Increasing Contact Time And Reducing Short-Circuiting In A Clearwell Tank

By SolarBee, Inc.

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 a 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 in the range of 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 the inlet across the tank to the outlet. Contact times (CT) were about 30 minutes.

Several steps were undertaken to address the concerns. These were:

1. Installed a SolarBee circulator in the center of the tank inline between the inlet and outlet to increase CT and break up thermal stratification.
2. Undertook a tracer study to verify the CT with the circulator operational to ensure regulatory compliance.
3. Conducted a computational fluid dynamics (CFD) model of the flow conditions with and without the SolarBee circulator.