A series of measurements of flow rates and corresponding head losses through two ultraviolet disinfection units at the Sanitary District of Decatur provided data that were used to describe head- loss coefficients for these units and to determine the maximum capacity of these units without overtopping the channels in which they are installed. Two UV units have been installed in the chlorine contact tanks at the SDD. Operation records indicated that the maximum head loss across these units specified in the original procurement documents (2.2 ft) occurs at a flow rate of 105 million gallons per day (MGD), which is 16 percent below the maximum flow rate of 125 MGD specified in the original procurement documents. Stage and discharge at the disinfection units were measured between June 27 and August 23, 2002 for discharges ranging from 30-70 MGD). Results from these measurements indicate that the maximum flow that can be passed with head loss limited to 2.2 ft is between 55.4 and 60.8 MGD, depending on assumptions made to interpolate the head-loss coefficient between measured flow rates.

The measured head loss coefficients for the UV reactors indicated that the head-loss coefficient is not constant but varies with the flow rate. On-site measurements are limited to the discrete flow conditions that are possible for the plant so the relation between the head-loss coefficient and the flow cannot be determined from on-site measurements. Furthermore, lateral and longitudinal displacement of the arms of the UV reactors, which may affect the relation between the flow rate and head-loss coefficient have been observed at high flows. Measurements with the UV reactor lamps removed from the flow indicated that potential improvements to the efficiency of the system could be attained through modification of the approach and exit sections for the reactors.