Increasing Contact Time and Reducing Short-Circuiting In a Clearwell Tank
In 2005 the City of Bend, Oregon’s Water Division commissioned a new reservoir, known as Outback Reservoir #2, which functions as a clear well. The reservoir is welded steel above ground tank, which is 120 feet in diameter and has a maximum depth of 35.4 feet. The reservoir has a capacity of 2.9 million gallons (MG), however typical operating depth is about 26.6 feet or approximately 2.23 MG. The well was designed with separate inlet and outlet pipes, which are located directly opposite each other at the bottom of the tank. There are no baffles and the disinfectant used is chlorine.
In summer conditions, the maximum inflow from surface water sources is about 7,000 gallons per minute (gpm). Water is chlorinated just prior to inflow to the clear well. At this flow rate, the water in the tank would optimally turn over more than four (4) times per day. However, disinfection efficiency was unacceptably low. Thermal stratification and high flow rates were causing short-circuiting of the flow directly from inlet across the tank to the outlet. Contact times (CT) were about 30 minutes.