Intermittent Heat Shocks Can Reduce Methanogenesis and Increase Generation of Longer-Chain Volatile Fatty Acids in Anaerobic Bioreactors

Production of volatile fatty acids (VFAs) from organic wastes in anaerobic bioreactors can be increased if methanogenesis is inhibited. Pretreating bioreactor inocula at elevated temperatures slows methanogenesis in the short term, but over the long term, methanogenic activity often recovers. Here, we examined whether elevated temperatures or “heat shocks” (HSs) applied at the onset of CH₄ production can inhibit methanogenesis and increase VFA generation. The effects of multiple 15–30 min intermittent HSs at 50, 65, or 80 °C on mesophilic bioreactors compared to controls at 37 °C were studied. All HS temperatures significantly reduced CH₄ production (70–90%) without decreasing VFA production. After 135 days, total VFA concentrations in the HS treatments were around four times larger than the controls. The HSs led to appreciable shifts in the VFA profiles. Longer-chain VFAs, especially caproate, increased more than 6-fold in the 65 °C treated bioreactors. The microbial communities in the HS bioreactors were significantly different than the controls. The relative abundances of putative chain-elongating bacteria increased and those of syntrophic acetate-forming bacteria decreased when the HSs were applied. Our findings show that intermittent HSs may provide a chemical-free methanogen-specific strategy to improve the production of VFAs, especially longer-chain species.