Modeling of Drinking Water Biofilm Thickness

Abstract

Biofilm development in drinking water distribution systems (DWDS) affects water quality, hydraulic performance, and microbial risk, yet its spatial distribution and structural properties remain poorly characterized. Existing assessment methods rely on microbiological or bulk water indicators that are difficult to interpret at the system scale and do not directly reflect biomass accumulation on pipe walls. This study presents the first model to predict the mean biofilm thickness in drinking water pipes using routinely measurable operational variables under controlled laboratory conditions. Biofilms were grown in a controlled pipe facility over ten months, and thickness was quantified using a hydraulic residence time method. A random forest model using seven variables describing hydraulic, thermal, and limited chemical conditions achieved high prediction accuracy (R2 = 0.91 on unseen data) and identified flow rate, water temperature, and environmental stability as dominant factors. Feature importance and SHAP analyses highlighted the influence of conditioning shear stress and recovery time, while meta-analysis showed how operational conditions govern the formation of a stable biofilm base and a more easily removable outer layer. By linking operational conditions to biofilm thickness, this work provides a foundation for assessing and managing biofilm accumulation in drinking water systems subject to further validation.