Impact of flow rate, bacterial load, and hydraulic modification on UV disinfection performance of wastewater

This study assesses ultraviolet (UV) disinfection for municipal wastewater treatment in Baghdad, Iraq, using a pilot-scale reactor under different operational conditions. The research examines the effects of flow rate, bacterial load, total suspended solids (TSS), turbidity, UV absorbance, and hydraulic modifications. Unlike computational fluid dynamics (CFD) models or idealized laboratory tests, this work combines pilot-scale experiments with real municipal wastewater and Point Source Summation Method (PSSM) modeling to evaluate the combined effects of hydraulics, particle shielding, and UV fluence distribution. At low microbial loads, complete fecal coliform inactivation was achieved at a flow rate of 4 L/min, meeting WHO (≤ 3.0 log10 and Iraqi (≤ 3.3 log10 standards. Higher flow rates (≥ 12 L/min) decreased disinfection efficiency due to shorter exposure times, especially at high bacterial concentrations. Elevated total suspended solids (TSS > 398 mg/L), turbidity (> 270 NTU), and UV absorbance (> 1.0 cm−1) significantly impeded UV penetration, confirming the impact of particle shielding. Adding baffles improved hydraulic mixing and increased pathogen inactivation by up to 70%. PSSM results emphasized the importance of reactor geometry and optical attenuation in UV dose distribution. Overall, the findings offer practical guidance for optimizing UV disinfection in real wastewater treatment systems. UV disinfection achieved complete removal of fecal coliforms at a flow rate of 4 L/min under low load conditions. At high loads, fecal coliform levels rose from 2.50 to 3.70 log10 as the flow rate increased from 4 to 20 L/min. TSS levels exceeding 398 mg/L, turbidity higher than 270 NTU, and absorbance above 1.0 cm−1 reduce disinfection efficiency. Adding baffles enhanced microbial inactivation by approximately 0.5 log10, corresponding to about a 70% reduction. PSS modeling confirmed that reactor geometry has a strong influence on UV dose distribution.