Hygienic Effects of Plastic Bio-digesters under Tropical Conditions

Plastic plug-flow bio-digesters have been promoted in many developing countries although minimal information has been published on their hygienic status. This bench-scale study replicates bio-digester conditions to evaluate pathogen reduction at three different hydraulic retention times (HRT) in the anaerobic treatment of pig manures at 30 C for 50 days. Results showed that physio-chemical values differed between treatments. Gas production efficiency was better for 15 and 30 days retention than for 3 days. The accumulated sludge at the reactor’s base increased with longer HRT. The phages and bacteria examined were all reduced, but none was completely eliminated. Log10 reduction of bacteria ranged from 0.54 to 2.47. Phages ranged from 1.60 to 3.42. The reduction of organisms at HRT=30d was about one log10 unit higher than HRT=15d and about two log10 units higher than HRT=3d. The results indicate that the reduction of tested organisms increases with HRT. However the hygienic quality of the liquid effluent does not meet required quality values for surface and irrigation water. Longer HRTs are recommended to increase gas yield and achieve higher pathogen reduction. Hence more barriers should be applied while handling biodigester outputs to minimise risks to environmental and human health