Unlocking the potential: Sustainable land use of urban septic tank slurry as granules with emphasis on soil improvement and safety consideration

Resource recovery of the slurry in septic tanks around the cities should be a promising and sustainable approach. However, the amorphous form of the slurry and safety of use put a big challenge on land use of the wastes. This study aimed to transform the slurry into sanitary, nutrient-rich granules and assess the environmental and health risks from land use in agriculture. The granules' impact on soil fertility, plant growth, heavy metal accumulation, pathogenic bacteria and soil bacterial communities was evaluated in field experiments with radish as a test crop. The granules were found to be rich in essential nutrients and hygienically safe; Pb, Cu, Ni, and Zn concentrations were much lower than regulatory limits for land use of sewage sludge. Cd, Hg, As, and Cr were not detected in granules. Application of granules at 30 t/ha and 60 t/ha significantly improved soil quality and increased crop yield by up to 176 ± 14%. The supplement ratio was significantly negatively correlated with the soil bulk density and positively correlated with the soil porosity, organic matter, and humus in soil. Heavy metals in crops remained within safe limits, with minor variations observed. Soil porosity, ammonium nitrogen, and organic matter were the main environmental factors driving bacterial community differences and causing significant shifts towards beneficial bacterial families, enhancing soil health by suppressing harmful pathogens. This research underscores the potential of septic tank slurry granules to improve soil quality and support sustainable land use, offering valuable insights for urban waste management strategies.