Bacteria removal in septic effluent: influence of biofilm and protozoa

Numerous biological, physical and chemical parameters are involved in the retention and removal of bacteria in wastewater treatment systems. Biological parameters, such as biofilms and protozoa grazing activity, are often mentioned but few studies provide a better understanding of their influence. In this study, the effect of bacterivorous protozoa on pathogenic indicator bacteria removal was investigated in septic effluent and in the presence of a biofilm coating glass slides. Endogenous bacteria from septic effluent were quantified. First, bacteria removal was compared between septic effluents treated or not with an inhibitor of protozoa (cycloheximide). The mortality rates were 10 times lower in treated effluent (96 CFU mL(-1) d(-1)) than in untreated effluent (1100 CFU mL(-1) d(-1)). Secondly, the efficiency of bacteria removal was studied (i) with a biofilm surface and active protozoa, (ii) with a biofilm surface and inactivated protozoa, (iii) with a clean surface. Protozoa in the presence of a biofilm were responsible for 60% of bacteria removal. Biofilm without protozoa and a clean surface each removed similar quantities of bacteria. Grazing by protozoa could be an important biological mechanism for bacterial elimination in wastewater treatment systems.