A Review, Clues, and Hypothesis to Explain Regrowth Issues

Anaerobic digestion of wastewater sludges relies on complex microbial communities for the hydrolysis and degradation of organic matter. Methanogenesis is the rate limiting step for stable anaerobic digestion, and about two-thirds of the methane is generated from the conversion of acetate. The acetate-utilizing methanogens have a fundamental role in digester capacity and stability. The acetate-utilizing methanogens are comprised principally of two genera, Methanosaeta and Methanosarcina. Typically Methanosaeta dominates mesophilic anaerobic digestion; however Methanosarcina is capable of higher growth and acetate-utilization rates and confers greater stability to anaerobic digestion through its ability to better accommodate transient loadings.A laboratory method was used to measure the maximum acetate-utilization capacity, termed Vmax, for several full-scale anaerobic digesters, and molecular methods, specifically quantitative polymerase chain reaction (qPCR), were applied to the digester samples to enumerate the acetate-utilizing Methanosaeta and Methanosarcina populations. Previous work and these results demonstrate that monitoring Vmax of an anaerobic digester is a sensitive tool for measuring digester stability, predicting digester upsets, and monitoring recovery. The plant evaluations showed that thermophilic digesters have greater ability to process acetate than mesophilic digesters, and molecular methods indicate some thermophilic digesters have a greater population of Methanosarcina than mesophilic digesters. While the Methanosaeta population numbers varied greatly between different digesters, this genera clearly predominated in all mesophilic digesters.Recent literature and studies have reported high concentrations of indicator organisms such as fecal coliforms following centrifuge dewatering of anaerobically digested municipal sludges. Pathogen regrowth has been shown to closely trend with odor generation and reduction in the dewatered cake. The reduction of odors, and fecal coliforms, in the dewatered cake can be associated with activity of the methanogens. Reviews of current findings indicate a strong methanogenic population in dewatered cake will repress the generation of odors and fecal coliforms. The ability of the methanogenic population to survive and recover from the stresses of dewatering activities, in particular centrifugation, will greatly impact the size and health of the methanogenic population in the dewatered cake. This ability may be genera specific, and currently findings and literature indicate Methanosarcina genera may have a greater ability to survive dewatering activities and thus repress fecal coliform regrowth.