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.