Quantification of zoonotic pathogenic bacteria in manures and raw digestates of three biogas plants
Résumé
Introduction
On-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 on
the level of pathogenic bacteria. The study aims to evaluate the impact of mesophilic AD on the
concentrations of zoonotic pathogenic bacteria (Listeria monocytogenes, Salmonella spp,
thermotolerant campylobacters) and on indicator bacteria (E. coli and enterococci) in manures and in
raw digestates sampled in three biogas plants (BGP).
Materials and Methods
Samples were collected in storage tank (manures) and at the outlet valve of the digester (raw
digestates) from three BGP during a one-year period (8 sampling dates per BGP, 3 replicates per
sampling date). Two digesters were fed with pig manure. The third digester was essentially fed with
cattle manure. Indicator bacteria were enumerated on specific agar media. Pathogenic bacteria were
quantified after an enrichment step by MPN. A total of 144 samples (72 inputs, 72 digestates) were
analyzed.
Results
The pathogens have been detected in manure at a frequency ranged between 92 and 96%. They
were 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 than
in manures. The average concentrations of thermotolerant campylobacters, L. monocytogenes and
Salmonella spp. in raw digestates were 2, 23 and 5 MPN/g, respectively. The average levels of E.coli
and enterococci (104-105 cfu/g in manure) decreased by 1 to 2 Log10 during AD.
Conclusion
Although 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 the
digestate onto agricultural land.
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