Pathogen Reduction in Small-Scale Biogas Plants in a Tropical Region - Bench-Scale Experiments

Small-scale biogas plants in tropical regions are cost-effective and therefore a popular way to treat animal slurries and produce cooking gas (Foresti, 2001). However their pathogen reduction efficacy is not well documented. Some site studies found Escherichia coli, Salmonella spp. with high concentrations in the effluents of these plants (e.g. Kobayashi et al., 2003). This research aims to evaluate the pathogen reduction in this sanitation solution. This paper presents findings from preliminary laboratory tests for the reduction of pathogens and indicator microorganisms E.coli, Salmonella spp., Enterococci, Clostridium perfringens, somatic coliphages and Ascaris suum eggs from fresh cattle and swine slurry substrates. Temperature conditions replicated an average tropical biogas plant (30oC) but a higher retention time of 45 days was chosen. Substrates were spiked with low (103–104 CFU/mL) and high (106–107 CFU/mL) microbial concentrations. Ascaris suum eggs were placed in bags prior to incubation (10,000 eggs per bag). Substrates without inoculates were used as controls. Results showed that the examined coliphages and bacteria survived longer at low initial concentrations. The slurry type affected the survival of some organisms. After a lag phase of 1-2 days E. coli and Salmonella spp. concentrations decreased rapidly regardless of substrate and treatment. None were present after 8 days indicating a T90 of 10 days is needed for acceptable (>log 4) removal of these zoonotic organisms. Other organisms tested require a longer retention time to be noticeably reduced. These findings show the reduction efficacy of key pathogens in small-scale tropical biogas plants is promising