Unsafe containment: Public health risks of septic tanks discharging to drains in Dhaka Bangladesh

Sanitation approaches in low-income urban areas are predominately on-site sanitation, with septic tanks promoted as an improved sanitation solution. While a septic tank system is designed to contain sludge in the tank and discharge effluent to a soil infiltration system, in many urban contexts effluent from tanks discharge directly to open drains or surface waters. This research addresses the paucity of data on pathogen removal and discharge from septic tanks as operated in low-income contexts and the significance of this public health hazard. This study assessed the performance and risks of “septic tanks” discharging to open drains in a low-income neighbourhood in Dhaka, Bangladesh, considering the influence of usage and tank operation on concentrations of Norovirus GII, Salmonella Typhi, Vibrio cholerae, Giardia, Shigella and E. coli in the effluent and receiving drains. While 66% of septic tanks were functioning within design limits, multiple pathogens were detected in all effluent samples, with a mean concentration of 7.6 log10 MPN/100mL for E. coli and 4.2–5.6 log10 genome copies/100mL for pathogens, excluding S. Typhi which was not detected. Children’s exposure to septic tank discharge in drains could result in an 18% risk of illness from Norovirus GII and 3% from Giardia annually. The pathogen reduction between the estimated septic tank inflows and measured effluent concentration ranged from 1.3 log10 MPN reduction for E. coli to 2.2 log10 genome copies reduction for Giardia. Increased coverage of septic tanks was significantly associated with reduced concentrations of Shigella in open drains compared to direct discharge from toilets, with increased reduction for septic tanks operating within design standards. Implementing septic tanks without sub-surface infiltration or treatment of effluent is a major concern. The potential health risk of exposure to septic tank effluent warrants increased attention to appropriate technical design, effluent management and alternatives such as networked sanitation.