Biological Removal, Membrane Separation, and Thermal Destruction: A Multi-Barrier Approach to Potable Water Reuse and Waste Heat Recovery

Onsite wastewater treatment and potable reuse technologies represent significant electrical energy consumption and operational costs at Department of Defense (DoD) fixed installations. In this work, a membrane bioreactor – membrane distillation (MBR-MD) system is proposed to accomplish high quality wastewater reuse objectives and represent an opportunity for energy independence and water supply security. This MBR system aims to accomplish treatment of wastewater to potable reuse standards while maintaining low electrical energy demand. Compared to conventional wastewater treatment methods this system offers the following advantages, 1) higher effluent quality, 2) smaller footprint, 3) increased sludge concentration, and 4) lower excess sludge waste. In MBR applications, external pressure (or vacuum) induces the flow of water across a porous membrane where suspended solids and other macromolecules are rejected by the membrane. In the proposed MBR system (Figure 1), wastewater is fed into a bioreactor and an ultrafiltration (UF) membrane provides high rejection of contaminants in the wastewater. The MBR filtrate is then processed by the membrane distillation (MD) stage that generates high-quality product water. MD involves the transport of mass and heat through a hydrophobic, microporous membrane. In MD, a feed solution at elevated temperature is in contact with one side of the membrane and the permeate is collected on the opposite side of the membrane; the temperature difference across the membrane induces a vapor pressure gradient for mass transfer. Compared to conventional distillation methods, MD requires only small temperature differences achievable using low-grade or waste heat sources. This treatment scheme represents a multi-barrier system (Figure 2) that includes biological removal, membrane separation, and thermal destruction. This is particularly important when microbial and trace organic contaminants (pharmaceuticals or personal care products) are present. Using waste heat existing on-site at DoD installations, this MBR-MD system can produce potable-quality water with minimal electrical energy input and minimal waste, resulting in lower operational costs as well as water and energy savings. The demonstration of this engineering-scale system is composed of two phases. The first phase is in in-house testing at the WEST Center in Tucson, AZ. This is a state-of-the-art facility collocated with a wastewater reclamation facility which allows for access to direct sources of wastewater and reclaimed water. The second phase is field testing and will be conducted at the Ft. Huachuca army base wastewater treatment facility. This location allows for real-world testing at a facility with wastewater qualities that capture daily and seasonal fluctuations. The goal of the demonstration is to construct and validate the designed MBR-MD system and to investigate its efficiency at producing high quality water that is suitable for potable reuse at DoD fixed installations. The system will be considered successful if the following goals are met: - Overall specific energy consumption (SEC) below 4.5 kWh/m3 - Total organic carbon (TOC) removal to levels below 0.5 mg/L - Log removal of no less than 12 for viruses, 10 for Giardia, and 10 for Cryptosporidium - Better removal of pharmaceutical and personal care products (PCCPs) than other technologies - System robustness demonstrated by minimal operator intervention and system downtime Currently this work is in the first phase and in-house testing and characterization of the MD unit is of primary focus. The MD system has been shown to be capable of producing up to 1030 L/day of product water (Figure 3) and providing up to 2-log removal of selected ions and total organic carbon (Figures 4) of a treated wastewater stream. The system also showed gain-output-ratio (GOR) values between 7-9 during initial testing. A high GOR values is important because it is a measurement of how efficient the system is at recovering and reusing heat supplied to the MD. The GOR values reached in this testing are considerably high. Water quality preliminary results show promising potential to address rejection of inorganic and organic matter for producing potable-quality water while maintaining low energy demand. During the presentation, pathogen and trace organic rejection and energy efficiency of the whole MBR-MD system operating on the field will be analyzed and discussed.