Bacteria and virus removal in denitrifying bioreactors: Effects of media type and age

Abstract

Denitrifying bioreactors are simple, low-cost ecotechnologies designed to reduce nitrate (NO3−) present in septic tank effluent and drainage water. Recent studies indicate that, in addition to significant reduction in NO3− loads, these systems are also able to remove microbial contaminants from municipal wastewater. However, the removal of microbial contaminants in denitrifying bioreactors remains poorly characterised and factors that control removal in denitrifying bioreactors remain unexplored. In this study, the removal efficiency of faecal indicator bacteria Escherichia coli (E. coli) and total coliforms (TC) as a model for bacterial pathogens and F-specific RNA bacteriophage (FRNA bacteriophage) as a model for viruses was assessed for mesocosm-scale (∼700 L) bioreactors receiving municipal wastewater. Systems were filled with two different slow-release carbon sources: woodchip and coconut husk. The effect of media age on attenuation of microbial contaminants was assessed by comparing the performance of 8-year old systems with equivalent newly constructed woodchip and coconut husk bioreactors. Additionally, removal performance of these carbon substrates was compared to that of gravel, a non-carbon substrate commonly used in subsurface flow (SSF) constructed wetlands. Substantial reduction of E. coli, TC and FRNA bacteriophage from primary treated municipal wastewater was achieved in all bioreactors. Mean annual log10 removal efficiencies were similar between microbial indicators ranging from 1.4 to 1.9 for TC, 1.3 to 1.8 for E. coli and 1.3 to 2.0 for FRNA bacteriophage. All denitrifying bioreactors showed consistent year-round performance and long-term performance which did not markedly change in the ninth year of operation. The woodchip or coconut husk bioreactors achieved microbial effluent quality within the same range of log10 removal rates achieved in gravel-based systems. This suggests that denitrifying bioreactors, as well as reducing N loads, can effectively reduce microbial contaminants in wastewater, providing a complimentary disinfection role. Further research is needed to increase understanding of factors affecting removal of microbial contaminants in denitrifying bioreactors to support design of these systems for microbial contaminant removal.