Hypoxic Methane Oxidation Coupled to Denitrification in a Membrane Biofilm Reactor

Abstract

Nitrate removal has become a necessity in Canada and around the globe as a means to mitigate harmful effects from nitrogen compounds, such as eutrophication, which pose toxic hazards to both aquatic life as well as human health. Nitrogen compounds, among other contaminants, will be or already have been regulated for drinking water and wastewater effluent. Thus, to aid in compliance with regulations, biological nitrogen removal has been thoroughly investigated using numerous bioreactor configurations and electron donors with consideration given to minimizing carbon footprint and finding cost-efficient methods. The membrane biofilm reactor (MBfR) is a bio-technique that has been investigated for the removal of various contaminants from water bodies and wastewater. This approach uses a gaseous substrate that serves as an electron donor (e.g., methane or hydrogen), which is supplied via a hollow-fiber membrane lumen to the acclimated biofilm on the outer surface. This technology is an option for efficient nitrate removal with methane as the primary electron donor and carbon source. This treatment is both practical and eco-friendly because methane can be produced on-site in wastewater treatment plants at a lower cost than methanol that is widely used for denitrification. Hence, the objectives of this study was to gain an advanced understanding of the process of methane oxidation coupled to denitrification (MOD) in a methane-based membrane biofilm reactor. The study also aimed to systematically characterize and optimize the effects of operating conditions, hydraulic retention time (HRT), nitrate loading rate (NLR), methane flux at high and low methane pressures. In addition, nitrite removal via methane-based MBfR was also explored to evaluate the sustainability of nitrite reduction compared to nitrate reduction in the denitrification process under hypoxic conditions. In addition, the microbial population in the MBfR was studied under different operating scenarios. An MBfR with gas-permeable hydrophobic polyethylene fibers, enriched with MOD culture was operated while applying high methane pressures (7, 5, and 2 psig), and total specific surface area of 35 m2/m3, have showed a relatively low nitrate removal rate, with 1.2 – 1.3 mg N/L-h at methane pressure of 2-7 psig and HRT 12 h, while the dissolved methane was as high as (8 -13 mg CH4/L). These results suggest that the methane oxidation and nitrate reduction are limited by the microorganism’s kinetics, rather than methane transfer to the biofilm. The sequencing analysis showed Methylocystaceae was dominant, with 21% of bacterial SSU rRNA genes. Moreover, there were no traces of archaea in the community in either biofilm or planktonic samples. An MBfR enriched with Methylocystaceae using a hydrophobic polyvinylidene difluoride (PVDF) gas permeable membranes, with total surface area of 188.5 cm2, was operated at low methane pressures (0.05 – 0.25 psig) to monitor the impact on the dissolved methane concentrations and nitrate reduction. The nitrate concentration in the effluent was 4.0 mg NO3 - /L, with a minimal methane concentration in the effluent of 3.3 mg CH4/L with a hydraulic retention time of 4 hours. These results imply that the implication of this type of gas permeable membranes have proven to be more efficient in methane gas delivery, in which the nitrate removal flux was as high as 412.2 mg N/m2-d, and successfully managed to decrease the dissolved methane in the effluent. The dissolved oxygen in the MBfR recorded an average of 0.04 mg O2/L. The 16S rRNA gene sequencing analysis detected Methylococcus bacteria that are able to oxidize methane coupled to denitrification. The results indicated syntrophic microbial interaction in a consortium of aerobic methanotrophs and denitrifiers in the active biofilm of a methane-based MBfR under hypoxic conditions. A methane-based MBfR inoculated with 21% Methylocystaceae in bacterial SSU rRNA genes, evidenced to have the ability to remove nitrite; with a removal flux was up to 885 mg N/m2-d, at a relatively low methane pressure of 2.4 kPa (0.35 psig); indicating that nitrite reduction step is not the main rate limiting step in the denitrification process inside the biofilm. The microbial community sequencing showed the existence of Methylococcus capsulatus, a Type I methanotroph, at relative abundance of 78% for the maximum HRT of 12 hours and a methane pressure of 0.35 kPa (0.05 psig), while the relative abundance decreased when the HRT was 4 and 2 hours at the same methane pressure. This implies that we cannot correlate Methylococcus capsulatus abundance to the electron donor (methane) flux in this particular experimental set. The study outcomes support that methanebased MBfRs can be a proficient and sustainable biotechnology approach to meet strict nitrogen standards in wastewater effluent, under hypoxic conditions with insignificant methane buildup.