Enhanced Dewatering of Anaerobically Digested Sludge: Unveiling the Impact of Substrate Composition, Microbial Activity, and Pretreatment Strategies

Abstract

Effective dewatering can greatly benefit partially stabilized liquid sludge generated from the two-stage anaerobic digestion of various solid fractions by reducing the volume of digested material, lowering storage and conditioning costs, and enhancing product stability. Despite the capacity of anaerobic digestion to reduce the initial solid content, the resulting sludge often presents greater challenges in dewatering. This study investigates the factors influencing the dewatering process, focusing on the digestion stage and microbial activity, which are influenced by operational parameters such as the inoculum-to-substrate ratio, incubation conditions, and particularly substrate composition. Enhancing process performance typically involves improving substrate hydrolysis, a step that is often facilitated by thermal, chemical, or mechanical pretreatments designed to boost microbial activity and, consequently, biogas production. In this context, the study examined sludge dewatering for codigested sludge recovered from batch reactors operating under thermophilic conditions. These reactors treated food waste (FW), both in its raw form and chemically pretreated with 3% oxygenated water, along with orange peel (OP) as a cosubstrate, with adjustments in the volumes of both components. Dewatering efficiency was evaluated via centrifugation, and the digested sludge was characterized in terms of mineralization rate, protein and polysaccharide content, viscosity, capillary suction time, and morphological traits. The highest cumulative biogas production, 1,709 mL/g TVS, was observed with the ADRP5 combination (50% FW and 50% OP). Centrifugation yielded solid cake contents ranging from 12.1% to 14.32% for raw FW-digested sludge and from 13.67% to 15.30% for pretreated sludge, with improved dewaterability attributed to a decrease in the protein-to-polysaccharide ratio, which dropped from 1.96 to 4.09 in raw FW-digested sludge to 0.53–2.21 in the pretreated substrate sludge.