Use of Indicators for monitoring the Reduction of Pathogens in Animal Waste treated in Biogas Plants

The possibility of including bacterial indicators in programmes for monitoring the disinfection of animal waste treated in biogas plants was examined. Five full-scale plants receiving waste from several farms were monitored. Of these, two used thermophilic and one mesophilic digestion exclusively whereas two employed a two stage process involving thermophilic pretreatment followed by mesophilic digestion. For each plant the reducing effect on bacterial pathogens present in the incoming waste and on E. coli and enterococci indigenous to the waste was followed over long periods of time (six to 16 months). It was found that both E. coli and enterococci could be used as indicators to monitor the reduction of vegetative bacterial pathogens, but the use of enterococci would give the broadest "safety margin" in this respect. It is concluded that thermophilic as well as mesophilic digestion with thermophilic pretreatment will result in a sufficient reduction of both vegetative pathogenic bacteria and intestinal parasites occurring in concentrations usually found in animal waste to allow for unrestricted use of the de-gassed material in agriculture. This cannot be said for mesophilic digestion used alone. To achieve a sufficient reduction in the number of pathogens during the digestion process, a decrease in the enterococcal concentration of at least 3-4 log10 units should be accomplished, which for normal waste would correspond to a maximum concentration of these bacteria in treated material in the order of 10(2) cfu/ml. It is suggested that enterococcal counts in newly digested material should be one of the parameters in a monitoring programme aimed at controlling the hygienic state of the end-product from large biogas plants.