Phosphorus recovery from Indian sewage sludge by acidification and precipitation

Abstract

Phosphorus (P) recovery from sewage sludge offers potential for nutrient recycling in agriculture and is supported by India’s 2023 Draft National Policy on Fertilizer Management. This study investigates P recovery from five sewage sludge samples collected from sewage treatment plants in Kanpur and Delhi, with P concentrations ranging from 2.34 ± 0.03 to 15.80 ± 2.05 g P/kg dry matter. However, heavy metal concentrations, particularly cadmium (Cd) and chromium (Cr), exceeded Indian compost regulations, making direct land application unsuitable without further treatment. To assess recovery potential, sludge samples underwent 1-hour acidification. Between 17 and 48% of P was released at pH 3.5–4.5, considered optimal for minimizing chemical input. However, Cd, nickel (Ni), and zinc (Zn) were solubilized below pH 5, and Cr, copper (Cu), and lead (Pb) below pH 3. Subsequent precipitation from pH 3 leachates achieved 80–88% P recovery, but the final products exceeded international limits for Cr, Cd, arsenic (As), Ni, and Zn, even after citric acid treatment for heavy metal complexation. While the process demonstrates technical feasibility, as established in European contexts, the lower P concentrations in Indian sludge and the co-leaching of heavy metals present major challenges. With chemical costs estimated at 37–99$/kg of recovered P, the process is not yet economically viable compared to subsidized synthetic fertilizers at 0.76 $/kgP. Additionally, the absence of regulatory standards for mineral recycling fertilizers in India further limits the potential for scaling, underscoring the need for clear guidelines to foster product safety and market acceptance.