Forward Osmosis Membrane Bioreactor Performance for Wastewater Treatment Applications
AuthorFreitas, Ally M.
Civil and Environmental Engineering
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Global water shortage and scarcity has spurred worldwide efforts to explore alternative approaches to augment the existing water supply. Recent consideration for the utilization of wastewater as a viable water source has been adopted in several efforts to develop advanced treatment methods capable of providing adequate treatment for water reuse. The forward osmosis membrane bioreactor (OMBR) offers one such alternative, coupling conventional wastewater treatment processes with the promising high rejection properties of the forward osmosis (FO) membrane. The OMBR offers a low-energy alternative to conventional wastewater treatment and other energy intensive membrane processes (i.e., reverse osmosis), while still achieving advantageous wastewater treatment as well as high-quality salty product water obtained through the FO membrane. This preliminary FO process serves as a high-quality pretreatment step that has far-reaching expansion potential, as the product water may be further treated through an additional membrane treatment process to produce a potable and recyclable water source. This makes the OMBR an effective pretreatment device for recycled water applications. The single-stage reactor design employed here combines anoxic and aerobic processes to reduce the footprint and decrease energy costs of continuous aeration. The aeration cycling, solids and hydraulic retention times, and membrane cleaning strategies were examined for adequate wastewater treatment with optimal flux through the FO process. One of the major challenges facing the continuous operation of the OMBR system was maintaining adequate biomass concentrations to achieve denitrification when treating primary effluent from a local wastewater treatment facility. Efforts to address this focused on having sufficient carbon substrate present to facilitate microbial growth and denitrification. Spiking the primary effluent with an additional 300 mg/L glucose resulted in a noticeable decrease in NOx species, consistent with enhanced denitrification, and the reactor MLSS concentrations stabilized. Subsequent batch experiments using three subset reactor configurations with OMBR mixed liquor, OMBR mixed liquor spiked iiwith 30 mg/L NO3-, and OMBR mixed liquor spiked with 30 mg/L NO3- and 60 mg/L glucose validated the carbon limited status, as the denitrification rate increased from 0.102 to 0.214 to 0.523 mg-N/L-hr, respectively. The fivefold increase in denitrification observed in the reactor spiked with both NO3- and glucose further confirmed the influence additional carbon and nitrogen substrates have on increased reactor performance.A high-strength wastewater was eventually implemented in the continuously operating OMBR system to increase both carbon and nitrogen loading. Higher MLSS levels and denitrification rates were observed; however, the FO membrane flux decreased more rapidly and required more frequent membrane cleaning. These results demonstrate the complexity and interconnectedness of the operational parameters and the need for a fine-tuned balance when combining the two treatment processes. The fate of solute transport into the draw solution from the OMBR was also investigated. Solute behavior and fate in the draw solution stream is important because this draw solution will serve as the feed stream for the secondary membrane process. This coupled membrane system utilizes the OMBR as a pretreatment step, whose product water serves as the feed solution for the final treatment process to produce potable quality water while reconcentrating the FO draw solution in the process. Examining solute transport from the OMBR to the draw solution, and whether the secondary membrane process provides near complete rejection, gives insight into whether draw solution treatment or replacement will be required for optimal water quality production in the coupled membrane system. A series of abiotic studies examined the behavior of several solutes relevant to OMBR systems in an effort to determine the implications solute passage and buildup have for long-term and continuously operated OMBR systems.