Fate of Manure‐Borne Pathogens during Anaerobic Digestion and Solids Separation

Burch, Tucker R.; Spencer, Susan K.; Borchardt, Spencer S.; Larson, Rebecca A.; Borchardt, Mark A.

Anaerobic digestion can inactivate zoonotic pathogens present in cattle manure, which reduces transmission of these pathogens from farms to humans through the environment. However, the variability of inactivation across farms and over time is unknown because most studies have examined pathogen inactivation under ideal laboratory conditions or have focused on only one or two full‐scale digesters at a time. In contrast, we sampled seven full‐scale digesters treating cattle manure in Wisconsin for 9 mo on a biweekly basis (n = 118 pairs of influent and effluent samples) and used real‐time quantitative polymerase chain reaction to analyze these samples for 19 different microbial genetic markers. Overall, inactivation of pathogens and fecal indicators was highly variable. When aggregated across digester and season, log‐removal values for several representative microorganisms—bovine Bacteroides, Bacteroidales‐like CowM3, and bovine polyomavirus—were 0.78 ± 0.34, 0.70 ± 0.50, and 0.53 ± 0.58, respectively (mean ± SD). These log‐removal values were up to two times lower than expected based on the scientific literature. Thus, our study indicates that full‐scale anaerobic digestion of cattle manure requires optimization with regard to pathogen inactivation. Future studies should focus on identifying the potential causes of this suboptimal performance (e.g., overloading, poor mixing, poor temperature control). Our study also examined the fate of pathogens during manure separation and found that the majority of microbes we detected ended up in the liquid fraction of separated manure. This finding has important implications for the transmission of zoonotic pathogens through the environment to humans. CORE IDEAS: Pathogen inactivation is highly variable among full‐scale anaerobic digesters. Pathogen inactivation by full‐scale digesters on cattle farms needs optimization. Most microbes end up in the liquid fraction during solids separation of manure.