Testing an Underdrain System under Backwash Flow Conditions for the Chaparral Water Treatment Plant

Johnsons Screens was required to test its underdrain system under backwash flow conditions to assess the uniformity of flow discharge throughout the manifold system. To test the system, a basin was built in St. Anthony Falls Laboratory and the underdrain system components were provided and installed by Johnsons Screens. To assess the uniformity of flow of the manifold system, a measurement technique was developed to determine the flow at a number of locations at design flows. The technique was a flow-capturing apparatus with an open-topped tank, two tubes, a hose and a pump. Using this apparatus, the flow was captured over a period of time and weighed to determine the flow rate from each triplet of orifices. The system error under high system flow rate (about 19 cfs) was less than 0.7%. Six locations were determined by Johnsons Screens for the testing. At each location, a total of five measurements were taken and averaged under the high system flow rate (about 19 cfs) and an additional five measurements for the low system flow rate (about 4.4 cfs). The results of the measurements showed that the deviation of the flow measurements from the mean were within ±5% under high system flows and ±7% under low system flows.