460–474. doi: 10.1007/s13280-019-01217-7,
Ramm, K., and Smol, M. (2024). The potential for water recovery from urban wastewater-the perspective of urban wastewater treatment plant operators in Poland. J. Environ. Manag. 358:120890. doi: 10.1016/j.jenvman.2024.120890,
Ramm, K., and Smoll, M. (2023). Water reuse – analysis of the possibility of using reclaimed water depending on the quality class in the European countries. Sustainability 15:12781. doi: 10.3390/su151712781
Raudsepp, U., Maljutenko, I., Kõuts, M., Granhag, L., Wilewska-Bien, M., Hassellöv, I. M., et al. (2019). Shipborne nutrient dynamics and impact on the eutrophication in the Baltic Sea. Sci. Total Environ. 671, 189–207. doi: 10.1016/j.scitotenv.2019.03.264,
Ricart, S., and Rico, A. M. (2019). Assessing technical and social driving factors of water reuse in agriculture: a review on risks, regulation and the yuck factor. Agric. Water Manag. 217, 426–439. doi: 10.1016/j.agwat.2019.03.017
Richards, S., Bidgood, L., Watson, H., and Stutter, M. (2022). Biogeochemical impacts of sewage effluents in predominantly rural river catchments: are point source inputs distinct to background diffuse pollution? J. Environ. Manag. 311:114891. doi: 10.1016/j.jenvman.2022.114891,
Rinne, H., Björkman, U., Sjöqvist, C., Salovius-Laurén, S., and Mattila, J. (2018). Morphological and genetic variation of Fucus in the eastern Gulf of Bothnia, northern Baltic Sea. Eur. J. Phycol. 53, 369–380. doi: 10.1080/09670262.2018.1453089
Rout, P., Shahid, M., Dash, R., Bhunia, P., Liu, D., Rout, P. R., et al. (2021). Nutrient removal from domestic wastewater: a comprehensive review on conventional and advanced technologies. J. Environ. Manag. 296:113246. doi: 10.1016/j.jenvman.2021.113246,
Rydberg, L., Ærtebjerg, G., and Edler, L. (2006). Fifty years of primary production measurements in the Baltic entrance region, trends and variability in relation to land-based input of nutrients. J. Sea Res. 56, 1–16. doi: 10.1016/j.seares.2006.03.009
Saliba, R., Callieris, R., D’Agostino, D., Roma, R., and Scardigno, A. (2018). Stakeholders’ attitude towards the reuse of treated wastewater for irrigation in Mediterranean agriculture. Agric. Water Manag. 204, 60–68. doi: 10.1016/j.agwat.2018.03.036
Saliu, T., and Oladoja, N. (2021). Nutrient recovery from wastewater and reuse in agriculture: a review. Environ. Chem. Lett. 19, 2299–2316. doi: 10.1007/s10311-020-01159-7
Santos, A. F., Mendes, L. S., Alvarenga, P., Gando-Ferreira, L. M., and Quina, M. J. (2024). Nutrient recovery via struvite precipitation from wastewater treatment plants: influence of operating parameters, coexisting ions, and seeding. Water 16:1675. doi: 10.3390/w16121675
Saraiva, S., Meier, H., Andersson, H., Höglund, A., Dieterich, C., et al. (2018). Baltic Sea ecosystem response to various nutrient load scenarios in present and future climates. Clim. Dyn. 52, 3369–3387. doi: 10.1007/s00382-018-4330-0
Sarrazin, F., Attinger, S., and Kumar, R. (2024). Gridded dataset of nitrogen and phosphorus point sources from wastewater in Germany (1950–2019). Earth Syst. Sci. Data 16, 4673–4708. doi: 10.5194/essd-16-4673-2024
Savchuk, O. (2002). Nutrient biogeochemical cycles in the Gulf of Riga: scaling up field studies with a mathematical model. J. Mar. Syst. 32, 253–280. doi: 10.1016/s0924-7963(02)00039-8
Sayin, E., and Krauss, W. (1996). A numerical study of the water exchange through the Danish straits. Oceanogr. Lit. Rev. 48, 324–341. doi: 10.3402/tellusa.v48i2.12063,
Schernewski, G., Friedland, R., Carstens, M., Hirt, U., Leujak, W., Nausch, G., et al. (2015). Implementation of European marine policy: new water quality targets for German Baltic waters. Mar. Policy 51, 305–321. doi: 10.1016/j.marpol.2014.09.002
Shortle, J., Mihelcic, J., Zhang, Q., and Arabi, M. (2020). Nutrient control in water bodies: a systems approach. J. Environ. Qual. 49, 517–533. doi: 10.1002/jeq2.20022,
Sibrell, P., Montgomery, G., Ritenour, K., and Tucker, T. (2009). Removal of phosphorus from agricultural wastewaters using adsorption media prepared from acid mine drainage sludge. Water Res. 43, 2240–2250. doi: 10.1016/j.watres.2009.02.010,
Silva, J. (2023). Wastewater treatment and reuse for sustainable water resources management: a systematic literature review. Sustainability 15:10940. doi: 10.3390/su151410940
Śniatała, B., Al-Hazmi, H. E., Sobotka, D., Zhai, J., and Mąkinia, J. (2024). Advancing sustainable wastewater management: a comprehensive review of nutrient recovery products and their applications. Sci. Total Environ. 937:173446. doi: 10.1016/j.scitotenv.2024.173446,
Solovieva, E. A. (2021). Technology for biological treatment of urban wastewater and sludge treatment with deep removal of nitrogen and phosphorus. IOP Conf. Ser. Earth Environ. Sci 677:052018. doi: 10.1088/1755-1315/677/5/052018
Soo, A., and Shon, H. K. (2024). A nutrient circular economy framework for wastewater treatment plants. Desalination 592:118090. doi: 10.1016/j.desal.2024.118090
Stakėnienė, R., Jokšas, K., Kriaučiūnienė, J., Jakimavičius, D., and Raudonytė-Svirbutavičienė, E. (2023). Nutrient loadings and exchange between the Curonian lagoon and the Baltic Sea: changes over the past two decades (2001–2020). Water 15:4096. doi: 10.3390/w15234096
Stålnacke, P., Grimvall, A., Sundblad, K., and Tonderski, A. (1999). Estimation of riverine loads of nitrogen and phosphorus to the Baltic Sea, 1970–1993. Environ. Monit. Assess. 58, 173–200. doi: 10.1023/a:1006073015871
Stålnacke, P., Vagstad, N., Tamminen, T., Wassmann, P., Jansons, V., and Loigu, E. (1999). Nutrient runoff and transfer from land and rivers to the Gulf of Riga. Hydrobiologia 410, 103–110. doi: 10.1023/a:1003736620843
Stanev, E., Pein, J., Grashorn, S., Zhang, Y., and Schrum, C. (2018). Dynamics of the Baltic Sea straits via numerical simulation of exchange flows. Ocean Model 131, 40–58. doi: 10.1016/j.ocemod.2018.08.009
Stenekes, N., Colebatch, H. K., Waite, T. D., and Ashbolt, N. J. (2006). Risk and governance in water recycling: public acceptance revisited. Sci. Technol. Hum. Values 31, 107–134. doi: 10.1177/0162243905283636
Stockholm University Baltic Sea Centre (2024). Effects of the new UWWTD on nutrient inputs to the Baltic Sea. Available online at: https://www.diva-portal.org/smash/get/diva2:1879700/FULLTEXT01.pdf (Accessed June 3, 2025).
Strizhenok, A., Korelskiy, D., and Kuznetsov, V. (2019). The wastewater disposal system modernization during processing of Amber deposit as a way to reduce the anthropogenic load on the Baltic Sea ecosystem. J. Ecol. Eng. 20, 30–35. doi: 10.12911/22998993/99731
Sun, C., Wang, S., Wang, H., Hu, X., Yang, F., Tang, M., et al. (2022). Internal nitrogen and phosphorus loading in a seasonally stratified reservoir: implications for eutrophication management of deep-water ecosystems. J. Environ. Manag. 319:115681. doi: 10.1016/j.jenvman.2022.115681,
Tamminen, T., and Seppälä, J. (1999). Nutrient pools, transformations, ratios, and limitation in the Gulf of Riga, the Baltic Sea, during four successional stages. J. Mar. Syst. 23, 83–106. doi: 10.1016/S0924-7963(99)00052-4
Tortajada, C., and Bindal, I. (2020). “Water reuse in Singapore: the new frontier in a framework of a circular economy?” in Water reuse within a circular economy context. eds. E. Shin, S. H. Choi, A. Makarigakis, O. Sohn, C. Clench, and M. Trudeau (Seoul: UNESCO), 55–67.
Ugwuanyi, E., Queen, Z., Nwokediegwu, S., Dada, M., Majemite, M., et al. (2024). Review of emerging technologies for nutrient removal in wastewater treatment. World J. Adv. Res. Rev. 21, 1737–1749. doi: 10.30574/wjarr.2024.21.2.0520
Vaneeckhaute, C., and Fazli, A. (2020). Management of ship-generated food waste and sewage on the Baltic Sea: a review. Waste Manag. 102, 12–20. doi: 10.1016/j.wasman.2019.10.030
Vergine, P., Lonigro, A., Salerno, C., Rubino, P., Berardi, G., and Pollice, A. (2017). Nutrient recovery and crop yield enhancement in irrigation with reclaimed wastewater: a case study. Urban Water J. 14, 325–330. doi: 10.1080/1573062x.2016.1141224
Viitasalo, M., and Bonsdorff, E. (2022). Global climate change and the Baltic Sea ecosystem: direct and indirect effects on species, communities and ecosystem functioning. Earth Syst. Dynam. 13, 711–747. doi: 10.5194/esd-13-711-2022
Vo, P. T., Ngo, H. H., Guo, W., Zhou, J. L., Nguyen, P. D., Tram VO, P., et al. (2014). A mini-review on the impacts of climate change on wastewater reclamation and reuse. Sci. Total Environ. 494, 9–17. doi: 10.1016/j.scitotenv.2014.06.090,
Walve, J., Sandberg, M., Larsson, U., and Lännergren, C. (2018). A Baltic Sea estuary as a phosphorus source and sink after drastic load reduction: seasonal and long-term mass balances for the Stockholm inner archipelago for 1968–2015. Biogeosciences 15, 3003–3025. doi: 10.5194/bg-15-3003-2018
Wang, J., Zhang, T., Dao, G., Xu, X., Wang, X., Wang, J. H., et al. (2017). Microalgae-based advanced municipal wastewater treatment for reuse in water bodies. Appl. Microbiol. Biotechnol. 101, 2659–2675. doi: 10.1007/s00253-017-8184-x,
Wei, X., Viadero, R., and Bhojappa, S. (2008). Phosphorus removal by acid mine drainage sludge from secondary effluents of municipal wastewater treatment plants. Water Res. 42, 3275–3284. doi: 10.1016/j.watres.2008.04.005,
Weidner, E., Stranne, C., Sundberg, J., Weber, T., Mayer, L., Sundberg, J. H., et al. (2020). Tracking the spatiotemporal variability of the oxic–anoxic interface in the Baltic Sea with broadband acoustics. ICES J. Mar. Sci. 77, 2814–2824. doi: 10.1093/icesjms/fsaa153
Werner, W., and Wodsak, H. (1994). Germany: the Baltic Sea and its agricultural environmental status. Mar. Poll. Bull. 29, 471–476. doi: 10.1016/0025-326X(94)90673-4
WISE Freshwater (2024). Freshwater information system for Europe. Countries. Available online at: https://water.europa.eu/freshwater/countries (Accessed June 20, 2025).
Wojciechowska, E., Pietrzak, S., Matej-Łukowicz, K., Nawrot, N., Zima, P., Kalinowska, D., et al. (2019). Nutrient loss from three small-size watersheds in the southern Baltic Sea in relation to agricultural practices and policy. J. Environ. Manag. 252:109637. doi: 10.1016/j.jenvman.2019.109637,
Wu, J., Franzen, D., and Malmström, M. (2016). Anthropogenic phosphorus flows under different scenarios for the city of Stockholm, Sweden. Sci. Total Environ. 542, 1094–1105. doi: 10.1016/j.scitotenv.2015.09.024,
Wulff, F., Humborg, C., Andersen, H., Blicher-Mathiesen, G., Czajkowski, M., Andersen, H. E., et al. (2014). Reduction of Baltic Sea nutrient inputs and allocation of abatement costs within the Baltic Sea catchment. Ambio 43, 11–25. doi: 10.1007/s13280-013-0484-5,
Wurtsbaugh, W., Paerl, H., and Dodds, W. (2019). Nutrients, eutrophication and harmful algal blooms along the freshwater to marine continuum. WIREs Water 6:e1373. doi: 10.1002/wat2.1373
Yadav, A., Rene, E., Sharma, M., Jatain, I., Mandal, M., Rene, E. R., et al. (2022). Valorization of wastewater to recover value-added products: a comprehensive insight and perspective on different technologies. Environ. Res. 214:113957. doi: 10.1016/j.envres.2022.113957,
Yang, S., Büttner, O., Kumar, R., Jäger, C., Jawitz, J., Jawitz, J. W., et al. (2019). Spatial patterns of water quality impairments from point source nutrient loads in Germany's largest national river basin (Weser River). Sci. Total Environ. 697:134145. doi: 10.1016/j.scitotenv.2019.134145
Ye, Y., Ngo, H., Guo, W., Chang, S., Nguyen, D., Ngo, H. H., et al. (2020). Nutrient recovery from wastewater: from technology to economy. Bioresour. Technol. Rep. 11:100425. doi: 10.1016/j.biteb.2020.100425
Ylöstalo, P., Seppälä, J., Kaitala, S., Maunula, P., and Simis, S. (2016). Loadings of dissolved organic matter and nutrients from the Neva River into the Gulf of Finland – biogeochemical composition and spatial distribution within the salinity gradient. Mar. Chem. 186, 58–71. doi: 10.1016/j.marchem.2016.07.004
Keywords: Baltic Sea eutrophication, environmental governance, nitrogen recovery, nutrient pollution, phosphorus recovery, wastewater treatment
Citation: Krumins J, Ramm K, Purmalis O, Mezulis M and Klavins M (2026) Reclaimed water reuse as a tool to reduce nutrient loads in the Baltic Sea. Front. Water. 7:1676213. doi: 10.3389/frwa.2025.1676213
Edited by:
Reza Kerachian, University of Tehran, IranReviewed by:
Justus Van Beusekom, Helmholtz Centre for Materials and Coastal Research (HZG), GermanyMirela Alina Sandu, University of Agronomic Sciences and Veterinary Medicine, Romania
Copyright © 2026 Krumins, Ramm, Purmalis, Mezulis and Klavins. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Janis Krumins, a3J1bWlucy5qYW5pc0BsdS5sdg==