Decentralized Wastewater Treatment Technologies

Decentralized wastewater management is a significant area of focus for many researchers and a highly topical global issue. Decentralized treatment systems have different challenges than centralized processes, and in recent years, several new technologies have been developed worldwide, which demonstrate more effective and reliable processes for water and nutrient recycling, as well as the safe end-use of the treated sewage sludge and water fractions. The researches on decentralised wastewater treatment systems can cover all the fractions as well as only focus on one of the fractions, and for implementation the different treatment alternatives need to be combined together to enable the treatment of all the wastewater flows generated in a household. Many of the decentralized wastewater treatment systems are source separating, dealing with the individual flows of wastewater according to their composition. Source separating system has a great opportunity for more effective resource recovery in that they can treat the different fractions according to their composition. In this research topic we received seven contributions looking at different aspects of decentralized wastewater treatment methods.One contribution by (Engstler et al., 2022) focused on mixed wastewater and evaluated the performance of small scale wastewater treatment plants in Austria. They evaluated the treatment performance of about 2,500 small wastewater treatment plants with the capacity of ≤ 50 PE (population equivalent) using external monitoring data from 2009 to 2018. The main technologies represented were Sequencing Batch Reactor plants, conventional activated sludge plants and vertical flow wetlands. The evaluation showed that all technologies meet the legal requirements regarding BOD 5 , COD and NH 4 -N effluent concentrations. There was no evidence of a significant deterioration in treatment performance with the age of the plant for any technology, but many of the plants showed an increased variation in the performance in relation to increased time of operation. The vertical flow wetlands proved to have considerable more stable performance over time compared to the other treatment systems.Several studies focused on treatment of one of the fractions. (Aicher et al., 2022) focused on treatment by vertical green systems for greywater treatment. They made a field study with 4 modules using synthetic greywater hourly fed for 18 weeks, and monitored the performance in regards of COD, total P, ortho P, total N, NH 4 -N and NO 3 -N removal. The modules using agricultural residues for treatment are promising with up to 92% COD reduction together with high reduction of N and P. The modules with wooden chips had lower reduction, only reaching 67% COD removal and lower reduction of N and P compared to the agricultural residue filter.Two studies (Randall et al., 2022;Simha et al., 2022) focused on source separated human urine and the effect of alkaline stabilization and the effect from selected alkalizing chemical and how the drying conditions affect the pH of the solution. (Randall et al., 2022) presented the chemical kinetics of CO 2 entering the alkalized urine from the evaporation air during dehydration. The rate constant increased with temperature and the effect of pH was negligible on the chemical urea hydrolysis rate in alkalized urine (>11). Their computer simulations showed that CO 2 dissolution was the main cause of the pH decrease, but CaCO 3 precipitation, and NH 3 volatilization foster the pH decrease. Residual undissolved Ca(OH) 2 was shown to substantially delay the pH decrease. Overall, this work provides valuable insights into the CO 2 related decrease in pH and the mechanisms of urea hydrolysis in alkaline urine dehydration. Simha et al (Simha et al., 2022) used Alkaline earth hydroxides to prevent urease activity in source separated urine. They investigated and simulated the solubility of Mg(OH) 2 and the factors affecting the dissolution in different types of urine both real and synthetic. They found that it took 6-16 min for Mg(OH) 2 to dissolve. When fresh urine was supersaturated with Mg(OH) 2 (650mgL -1 ), the pH increased to >10.5. The high pH inhibited enzymatic degradation of urea for >14 days. When 95% of the water was removed the solubility of Mg(OH) 2 increased to 16,240 mg L -1 and the pH dropped below ten. (Wang et al., 2023) conducted a review of technologies for recovering nutrients from blackwater. Nutrient recovery and recycling are of great importance in sustainable development. Blackwater (BW) refers to wastewater from toilets, which contains faeces, urine, water, and toilet paper from flush toilets. The highly concentrated nutrients of BW could be collected through source separation and treated adequately to recover nutrients efficiently and economically. The review gave an overview of the characteristics of BW and different techniques to recover nutrients and other valuable products. A number of these technologies are currently under development or being tested at laboratory or pilot scale. The perspective for BW nutrient recovery technologies is very positive due to their great potential for resource recovery. For application of resource-oriented sanitation infrastructure and systems, there is still a long way to go for development of commercial technologies and valuable products. (Häfner et al., 2023) used field experiments to evaluate the fertilizer potential and nitrogen value of three novel and safe recycling products. They fertilised white cabbage, cultivated in three soil types (sand, loam or silt) with two nitrified urine fertilizers and one faecal compost, applied alone or in combination. The control was fertilised with organic fertilizer vinasse. In addition to the growth, the uptake of pharmaceuticals was assessed for treatments with compost application. The two nitrified urine fertilisers resulted in similar effect to vinasse, producing up to 72 t ha -1 . Plant uptake of pharmaceuticals (Carbamazepin) was higher in sand than in loam with lower concentration in the edible part compared to outer leaves. They found that nitrified urine fertiliser was a promising fertilizer substitute in horticultural food production.The final paper wrapping up this research topic looked into the "Emerging dynamics and prospects in France". In this paper (Joveniaux et al., 2022) presented a study looking into source separating sanitation as an alternative to conventional sanitation management. They looked into environmental terms and agricultural use. In France, systems for the source separation have previously been found mainly in single households in rural areas. However, since the 2010s source separating have been introduced to urban areas in larger scale. They investigated the innovative projects for the source separation in urban areas, performing a cross-sectional analysis of experimental projects in three cities (Paris, Bordeaux and Grenoble). Their analysis showed that source separation is still in an emerging phase in France, currently experimented in cities through diversified projects and approaches, both to socio-technical choices and territorial embeddedness' forms. They also point out that beyond technical issues, shared social values, stakeholder alignment issues and organizational challenges are essential. They also considered possible directions for future development, ranging from the spread of a homogeneous solution to the continuation of a diversity of approaches on different territories.To sum up this research topic we can see that the contributions covered several aspects of the decentralized wastewater treatment systems and that most of them were focusing on systems with separated treatment of the different wastewater fractions with resource recovery in mind and the challenges with the different treatment alternatives that are available when decentralizing sanitation.