Anaerobic biological phosphate remobilization from activated sludge: the case study of the wastewater treatment plant in Braunschweig

Phosphorus (P) is an essential nutrient for biological cell growth, vital for agricultural productivity. The fertilizer industry is a major consumer of raw phosphates, which are largely imported in Germany. This underscores the need for efficient phosphorus recycling from municipal wastewater as part of a sustainable circular economy. Activated sludge (AS) in wastewater treatment plants (WWTPs) serves as a significant phosphorus sink, necessitating an effective extraction process to mobilize the phosphorus for reuse. Enhanced biological P-remobilization offers a promising solution, capable of releasing up to 75% of total P from AS when supplied with an easily biodegradable carbon source. Phosphate accumulating organisms (PAOs) play a crucial role in this process, transforming fixed phosphorus into a dissolved form, with additional contributions from facultative anaerobic microorganisms producing organic acids, reducing pH, and enabling iron reduction. This thesis investigates the anaerobic biological phosphorus remobilization process at the Braunschweig WWTP, examining the impact of operational conditions on remobilization efficiency. Key parameters monitored include temperature, pH, electric conductivity, and redox potential. The study also characterizes the remobilized AS by analyzing total and dissolved concentrations of P, Fe (Fe2+/Fe3+), Mg, Mn, NO3-, and NH4+, and evaluates sulfur dynamics by assessing Stot, S2-, and SO42- in AS and H2S in biogas. The impact of hydraulic retention times (HRT) exceeding 15 days due to flood incidents on P-remobilization and methanogenic activity is also explored.Results demonstrate that optimal P-remobilization is achieved under controlled anaerobic conditions with appropriate co-substrate dosage, preventing methanogenic inhibition. High H2S concentrations observed in biogas indicate significant sulfur reduction, necessitating strategies to manage H2S emissions. The findings provide insights into optimizing P-remobilization processes and suggest practical recommendations for handling extended HRT incidents. This study contributes to the development of sustainable phosphorus recovery strategies, enhancing resource efficiency in wastewater management.