Alkaline stabilization of sewage sludge using quicklime: A process-based approach for reducing environmental risk and enhancing safe reuse

Abstract

Municipal sewage sludge, while rich in nutrients, poses significant environmental and operational safety concerns due to the presence of ammonia, organic nitrogen, heavy metals, and pathogenic microorganisms. This study investigates the process-based alkaline stabilization of anaerobically digested sludge using quicklime (CaO) at four solids concentrations (5%, 10%, 15%, 20%) and three lime doses (10%, 20%, 30% w/w of dry solids) in a 50-L pilot-scale in-vessel reactor. Over a 72-h period, chemical, microbiological, and biological stability parameters were evaluated, including nitrogen speciation, phosphorus retention, pathogen indicators, phytotoxicity, and heavy metal leachability. Lime addition elevated pH and significantly reduced total Kjeldahl nitrogen (by up to 61%) and ammonium (by 93%) via pH-driven ammonia volatilization. Organic nitrogen declined most notably in lower-solids sludge, while nitrate levels remained largely unchanged. Slight reductions in total phosphorus were attributed to calcium phosphate precipitation. Phytotoxicity bioassays revealed species-specific responses, with Brassica alba maintaining up to 90% germination, while Sorghum saccharatum was more sensitive (down to 52%). Microbiological analyses confirmed complete inactivation of total and fecal coliforms and Salmonella spp. to below U.S. EPA Class A limits. TCLP tests indicated a marked decrease in leachable cadmium, lead, copper, and nickel—often below detection limits. The results demonstrate that optimized quicklime stabilization provides a robust and scalable approach for ensuring microbial safety, minimizing environmental risk, and enhancing the safe reuse of sewage sludge.