Coupling CFD with the Bioassay a Cost-Effective Method for In-Vessel UV Reactor Certification and Design

For reclaimed water disinfection, each ultraviolet (UV) light reactor (lamp type, spacing, orientation) must undergo independent third-party performance certification under the UV Guidelines for Drinking Water and Water Reuse, published by the National Water Research Institute and the American Water Works Association Research Foundation (2003), hereafter called the 2003 UV Guidelines. Detailed reactor tests must be run to document the UV reactor performance over a range of hydraulic conditions (flow), water quality (UV transmittance [UVT]), and lamp intensity. The certification effort is costly, commonly ranging from $50,000 to greater than $120,000 for a single reactor. For open channel reactors with a common lamp and lamp spacing, it is possible to certify one small reactor and utilize the data set for the design and operation of much larger reactors, thus open channel systems require a reasonable certification effort that allows for wide ranging applications. This is not equally true for in-vessel UV reactors.Commercially available in-vessel UV reactors for wastewater applications are not standardized with one lamp type and lamp spacing. One UV manufacturer may, for example, have one reactor diameter, lamp type (intensity per length of lamp) and lamp density for one application and another completely different reactor (different diameter, lamp intensity, and lamp density) for another application based on varying flow and water quality conditions such as UVT. Thus test results for one reactor are obviously not valid for the second reactor. Aquionics Inc. (Aquionics), for further illustration, currently markets 7 entirely different UV reactors for various reclaimed water disinfection applications. Certification of all 7 reactors is cost prohibitive, with an estimated cost of greater than $700,000, which does not include the substantial costs of pilot construction.Carollo, working with Aquionics and the California Department of Health Services (CDHS) has found a solution to costly in-vessel UV reactor certification and design uncertainty, the coupling of detailed computational fluid dynamics (CFD) with the bioassay. As part of the project, detailed CFD models have been prepared for each of the 7 UV reactors, with flow capacities ranging from 5.0 mgd per reactor. To best calibrate the CFD results, 3 of the 7 reactors were rigorously bioassayed, with the bioassayed reactors spanning the lamp reactor diameter, lamp density, and lamp intensity range of the 7 reclaimed water reactors.This paper will detail the CFD modeling approach, the method of CFD model calibration with the bioassay, and the accuracy of the CFD when compared to the three bioassays. This paper will then conclude with an example of how to properly utilize the CFD/bioassay results for the proper design of an in-vessel reclaimed water UV system.