Enrichment of sulfate-reducing microbial communities for sewage sludge treatment

Industrial waste gypsum is perpetually stored or released into the ocean. Sulfate-reducing microbial communities couple the oxidation of organic matter from municipal sewage sludge with the reduction of sulfate from gypsum. This process generates sulfidic alkaline solutions enabling carbonate precipitation. To establish a stable microbial community mediating this process on timescales comparable to anaerobic digesters, we enriched a community that removed organic matter from municipal sewage sludge and reduced sulfate over 17–32 days. Three cycles of enrichment were sufficient to achieve a stable community composition and increase sulfate reduction rates more than fourfold (44 ± 12 mg sulfate/L·day) relative to unenriched communities. The community organized around fermenting Bacteroides sp., sulfate-reducing bacteria (SRB), and methylotrophic methanogens at low abundances. Diverse sequences of SRB in the enrichments reflected the presence of multiple SRB families that cannot oxidize acetate, whereas those of the fast-growing Desulfovibrio and Desulfobulbaceae prevailed in unenriched communities. To assess the impact of sludge pretreatments on sulfate reduction and organic matter removal, we incubated the enriched microbial community with gypsum and either dilute, partially digested sewage sludge or undigested sewage sludge. The enriched communities and those native to the sludge exhibited similar sulfate reduction rates and contributed to a 63–77% decrease in chemical oxygen demand (COD), but the enrichment incubated on partially digested sludge produced 88.9% less methane compared to all other groups. These experiments establish a procedure to enrich communities that concurrently degrade waste organics and reduce sulfate from gypsum, lowering greenhouse gas emissions and producing alkalinity.