Phosphorus recovery from wastewater sludges by acid and alkaline leaching

Phosphorus (P) is a non-renewable and irreplaceable resource integral to all life on Earth. It plays a key role in upkeeping food systems and serves as a valuable raw material for industrial processes. Current P reserves are expected to be depleted within the next century, prompting increased legislative efforts to find sustainable al-ternative sources, such as the municipal wastewater sludge, where P is precipitated to prevent eutrophication in waterbodies. Wet-chemical leaching has proven effective in dissolving P and recovering it as a product, but the results are inconsistent in iron treated sludges.This thesis seeks to explore the reasons behind these inconsistencies and find an effective method for P recovery through chemical dissolution combined with iron re-duction, by experimenting with leaching at different pH levels using four different sludge materials. Alkaline method was more effective in solubilizing phosphate from digester feed (53.8%) and dewatered sludge (11.2%), likely due to the higher dry weight and organic matter content of these sludges, as well as a greater release of organic phosphorus compared to digested sludge. Acid treatment was more effective for digested sludge (39.4%) and liquid from dewatering (35.6%), which have more inorganic material. Optimizing the leaching methodology and conditions, such as ad-justing the liquid/solid ratio or employing sequential washing, could further enhance P leaching potential for dewatered sludge. For theoretical struvite production digested sludge or dewatered sludge are ideal due to their high P release at pH 12 and 2 and substantial ammonium concentrations. This study aids in selecting suitable P recov-ery methods and conditions for different sludge materials and examines the potential reasons for differences between acid and alkaline leaching. The current limited char-acterization of P, iron and sulphur prevents a complete understanding of P behaviour and fate in wastewater treatment plants, as well as the development of effective P recovery strategies.