Full-Scale RO 3+ Log Removal of Virus Surrogates Using Native and Spiked Molecular Markers and Condusctivity Profiling

Advanced treatment trains for potable reuse in California are required to demonstrate high log removal values (LRVs) for Giardia, Cryptosporidium, and enteric viruses. Reverse osmosis (RO) is a key treatment step that contributes to the total required LRV credits. The RO system at the Orange County Water District (OCWD) Groundwater Replenishment System (GWRS) currently employs redundant online total organic carbon (TOC) analyzers that continuously demonstrate 2+ log removal of TOC. The measured rejection of TOC across RO feed to permeate is applied as the pathogen credit value based on the daily average LRV. Electrical conductivity (EC) is used as a backup to demonstrate a 1.5+ log removal. OCWD is interested in obtaining greater LRV credit for GWRS. The objective of this study was to demonstrate higher LRV credit for RO at full scale using marker-based and conductivity profiling-based approaches. One approach for testing RO integrity is to spike uranine (a fluorescent compound) into the feed and trace its passage into the RO permeate. Sulfate and strontium were also sampled because they are known to natively occur at high enough concentrations in feed (i.e., these were not spiked). The conductivity profiling-based approach is a previously developed statistical analysis framework that can potentially be used to demonstrate greater LRV credit than traditional EC LRV by isolating “diffusion” versus “defect” related measurements. The method relies on conductivity profiling to identify integrity-related issues and to determine outliers (i.e., RO pressure vessels with relatively high EC in the permeate). The statistical analysis method applied to weekly RO permeate conductivity profiles (over six months) from a full-scale 5-MGD RO unit demonstrated an average of 3.6-log removal value. Uranine spiking tests in the same RO unit resulted in LRV up to 3.5-log. The observed removals for strontium and sulfate were similar, ranging between 2.9 – 3.7 LRV for sulfate and 3.1-3.9 LRV for strontium.The testing results from this study provide alternative methods for pathogen LRV crediting and monitoring of RO systems, which may be particularly attractive for direct potable reuse systems having the greatest LRV requirements.

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