Removal of MS‐2 coliphage in full‐scale reverse osmosis systems

Abstract

AbstractRecent activity in the potable reuse field will incorporate reverse osmosis (RO) processes that will be required to provide pathogen removal. The California State Water Resources Control Board has implemented a 12‐10‐10 log removal value (LRV) requirement for viruses, Giardia, and Cryptosporidium for groundwater recharge projects. Depending on the details of a Surface Water Source Augmentation Project, new regulations in 2018 specify an additional log reduction requirement up to a total of 14‐12‐12. Regulatory requirements that specify a treatment technique (e.g., RO) and establish log removal requirements for treatment trains are being implemented. Surrogate indicators such as electrical conductivity (EC) and total organic carbon (TOC) reduction are currently being used as online indicators to determine compliance of RO systems. The actual virus removal through a full‐scale operating system is something that has not been extensively characterized. Most of the information that exists for testing with viruses is associated with bench‐ or pilot‐scale evaluations. Evaluations of permeate from full‐scale RO systems operating with wastewater effluent as the source water indicate that viruses are not present; however, concerns remain about the potential loss of integrity through the RO system. Under the regulatory criteria established in the US Environmental Protection Agency Membrane Filtration Guidance Manual for drinking water, bulk surrogate indicators such as EC or TOC lack the sensitivity to accurately measure the actual log reduction for pathogens. It is generally recognized that the feed water quality matrix can be variable in composition, which can also affect the measurement. Bulk surrogate indicators do not satisfy the criteria for being discretely quantifiable, a term that is synonymous with the indicator having a specific molecular weight or defined composition. While alternative chemical markers such as fluorescence compounds, sulfate, or strontium yield higher LRVs, the methods are not easily implemented. An underlying issue with these indicators is that, because of their small size, they will also diffuse through the membrane and understate the pathogen removal that is occurring. An article presented in Journal AWWA proposes that the results of a conductivity profile can be used to increase the sensitivity and calculate a higher LRV. Using the dilution model as the underlying approach, the technique differentiates conductivity that would be associated with diffusion from conductivity that would be associated with a defect. Literature review of prior work with MS‐2 challenge studies has been limited to small‐scale pilot evaluations. Information regarding the removal of MS‐2 coliphage in a typical production system was not present in literature. Conceptually, there should be no difference in MS‐2 removal between a pilot‐ and a full‐scale system; however, loss of rejection due to membrane aging may be a consideration. This article provides the challenge test results of MS‐2 bacteriophage (virus) through a full‐scale RO train operating in a water recycling facility. Challenge testing was performed at various operating conditions and includes the conductivity profiling results to characterize removal under integral, as well as intentionally compromised, conditions.