Microbial contaminant removal and alternative nitrogen removal pathways in denitrifying bioreactors
Denitrifying bioreactors, simple treatment systems consisting of a container filled with a particulate organic carbon source, are an effective and low-cost technology for the effective removal of nitrogen (N) from water by enhancing denitrification (i.e. the bacterial conversion of nitrate [NO3-] to N gas). To date, studies on denitrifying bioreactors have mainly focused on the ability of, and factors influencing, the removal of NO3- within these systems. In the research presented in this thesis, a more holistic view of denitrifying bioreactors was taken in which N removal was assessed in conjunction with the removal of faecal microbial contaminants. The objectives of this research were: (1) to assess how well denitrifying bioreactors can remove microbial contaminant loads from wastewater and (2) to assess the potential role of alternative N removal pathways, namely anaerobic ammonium oxidation (anammox) and codenitrification for N removal in denitrifying bioreactors. The first part of this thesis focused on the removal of microbial contaminants in denitrifying bioreactors. Removal of microbial contaminants from wastewater is a key factor in wastewater treatment, since the contamination of receiving waters with inadequately treated wastewater can contribute to the transmission of infectious disease caused by waterborne pathogenic microorganisms. First, the removal of bacterial and viral indicators (Escherichia coli [E. coli] and F-specific RNA bacteriophage [FRNA bacteriophage]) was assessed along the longitudinal transect of a full-scale operating woodchip bioreactor (~114 m3 in size with 9 sampling wells along the length of the bioreactors) loaded with nitrified septic tank effluent. In addition to significant reduction in NO3- loads, the bioreactor demonstrated consistent and substantial reduction of E. coli (2.9 log10 reduction) and FRNA bacteriophage (3.9 log10 reduction) despite receiving highly fluctuating inflow concentrations (up to 3.5 × 105 MPN/100mL and 1.1 × 105 PFU/100 mL,respectively). In a follow-up experiment, removal of E. coli, total coliforms (TC) and FRNA bacteriophage was analysed in fifteen mesocosm scale bioreactors (~700 L each) filled with two different carbon sources: woodchip or 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 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 that was not greatly dependent on age of carbon material. The results from both studies suggested that denitrifying bioreactors, as well as reducing N loads, can effectively reduce microbial contaminants in wastewater, providing a complimentary disinfection role. Denitrification has generally been considered the major pathway converting NO3- to dinitrogen gas (N2) in denitrifying bioreactors. In the second part of the thesis, the importance of anaerobic ammonium oxidation and codenitrification (jointly referred to as An/coD), was assessed by monitoring the removal of N species from partially nitrified municipal wastewater passing through the mesocosm scale bioreactors described above. Lab experiments using a 15N isotope-pairing technique were also performed to partition production of N2 to these different microbial processes. Results obtained from this study altered our understanding of the potential mechanisms responsible for N loss in these systems. The effective removal of both NO3- and ammonium (NH4+) and the formation of hybrid N2 (i.e. 29N2) observed in bioreactors demonstrated that the An/coD pathway was an effective pathway for N removal when both NO3- and NH4+ were present. An/coD removal rates ranged from 0.6 to 3.8 g N per m3 reactor volume per day while denitrification rates ranged from 0.7 to 2.6 g N per m3. The contributions of An/coD to N removal was dependent on media, with An/coD becoming more dominant in bioreactors where denitrification was carbon limited. The research presented in this thesis has important implications for the use of denitrifying bioreactors for domestic wastewater treatment, since it demonstrates that in addition to removing NO3-, denitrifying bioreactors can also remove microbial contaminants and NH4+, both commonly present in domestic wastewater. A greater understanding of factors controlling microbial contaminant removal, denitrification and An/coD activity in these systems would allow improved design of bioreactors with the capacity to treat a broader range of wastewater contaminants. The thesis opens a discussion on the potential of denitrifying bioreactors to evolve into a reliable treatment technology for different types of wastewater contaminants.
wastewater treatment, nitrogen removal, nitrate, ammonium, viruses, bacteria, F-specific RNA bacteriophage, Escherichia coli, woodchip, coconut husk, gravel, treatment wetland
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