Addressing Low Molecular Weight Organics in DPR using Membrane Bioreactors

The Pure Water Southern California Program is a partnership between the Metropolitan Water District of Southern California and the Los Angeles County Sanitation Districts. The program is targeted to create a new water supply to help meet the region's needs by providing up to 150 million gallons of water daily through indirect and direct potable reuse. The Pure Water SoCal Demonstration Plant has been evaluating treatment with membrane bioreactors (MBRs), reverse osmosis (RO), and ultraviolet light (UV) advanced oxidation (AOx) to comply with evolving state regulations for potable reuse. Draft criteria for direct potable reuse were first released in March 2021, requiring the use of ozone and biologically active carbon (BAC) upstream of RO and AOx to reduce the risk of low molecular weight chemical spikes passing through the advanced treatment processes. The final version of the regulations, released in December 2023, requires that an ozone-BAC system achieve a 1-log reduction of acetone, formaldehyde, carbamazepine, and sulfamethoxazole, or that the facility include an alternative treatment process providing an equivalent level of chemical reduction and public health protection using diverse treatment mechanisms. MBRs provide an opportunity for the reduction of organic chemical spikes using three diverse treatment mechanisms: air stripping, adsorption, and biological degradation. MBRs are expected to provide enhanced treatment, when compared to a conventional wastewater treatment system, due to considerably higher solids concentrations, microbial densities, solids retention times, and aeration rates. To evaluate MBRs as a treatment process for chemical reduction, the demonstration facility feed water was injected with varying doses of acetone and formaldehyde over four to eight-hour durations, evaluating the reduction of the chemicals through the MBR, RO, and UV/AOx processes. Spiked concentrations ranged from 2.5 mg/L to 33 mg/L. Removal rates were compared to removal rates seen with background levels of both chemical and with removals seen at the adjacent wastewater treatment facility, indicating that the biological treatment system in the MBR was able to adapt to the elevated concentrations, significantly increasing the rate of chemical removal after three to four hours of elevated feed concentrations. Removals were considerably higher than the 1-log requirement for ozone-BAC, exceeding 2.2-log for both formaldehyde and acetone. This paper explores chemical removals accomplished through different processes at the Demonstration Plant. It delves into the challenges posed by specific chemicals in each treatment process and removal mechanism. Additionally, the paper discusses the potential application of the integrated treatment train in a future direct potable reuse system. This system aims to comply with new regulations while ensuring the delivery of safe and reliable water to supplement existing supplies.