Production of volatile fatty acids through microbial conversion of waste sludges: influence of substrates

Inadequate sludge treatment poses a significant risk of environmental pollution. To reduce pollution, utilizing waste sludge as an energy source offers a sustainable solution to mitigate pollution. Since sludges contain abundant organics, they are expected to produce more valuable organics, such as volatile fatty acids (VFAs). Although sludge fermentation has been widely studied, direct yields of VFA production from different sludge types are still limited, particularly for sludge materials originating from Finnish wastewater treatment plants. In this study, laboratory-scale bioreactors were built to examine VFA production from digester feed sludge, digested sludge, and ammonia removal sludge, which were collected from local wastewater treatment plants. Among the tested substrates, digested feed sludge resulted in the highest VFA yield, reaching 171.6 ± 6.3 mg/g volatile solids after 11 days of incubation, which indicated its superior potential for VFA generation. In contrast, bioreactors fed with digested sludge or ammonia removal sludge showed no clear increasing trend in VFA production. The addition of sawdust led to lower overall VFA yields, approximately half of those obtained with digested feed sludge alone, and increasing the sludge proportion did not result in further yield enhancement. Acetic acid was the dominant VFA in all bioreactors, accounting for 61.2-97.9% of total VFAs. Microbial community analysis indicated the prevalence of phyla Bacillota and Pseudomonadota, with Lactobacillus being relatively abundant in bioreactors exhibiting higher VFA production. These findings suggest that sludge type plays an important role in determining VFA production performance and may support more informed selection of sludge substrates for VFA-oriented sludge valorisation.