Volatile solids demonstrate better performance and reliability as a normalization parameter in upper sewershed wastewater surveillance.

Normalization of viral concentrations in wastewater aims to account for variability in system conditions and contributing populations, yet commonly used parameters often behave inconsistently across locations and time. This study assessed the use of volatile solids (VS) as a normalization parameter at upper sewershed locations and compared its performance with other commonly employed normalization parameters, including total solids (TS), pepper mild mottle virus (PMMoV), and average daily flow. SARS-CoV-2 RNA was quantified in both liquid and solid fractions. Instead of comparing with unreliable clinical case data, the study assessed how each normalization parameter affected trends relative to raw SARS-CoV-2 concentrations and identified site-specific factors that influence normalization reliability. Even after intense weather events, VS remained relatively unaffected, whereas TS was influenced by inorganic solids, particularly in areas with combined sewers. PMMoV exhibited the highest variability due to changes in seasonal trends and dietary habits. Sewer travel times affected PMMoV normalization as differences in partitioning and decay of PMMoV and SARS-CoV-2 over time led to discrepancies. Flow normalization proved useful in precipitation-impacted systems but is limited by measurement challenges. Among the parameters evaluated, VS proved more reliable for normalizing SARS-CoV-2 concentrations, remaining largely unaffected by weather events, inorganic solids, and sewer travel times, unlike TS, PMMoV, or flow. It enables more accurate interpretation of wastewater surveillance data, supporting better detection of temporal trends and informing public-health decision-making.