Hydraulic Effects on Ultraviolet Disinfection: Modification of Reactor Design

Abstract

Results from several recent studies of UV disinfection of reclaimed wastewater led to the conclusion that disinfection effectiveness is likely to be reduced at low flow velocities. Considerations from basic hydraulics make this likely because the formation of boundary layers along wetted surfaces in lamp banks reduces the UV dose absorbed by most of the water. Boundary layers grow thicker with decreasing mean flow velocity through the lamp bank. This analysis of disinfection is supported by two reports of hydraulic tests and a study that changed flow rates to change the estimated dose per bank of lamps and found that the proportion of viruses killed in each bank was independent of flow velocity over the observed range. Additional laboratory and computational work is suggested to test this hypothesis. If, as expected, results confirm the hypothesis, then arrays of small vanes, similar to those used in aeronautical engineering, may provide a useful countermeasure. This recommendation can be tested by the same methods used to test the hypothesis.