Investigating the impacts of dewatering and drying on the biological activity and nutrient availability of sludge from wastewater treatment plants

Dewatered sludge is the residual solid remaining after moisture is removed from wastewater treatment plants. This study assessed the physicochemical, elemental, microbial, and heavy-metal properties of fresh and 2-month-old dewatered sludges and 2-and 6-month-old non-dewatered sludges from two wastewater treatment plants, and used molecular techniques to identify bacterial strains. Sludge pH ranged from slightly acidic to alkaline (6.43–8.13). Evaporation in the drying bed reduced moisture in dewatered sludge from 86% to 0.6% in two months, versus 83% to 1.6% in non-dewatered sludge over six months; this increased conductivity in both older sample types. TOC declined due to microbial stabilization and conversion of organic matter to CO2; thus, older sludge samples had reduced microbial counts, reflecting aging and treatment effects. Most heavy metals met safe limits except for Cd, Zn and Cu, warranting cautious use to prevent soil and crop contamination. Molecular analysis identified thermophilic and mesophilic bacteria, including Streptomyces thermolineatus and Litorilinea aerophila, associated with aerobic treatment of non-dewatered samples, indicating their potential for biofertilizer production. These findings guide best practices, but routine management and toxicological analysis are needed to mitigate environmental risks and ensure sustainable reuse.