Abstract
Introduction:
The sustainable recovery and reuse of nutrients from aquaculture waste represent a promising approach to reducing the environmental impact of fish farming while supporting cost-effective microalgal cultivation.
Materials and methods:
This study investigated the potential of aquaculture sedimented sludge as a nutrient source for cultivating Arthrospira platensis. Chemical and thermochemical treatments were applied to enhance the solubilization of essential nutrients, particularly phosphorus.
Results and discussion:
Among the tested reagents, 1 M HCl and 2 M NaOH resulted in the highest phosphate release, with increasing sludge dosage up to 20 g L-1 further improving solubilization efficiency. Thermochemical treatments combining ultrasound or autoclaving with chemical conditioning significantly enhanced nutrient recovery, with the acid–autoclave (AUA) and autoclaved water (AUW) treatments achieving the highest phosphate concentrations (270.6 and 210.8 mg L-1, respectively). Recovered nutrient extracts were subsequently tested as culture media for A. platensis, with six treatments evaluated (acidic, alkaline, and water-based extracts with ultrasound and autoclaving) compared to a standard synthetic medium (Spirulina SAG). The AUW extract supported robust biomass accumulation, comparable to that in SAG medium, when supplemented with 2 g L-1 KNO3 and 10 g L-1 NaHCO3. Further optimization of nitrogen supplementation (0–2 g L-1 KNO3) identified 1 g L-1 KNO3 as optimal, yielding a biomass of 1.45 g L 1 and a protein content of 559.3 ± 21.6 mg L-1. Phycobiliprotein yields (C-phycocyanin: 25.12 mg g-1; allophycocyanin: 6.25 mg g-1) were comparable to those in synthetic medium. Nutrient analysis revealed substantial reductions in 1 g L-1 KNO3 medium: PO43- (90.90%), NH4+ (99.2%), NO3- (68.2%), and K+ (79.7%) during cultivation, indicating efficient nutrient assimilation by A. platensis. This study demonstrates a novel, sustainable approach to transforming aquaculture sedimented sludge into an effective culture medium for A. platensis, thereby reducing reliance on synthetic nutrients and advancing circular bioresource utilization in aquaculture systems.
1 Introduction
Aquaculture is the fastest-growing food production sector worldwide, supplying an increasing proportion of global fish consumption as demand for protein rises, thereby significantly increasing its contribution to food security, income, and trade (Verdegem et al., 2023). However, traditional pond-based aquaculture, which contributes over 50% of global aquaculture production volume, faces significant climate-related production risks, including temperature extremes, drought, eutrophication events, and oxygen depletion, leading to production losses and reduced reliability in many regions (). These climatic vulnerabilities, along with water scarcity and deteriorating water quality, pose serious challenges to extensive pond systems, which depend on ambient environmental conditions.
Recirculating aquaculture systems (RAS) are emerging as a climate-resilient alternative. RAS operate in controlled environments and recycle water internally, reducing the need for freshwater inputs and lowering susceptibility to external climatic variability such as drought, floods, and temperature fluctuations (). The ability to decouple fish production from ambient water bodies reduces vulnerability to thermal stress and pollution events that frequently affect open-pond farms. By capturing and treating waste streams on-site, RAS also offers opportunities for nutrient recycling and reduced eutrophication risk compared to traditional systems (; ). In conventional ponds or cage systems, approximately 79% of feed nitrogen and 83% of feed phosphorus are lost to the surrounding environment (), contributing significantly to eutrophication and aquatic pollution. By contrast, RAS capture and treat wastes on-site and substantially reduce environmental discharge (; ; ).
Despite this advantage, the adoption of RAS remains limited due to high initial investment costs, energy requirements for system operation, and other economic constraints. Paradoxically, one of the key advantages of RAS, i.e. its reduced impact on eutrophication, becomes an economic challenge. Sludge management, the byproduct of nutrient retention, accounts for approximately 2% of total production costs (Mannina and Mineo 2024). Growing RAS inevitably generates large volumes of sludge. For example, in 2017 alone, Norway’s salmon and rainbow trout production reached 1.3 million tons, generating approximately 2.1 million tons of sludge (). Projections indicate that by 2050, the country’s sludge output could rise to as much as 11 million tons (). Since RAS facilities are often located far from nutrient valorization companies, transporting sludge over long distances requires additional dewatering and drying, further increasing costs.
This sludge contains phosphorus, nitrogen, and trace elements that are only partially assimilated by fish and concentrated due to the enclosed water circulation (; , ). Phosphorus, in particular, is a critical nutrient with finite global reserves and is essential for both aquaculture productivity and broader agricultural systems. Recovering phosphorus from waste streams aligns with circular economy goals and helps close nutrient loops that would otherwise contribute to environmental degradation.
A major challenge in utilizing sludge, e.g., for micro- or macroalgae production, is that the nutrients it contains are not readily available and must be mineralized before use as fertilizer. Previous work examined the chemical and ultrasound recovery of phosphorus from RAS sludge, demonstrating that significant fractions of phosphorus can be solubilized and precipitated for reuse (e.g., via acid redissolution techniques) (; ; ). However, the efficient transformation of sludge into biologically available nutrients for productive reuse remains a research challenge. Thermal disintegration methods, such as autoclaving or hydrothermal treatment, can break down organic matrices and liberate bound nutrients, and are widely used in other sludge treatment contexts (e.g., sewage sludge thermal processes) (Zhu et al., 2022). This disrupts cell walls and floc structures in sludge, releasing intracellularly bound and organically complexed nutrients particularly phosphorus and nitrogen into the soluble phase (). This solubilization is necessary because microalgae can only assimilate dissolved inorganic nutrients; organically bound or mineral-precipitated forms are largely unavailable for direct uptake (). Yet their application to aquaculture sedimented sludge and subsequent use in algal cultivation has not been fully explored.
In addition, any disintegration method must be economically viable, and the recovered nutrients must be competitive with mineral-based phosphorus and nitrogen fertilizers. This study evaluated the effects of sludge dose, pH, and thermal energy input on nutrient recovery from RAS sludge. These factors are considered the main drivers for the economic recovery of phosphorus and nitrogen. The recovered nutrients were subsequently tested for their suitability as fertilizers for microalgae cultivation, providing a potential pathway for sustainable nutrient recycling in aquaculture systems. A preliminary look into the economics and sustainability of sludge-based nutrient recovery and application as a microalgae fertilizer is provided.
2 Materials and methods
2.1 Sludge collection and composition
Aquaculture sludge was obtained from a commercial recirculating aquaculture system (RAS) producing Nile tilapia (Oreochromis niloticus). The RAS is located at the Acheron GmbH in Bremen, Germany (53° 6.7099’ N, 8° 48.6344’ E). The system consisted of five cylindrical tanks, each with a working volume of 2 m3, stocked at an average density of 63 kg m−3. Fish were fed daily with a commercial floating feed (Aller Primo Float, 4 mm) at a rate of 0.68% of total fish biomass.
Sludge was collected after drum filtration (Tiny Drum L4, Spranger) and stored in 50 L drums for 24 h, during which the sludge separated into two fractions: a floating layer consisting of light organic particles (uneaten feed, lipids) and a sedimented layer of denser fecal solids. Subsequently, the sludge was transferred to a 100 L conical container for additional settling. The sedimented layer was collected and homogenized for experimental use.
Untreated, homogenized sludge was analyzed for physicochemical and elemental properties. Total solids (TS) were determined by drying 25 g of sludge in 90 mL ceramic crucibles, covered with aluminum foil to prevent loss of particulate matter during boiling. Samples were dried at 105 °C in a drying oven (Model 400, Memmert GmbH + Co. KG, Schwabach, Germany) until constant weight was achieved, then weighed. Volatile solids (VS) were measured by incinerating the dried samples at 550 °C for two hours in a furnace (Nabertherm Controller P 320) and calculating the mass loss.
Electrical conductivity (EC) and pH were measured using a multiparameter meter (HQ1110, Hach-Lange GmbH, Dusseldorf, Germany), which was calibrated according to the manufacturer’s instructions prior to analysis. Total nitrogen (TN) was determined in accordance with DIN 51732:2014-07 (Testing of solid mineral fuels – Determination of total carbon, hydrogen and nitrogen – Instrumental methods), adapted for dried aquaculture sedimented sludge samples. Elemental composition was analyzed following DIN EN 16171:2017-01, using microwave-assisted acid digestion and inductively coupled plasma mass spectrometry (ICP-MS; Agilent MS 7850).
2.2 Phosphate release under chemical treatment
To evaluate phosphate release under different chemical conditions, 10 g of fresh weight sedimented sludge (approximately 2 g dry weight) was added to 100 mL of treatment solution (distilled water, 1 M NaOH, 2 M NaOH, 1 M HCl, or 2 M HCl) in Duran glass bottles. Each bottle was sealed and agitated at 100 rpm on a rotary shaker at 25 °C for 24 h (Supplementary Table 1).
After incubation, 10 mL aliquots were collected from each treatment and filtered through 0.45 µm membranes to remove suspended solids. The filtrates were analyzed for dissolved total phosphorus (TP). Phosphorus concentrations were determined spectrophotometrically using a Hach spectrophotometer (DR3900) at 490 nm using the PhosVer® 3 Phosphate Reagent Powder Pillows, 10 mL, 0.02 - 2.50 mg/L PO4 (Hach Company, USA) according to the manufacturer’s protocol.
2.3 Phosphate release under different doses
Based on the results of the chemical treatment experiment, the most effective chemical reagent was selected (1 M HCl, 1 M NaOH, and water) for further investigation of phosphate release at varying fresh weight dosages. The selected reagent was applied at concentrations equivalent to 2, 6, 10, and 20 g L−1 of sludge dry weight (Supplementary Table 1).
For each treatment, the predetermined amount of sludge (calculated based on dry weight) was mixed with the selected reagent solution in Duran glass bottles. To ensure consistent mixing dynamics and a constant solid-to-liquid contact environment, the final volume of each mixture was standardized to 100 mL. The mixtures were shaken at 100 rpm at 25 °C for 24 hours to ensure thorough interaction between the sludge and the reagent.
After incubation, the supernatant was collected and filtered through 0.45 µm membranes. The filtrates were analyzed for dissolved total phosphorus (TP) concentration as described previously (Section 2.2).
2.4 Nutrient release under thermochemical treatments
Based on the results of the previous experiment, identifying the optimal chemical reagent and dosage, thermochemical pre-treatments were conducted to evaluate the release of major nutrients for the potential formulation of microalgae cultivation medium. Sludge samples were subjected to combined chemical and thermal pre-treatments, including ultrasound irradiation (Hielscher UP100H, Germany) and autoclaving. The ultrasonic treatment was applied for 1 hour at 0.5 cycle and 100% amplitude, while autoclaving was performed for 1 hour at 121 °C and 1.5 atm.
Following pre-treatment, the concentrations of ammonium (NH4+), nitrite (NO2−), nitrate (NO3−), and phosphate (PO43−) in the supernatant were determined using a QuAAtro Continuous Flow Analyzer (CFA) (SEAL Analytical, Norderstedt, Germany). All treatments were conducted in duplicate (Supplementary Table 1).
Potassium (K+) concentrations were measured using a Hach spectrophotometer (DR3900, Hach Company, USA) with pre-dosed cuvette test kits (LCK328 for 8–50 mg L−1 K+ or LCK228 for 5–50 mg L−1 K+), following the manufacturer’s analytical procedures.
In addition, major elements (Na, Mg, S, Ca, and Sr) and trace elements (Li, Al, Si, P, Cr, Mn, Fe, Ni, Cu, As, and Cd) were quantified in the treated samples using inductively coupled plasma optical emission spectroscopy (ICP-OES). Analyses were performed at the ATI Aquaristik Laboratory (Hamm, Germany) using a Spectro Arcos 2 ICP-OES analyzer.
2.5 Arthrospira platensis cultivation
2.5.1 Microalgal strain and inoculum preparation
The cyanobacterium Arthrospira platensis (UTEX 2340) was sourced from the Culture Collection of Algae at the University of Texas, USA. While this species has been recently proposed for reclassification as Limnospira platensis (), the name A. platensis is retained here for consistency with the strain’s source and established literature.
Stock cultures were maintained in spirul SAG medium () (Supplementary Table 2) under photoautotrophic conditions. To prepare the inoculum, 50 mL of the stock culture was transferred into a 1 L glass bottle containing 450 mL of fresh SAG medium. Cultures were grown under a 12:12 light/dark cycle at a light intensity of 100 ± 2 μmol m−2 s−1, provided by daylight fluorescent tubes.
2.5.2 Experimental design
Six pre-treatments were performed to evaluate the suitability of chemically and thermochemically treated aquaculture sedimented sludge extracts as nutrient sources for Arthrospira platensis cultivation. Treatments included 3 chemical conditions (acid [A], base [B], and water [W]) with 2 thermal methods (autoclave [AU] and ultrasound [US]), yielding: AUA (acid-autoclave), AUB (base-autoclave), AUW (water-autoclave), USA (acid-ultrasound), USB (base-ultrasound), and USW (water-ultrasound) (Supplementary Table 1).
After treatment, suspensions were filtered, and the pH was adjusted to 8.0. In all treatments, phosphate and other macro- and micronutrients were fully substituted by those present in the treated sludge extract. Nitrogen and potassium were supplemented by adding 2 g L−1 KNO3. The performance of these treatments was compared to that of a synthetic control medium (Spirulina SAG medium; ).
Cultures were continuously aerated using filtered air delivered through autoclaved silicone tubing connected to a 0.2 µm PTFE air filter (Sartorius AG). Cultivation was carried out at 30 ± 2 °C under a
2.5.3 Optimization of KNO3 content
The most effective treatment identified in the previous experiment was selected for further optimization of nitrogen concentration. A. platensis was cultivated using three different KNO3 levels (0, 1, and 2 g L−1). All cultures were maintained under the same environmental and aeration conditions described above, and each treatment was performed in triplicate (Supplementary Table 1).
2.5.4 Growth and proximate estimation
Growth was monitored spectrophotometrically by measuring optical density at 680 nm (OD680) using a Libra S70 Double-Beam Spectrophotometer (Biochrom Ltd., UK). The specific growth rate (μ) was calculated according to Wang et al. (2025) using (Equation 1):
where ODTf and ODT0 are the final and initial OD680, and ΔT is the cultivation period (days).
Dry weight (DW) was determined by centrifuging 5 mL of culture at 4,000 rpm for 2 min, washing the pellet twice with distilled water, and filtering it onto pre-weighed Whatman GF/F filters. Filters were dried at 60 °C until constant weight was achieved. Biomass concentration (g DW L−1) was determined gravimetrically.
Protein content was quantified using the Bradford method (). This involved hydrolyzing the cell pellet with 1N NaOH in a water bath at 90°C for 2h, after which 0.1 mL of the hydrolysate was mixed with 1 mL of Coomassie Brilliant Blue reagent. The resulting mixture was then measured spectrophotometrically at 595 nm, with a bovine serum albumin dilution series used as a typical reference.
Extraction of phycobiliproteins was performed using a triple freeze–thaw procedure. Cell pellets at the end of the cultivation period were pelleted and resuspended in phosphate buffer (pH 8) at a biomass-to-buffer ratio of 1:10, incubated at room temperature for 2 h, and mechanically disrupted using a Thermomixer (1000 rpm) with 0.5 mm glass beads. After a second freeze–thaw cycle at −80 °C, the samples were centrifuged at 4000 rpm for 20 min at 4 °C. The absorbances of the supernatants were then measured at 615 and 652 nm using a Libra S70 Double-Beam Spectrophotometer (Biochrom Ltd., UK).
Phycocyanin and allophycocyanin concentrations (mg mL−1) were calculated using (Equations 4) and (5) (), and by applying the slope equations derived from the standard curves for each pigment:
2.6 Nutrient-exhausting
Following cultivation, the concentrations of NH4+, NO2−, NO3−, and PO43− in the remaining media were determined using a QuAAtro Continuous Flow Analyzer (CFA) (SEAL Analytical, Norderstedt, Germany). All treatments were conducted in duplicate, yielding a total of 8 samples. Potassium (K+) concentrations were measured using a Hach spectrophotometer (DR3900, Hach Company, USA) with pre-dosed cuvette test kits, as mentioned previously. The nutrient removal percentages were calculated as (Equation 6):
where C0 is the initial nutrient concentration and C1 is the final concentration after cultivation (tracked change).
2.7 Statistical analysis
All experimental data were obtained from at least 2–3 replicates per treatment. Data are presented as mean ± SD. Statistical differences among groups were assessed using one-way ANOVA followed by Tukey’s post hoc test for multiple comparisons (GraphPad Software, 10.4.1. San Diego, CA, USA). A p-value< 0.05 was considered statistically significant. Different letters above data points or bars indicate significant differences according to Tukey’s test. Prior to one-way ANOVA, normality of data distribution was assessed using the Shapiro-Wilk test, and homogeneity of variances was evaluated with Levene’s test. Data met the assumptions of normality and homoscedasticity in all cases.
3 Results
3.1 Phosphate release under chemical treatment
Characterization of the raw aquaculture sedimented sludge revealed total phosphorus (TP) and total nitrogen (TN) contents of 16.2 mg g 1 DW and 37.5 mg g−1 DW, respectively (Table 1). In addition to phosphorus and nitrogen, the dominant cation was calcium (93.3 mg g−1 DW), far exceeding magnesium (1.9 mg g 1 DW), iron (1.2 mg g 1 DW), potassium (2.9 mg g 1 DW), and sodium (0.3 mg g−1 DW).
Table 1
| Parameters | Value |
|---|---|
| TS (%) | 2.8 ± 0.03 |
| VS (%TS) | 69.41 ± 0.14 |
| pH | 7.10 ± 0.02 |
| EC | 1.19 ± 0.04 |
| TP (mg/g) | 16.2 ± 1.2 |
| TN (mg/g) | 37.5 ± 2.1 |
| Ca2+ (mg/g) | 93.3 ± 3.2 |
| Mg2+ (mg/g) | 1.9 ± 0.05 |
| Al3+ (mg/g) | 0.2 ± 0.01 |
| Fe2+ (mg/g) | 1.2 ± 0.02 |
| Na+ (mg/g) | 0.3 ± 0.001 |
| K+ (mg/g) | 2.9 ± 0.01 |
Physicochemical composition of the solid phase of centrifuged aquaculture sedimented sludge. .
The release of phosphate (PO43−), a key nutrient fraction of aquaculture sedimented sludge for potential application in algal cultivation, was evaluated under both acidic and alkaline treatments (Figure 1). Chemical treatment significantly influenced phosphate solubilization (Figure 1). Distilled water (control) released the lowest phosphate concentration (15.5 mg L−1). Alkaline treatment with NaOH produced modest increases, yielding 18.0 and 27.6 mg L−1 for 1 M and 2 M NaOH, respectively. Acidic conditions resulted in substantially higher phosphate release: 1 M HCl achieved the highest concentration (33.1 mg L−1), followed by 2 M HCl (29.1 mg L−1). Based on these results, 1 M HCl and 2 M NaOH were selected as the most effective reagents for subsequent dose-response experiments.
Figure 1
The release of phosphate (PO43−) from aquaculture sedimented sludge was evaluated under different chemical treatments (Figure 1). Distilled water, serving as the control, resulted in the lowest phosphate concentration (15.5 mg L−1). Treatment with alkaline solutions (NaOH) slightly enhanced phosphate solubilization, with values of 18.0 mg L−1 for 1 M NaOH and 27.58 mg L−1 for 2 M NaOH.
3.2 Phosphate release under different aquaculture sedimented sludge doses
Phosphate release increased with increasing sludge dosage across all three treatments (water, 2 M NaOH, and 1 M HCl; Figure 2). In the water control, PO43− concentration rose from 15.0 mg L−1 at 2 g L−1 DW to 66.0 mg L−1 at 20 g L−1 DW. Under 2 M NaOH, concentrations ranged from 21.5 to 84.0 mg L−1 across the same dose range. The 1 M HCl treatment consistently yielded the highest phosphate release at all doses (33.5, 54.3, 66.8, and 99.7 mg L−1 at 2, 6, 10, and 20 g L−1 DW, respectively). These findings confirm that increasing sludge dosage enhances phosphate release Based on these findings, 20 g L−1 DW was selected as the standard sludge dose for all subsequent thermochemical experiments.
Figure 2
3.3 Nutrient release under thermochemical treatments
All thermochemical treatments released substantially more phosphate than the water-only control at 20 g L−1 DW (66.9 mg L−1; Figure 3). The acid–autoclave combination (AUA) achieved the highest phosphate recovery (270.6 mg L−1; 83.5% of total P), followed by the autoclaved water extract (AUW; 210.8 mg L−1; 65.6%), the alkaline–ultrasound treatment (USB; 198.5 mg L−1; 61.3%), and the acid–ultrasound treatment (USA; 196.5 mg L−1; 60.7%). The base–autoclave treatment (AUB) and water–ultrasound treatment (USW) yielded intermediate phosphate concentrations.
Figure 3
Ammonium (NH4+) release varied markedly among treatments. The highest NH4+ concentration was recorded in AUB (84.0 mg L−1), followed by AUW (52.8 mg L−1) and AUA (50.5 mg L−1). The acid–ultrasound treatment (USA) produced the lowest ammonium release (6.8 mg L−1). Nitrite (NO2−) and nitrate (NO3−) remained minor fractions across all treatments: nitrite ranged from 0.16 to 5.51 mg L−1, and nitrate was negligible in most cases except for AUW (9.71 mg L−1). Potassium concentrations were relatively stable across all treatments, ranging from 26 to 30 mg L−1.
Although alkaline treatments (AUB and USB) achieved comparatively high nutrient recovery, both produced extracts that were intensely dark-colored and would severely limit light penetration in algal cultures. Subsequent cultivation experiments were therefore restricted to the water-based (AUW, USW) and acid-based (AUA, USA) extracts.
3.3.1 Major and trace element composition of treated sludge extracts
Elemental profiling of the four extracts selected for cultivation (AUA, USA, AUW, USW) revealed consistently higher concentrations of major ions and trace elements in acid-treated compared with water-treated fractions (Table 2). Chloride was by far the most abundant ion in acid extracts (AUA: 4,670 mg L−1; USA: 4,735 mg L−1), compared with 91–121 mg L−1 in water-based extracts. Calcium was also substantially higher in acid treatments (~322 mg L−1) relative to water extracts (104–159 mg L−1). Similar trends were observed for magnesium, sulfur, and sodium.
Table 2
| Element | AUA | USA | AUW | USW |
|---|---|---|---|---|
| Cl mg L-1 | 4670.33 ± 15.63a | 4735.33 ± 16.74a | 91.23 ± 12.55b | 120.67 ± 72.81b |
| Na mg L-1 | 42.26 ± 24.67a | 29.65 ± 0.05b | 14.73 ± 0.05c | 30.35 ± 0.29b |
| S mg L-1 | 14.85 ± 0.224a | 0.42 ± 0.00b | 0.42 ± 0.00b | 0.42 ± 0.00b |
| Mg mg L-1 | 24.35 ± 14.85a | 23.05 ± 0.42a | 6.06 ± 0.42c | 8.21 ± 0.42b |
| Ca mg L-1 | 322.23 ± 0.48a | 321.13 ± 0.74a | 103.70 ± 1.56b | 163.20 ± 5.25b |
| B mg L-1 | 0.19 ± 0.03a | 0.13 ± 0.02b | 0.11 ± 0.01b | 0.04 ± 0.01c |
| Fe mg L-1 | 10.14 ± 0.43a | 6.12 ± 0.17b | 0.14 ± 0.06d | 0.51 ± 0.025c |
| Cu mg L-1 | 0.63 ± 0.13 a | 0.53 ± 0.1a | 0.03± 0.01b | 0.01 ± 0.00c |
| Zn mg L-1 | 6.79 ± 0.24b | 7.58 ± 0.07a | 0.90 ± 0.002d | 1.16 ± 0.01c |
| Si mg L-1 | 6.79 ± 0.06a | 2.07 ± 0.02c | 5.04 ± 0.07b | 1.63 ± 0.01d |
| Pb μg L-1 | 9.66 ± 3.86a | 8.41 ± 1.32a | 0.14 ± 0.01c | 0.60 ± 0.01b |
| Mo μg L-1 | 5.46 ± 0.63a | 2.60 ± 0.06b | 1.76 ± 0.07c | 0.56 ± 0.2d |
| Cd μg L-1 | 6.79 ± 0.5b | 7.58 ± 0.09a | 1.17 ± 0.37c | 0.90 ± 0.15d |
| As μg L-1 | 5.24 ± 0.05a | 2.57 ± 0.08b | 1.30 ± 0.16cc | 1.42 ± 0.17 |
| Co μg L-1 | 13.82 ± 0.16a | 12.15 ± 0.38a | 2.57 ± 0.06b | 1.49 ± 0.03c |
Elemental composition (mean ± SD) of major ions and trace/heavy metals of the aquaculture sedimented sludge under different treatments (AUA, USA, AUW, and USW).
Different superscript letters within the same row indicate statistically significant differences among treatments according to Tukey’s HSD post-hoc test (p< 0.05) following one-way ANOVA, while values sharing the same letter are not significantly different.
Trace element concentrations followed the same pattern. Iron reached 10.1 mg L−1 in AUA and 6.1 mg L−1 in USA, compared with ≤0.5 mg L−1 in water-based extracts. Zinc was similarly elevated in acid treatments (AUA: 6.8 mg L−1; USA: 7.6 mg L−1) relative to AUW (0.9 mg L−1) and USW (1.2 mg L−1). The non-essential toxic metals lead and cadmium were also markedly higher in acid extracts (Pb: 8.4–9.7 µg L−1; Cd: 7.0–7.6 µg L−1) compared with water-based extracts (≤2 µg L−1 for both). Molybdenum, arsenic, and cobalt were similarly enriched in acid treatments. Overall, water-based extracts (AUW, USW) presented a substantially lower ionic load and trace metal burden than acid-treated extracts.
3.4 Arthrospira platensis cultivation on treated sludge extracts
Significant differences in growth were observed among the six pre-treatment groups over the 17-day cultivation period (Figure 4). Since the phosphate content in all treatments was within the range of the PO43− content in SAG (272.6 mg L-1), the recovered nutrient extracts were directly applied without additional supplementation except 2 g L−1 KNO3 and 10 g L−1 NaHCO3 to simulate the SAG medium to cultivate A. platensis under six treatment conditions (acidic, alkaline, and water-based extracts with ultrasound and autoclaving) (Supplementary Figure 1).
Figure 4
Water-based extracts (AUW and USW) supported the highest biomass accumulation among sludge-derived treatments. AUW achieved the highest dry weight (0.98 g L−1), which was not statistically significant to the SAG synthetic medium control (0.93 g L−1; p > 0.05). Both AUW and SAG maintained OD680 values above 2.0 at the final time point. USW followed closely, with similarly sustained growth throughout the cultivation period.
In contrast, all acid-treated (AUA and USA) and alkaline (AUB and USB) extracts produced significantly lower final dry weights. AUB and USB cultures showed immediate decreases in OD680 following inoculation, and did not recover during the 17-day period. AUA and USA cultures showed initial growth that subsequently arrested, with final dry weights well below those of water-based treatments and the SAG control.
To further elucidate the relationship between nutrient availability and biomass formation, nutrient concentrations in the AUW medium were analyzed before and after cultivation. Nutrient concentrations in AUW before and after cultivation showed significant reductions in PO43− (>90% removal), NH4+ (>99% removal), NO3−, and K+, confirming active uptake by A. platensis. A slight increase in NO2− was observed after cultivation. The residual KNO3 concentrations indicated that the 2 g L−1 supplement modestly exceeded assimilation demand, providing the rationale for the subsequent nitrogen optimization experiment.
3.5 Optimization of KNO3 concentration
3.5.1 Growth performance
To determine the optimal nitrogen and potassium supplementation required for A. platensis growth, cultures were cultivated in AUW-derived medium with three KNO3 concentrations (0, 1, and 2 g L−1). A control culture using standard Spirulina synthetic medium (SAG) was included for comparison. Growth performance was monitored based on optical density at 680 nm (OD680) over 24 days (Figure 5).
Figure 5
Growth was strongly influenced by KNO3 concentration over the 24-day cultivation period (Figure 5). Cultures without KNO3 supplementation showed minimal growth throughout, with a final dry weight of 0.32 g L−1 and a specific growth rate of 0.01 day−1. Supplementation with 1 g L−1 KNO3 markedly increased biomass accumulation, reaching 1.45 g L−1 DW with a specific growth rate of 0.06 day−1 a 4.5-fold improvement over the unsupplemented treatment. The OD680 growth curve of the 1 g L−1 KNO3 treatment closely paralleled that of the SAG control throughout the cultivation period, and final dry weight exceeded the SAG control by 1.3-fold.
Increasing KNO3 to 2 g L−1 did not further improve biomass yield (1.40 g L−1 DW) and resulted in a perceptible delay in the onset of exponential growth compared with the 1 g L−1 treatment. These results identify 1 g L−1 KNO3 as the optimal nitrogen supplement for AUW-based cultivation of A. platensis.
3.5.2 Pigment accumulation under different KNO3 concentrations
The effects of varying KNO3 supplementation (0, 1, and 2 g L−1) on A. platensis pigment synthesis were evaluated in AUW-derived medium and compared to the synthetic SAG medium (Figure 6).
Figure 6
As shown in Figure 6A, chlorophyll an accumulation was markedly influenced by nitrogen availability. In the absence of KNO3, Chl a concentration remained below 3 mg L−1 throughout the cultivation period, indicating severe nitrogen limitation. In contrast, AUW supplemented with 1 g L−1 and 2 g L−1 KNO3 substantially enhanced pigment synthesis, with maximum Chl a level. The culture grown in AUW supplemented with 1 g L−1 KNO3 exhibited Chl a dynamic closely resembling those of the standard SAG medium, both reaching around 15.5 and 14.5 mg L−1 at the 24th day, respectively.
Carotenoid accumulation followed a trend similar to Chl a (Figure 6B). Cultures without KNO3 showed negligible carotenoid production (< 1 mg L−1), whereas those supplemented with 1 g L−1 and 2 g L−1 KNO3 exhibited a steady increase, reaching 4.17 mg L−1 and 3.8 mg L−1, respectively, by day 24. The 1 g L−1 KNO3 treatment demonstrated the highest carotenoid accumulation, slightly exceeding that of SAG and AUW control cultures.
3.5.3 Protein and phycobiliprotein content
The total protein and phycobiliprotein (C-phycocyanin and allophycocyanin) contents of Arthrospira platensis were quantified following cultivation in AUW-based media supplemented with varying KNO3 concentrations (0, 1, and 2 g L−1) and compared with the standard synthetic medium (SAG) by the end of the cultivation period. Results are summarized in Table 3.
Table 3
| Treatments | Protein | C-Ph | A-Ph | |||
|---|---|---|---|---|---|---|
| mg L−1 | mg g−1 | mg L−1 | mg g−1 | mg L−1 | mg g−1 | |
| SAG | 580.16 ± 12.8a | 568.78 ± 10.5a | 28.03 ± 4.04a | 27.47 ± 0.03a | 9.59 ± 0.02a | 9.40 ± 0.01a |
| 0 g L−1 KNO3 | 540.15 ± 3.1b | 208.26 ± 9.8c | 2.55 ± 0.01c | 9.8 ± 0.02b | 1.01 ± 0.01c | 3.88 ± 0.01b |
| 1 g L−1 KNO3 | 559.34 ± 15.2a | 466.12 ± 11.3b | 25.12 ± 2.05b | 20.93 ± 0.04ab | 7.5 ± 0.03ab | 6.25 ± 0.02ab |
| 2 g L−1 KNO3 | 556.57 ± 18.4ab | 441.72 ± 14.7b | 25.51 ± 2.06b | 20.25 ± 0.05ab | 6.06 ± 0.02b | 4.80 ± 0.02ab |
Protein and phycobiliprotein concentrations of A. platensis cultivated under different KNO3 concentrations (mean ± SD, n = 3).
Different letters within a column indicate significant differences (p < 0.05).
Protein concentrations varied significantly among treatments (p < 0.05). The highest volumetric protein content was recorded in cultures grown in the SAG medium (580.16 mg L−1), followed closely by the 1 g L−1 (559.34 mg L−1) and 2 g L−1 KNO3 (556.57 mg L−1) treatments. The lowest protein concentration was observed in the culture without KNO3 addition (540.15 mg L−1). Biomass-specific protein content exhibited a similar pattern, ranging from 208.26 mg g−1 in the nitrogen-free treatment to 568.78 mg g−1 in the SAG control.
C-phycocyanin (C-Ph) concentrations ranged between 2.55 mg L−1 (0 g L−1 KNO3) and 28.03 mg L−1 (SAG). Comparable values were obtained for cultures supplemented with 1 g L−1 and 2 g L−1 KNO3 (25.12 mg L−1 and 25.51 mg L−1, respectively). Biomass-specific C-Ph content ranged from 9.8 mg g−1 to 20.93.10 mg g−1 of 0 g L−1 KNO3 and 1g L−1 KNO3, respectively.
Allophycocyanin (A-Ph) concentrations followed a similar trend, with the highest values obtained in SAG (9.4 mg g−1), followed by 1 g L−1 KNO3, and the lowest in the KNO3-free medium (3.88 mg g−1).
3.6 Residual nutrient concentrations before and after cultivation
Residual concentrations of phosphate (PO43−), ammonium (NH4+), nitrite (NO2−), nitrate (NO3−), and potassium (K+) were determined before and after cultivation of Arthrospira platensis using AUW medium supplemented with 0, 1, and 2 g L−1 KNO3. (Table 4).
Table 4
| KNO3 (g L−1) | Sampling | PO43− (mg L−1) | NH4+ (mg L−1) | NO3− (mg L−1) | K+ (mg L−1) |
|---|---|---|---|---|---|
| 0 | Before | 210.35 ± 1.50 | 54.20 ± 0.42 | 77.10 ± 0.73 | 195.40 ± 1.12 |
| After | 20.23 ± 0.31 | 0.38 ± 0.03 | 31.00 ± 1.10 | 0.00 ± 0.00 | |
| Reduction % | 90.38 ± 2.1b | 99.3 ± 0.3a | 59.8 ± 2.9b | 100.0 ± 0.0a | |
| 1 | Before | 211.35 ± 0.20 | 54.20 ± 0.45 | 716.70 ± 3.15 | 582.10 ± 2.80 |
| After | 20.00 ± 0.00 | 0.43 ± 0.02 | 225.03 ± 2.12 | 118.50 ± 1.65 | |
| Reduction % | 90.90 ± 0.0a | 99.2 ± 0.4a | 68.6 ± 3.4b | 79.7 ± 4.2b | |
| 2 | Before | 210.05 ± 1.00 | 54.20 ± 0.50 | 1328.95 ± 4.22 | 968.80 ± 4.80 |
| After | 20.05 ± 0.28 | 0.25 ± 0.01 | 974.33 ± 3.95 | 452.50 ± 3.20 | |
| Reduction % | 90.45 ± 1.8b | 99.5 ± 0.2 a | 26.7 ± 1.7d | 53.3 ± 3.8c |
Nutrient concentrations (mg L−1) before and after A. platensis cultivation under different KNO3 levels and corresponding reduction efficiencies (%).
Results are expressed as mean ± SD (n = 3). Different superscript letters in the same column indicate significant differences (p< 0.05).
Phosphate concentrations decreased significantly after cultivation in all treatments, with depletion exceeding 90%. Ammonium (NH4+) removal exceeded 99% in all treatments, with no significant difference (p > 0.05). Nitrate (NO3−) removal was highest at 1 g L−1 KNO3 (68.6%), followed by 0 g L−1 (59.8%), while 2 g L−1 exhibited the lowest reduction (26.7%).
Potassium removal was also strongly affected by KNO3 concentration, with complete depletion (100) at 0 g L−1, followed by 79.7% and 53.3% at 1 g L−1 and 2 g L−1, respectively.
Overall, nutrient depletion efficiency showed that the AUW medium supplemented with 1 g L−1 KNO3 treatment provided the most balanced assimilation of nitrogen and phosphorus.
4 Discussion
4.1 Phosphate release under chemical treatment
Phosphorus is essential for all living organisms, but excessive release into water bodies can cause eutrophication and environmental damage (). Untreated aquculture sludge, composed of feces, leftover feed, and associated microbial biomass rich in organic matter and nutrients, can lead to severe environmental pollution (). Phosphorus recovery has become a key focus for sustainability and environmental protection (). The initial characterization of RAS fish farm sludge in the present study revealed a high phosphate content of 16.2 mg g−1, indicating that this waste stream could be a valuable secondary phosphorus source. Comparable concentrations have been reported in aquaculture and livestock-derived biosolids, where phosphorus is largely associated with mineral and organic particulate fractions. The presence of multivalent cations such as Ca2+, Fe3+, and Al3+ is particularly relevant, as these elements can strongly bind phosphate via precipitation and adsorption, especially Ca2+, which represented a high calcium-to-phosphorus mass ratio (~25:1), thereby reducing its immediate solubility but potentially enabling controlled release under changing pH conditions (Zhang and Kuba, 2014). This mineral association is a critical interpretive context: calcium phosphates have solubility products that decrease sharply with increasing pH, explaining the limited release under the water control and the only modest gain under NaOH treatment (Wang et al., 2022). Acid leaching is widely reported to mobilize labile and bound phosphorus (P) fractions more effectively than neutral or alkaline extracts, which often release P through hydroxide-mediated dissolution or complexation mechanisms, but to a lesser extent than strong acids. These mechanistic patterns match recent optimizations of wet acid extraction for phosphorus recovery from sludges and biosolids (). Acid redissolution and subsequent precipitation achieved up to 86% P recovery from RAS sludge, emphasizing acid-driven solubilization of metal phosphates (). The paradoxical reduction in phosphate release from 33.1 to 29.1 mg L−1 when increasing HCl from 1 M to 2 M is mechanistically interpretable as competitive re-precipitation: at lower pH, co-released Fe3+ and Al3+ favor formation of insoluble iron and aluminum phosphate complexes, effectively sequestering a fraction of the liberated phosphate before measurement (; ) This behavior is consistent with sequential extraction studies on biological sludges demonstrating growth of Fe/Al-bound phosphorus fractions at progressively lower pH (; Wang et al., 2024).
4.2 Dose response for phosphate release
For further phosphate release, different aquaculture sludge dosages were used. The positive correlation between aquaculture sludge dose and soluble PO43− (peaking at 99.7 mg L−1 for 1 M HCl at 20 g L−1) indicates that increasing solids loading raises the absolute pool of leachable phosphorus available for solubilization. This scaling behavior is expected for batch leaching systems until solute saturation or reagent limitation occurs; the robust response under acid treatment supports the conclusion that a substantial fraction of P in aquaculture sludge is present in acid-labile forms. These observations align with recent work demonstrating that both reagent strength and solids loading are key levers for maximizing P extraction from waste solids (). Acid-assisted hydrothermal and chemical extraction processes achieved up to 91.8% P recovery from sewage sludge with stronger acids, whereas NaOH leaching reached 85.2%, indicating that the agent strength and solid–liquid ratios jointly control P solubilization efficiency (Vo et al., 2025).
4.3 Nutrient and mineral release under thermochemical pre-treatments
Thermal pre-treatment dramatically enhanced phosphate release from 99.7 mg L−1 (chemical-only at 20 g L−1) to 270.6 mg L−1 under AUA conditions, achieving 83.5% P recovery. Heat disrupts the organic matrix and mineral associations that constrain chemical extraction alone. Autoclaving at 121 °C drives thermal hydrolysis of ester-linked and organically bound phosphate, ruptures microbial and fecal cell walls to liberate intracellular polyphosphate granules, and accelerates mineral dissolution kinetics (; Wang et al., 2024). Wang et al. (2024) showed thermal hydrolysis of activated sludge increased soluble phosphorus by 53.8% over three days. Ultrasound produced comparable phosphate release via acoustic cavitation, which physically fragments floc structure and cell membranes without sustained heating (Zhang et al., 2021; ), consistent with 13–38% increases in supernatant total phosphorus reported for excess activated sludge. This convergence indicates that physical disintegration of the sludge matrix is the principal rate-limiting step, independent of energy-delivery mode.
Ammonium patterns further reveal mechanistic differences. Markedly elevated NH4+ in AUB (84.0 mg L−1) arises from base-catalysed deamidation of proteins and hydrothermal ammonification at elevated temperature (; Wang et al., 2024). In contrast, USA suppressed NH4+ (6.8 mg L−1) through acid inhibition of enzymatic ammonification and possible NH3 volatilization. Ammonium dominated nitrate and nitrite across all thermochemical treatments, as expected under anaerobic, non-nitrifying conditions (Wang et al., 2024).
Alkaline extracts were excluded from cultivation experiments because alkaline thermal conditions generate Maillard-type melanoidins dark, recalcitrant compounds formed from amino acids and reducing sugars. These compounds attenuate photosynthetically active radiation and exert antimicrobial effects (Yang et al., 2022; Wang et al., 2023; ). Yang et al. (2022) quantified their formation as strongly temperature- and alkalinity-dependent, with confirmed relevance to microalgal inhibition ().
Elemental profiling showed that the treated extracts provided a balanced mix of major and trace elements (Na, Mg, K, Ca, Fe, etc.) that met the requirements of A. platensis. Acid-treated extracts (AUA and USA) contained substantially higher concentrations, confirming that acid-assisted thermochemical processes enhance solubilization by disrupting floc structure and dissolving metal–phosphate and metal–organic complexes (). Appropriately conditioned sludge extracts can therefore furnish essential minerals for cyanobacterial growth, provided toxic elements remain below safety thresholds and ionic balance is optimized through targeted supplementation ().
4.4 Arthrospira platensis growth performance and nutrient uptake under different aquaculture sludge pre-treatments
All aquaculture sludge extracts provided a balanced profile of macro- and micronutrients (phosphorus, potassium, and essential trace elements) suitable for A. platensis cultivation (Yu et al., 2019; ; ). Nitrogen availability in AUW alone was insufficient for optimal growth and biomass accumulation; supplementation with KNO3 enabled efficient utilization of existing nutrients while enhancing pigment, protein, and phycobiliprotein synthesis.
Water-based extracts, particularly AUW, effectively replaced synthetic SAG medium without compromising productivity. Comparable biomass yields (AUW: 0.98 g L−1 vs SAG: 0.93 g L−1) demonstrate that mild hydrothermal treatment recovers sufficient bioavailable macro-elements to sustain robust cyanobacterial growth (Singh and Dhar, 2019; ). AUW and USW outperformed acid- and alkali-treated extracts despite the latter’s higher PO43− and NH4+ recovery. Extreme pH pre-treatments likely generated inhibitory by-products, altered nutrient speciation, and produced darker extracts that reduced light penetration and photosynthetic efficiency (; ). In alkaline treatments (AUB, USB), a rapid decline in OD680 was associated with elevated free ammonia toxicity at high pH ().
Acid and base pre-treatments further impaired growth by increasing heavy-metal enrichment and elevating ionic strength. Copper and zinc reached concentrations known to inhibit cyanobacteria (), while chloride concentrations rose to 4670–4735 mg L−1 (vs 91–121 mg L−1 in water treatments), imposing osmotic stress (). Iron levels reached 10.14 mg L−1 (AUA), potentially promoting oxidative stress. In contrast, water-based treatments maintained a more favorable nutrient-toxicity balance. Harsh chemical pre-treatments release excessive soluble organic matter, free ammonia, or toxic compounds, whereas milder aqueous extractions better preserve nutrient equilibrium (; Yu et al., 2019).
Post-cultivation analysis of AUW confirmed active nutrient assimilation, with substantial reductions in PO43−, NH4+, NO3−, and K+. The observed increase in NO2− may indicate partial nitrification or incomplete nitrate reduction (). Persistent high residual nitrate and potassium suggest that the supplied KNO3 (2 g L−1) exceeded assimilation demand, consistent with evidence that nitrogen oversupply does not proportionally increase biomass and lowers nutrient-use efficiency (; ). Optimization of KNO3 dosage is therefore necessary to improve nitrogen recovery and minimize residual discharge.
Overall, autoclaved water-extracted sludge (AUW) constitutes a sustainable and effective alternative to synthetic media for A. platensis cultivation, delivering high biomass productivity alongside substantial nutrient recovery. Refining nitrogen supplementation will further enhance process efficiency in integrated waste-to-biomass biorefineries.
4.5 Optimization of KNO3 concentration for enhanced biomass productivity
The growth responses clearly demonstrate that nitrogen availability was the principal limiting factor in AUW-derived medium, as cultures without KNO3 supplementation exhibited minimal biomass accumulation (0.32 g L−1; 0.01 day−1), consistent with the well-established nitrogen dependence of A. platensis for protein-rich biomass synthesis (; ). Nitrogen limitation in cyanobacteria typically leads to reduced growth rates and chlorophyll content, which explains the persistently low OD680 observed in the non-supplemented treatment.
Supplementation with 1 g L−1 KNO3 markedly enhanced growth performance (1.45 g L−1; 0.06 day−1), yielding biomass levels 1.3-fold higher than the SAG control. This finding aligns with previous reports showing that optimized nitrate concentrations significantly improve A. platensis productivity while maintaining efficient nitrogen assimilation and protein synthesis (; ). The comparable OD profiles between AUW + 1 g L−1 KNO3 and SAG further confirm that properly balanced waste-derived media can substitute synthetic formulations when nitrogen is adequately adjusted.
In contrast, increasing KNO3 to 2 g L−1 did not produce additional biomass gains and slightly delayed exponential growth, suggesting that excess nitrate did not translate into improved productivity. Similar observations have shown that nitrate oversupply reduces nitrogen use efficiency and may induce osmotic or ionic stress in algal cultures (; ). Elevated nitrate concentrations may also disturb ionic equilibrium in alkaline systems, limiting further enhancement of biomass accumulation (; Zegers et al., 2025).
Overall, these results indicate that 1 g L−1 KNO3 represents the optimal supplementation level for AUW-based cultivation, effectively alleviating nitrogen limitation while avoiding inefficiencies associated with excessive nitrate input. Careful nitrogen optimization is therefore essential to maximize biomass yield and improve the sustainability of waste-derived cultivation systems.
4.6 Pigment and protein yields: nitrogen as master regulator
The consistent and significant depression of all measured biosynthetic endpoints, chlorophyll a, carotenoids, total protein, C-phycocyanin (C-PC), and allophycocyanin (A-PC) under nitrogen-depletion conditions of only AUW is mechanistically explained by the coordinated nitrogen-saving programme that non-diazotrophic cyanobacteria activate under nitrogen starvation. This programme involves the ordered degradation of phycobilisomes the nitrogen-rich primary light-harvesting complexes that can constitute up to 50% of total cellular protein to replenish internal nitrogen pools and sustain essential metabolic functions (). Chlorophyll catabolism accompanies phycobilisome breakdown as part of the same regulatory response, resulting in the characteristic bleaching or chlorosis phenotype evident in the nitrogen-free cultures here (; ). Phycobiliproteins are nitrogen-rich light-harvesting complexes, and their degradation is a common physiological response to nitrogen stress in cyanobacteria (). recently demonstrated that NblAB-mediated phycobilisome degradation is an essential adaptive mechanism for maintaining amino acid pools during nitrogen starvation in cyanobacteria, providing the mechanistic basis for the depressed C-PC and A-PC observed in the KNO3-free treatments.
The full restoration of all biochemical parameters at 1 g L−1 KNO3 with values statistically indistinguishable from the SAG control confirms that AUW supplies all necessary cofactors, micronutrients, and carbon precursors, and that nitrogen was the sole supplementation requirement. The biomass yields achieved under optimized AUW cultivation (DW: 1.45 g L−1; μ: 0.06 day−1; protein: 559.3 mg L−1) compare favorably with published values for A. platensis grown on wastewater-derived media (; ). Increasing KNO3 to 2 g L−1 produced no statistically significant further improvement in any parameter, confirming physiological nitrogen saturation at the lower dose and reinforcing 1 g L−1 as both the minimum effective and economically optimal supplement.
4.7 Nutrient removal efficiency and circular economy implications
The high nutrient removal efficiencies under optimized AUW cultivation (PO43−: >90%; NH4+: >99%; NO3−: 68.6% at 1 g L−1 KNO3) position this system as a viable dual-function platform simultaneously producing commercially relevant cyanobacterial biomass and treating aquaculture wastewater to levels that approach European discharge standards for P and N (; ). The preferential and near-complete uptake of NH4+ across all nitrogen supplementation levels is consistent with the energetic advantage of ammonium assimilation via GS-GOGAT over the ATP-costly reduction of nitrate a metabolic preference well-established in A. platensis (Yutthanasirikul et al., 2024). The absence of significant differences in NH4+ removal suggests that ammonium was rapidly assimilated regardless of additional nitrate supplementation.
The inverse relationship between KNO3 dose and nitrate removal efficiency (59.8% → 68.6% → 26.7% for 0, 1, and 2 g L−1 respectively) has direct implications for effluent management: oversupplying nitrogen not only fails to improve biomass yield but actively degrades the wastewater treatment function by leaving substantial residual NO3− in the post-cultivation effluent, reducing net nutrient recovery per unit of supplement used ().
Potassium dynamics followed a similar pattern, with complete depletion at 0 g L−1 KNO3 and progressively lower removal at higher supplementation levels. Since K+ plays essential roles in osmotic regulation and enzyme activation, its uptake generally parallels biomass production; however, excessive external supply can surpass physiological demand (). The reduced percentage removal at 2 g L−1 therefore reflects surplus input rather than limited uptake capacity.
Overall, the nutrient balance data confirm that supplementation with 1 g L−1 KNO3 achieved the most efficient and proportional assimilation of nitrogen and phosphorus, maximizing nutrient recovery while minimizing residual discharge. This further supports the conclusion that moderate nitrogen dosing optimizes both biomass productivity and nutrient removal efficiency in AUW-based cultivation systems.
From a safety and circular bioeconomy perspective, although dissolved heavy metal concentrations in the optimized AUW treatments remained low, the potential for bioaccumulation cannot be excluded. Microalgae are known to accumulate trace metals in biomass depending on exposure conditions (; ). Long-term reuse within circular production systems may therefore promote gradual redistribution of contaminants across trophic levels, as discussed in circular food system assessments (). Consequently, while pigment and protein yields under 1 g L−1 KNO3 demonstrate strong biotechnological potential, verification of heavy metal concentrations in harvested biomass and continuous monitoring are essential before confirming suitability for feed, food, or integrated aquaponic applications.
4.8 Economic considerations of thermal and thermal sludge pre-treatment
Despite its effectiveness, thermal disintegration requires substantial energy input, which may offset environmental benefits if derived from non-renewable sources. Although energy consumption and scale-up were not experimentally assessed, a preliminary evaluation can be made. Extrapolating to a commercial scale, heating a 10,000 L autoclave to 121 °C for 1 h would require approximately 200–220 kWh. This corresponds to energy costs of ~€10–11 at subsidized industrial electricity prices (5 ct kWh−1) or ~€25–31 at typical industrial rates (11–15 ct kWh−1). Based on measured concentrations (211 mg L−1 P and 53 mg L−1 NH4+), approximately 2 kg P and 0.53 kg NH4+ could be recovered per batch, with an estimated market value of ~€3–4. The price of industrial sourced N and P outcompetes the costs for energy required alone even when paying the industrial electricity price 5 ct kW h-1, not considering other associated costs such as CAPEX and OPEX costs for nutrient recovery units (; ; ).
Thus, even at low electricity prices, energy costs alone exceed the economic value of the recovered nutrients, excluding CAPEX and OPEX. From an ecological perspective, the process is only justifiable if renewable energy is used (; ). With a 50:50 renewable–conventional energy mix, emissions would amount to roughly 27 kg CO2e per batch, exceeding the carbon footprint of conventionally produced fertilizers. For comparison, urea production emits 1.5–1.8 kg CO2e kg−1 (), and mineral P2O5 production about 0.375 kg CO2e kg−1 (). However, coupling thermal treatment with industrial waste heat or renewable energy sources could substantially reduce the effective energy burden and improve sustainability (; ). When integrated into circular nutrient recovery systems, thermally treated sludge may contribute to reduced dependence on mineral fertilizers and support biomass production, including algal cultivation (; Vo et al., 2025). Despite this effectiveness, thermal disintegration requires substantial energy input, and if sourced from non-renewable electricity or fuels, energy demands could negate some environmental benefits. Experimental data on energy consumption or upscaling were not conducted in this study. However, present results and settings explored (yield per volume, and sludge concentration, etc…) allow a preliminary evaluation on whether thermal treatment may be a means to further explore this sludge utilization pathway.
5 Conclusion
This study demonstrated that autoclaved water-extracted aquaculture sludge (AUW) can effectively replace synthetic SAG medium for Arthrospira platensis cultivation, achieving comparable biomass yield, pigment content, and protein accumulation. Among all pre-treatments evaluated, AUW provided the most balanced nutrient profile, with substantially lower ionic strength and heavy metal concentrations than acid- or alkali-assisted extracts, which inhibited growth despite higher absolute nutrient concentrations. Although AUW supplied adequate macro- and micronutrients, nitrogen was the primary growth-limiting factor; supplementation with 1 g L−1 KNO3 was identified as optimal, maximizing biomass productivity and nutrient removal efficiency without the diminishing returns observed at 2 g L−1. High removal efficiencies of phosphate (>90%) and ammonium (>99%) confirm the strong nutrient recovery potential of this system. However, the detectable presence of trace metals in the sludge extract necessitates verification of metal accumulation in harvested biomass before implementation in food, feed, or integrated aquaculture applications. These results support the use of thermally treated aquaculture sedimented sludge as a sustainable nutrient source for microalgal production, contributing to waste valorisation and circular bioeconomy objectives, while underlining the need for techno-economic and life cycle evaluation prior to large-scale deployment.
Statements
Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author/s.
Ethics statement
Ethical approval was not required for the study involving animals in accordance with the local legislation and institutional requirements.
Author contributions
ME: Conceptualization, Investigation, Methodology, Software, Validation, Visualization, Writing – original draft, Writing – review & editing. LG: Conceptualization, Investigation, Methodology, Validation, Visualization, Writing – original draft, Writing – review & editing. TS: Methodology, Software, Validation, Visualization, Writing – review & editing. AN: Funding acquisition, Writing – review & editing. SE: Conceptualization, Formal analysis, Investigation, Validation, Writing – original draft, 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 German Federal Ministry of Education and Research (BMFTR) within the program “Bioökonomie auf Marinen Standorten (BaMS)”, project “UrbanAqua” (grant number: 031B0915U2).
Acknowledgments
Authors acknowledge the support by the Open Access publication fund of Alfred-Wegener-Institut Helmholtz-Zentrum für Polar-und Meeresforschung.
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.
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Supplementary material
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Summary
Keywords
aquaculture sediment sludge valorization, Arthrospira platensis, circular aquaculture, nutrient recycling, phosphate recovery, sustainable cultivation, thermochemical treatment
Citation
Elshobary ME, Gerdes genannt Janßen L, Schwenkler T, Noke A and Ende SSW (2026) From waste to resource: nutrient recovery from aquaculture sedimented sludge for sustainable Arthrospira platensis cultivation in circular aquaculture systems. Front. Aquac. 5:1822151. doi: 10.3389/faquc.2026.1822151
Received
03 March 2026
Revised
25 May 2026
Accepted
03 June 2026
Published
26 June 2026
Volume
5 - 2026
Edited by
Aya S. Hussain, Purdue University, United States
Reviewed by
Santlal Jaiswar, Central Salt & Marine Chemicals Research Institute (CSIR), India
Gustavo Enrique Olivos Ramirez, Polish Academy of Sciences, Poland
Updates
Copyright
© 2026 Elshobary, Gerdes genannt Janßen, Schwenkler, Noke and Ende.
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: Mostafa E. Elshobary, mostafa_elshobary@science.tanta.edu.eg
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