A pilot study was carried out at the Fargo wastewater treatment plant to determine the impacts that flow rate, water quality, and system fouling may have on the efficiency of UV disinfection. A second-order model successfully explained the results obtained with the collimated beam. The second-order model was used to study the impact of water quality and initial microorganism concentration on E. coli inactivation rates. Fouling material was mostly made of precipitated metal salts and its impact on UV intensity reduction was explained with the application of the Beer-Lambert law. E. coli inactivation in the pilot unit was found to be dependent on UVT, flow rate, and UV intensity. A first-order plug-flow model successfully explained the inactivation data obtained in the pilot unit. No significant seasonal water quality changes that may affect system operation were identified. However, UVT changes caused by storm events had short-term adverse impacts on system performance.