Suspended solids dominated tailing effect in UV disinfection of livestock wastewater: Mechanistic deciphering and modelling prediction for epidemic control

During epizootic outbreaks in intensive livestock operations, wastewater serves as a critical transmission vector for zoonotic pathogens, necessitating advanced disinfection to mitigate public health hazards. Ultraviolet (UV) disinfection is commonly utilized in wastewater treatment, but the tailing phenomenon poses a significant challenge in suspended solids (SS)-enriched livestock wastewater. The mechanistic relationship between SS particle size and tailing dynamics remains poorly characterized. This study investigated the role of different-sized SS on the tailing levels of two typical fecal indicator bacteria (FIB) during UV disinfection of dairy wastewater through a combined experimental-modeling approach. Inactivation efficiency increased with initial increasing UV dose, but tailing effects emerged at UV dose beyond 513 mJ/cm², resulting in residual FIB concentrations of 5.07–5.21 log₁₀ CFU/L with enterococci exhibiting higher tailing level than total coliforms. Despite SS >1000 μm accounting for 43.9% of total solids, >85% of FIB colonized on SS <200 μm. This preferential association was linked to higher concentrations of extracellular polymeric substances, predominantly polysaccharides, on smaller particles. The critical SS size was identified at 337 μm for total coliforms and 233 μm for enterococci, respectively, which dominated the tailing level due to dual inhibitory effects on UV disinfection: (1) optical attenuation through UV absorption, and (2) physical shielding of embedded microorganisms. A novel disinfection tailing level prediction model for wastewater with high SS was developed by combining empirical SS size-tailing relationships with the principle of integration. The model demonstrated high accuracy (mean absolute error = 0.116 and Nash-Sutcliffe efficiency = 0.908) and precision (standard deviation = 0.134). These findings advance mechanistic understanding of SS-mediated tailing phenomena and offer actionable insights for biosecurity management of wastewater during epizootic events of animal farms.