Organic mass and protein extraction from secondary sewage sludge via multi-step physical alkali- and acid-based treatment

Abstract

The perception of sewage sludge has been shifting from waste to resource, leading to various technological proposals for its management and resource recovery. This study explores a two-step sewage sludge treatment using different pathways—physical-alkali followed by physical-acid, and physical-acid followed by physical-alkali hydrolysis—to understand the efficiency of organic matter (OM) and Kjeldahl nitrogen extraction, and protein solubilization. Hydrolysis of the sewage sludge was performed with 3 M H2SO4 and 2.8 M NaOH and combined with physical treatment—thermal, ultrasonication, microwave irradiation, and cavitation. The results showed that cavitation chemical hydrolysis in an alkaline environment (CCH-alkali) extracted the highest amount of OM—up to 79.0%. When further cavitation chemical hydrolysis in an acid environment (CCH alkali–acid) was performed, OM extraction reached 90.2%. Physical-alkali treatment showed better performance in resource recovery from secondary sludge (SS) in both treatment steps. The highest protein extraction rate of 23,046 mg/L in the supernatant was obtained using SS treatment with microwave chemical hydrolysis in an alkaline environment (MCH-alkali). Although physical-acid treatment resulted in reduced protein solubilization and OM extraction, it provides a higher protein hydrolysis rate. Organic nitrogen compounds were better extracted with thermal-alkali treatment, reaching 95.3% removal. The study showed that different physical treatment methods demonstrate selective resource recovery or extraction performance.