Use of CFD Modeling for Predicting Contact Time and Improving Hydraulic Conditions in a Chlorine Contact Tank

Traditionally, contact time of a chlorine contact tank (CCT) is determined by performing field tracer tests. This paper presents an alternative of using computational fluids dynamics (CFD) modeling to predict the contact time. This method is known as numerical tracer testing. Field tracer tests require maintaining constant flow through the CCT, chemical injection at the inlet, and continuous sampling. Typically three or more tests must be conducted at different flows to create flow-contact time curves. These requirements are difficult to meet in potable or wastewater treatment plants in operation; the tests are cumbersome and require significant operator effort. Numerical tracer testing facilitates the task without operation interruptions or complex set-ups. Numerical tracer testing was used for determining the contact time at the Irvine Ranch Water District's (IRWD) Michelson Water Reclamation Plant CCT in Irvine, California. The CCT is a two-million gallon, four-pass baffled tank capable of handling up to 20 mgd of tertiary filter effluent. CFD results were compared to field data obtained with a step-feed field tracer tests using lithium chloride. The comparison shows that CFD modeling is capable of accurately predicting contact times in the CCT. CFD modeling was used then to perform step-feed and slug-feed tracer test simulations at different flows in order to obtain T₁₀ and modal contact times. CFD modeling was also used for predicting flow patterns in the CCT. Results of CFD modeling, including flow streamlines, velocity contours and vectors, were used to improve velocity distribution and reduce solids deposition, short-circuiting and vortices with the inclusion of baffles. The study demonstrates that CFD modeling is a valuable modeling tool for the water and wastewater industry.