The case for credit: toward a mechanistic model of solids partitioning and virus removal for secondary biological wastewater treatment

Regulatory frameworks for potable reuse often include stringent log reduction value (LRV) targets for viruses and protozoa. To reliably protect public health while also considering the sustainability of advanced water treatment, it is critically important to accurately and rapidly assess pathogen removal and avoid under-crediting of any unit process in a potable reuse treatment train. This study systematically evaluates secondary biological wastewater treatment across solids retention times (SRTs) ranging from 2 to 20 days and uses culture and molecular methods to characterize attenuation of 12 viruses, including fecal indicators, surrogate bacteriophages, and enteric pathogens. This study also proposes a mechanistic model for physical removal based on a solids partitioning/adsorption framework incorporating mixed liquor suspended solids (MLSS) concentrations. Mean sample-specific partitioning coefficients (K_d) ranged from 2.1 log₁₀ mL/g for cucumber green mottle mosaic virus (CGMMV) to 3.8 log₁₀ mL/g for phiX174. K_d values were strongly correlated with LRVs, although the resulting adsorption models (sample-specific, linear, and Freundlich) could not fully explain the large variability in virus removal across all conditions. LRVs of 1-3 were observed for all viruses, but the 5^th percentile LRVs that drive regulatory determinations were often <0.5. This study proposes several approaches for awarding implicit LRVs of ≥1 using quantitative microbial risk assessment (QMRA) coupled with observed secondary treatment performance or secondary effluent concentrations.