(324b) An Integrated Membrane Bioreactor System for Low-Energy Wastewater Treatment

The main objective of this project is to develop a highly efficient wastewater treatment system to produce potable-quality reuse water with low energy input requirements. The designed system couples a novel osmotic membrane bioreactor (OMBR) with membrane distillation (MD). The OMBR consists of a forward osmosis (FO) membrane immersed inside a single bioreactor under alternating aerobic/anoxic conditions to promote complete nutrient treatment. The FO process utilizes a saline draw solution that will be reconcentrated using MD that will be powered by low-grade waste heat.

The focus of this presentation will be on the biological FO processes. The OMBR design 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 are being examined for adequate wastewater treatment with optimal flux through the FO process. One of the major challenges of continuous OMBR operation is maintaining adequate biomass concentrations to achieve denitrification. Efforts to address this have 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 NO_x 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 with 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 five-fold 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 is currently being used in the continuously operating OMBR system to increase both carbon and nitrogen loading. Higher MLSS levels and denitrification rates have been observed, however the FO membrane flux decreases more rapidly and requires 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 treatment processes.