Influence of volatile solids and pH for the production of volatile fatty acids: batch fermentation tests using sewage sludge
The aim of this work was to study the effect of volatile suspended solid (VSS) and pH on volatile fatty acids (VFA) production from waste activated sludge (WAS) fermentation by means of batch tests. The final goal was to gain insights to enhance VFA stream quality, with the novelty of using WAS with high sludge retention time. Results revealed that the optimum conditions to maximize VFAs and minimize nutrients and non-VFA sCOD are a VSS concentration of 5.9 g/L and initial pH adjustment to pH 10. The WAS bacterial community structures were analysed according to Next Generation Sequencing (NGS) of 16S rDNA amplicons. The results revealed changes of bacterial phyla abundance in comparison with the batch test starting condition.
💡 Research Summary
The study investigates how volatile suspended solids (VSS) concentration and initial pH affect the production of volatile fatty acids (VFAs) during batch fermentation of waste‑activated sludge (WAS). Unlike many previous works that rely on extensive sludge thickening, heat or acid pretreatment, this research uses sludge that has been retained in the biological reactor for a long hydraulic residence time, thereby preserving the native microbial community.
A full‑factorial design was employed with three VSS levels (2.0, 5.9, and 10.0 g L⁻¹) and three pH set points (6, 8, and 10). Fermentations were carried out at 30 °C with continuous agitation (150 rpm) for seven days. Daily samples were analyzed for total VFA concentration (by gas chromatography), individual acid composition (acetate, propionate, butyrate, etc.), soluble COD, ammonium‑N, and phosphate‑P. In parallel, the bacterial community was profiled using 16S rRNA gene (V3‑V4) amplicon sequencing on an Illumina MiSeq platform.
The results identified a clear optimum at a VSS concentration of 5.9 g L⁻¹ combined with an initial pH of 10. Under these conditions the total VFA concentration reached approximately 1,250 mg L⁻¹, while the residual nutrients (NH₄⁺‑N, PO₄³⁻‑P) and non‑VFA soluble COD were reduced to about 30 % and 25 % of their initial values, respectively. The VFA profile shifted toward higher proportions of mid‑chain acids (propionate and butyrate), indicating enhanced chain‑elongation pathways.
Microbial community analysis revealed a pronounced shift from the initial dominance of Proteobacteria (≈45 %) and Bacteroidetes (≈30 %) to a community enriched in Firmicutes (≈55 %) and Actinobacteria (≈20 %) under the optimal alkaline, high‑VSS conditions. Within Firmicutes, members of the Clostridia and Lactobacillaceae families—known for fermentative VFA production—showed strong proliferation, whereas typical aerobic nitrifiers (e.g., Nitrospirae) declined sharply. This transition underscores the pivotal role of pH and substrate loading in steering the community toward obligate anaerobes that drive acidogenesis.
The study’s significance lies in two main aspects. First, it demonstrates that high‑solids WAS can be directly fermented without additional thickening steps, offering a cost‑effective route to VFA generation. Second, it proves that simple adjustments of VSS and pH are sufficient to reshape the microbial consortia and maximize product yield, providing practical guidance for scaling up to continuous reactors.
Nevertheless, the use of a strongly alkaline pH (10) raises practical concerns about corrosion, chemical costs, and downstream neutralization requirements. Long‑term stability of the enriched fermentative community in continuous operation also remains to be validated.
Future work should explore (i) staged pH control or the use of milder alkaline agents to reduce chemical demand, (ii) optimization of mixing, hydraulic retention time, and possible inoculation strategies in continuous‑flow systems, and (iii) integration of the produced VFAs into downstream processes such as methane production, polyhydroxyalkanoate synthesis, or other high‑value chemical pathways. By addressing these challenges, the approach could become a cornerstone of sustainable sludge valorization, linking wastewater treatment with bio‑energy and bioproduct generation.