Assessing the basis for regulatory crediting of virus LRVs for secondary biological wastewater treatment: A systematic review.

Regulatory frameworks for potable reuse often include stringent log reduction value (LRV) targets to ensure public health protection against exposure to viruses and protozoa. To achieve overall LRV targets and reduce associated capital and operational costs, it is important to maximize LRV credits awarded to each unit process in a potable reuse treatment train. This may include processes that are historically uncredited or undercredited, such as secondary biological wastewater treatment incorporating activated sludge and secondary clarification. To identify gaps in knowledge and inform future efforts to justify virus crediting, this systematic literature review and meta-analysis focused on characterizing virus attenuation during secondary treatment and identifying any virus characteristics, operational conditions, or water quality parameters that could serve as predictors of virus LRVs. Out of a total of 1,341 search results from three databases, this review considered 44 peer-reviewed studies that met the criteria for inclusion. LRVs as high as 5.3 log were reported for F-specific coliphages, but some studies reported increases in rotavirus and norovirus GI concentrations across secondary treatment. Median LRVs ranged from 0.8 log for rotavirus and norovirus GI (both based on molecular methods) to 2.4 to 3.4 log for poliovirus (culture) and polyomavirus (molecular), respectively. 5 percentile LRVs were <1.0 log for multiple viruses. Study publication date was shown to have a significant impact, pooled virus LRVs exhibited a moderately strong correlation with reduction in biochemical oxygen demand (BOD), and mixed liquor suspended solids (MLSS) concentration demonstrated a threshold (∼1000 mg/L) beyond which there was no discernible relationship with virus LRV. In general, the lack of reported background data hindered identification of critical parameters that could inform model development or serve as surrogates for estimating virus LRV. More robust datasets that simultaneously evaluate a range of viruses and operational/water quality parameters are needed before LRV credits can be broadly and confidently awarded to conventional secondary biological wastewater treatment systems.