Nutrient Recovery from Urban Municipal Sewage Sludge: Production of a Liquid Fertilizer and Investigation of its Physicochemical and Biological Properties

Document Type : Research Paper

Authors

1 Department of Water and Environmental Engineering, Faculty of Water, Niroo Research Institute (NRI), Isfahan, Iran

2 Department of Civil and Environmental Engineering, Daneshpajoohan Pishro Higher Education Institute (DHEI), Isfahan, Iran,

Abstract

Objective: The principles of a circular economy and sustainable development emphasize the critical need for effective waste management, particularly for urban wastewater sludge. Although often viewed as a hazardous pollutant, this sludge is a rich source of essential nutrients for plants, presenting a significant opportunity for conversion into a valuable biofertilizer. This research aimed to address this challenge by presenting a comprehensive laboratory-scale investigation.
Method: In this research, dewatered sludge from the Tiran municipal wastewater treatment plant was subjected to alkaline hydrolysis, which involved chemical digestion using a 0.25 M sodium hydroxide solution. Following a controlled digestion period, the mixture was centrifuged to separate the solid residue from the nutrient-rich liquid fraction. The resulting liquid fertilizer was thoroughly characterized to assess its agricultural potential by meticulously measuring key physicochemical parameters (pH, EC, TOC, N, P, K), as well as quantifying micronutrients, heavy metals, and indicator pathogens in both the raw sludge and the final product. Finally, using sulfuric acid to adjust the high pH and potassium sulfate to enrich the low potassium content, a post-treatment step was implemented to optimize the fertilizer's quality and create a balanced nutrient profile.
Results: The alkaline hydrolysis process proved highly effective, yielding a liquid fertilizer with high concentrations of phosphorus and organic carbon. These components are crucial for enhancing soil fertility, improving soil structure, and stimulating microbial activity. Although the initial potassium concentration was found to be below the optimal level for plant growth, our post-processing step successfully enriched it to the desired range, thus demonstrating the feasibility of producing a complete and balanced fertilizer. Another significant finding of this research was the low concentration of both microbial load and heavy metals. Consequently, the final product met the stringent U.S. Environmental Protection Agency (EPA) Class A standards for pathogen safety, thereby ensuring its suitability for unrestricted use in agricultural applications.
Conclusions: This study successfully demonstrated a sustainable and effective solution for transforming urban wastewater sludge into a value-added liquid fertilizer. The developed method not only provides a viable pathway for the recovery of valuable nutrients but also ensures the production of an environmentally safe and agriculturally beneficial product. This approach contributes significantly to the development of sustainable agriculture by closing the nutrient loop and offers a reproducible model for urban waste management systems seeking to transition towards more circular and resource-efficient practices. 

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References

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Journal of Environmental Studies
Volume 51, Issue 3
December 2025
Pages 335-350

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