Advancing Anaerobic Membrane Bioreactors for Low Temperature Domestic Wastewater Treatment
Van Steendam, Caroline
2019
Abstract
Anaerobic membrane bioreactors (AnMBRs) use anaerobic microorganisms to convert organic compounds present in waste streams to biogas, a renewable energy source. They employ a membrane to remove suspended solids from treated wastewater and ensure excellent effluent quality, which allows for water reuse. The promise of treating wastewater while producing energy and water has increased interest in AnMBRs. The domestic wastewater temperature in temperate climates is often below 20°C, with lows around 5°C. Operation at these temperatures raises economic and environmental concerns associated with membrane fouling and the loss of methane through the effluent. This dissertation research developed and evaluated novel AnMBR designs to address these concerns and advance sustainable domestic wastewater treatment. First, we examined patents to achieve a deeper understanding of the AnMBR innovation landscape and its technological direction. We additionally aimed to determine if environmental concerns are being addressed by the field. Our review showed that only a fraction of AnMBR inventions address membrane fouling and methane loss mitigation, two impediments to sustainable AnMBR operation as concluded by previous life cycle assessment studies. We then evaluated methods focused on monitoring direct interspecies electron transfer (DIET) in anaerobic digesters. DIET has been suggested to enhance anaerobic digestion and we considered promoting DIET in biofilms in our novel AnMBR designs. Recent research has shown that DIET alone does not always explain observed performance enhancements. Our review indicated that a combination of methods is necessary to confirm the occurrence and expand our knowledge of DIET. Finally, we present the design and evaluation of two novel AnMBRs: the biofilm-enhanced AnMBR (BfE-AnMBR) and the MagnaTree reactor. The bioreactor of the BfE-AnMBR is separated into three compartments using two conductive meshes to support biofilm growth and DIET. The flow in the bioreactor is regularly reversed to avoid clogging of the meshes while allowing for substrate staging and partial biomass migration between the different compartments. The bioreactor is connected to an energy efficient membrane filtration unit containing a rotating ceramic disc. The BfE-AnMBR was operated at 15°C for approximately nine months, but the anticipated substrate staging was not accomplished. The concentration of organic compounds in domestic wastewater was likely too low to achieve localized bioreactor souring. Given these unanticipated outcomes and the complexity of BfE-AnMBR design and operation, its operation was discontinued. Subsequently, a second design, the MagnaTree reactor, which primarily relies on biofilm treatment, was evaluated. Biofilm growth in the MagnaTree reactor is accomplished through biofilm development on a tree-like structure, which contains branches with openings wrapped with meshes. Similar to the BfE-AnMBR, the MagnaTree contains conductive meshes to promote DIET. Influent wastewater and biomass mixed liquor are continuously recirculated through one set of meshes to maximize biofilm treatment, while another set of meshes provides filtration for permeate production. The MagnaTree reactor achieved 86% chemical oxygen demand removal after a startup of three months at 21°C. Future work with the MagnaTree reactor will determine its performance limits at lower temperatures. In conclusion, our work with the MagnaTree reactor confirms that biofilms can harness sufficient microbial activity to achieve adequate anaerobic treatment of domestic wastewater at 21°C. Future research is necessary to confirm if fouling and dissolved methane mitigation concerns with the MagnaTree reactor are sufficiently addressed to ensure domestic wastewater treatment with net positive energy and net greenhouse gas emission reductions.Subjects
anaerobic membrane bioreactor domestic wastewater treatment
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