Virus Log Reduction Values and Dominant Mechanisms in Full‐Scale Secondary Biological Wastewater Treatment Systems

Secondary biological wastewater treatment is a core component of potable reuse treatment trains, but it is often uncredited for virus attenuation. This study evaluated virus log reduction values (LRVs) at two full‐scale water resource recovery facilities in Southern Nevada: one with conventional activated sludge and another with lagoon treatment. Four human enteric viruses and four fecal indicator viruses were quantified using molecular assays; culture assays quantified F‐specific and somatic coliphages. Median LRVs were low (< 1.0) for plant viruses and generally higher (> 1.0) for adenovirus and crAssphage, consistent with predictions from a solids partitioning model. The fifth percentile LRVs that often drive regulatory determinations were < 0.5 for norovirus GI/GII. For conventional activated sludge, nucleic acid decay was a significant contributor to the molecular LRVs, whereas culturable coliphage data (median LRV = 2.5) highlighted solids attachment and subsequent physical removal as a dominant mechanism. In contrast, lagoon treatment sometimes achieved LRVs > 4.0 for culturable coliphages, primarily due to temperature‐dependent inactivation. Based on the collective insight from recent studies, adoption of a broad virus LRV crediting framework for conventional activated sludge systems in potable reuse applications will require better alignment between molecular and culture methods and a deeper understanding of virus attenuation mechanisms.