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Quantification of zoonotic pathogenic bacteria in manures and raw digestates of three biogas plants
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Edité par CCSD -
International audience. IntroductionOn-farm anaerobic digestion (AD) which has expanded in Europe, produced biogas and a by-product(digestate) used as fertilizer. However, very few studies have been devoted to the effect of AD onthe level of pathogenic bacteria. The study aims to evaluate the impact of mesophilic AD on theconcentrations of zoonotic pathogenic bacteria (Listeria monocytogenes, Salmonella spp,thermotolerant campylobacters) and on indicator bacteria (E. coli and enterococci) in manures and inraw digestates sampled in three biogas plants (BGP).Materials and MethodsSamples were collected in storage tank (manures) and at the outlet valve of the digester (rawdigestates) from three BGP during a one-year period (8 sampling dates per BGP, 3 replicates persampling date). Two digesters were fed with pig manure. The third digester was essentially fed withcattle manure. Indicator bacteria were enumerated on specific agar media. Pathogenic bacteria werequantified after an enrichment step by MPN. A total of 144 samples (72 inputs, 72 digestates) wereanalyzed.ResultsThe pathogens have been detected in manure at a frequency ranged between 92 and 96%. Theywere also present in raw digestate at a frequency of 38% (thermotolerant campylobacters), 83% (L.monocytogenes) and 88 % (Salmonella spp.). The concentrations were lower in raw digestates thanin manures. The average concentrations of thermotolerant campylobacters, L. monocytogenes andSalmonella spp. in raw digestates were 2, 23 and 5 MPN/g, respectively. The average levels of E.coliand enterococci (104-105 cfu/g in manure) decreased by 1 to 2 Log10 during AD.ConclusionAlthough mesophilic AD did not eliminate the pathogenic bacteria, it reduced their concentration.Raw digestates are not directly spread on agricultural land. It is thus likely that their post-treatment(eg. storage or post-digestion) also contribute to limit the spread of pathogens together with thedigestate onto agricultural land.
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