Enhanced production of short-chain fatty acids from sludge by thermal hydrolysis and acidogenic fermentation for organic resource recovery

Abstract

Acidogenic fermentation (FM treatment) converts organics in waste sludge to valuable short-chain fatty acids (SCFAs). To maintain a favorable condition for the production of SCFAs, an alkali is often added continuously to maintain an alkaline pH in the fermenter. However, this chemical adjustment is costly and biotic hydrolysis is slow. In this research, thermal hydrolysis (TH) was introduced as a pretreatment to enhance fermentation and SCFA production. The results were compared with those obtained from the untreated sludge that underwent fermentation with a daily pH 10 adjustment (NT–FMpH10). The TH pretreatment resulted in rapid abiotic hydrolysis within a short period (1 h), releasing more than 30.5% of organics into the liquid phase of the sludge. These dissolved organics in sludge promoted rapid acidogenesis and SCFA production. TH together with a one-time alkali pretreatment further increased the production of SCFAs during sludge fermentation (TH&Alk–FM): it produced 22.8% more SCFAs than the non-treated NT–FMpH10 sludge with alkaline pH control during fermentation. Semicontinuous fermentation further showed the advantage of the TH&Alk–FM process, as a rapid and high production of SCFAs was achieved when the fermentation time was shortened from 5 d to 2 d. The microbial community analysis revealed that TH&Alk–FM and NT–FMpH10 sludge samples had simple but varied microbial communities. The dominant genera in the TH&Alk–FM sludge were unclassified Ruminococcaceae (18.9%) and unclassified Porphyromonadaceae (22.3%), belonging to the classes Clostridia and Bacteroidia, respectively. NT–FMpH10 was dominated by Tissierella (23.7%) and Proteiniborus (13.5%), which belong to Clostridia. Compared with NT–FMpH10, the microbial consortia in TH&Alk–FM were supplied with sufficient soluble organics and performed better in fermentation and SCFA production, without the need for the daily alkali addition to control pH.