Spatial Configuration of Extracellular Organic Substances Responsible for the Biogas Conversion of Sewage Sludge

Abstract

The influence of the key structural features of sludge that are responsible for the low anaerobic conversion efficiency of sludge is poorly understood. In this study, sludge organic substances are reclassified into extracellular organic substances (EOSs) and cell biomass on the basis of sludge structure. The roles of EOSs in the biogas conversion of both sewage sludge (SS) and model sludge (MS) were investigated. It is observed that with increasing EOS content the net cumulative methane production (NCMP) of the sludge decreased by 36.4%, implying the crucial roles of EOSs in anaerobic sludge digestion. The experimental results showed that with increasing EOS content in sludge, the extracted EOS content decreased, indicating that the structural stability of EOSs in sludge was reinforced. Considering that the biodegradation of EOSs typically depends on structural stability, spatial configuration of EOSs has been hypothesized to account for the low anaerobic digestion efficiency. Further analyses of the spatial configuration of EOSs from the MS and SS revealed that the random-coil shape with extended chains in MS is more readily biodegradable than the dense globule shape with cross-linked chains in SS. These findings shed light on the underlying mechanism responsible for the low biogas conversion of sludge.