Challenges in optimizing anaerobic digestion of swine manure in PVC bioreactors

Abstract

This article evaluates the feasibility and operational challenges of using PVC bioreactors—operated in a stationary mode without mechanical agitation—for the anaerobic digestion of swine manure under mesophilic conditions (35 ± 2 °C). The experiment was conducted over 120 days, with weekly monitoring of pH, the FOS/TAC ratio, and biogas composition (CH₄, CO₂, O₂, H₂S, and CO). Although the reactors maintained a stable pH for the most part (between 7.6 and 8.9), fluctuations were observed in the FOS/TAC ratio; this ratio frequently exceeded 1.0, indicating organic overloading and a risk of acidification due to the absence of inoculum and agitation. The lack of mixing promoted substrate stratification, leading to instability in biogas composition and high hydrogen sulfide concentrations exceeding 1,000 ppm. It is concluded that, while PVC bioreactors offer a low-cost, easy-to-assemble alternative, they require modifications—such as agitation and thermal insulation systems—to ensure process stability and biogas quality.