Upgrading waste activated sludge into valuable biosolids via an integrated biochemical approach

Abstract

Sludge management is a significant challenge for water utilities worldwide. High costs are largely driven by poor digestibility and dewaterability, while elevated concentrations of pathogens and toxic metals limit the safe and beneficial reuse of sludge. This study proposes an innovative integrated biochemical method that concurrently enhances sludge digestibility, facilitates toxic metal removal, improves dewaterability, and achieves pathogen reduction. In a laboratory-scale aerobic sludge digester processing waste-activated sludge, in-situ sludge acidification was achieved within 35–40 days, reaching a pH of approximately 2.6. This acidification was driven by the natural cultivation of acid-tolerant ammonia-oxidizing bacteria (Ca. Nitrosoglobus), which generate protons by oxidizing ammonium released from the sludge. Sludge acidification resulted in significant improvements, including volatile solids (VS) destruction (49 ± 6 %), pathogen reduction (∼4 log reduction), enhanced dewaterability (demonstrated by three methods), and toxic metal solubilization. While most toxic metals were solubilized to meet the Grade A biosolid standard (the highest biosolids standard qualified for unrestricted and safe use), copper only met the Grade B standard. To address this, a low dose of nitrite (5 mg N/L) was added to the acidified sludge (pH 2.2, adjusted with a small amount of acid) for 5 h, successfully solubilizing copper and upgrading the sludge to Grade A standards. Overall, this study demonstrates the potential of in-situ sludge acidification combined with minimal nitrite and acid addition as an efficient approach for improving multiple aspects of sludge management.