Interpreting UV validation results: impact of key design parameters on performance of closed vessel reactors

The Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) dictates that for a DWTP to receive credit for UV inactivation of chlorine resistant pathogens such as Cryptosporidium and Giardia , validation testing must be performed to demonstrate that the UV reactor equipment delivers the required dose. This paper summarizes an extensive bioassay validation program undertaken at the DVGW testing center in Germany with the objective to evaluate the performance of two closed-vessel UV reactors for the disinfection of drinking water. A cross-flow in-line reactor and an axial-flow bottom-through-top reactor operating with 4-kWatt medium pressure lamps were tested using the UV intensity set-point method and Bacillus subtilis spores as a pathogen indicator. The paper reviews the impact of key design parameters on the disinfection performance as measured during bioassay validation testing. Key design parameters evaluated include sensor to sleeve distance, approach hydraulics, reactor inlet and outlet configuration and doped versus non-doped sleeves.

It was determined that optimization of sensor to sleeve distance is vital to yielding maximum reactor performance. Increasing the initial sensor to sleeve distance produced beneficial results. Under conditions where the inlet pipe diameter was smaller than the diameter of the cross-flow reactor, the resulting approach hydraulics yielded a high inlet velocity, described as a core jet, and subsequent degradation of performance. The implementation of a stilling plate restored performance to some extent particularly with low flow conditions. No significant impact on reactor performance was observed by using doped versus non-doped sleeves. Careful consideration and optimization of the design parameters outlined herein is a vital component of any reactor validation program, ensuring that UV reactors can deliver the proper UV dose over a wide range of water quality and flow conditions experienced at water treatment facilities.

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