Strategies for Gaining Pathogen Removal Credits using Reverse Osmosis Membranes

Past studies have demonstrated that reverse osmosis (RO) membranes can be highly effective in rejecting pathogens such as Cryptosporidium oocysts, Giardia cysts, and viruses. Nonetheless, most regulatory agencies have defaulted to a null log removal value (LRV) when using RO membranes. This disparity between regulatory acknowledgement and implementation is largely due to the challenge of developing a practical Direct Integrity Testing (DIT) method. Undervaluing the pathogen control efficacy from RO membranes necessitates additional treatment processes, e.g., chemical disinfection or physical separation, to meet pathogen LRV goals for potable reuse applications. This lack of acknowledgement leads to increased capital costs and redundant operational complexity. The United States Environmental Protection Agency (USEPA) Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) and USEPA Membrane Filtration Guidance Manual (MFGM) provides a framework in determining pathogen credit for Cryptosporidium, but it is then left at the discretion of State agencies to award credit for other pathogens such as viruses. Although RO technology transcends state boundaries, the current regulatory path to obtain accreditation in potable reuse applications is on a state-by-state basis. For example, the California State Water Resources Control Board granted the RO process of the Orange County Groundwater Replenishment System with a 2-log LRV for pathogens while Texas has not established any treatment credit. Methodology & Results Our study investigates the use of surrogates as an integrity testing method using the Wichita Falls brackish surface water treatment plant as a test site. Through parallel testing using a virus surrogate, i.e., MS2 bacteriophage, our results provide a better understanding of the variability in pathogen removal efficiency for RO membranes. Our testing utilized background concentrations of sulfate and strontium in the range of 800-1000 mg/L and 3000-5000 ug/L, respectively, with MS2 doses of 105 - 106 PFU/mL to compare rejection performance. Testing was performed on a variety of membrane conditions, including membranes that have been in continuous full-scale operations for +9 years, newly purchased membranes, and intentionally impaired and oxidized membranes. Multiple sample locations were analyzed, including samples from individual pressure vessels, combined permeate, and total concentrate. Lastly, samples were measured over the course of a 20-to-40-minute duration to identify effects of attenuation or accumulation that may occur with surrogates. To date, this is the only full-scale study of its kind and provides comprehensive data from a variety of practical conditions, such as varying membrane condition, multiple sample locations on a full-scale RO system, and measurements taken over a practical timescale. Study Implications Although prior studies with a similar scope have been implemented, our study has two novel elements that will expand industry and regulatory knowledge on this subject. First, our study utilizes a full-scale system to demonstrate practical RO performance and second, our analysis is performed on a variety of operating conditions. This data can be an integral tool for public and regulatory agencies to award pathogen LRVs for RO membranes while also providing guidance for systems to monitor and maintain their RO system's pathogen rejection performance. Our presentation will address the following objectives that can be valuable for reuse applications. 1.Learn whether ions are suitable surrogates in identifying virus-sized breaches or defects in RO membranes, including limitations and recommended practices of the proposed method. 2.Review statistical correlations between removal efficiencies of surrogates and other conventional water quality parameters, e.g., total organic carbon (TOC) or conductivity, and determine whether they can be used for indirect integrity monitoring. 3.Review the effects of membrane condition that impact DIT testing accuracy.