Phase separation as a strategy to prevent sulfide-related drawbacks in methanogenesis: performance and energetic...

The establishment of sulfate (SO₄^2-) reduction during methanogenesis may considerably hinder the efficient energetic exploitation of methane, once removing sulfide from biogas is obligate and can be costly. In addition, sulfide generation can negatively impact the performance of methanogens by triggering substrate competition and sulfide inhibition. This study investigated the impacts of removing SO₄^2- during fermentation on the performance of a second-stage methanogenic continuous reactor (R2), comparing the results with those obtained in a single-stage system (R1) fed with SO₄^2--rich wastewater (SO₄^2- of up to 400 mg L^-1, COD/SO₄^2- of 3.12-12.50). The organic load (OL) was progressively increased to 5.0 g COD d^-1 in both reactors, showing completely discrepant performances. Sulfate-reducing bacteria outperformed methanogens in the consumption for organic matter during the start-up phase (OL = 2.5 g COD d^-1) in R1, directing up to 73% of the electron flow to SO₄^2- reduction. An efficient methanogenic activity was established in R1 only after decreasing the OL to 0.625 g COD d^-1, after which methanogenesis prevailed by consuming ca. 90% of the removed COD. Nevertheless, high sulfide proportions (up to 3.1%) were measured in biogas. Conversely, methanogenesis was promptly established in R2, resulting in a methane-rich (> 80%) and sulfide-free biogas regardless of the operating condition. From an economic perspective, processing the biogas evolved from R2 would be cheaper, although the techno-economic impacts of managing the sulfur pollution in the fermentative reactor still need to be understood.