A Fully Integrated Membrane Bioreactor System for Wastewater Treatment in Remote Applications
Amy Childress | University of Southern California
Membrane bioreactors (MBRs) are proving to be effective and beneficial for wastewater reclamation due to their ability to produce high quality effluent. In a conventional MBR, microporous (microfiltration [MF] or ultrafiltration [UF]) membranes are immersed in a bioreactor and water is filtered through the membranes using vacuum; suspended solids are retained in the system and high levels of treatment can be achieved. MBRs replace the two stages of the conventional activated sludge process with a single, integrated process that has a much smaller footprint. MBR effluent may be suitable for use as irrigation water, process water, or potable water. For potable reuse, advanced treatment (e.g., reverse osmosis [RO] and chemical oxidation) is necessary after the MBR.
The advantages of MBRs over conventional treatment have been thoroughly reviewed and include consistently high product quality, ease of operation and automation, reduced footprint, reduced sludge production due to a high biomass concentration in the bioreactor, and complete suspended solids removal from the effluent. The main problem associated with MBRs is membrane fouling. Membrane fouling can occur in the MBR itself, and also in the downstream RO system. Specifically, high concentrations of dissolved organic compounds in the MBR effluent can cause severe fouling of RO membranes. Membrane fouling lowers productivity, increases energy requirements, increases frequency of membrane cleaning and replacement, and may result in deterioration of treated water quality.
In this project, a pilot-scale osmotic membrane bioreactor (OMBR) system consisting of a bioreactor, forward osmosis (FO) process, and membrane distillation (MD) process will be designed, constructed, and tested. MD reconcentration would be used in place of the RO process in order to significantly reduce energy requirements and make operation off-the-grid more feasible. The main objective of the investigation is to optimize the operating parameters necessary for developing a highly efficient wastewater treatment and water reuse system with a small footprint. In the investigation, University of Nevada, Reno (UNR) will work interactively with FO and MD membrane development companies to select and test FO and MD membranes and modules for OMBR system applications.
In satisfying the stated objectives, the following objectives of SERDP’s Statements of Need (SON) to develop innovative systems for on-site and sustainable wastewater treatment at the Department of Defense’s forward operating bases (FOBs) will be addressed: 1) development of a system capable of on-site treatment of gray and black wastewaters, 2) development of a system scalable in size, with a capacity to treat wastewater on site from approximately 250 personnel to many thousands, 3) development of a system capable of operating in a minimal external energy requirement (electricity), 4) development of a system capable of generating water for potable or non-potable reuse, and 5) development of a system capable of being transported to FOBs and requiring relatively minimal operator expertise, as well as having low operation and maintenance requirements.
The steps toward achieving the goals of the research are to systematically investigate the three major processes of the OMBR system (bioreactor [BIO], FO, and MD) and determine the energy requirements for each process. Subsequently, the processes will be integrated into a single system through the design and construction of a modular pilot-scale OMBR system that operates using renewable energy or with low-grade (“waste”) heat existing at FOBs. At each stage of the research, process performance will be evaluated based on specific indicators (e.g., water quality and quantity, energy and resources consumption, and frequency of hands-on modification to the system). From previous investigations, there is clear synergism in using MD to reconcentrate the draw solution used in the OMBR process in using low-grade heat and renewable energy to drive the MD process, and in using the bioreactor with influent wastewater as a heat sink. The abundance of low-grade heat produced by energy generators at FOBs make the OMBR-followed-by-MD system the ideal system for such locations.
The approach described below will demonstrate how the OMBR-followed-by-MD system is ideally suited to reclaim gray and black wastewaters for potable water supply at FOBs ranging from 250 to several thousand personnel. This innovative OMBR technology will reduce the burden of protecting the large fuel, water, and waste convoys through utilization of the natural processes of biological degradation and osmosis and while utilizing readily available low-grade heat and renewable energy to drive distillation.
Furthermore, due to the low-fouling nature of the FO and MD processes, cleaning requirements and membrane wear-and-tear will be minimal. More specifically, it is intended that the flux of the OMBR system be maintained by optimizing hydrodynamic operating conditions or using osmotic backwashing only, and thus, the FO process will likely require no cleaning chemicals. Because the MD membrane is essentially protected by the FO pre-treatment process, it is also likely to undergo minimal membrane fouling. Thus, costs associated with membrane chemical consumption and disposal and membrane replacement are expected to be low.
The OMBR system will also result in high removal of both traditional and emerging pollutants. The OMBR system treatment scheme not only represents a dual barrier system, but it represents complimentary utilization of the two most selective removal processes (distillation and osmosis). This is particularly important when low molecular weight organics are present and the product water needs to be utilized for direct or indirect potable reuse. Perhaps most important to SERDP is the ability to operate the system with low-grade heat and renewable energy. In this manner, wastewater reclamation would not contribute to the already high energy consumption of FOBs. It is well-known that fuel distribution faces frequent enemy attacks and the fully-burdened cost of fuel ranges hundreds of dollars per gallon of the delivered fuel. At many FOBs, support operations to power equipment, systems, and infrastructure represent a significant source of battlefield fuel demand, with the water heater for a field kitchen requiring more fuel than the AH-64D Apache attack helicopter. With one third of the Army’s total wartime fuel used for running electric generators, reducing electricity and energy demand at FOBs can result in significant fuel savings. (Anticipated Project Completion - 2018)
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