The main objective of this research was to design, fabricate, and test a pilot-scale osmotic membrane bioreactor (OMBR)-membrane distillation (MD) system consisting of biological reactor (BIO), forward osmosis (FO), and MD processes and driven by on-site waste heat. As part of this, the operating parameters necessary for highly efficient wastewater treatment and water reuse were determined. The biologically-catalyzed processes of the OMBR provide a combined treatment process for organic matter degradation and nutrient removal. When coupled to the osmotically driven mass transport mechanisms of FO, the system offers a low-fouling alternative to traditional membrane bioreactor (MBR) applications while also decreasing the electrical energy demands required for the process. The FO membranes also serve as a selective barrier system capable of rejecting recalcitrant trace organic compounds to prevent passage into the draw solution stream, thereby decreasing fouling in the downstream MD process.
The steps toward achieving the research goals were to systematically investigate the three major processes of the OMBR system (BIO, FO, and MD) and, at the same time, determine the energy requirements for each process. Subsequently, the processes were integrated into a single system through the design and construction of a modular pilot-scale OMBR system that operates using renewable energy or low-grade (“waste”) heat existing at forward operating bases (FOB). At each stage of the research, process performance was evaluated based on specific indicators (e.g., water quality and quantity, energy and resource consumption, and frequency of hands-on modification to the system).
To carry out the objectives, several tasks were performed including fundamental studies aimed at researching specific issues for each of the processes (BIO, FO, and MD) and the heat exchangers. With regard to each process, investigation of the critical phenomena enabled answering of fundamental questions about microbial ecology, FO membrane fouling, and MD membrane flooding that are essential for scale-up and integration of these technologies. The research team collaborated with Hydration Technologies, Inc. (HTI) (now Fluid Technology Solutions, Inc.), GE Water & Process Technologies, and Parker Performance Materials to acquire the most appropriate FO and MD membranes to characterize and test. Next, process models and controls for each of the subsystems were developed. Finally, OMBR and MD processes were integrated and a system-level model was developed and tested for water quality and membrane performance.
The project started by investigating fundamental aspects of the main components of the OMBR-MD system in Task 1: bioreactor design, instrumentation, and control; Task 2: bench-scale FO membrane fouling; Task 3: bench-scale MD process modeling; and Task 4: heat exchanger design and computational fluid dynamics (CFD) modeling for MD. Then it progressed toward the assembly of the integrated pilot-scale system in Task 5: lab-scale BIO+FO sub-system testing; Task 6: design of lab-scale MD + heat exchanger sub-system; Task 7: lab-scale FO-BIO and MD-heat exchanger sub-system assembly; Task 8: lab-scale OMBR system testing; and Task 9: OMBR system scale-up testing.
An integrated OMBR-MD system was designed, fabricated, and tested at the pilot-scale for wastewater treatment and production of high-quality reuse water. There were very clear synergies in using MD to reconcentrate the draw solution used in the OMBR process, in using low-grade heat/renewable energy to drive the MD process, and in recovering heat from the distillate solution to reduce the overall heat consumption of the system. The abundance of low-grade heat produced by diesel generators at FOBs make the OMBR-MD system an ideal system for such locations. During long-term OMBR operation, carbon and nitrogen removal was achieved in a single-reactor by alternating between aerobic and anoxic bioreactor conditions. Results from long-term testing using a high-strength wastewater showed 98.4% removal of chemical oxygen demand (COD) and 90.2% removal of NH4+-N was achieved.
Physical design aspects and process controls for system integration as well as procedures for wastewater treatment with the single-stage OMBR were established. Several areas for improvement were also identified throughout the development and testing phases of this system. Namely, passage of some dissolved, low molecular-weight contaminants into the closed-loop FO-MD solution led to MD membrane fouling and reduced MD water flux, and over time, the ammonium concentration in the distillate solution tended to increase. A suggested improved design will draw upon this and other lessons learned during the current project. Ammonium passage into the distillate solution is mitigated by improved biological ammonium removal (i.e., nitrification/denitrification) in the OMBR. To accomplish this, it is suggested to configure the OMBR with a dual-chamber tank with separate anoxic and oxic zones. Aeration in the oxic zone would be optimized for nitrification and membrane cleaning; the anoxic zone would be optimized for denitrification. The revised OBMR-MD system operates in a semi-batch configuration defined by two separate process loops. One loop operates the OMBR and the other operates multiple MD modules. Each loop operates individually until two triggering conditions are met; the draw solution reaches a target dilution in the OMBR loop and the MD feed solution reaches a target concentration in the MD loop. Once the triggering conditions are met, the solutions in each loop are exchanged; the diluted FO draw solution is then treated by MD and the concentrated MD feed solution is then diluted by FO. This semi-batch process scheme allows for independent and “continuous” operation of the FO and MD loops, reducing the complexity of the necessary control mechanisms to maintain steady-state operation of a dynamic, continuous process and thus implement a simplified operation that maximizes the performance of each process. Once the tank switch occurs, the diluted draw solution volume will pass to the MD loop and undergo further treatment through a fine screen, a cartridge filter, a carbon filter, and an ultraviolet (UV) reactor to reduce the possibility of membrane fouling, membrane damage, or passage through the MD membrane by small organic compounds or disinfection residual.
The suggested revised system will be programmed to periodically backwash the FO membranes to extend membrane life and improve performance. The backwash cycle will utilize a portion of the distillate stored in a standalone tank. When FO performance degrades, or after a predetermined time period has elapsed, the system will cease normal operation and change to the necessary valve configuration to route the distillate to the FO module. Backwashing in a counter-flow regime will reverse the impact of fouling and scaling that occurs over time on the FO membrane. The used backwash solution will be further treated with MD to maximize water recovery. Regarding the MD subsystem and based on testing subsequent to development of the current OMBR-MD system, it is suggested to consider use of an FO module in the revised system instead of the DCMD module that is used in the current system. In AGMD, heat recovery can be integrated into the module design.
Compared to conventional distillation methods, MD requires only small temperature differences - temperature differences achievable through the use of low-grade heat/renewable energy. Compared to RO, the driving force in MD is essentially not reduced by osmotic pressure and thus, MD operates at hydrostatic pressure and can be used to treat high salinity solutions or provide enhanced recovery through brine desalination. Also compared to RO, MD produces an even higher quality distillate; it does not allow the passage of small non-volatile compounds and provides high removal of emerging contaminants. Specifically, MD has been shown to reject greater than 97% of low molecular weight organics. For these reasons, MD may have substantial energy, recovery, maintenance, and water quality advantages over conventional methods. This technology was further tested and validated under an ESTCP project.
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Morrow, C.P., N.M. Furtaw, J.R. Murphy, A. Achilli, E.A. Marchand, S.R. Hiibel, and A.E. Childress. 2018. Integrating an Aerobic/Anoxic Osmotic Membrane Bioreactor with Membrane Distillation for Potable Reuse. Desalination, 432:46-54.
Morrow, C.P., A. McGaughey, S.R. Hiibel, and A.E. Childress. 2018. Submerged or Sidestream? The Influence of Module Configuration on Fouling and Salinity in Osmotic Membrane Bioreactors. Journal of Membrane Science, 548:583-592.