The primary objective of this pilot-scale project was to demonstrate and validate anaerobic membrane bioreactor (AnMBR) technology for domestic wastewater treatment. Specific objectives associated with this project included: 1) demonstrate the effectiveness of AnMBR at treating screened domestic wastewater at temperatures above 10°C to produce high-quality, re-usable water; 2) demonstrate that AnMBR technology for domestic wastewater treatment can be operated in an energy-neutral manner; 3) compare cost and performance of gas-sparged and granular activated carbon(GAC)-fluidized AnMBRs to conventional aerobic wastewater treatment systems; and 4) conduct a simplified lifecycle assessment of the technology in comparison to conventional technologies.
AnMBR technology is the marriage of anaerobic biological treatment and physical membrane separation. There are several different configurations of the AnMBR system. The main elements of the AnMBR system are a primary anaerobic bioreactor and a secondary membrane bioreactor. The primary anaerobic bioreactor contains microorganisms that convert organic carbon and associated five-day biochemical oxygen demand (BOD5) in wastewater into an energy-rich, biogas-containing methane and carbon dioxide. The biogas produced in the primary anaerobic bioreactor can be used to generate electricity, heat, or fuel for vehicles. The secondary membrane bioreactor contains ultrafiltration (UF) membranes that separate the microorganisms and other suspended solids from the treated effluent (permeate). The pilot systems included in this demonstration used different methods of UF membrane flux maintenance. One pilot system used gas-sparging (gas-sparged AnMBR), and the other pilot system used GAC‑fluidization (GAC-fluidized AnMBR). The primary bioreactor in the gas-sparged AnMBR was a suspended growth system. The primary bioreactor in the GAC-fluidized AnMBR was a GAC-fluidized bed reactor. Downstream processes were also evaluated including dissolved methane removal, sulfide and phosphorus removal, and ammonia removal.
From the 2015 NWRI Clarke Prize Conference: Anaerobic Fluidized Bed Membrane Bioreactor Treatment of Domestic Wastewater for Potential Reuse, presented by Perry McCarty, Sc.D., Stanford University.
Performance of the AnMBR systems was evaluated with respect to effectiveness, net energy production efficiency, and implementability. The effectiveness of the AnMBR technology was assessed with respect to treated water quality. Both AnMBR systems met or exceeded EPA secondary treatment objectives, but the GAC-fluidized AnMBR achieved better effluent quality at lower hydraulic residence times. Ammonia, total phosphorus, sulfide, and dissolved methane were also removed to varying extents. Energy consumption and production were calculated for a matrix of operating scenarios that included various net permeate fluxes and temperatures for the gas-sparged and GAC-fluidized AnMBRs. In general, energy-neutral or -positive operation was more likely at greater flux, temperature, and influent COD concentration. The AnMBR process has the potential to be cost-competitive with conventional treatment. The application of a hybrid process involving a GAC-fluidized bioreactor followed by a gas-sparged UF membrane process and a low cost process for dissolved methane removal appears to be promising. Alternative methods for sulfide removal such as biological oxidation should be evaluated because chemical coagulation is likely to be cost-prohibitive. In general, conventional treatment had the lowest overall environmental impact, followed by primary sedimentation in combination with a hybrid AnMBR comprised of a GAC-fluidized bioreactor, a gas-sparged UF membrane, a vacuum degasser for dissolved methane removal, and chemical coagulation for sulfide and phosphorus removal. Chemical consumption during sulfide and phosphorous removal are the primary environmental impact drivers. Considering that sulfide is probably more of a driver of chemical use than phosphorus (and that phosphorus removal may not always be necessary), alternative methods such as biological sulfide oxidation should be explored. Integration of alternative methods for sulfide removal alongside bioenergy recovery is necessary for developing an AnMBR treatment process that is be more sustainable than a conventional treatment approach.
The results of this demonstration and economic analysis support use of primary sedimentation followed by a bioreactor and a gas-sparged UF membrane system. Inclusion of primary sedimentation in the process is projected to provide a greater potential for energy-neutral or energy-positive operation. Based on the results of this demonstration, the recommendation is to use a hybrid AnMBR comprised of a GAC-fluidized bioreactor followed by gas-sparged UF membranes. Dissolved methane removal using vacuum-operated membrane contactors was determined to have potential of removing 90% dissolved methane, but the pressure loss through the contactors will result in high energy consumption. Therefore, alternative dissolved methane removal technologies, such as vacuum degassers, warrant evaluation as they have the potential for low-cost and low-energy consumption. Sulfide must be removed prior to discharge or reuse. Coagulation-flocculation-sedimentation is a standard process and was demonstrated to be capable of sulfide and total phosphorus removal. Chemical cost and environmental impact associated with sulfide removal were determined to be high. Alternative sulfide removal technologies, such as biological sulfide oxidation, may also be effective and less expensive. Further research into cost-effective and sustainable technologies for sulfide and phosphorus removal is recommended. Nitrogen removal requires further evaluation. Clinoptilolite was capable of removing ammonia in this demonstration, but the brine was not capable of being regenerated. Use of regenerable clinoptilolite downstream of an AnMBR is being evaluated further in ESTCP project ER-201728. Other options for nitrogen removal have also been evaluated and should be considered. (Project Completion - 2019)
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