Hypoxic Methane Oxidation Coupled to Denitrification in a Membrane Biofilm Reactor

Nitrate removal has become a necessity in Canada and around the globe as a means to mitigate \nharmful effects from nitrogen compounds, such as eutrophication, which pose toxic hazards to \nboth aquatic life as well as human health. Nitrogen compounds, among other contaminants, will \nbe or already have been regulated for drinking water and wastewater effluent. Thus, to aid in \ncompliance with regulations, biological nitrogen removal has been thoroughly investigated using \nnumerous bioreactor configurations and electron donors with consideration given to minimizing \ncarbon footprint and finding cost-efficient methods. The membrane biofilm reactor (MBfR) is a \nbio-technique that has been investigated for the removal of various contaminants from water \nbodies and wastewater. This approach uses a gaseous substrate that serves as an electron donor \n(e.g., methane or hydrogen), which is supplied via a hollow-fiber membrane lumen to the \nacclimated biofilm on the outer surface. This technology is an option for efficient nitrate removal \nwith methane as the primary electron donor and carbon source. This treatment is both practical \nand eco-friendly because methane can be produced on-site in wastewater treatment plants at a \nlower cost than methanol that is widely used for denitrification. Hence, the objectives of this \nstudy was to gain an advanced understanding of the process of methane oxidation coupled to \ndenitrification (MOD) in a methane-based membrane biofilm reactor. The study also aimed to \nsystematically characterize and optimize the effects of operating conditions, hydraulic retention \ntime (HRT), nitrate loading rate (NLR), methane flux at high and low methane pressures. In \naddition, nitrite removal via methane-based MBfR was also explored to evaluate the \nsustainability of nitrite reduction compared to nitrate reduction in the denitrification process \nunder hypoxic conditions. In addition, the microbial population in the MBfR was studied under \ndifferent operating scenarios. \nAn MBfR with gas-permeable hydrophobic polyethylene fibers, enriched with MOD culture was \noperated while applying high methane pressures (7, 5, and 2 psig), and total specific surface area \nof 35 m2/m3, have showed a relatively low nitrate removal rate, with 1.2 – 1.3 mg N/L-h at \nmethane pressure of 2-7 psig and HRT 12 h, while the dissolved methane was as high as (8 -13 \nmg CH4/L). These results suggest that the methane oxidation and nitrate reduction are limited by \nthe microorganism’s kinetics, rather than methane transfer to the biofilm. The sequencing analysis showed Methylocystaceae was dominant, with 21% of bacterial SSU rRNA genes. \nMoreover, there were no traces of archaea in the community in either biofilm or planktonic \nsamples. An MBfR enriched with Methylocystaceae using a hydrophobic polyvinylidene \ndifluoride (PVDF) gas permeable membranes, with total surface area of 188.5 cm2, was operated \nat low methane pressures (0.05 – 0.25 psig) to monitor the impact on the dissolved methane \nconcentrations and nitrate reduction. The nitrate concentration in the effluent was 4.0 mg NO3 \n- \n/L, with a minimal methane concentration in the effluent of 3.3 mg CH4/L with a hydraulic \nretention time of 4 hours. These results imply that the implication of this type of gas permeable \nmembranes have proven to be more efficient in methane gas delivery, in which the nitrate \nremoval flux was as high as 412.2 mg N/m2-d, and successfully managed to decrease the \ndissolved methane in the effluent. The dissolved oxygen in the MBfR recorded an average of \n0.04 mg O2/L. The 16S rRNA gene sequencing analysis detected Methylococcus bacteria that are \nable to oxidize methane coupled to denitrification. The results indicated syntrophic microbial \ninteraction in a consortium of aerobic methanotrophs and denitrifiers in the active biofilm of a \nmethane-based MBfR under hypoxic conditions. A methane-based MBfR inoculated with 21% \nMethylocystaceae in bacterial SSU rRNA genes, evidenced to have the ability to remove nitrite; \nwith a removal flux was up to 885 mg N/m2-d, at a relatively low methane pressure of 2.4 kPa \n(0.35 psig); indicating that nitrite reduction step is not the main rate limiting step in the \ndenitrification process inside the biofilm. The microbial community sequencing showed the \nexistence of Methylococcus capsulatus, a Type I methanotroph, at relative abundance of 78% for \nthe maximum HRT of 12 hours and a methane pressure of 0.35 kPa (0.05 psig), while the \nrelative abundance decreased when the HRT was 4 and 2 hours at the same methane pressure. \nThis implies that we cannot correlate Methylococcus capsulatus abundance to the electron donor \n(methane) flux in this particular experimental set. The study outcomes support that methanebased \nMBfRs can be a proficient and sustainable biotechnology approach to meet strict nitrogen \nstandards in wastewater effluent, under hypoxic conditions with insignificant methane buildup.

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