Rural blackwater treatment by a full-scale Brazilian Biodigester Septic Tank: microbial indicators and pathogen removal efficiency

Abstract

The Brazilian Biodigester Septic Tank (BBST) is an on-site appropriate technology for blackwater treatment, which was developed to yield an effluent suitable for agricultural use. Although several studies have proven its efficacy for secondary blackwater treatment, there are few published studies about the microbiological quality of its effluent, and most of them focus on the quantification of total or thermotolerant coliforms. This study evaluates the performance of a BBST for the removal of human adenovirus (HAdV), Enterococcus spp., Salmonella sp., and Escherichia coli. The results further clarify the safety and risks associated with the reuse of the obtained effluent. The full-scale system consists of three 1.2 m3 interconnected reactors, with a blackwater input of 0.045 m3/day, and hydraulic retention time of 80 days. Six sample campaigns were performed at different stages of the monthly operating cycle. The system presented an average removal efficiency of 5.09 log10 for E. coli, 3.22 log10 for Enterococcus spp., 1.2 log10 for Salmonella sp., and 3.0 log10 for HAdV. According to the World Health Organization standards, the obtained effluent is suitable for subsurface irrigation, and for use in crops that develop distant from the soil or highly mechanized crop systems.