Paris, France – City of “UV“ Light CFD Modeling as a Design Evaluation Tool for UV System Retrofit

The city of Paris, France is implementing Ultraviolet (UV) disinfection at the discharge of the Orly and Joinville drinking water treatment plants (WTPs) with the primary goal to limit the production of bromates (KBrO3, NaBrO3) during disinfection of the source water using the existing Ozonation process. The proposed UV system design consists of two closed vessel, cross-flow Medium Pressure (MP) UV reactors arranged in series for each filter cell at the Orly WTP and a single closed vessel, cross-flow MP UV reactor per filter cell at the Joinville WTP. Each UV treatment train is identical and equipped with both an upstream isolation valve and a downstream flow regulating valve.A Computational Fluid Dynamics (CFD) modeling study was conducted in order to analyze the predicted disinfection performance of the UV systems, noting that the proposed piping configuration differed from the bioassay validated configuration and typical recommendations from both the DVGW W294 and USEPA UVDGM guidelines. The CFD modeled dose values meet or exceed the design requirements for all conditions. The proximity of the isolation and flow regulating valves to the UV reactors does not compromise the disinfection performance of the UV trains. The addition of the UV reactors at the discharge of the filter cells does not increase headloss beyond the maximum limits specified for this project confirming that operation and performance of the upstream filters will not be disturbed.A sensitivity study was conducted encompassing parameters that are essential to the accuracy of the CFD modeling procedure employed. Based on the results of this sensitivity analysis the CFD modeled dose values initially obtained may be slightly over-estimated. However, the CFD predicted UV dose always underestimates the actual UV dose as obtained from bioassay validation testing. The proposed CFD modeling procedure used to model the UV systems for the Orly and Joinville WTPs yields a conservative estimate of the UV reactors bioassay validated performance irrespective of the settings that were changed during the parameter sensitivity study.