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Mixing Systems Solves Stagnation Problem In Finished Water Storage Tank
Mike Duer; P.E. Senior Engineer at Red Valve Inc.
The Old Bass River 1.3 million-gallon finished water standpipe.
The Dennis, MA, Water District recognized that it had a severe stagnation and stratification problem in its 50Õ diameter x 90Õ, 1.3 million-gallon Old Bass River standpipe. Water samples taken at the outlet would generally show no bacteria in the water; however, the sample was only representative of the fresh water at the bottom of the tank.
David Larkowski, Superintendent of the Dennis Water District, determined that a system was necessary to mix the incoming water with the finished stored water to improve the quality of the stored water and eliminate the stratification problems.
Finished water storage reservoirs have to contend with deterioration of water quality. The most common problem is the loss of disinfectant residual resulting from hydraulic short-circuiting, poor mixing and circulation, poor turnover rate and excessive detection time. Stagnation and dead spots can lead to serious water quality issues, such as the formation and multiplication of bacteria.
The Old Bass River tank utilized a common inlet/outlet pipe located near the bottom of the tank. This design was creating the short-circuiting and stratification problem. The 3D Computational Fluid Dynamics model shows that the mixing that took place over a 30-minute time period was limited to just a few feet around the inlet pipe.
According to Larkowski, "There was no mixing within the upper one-third of the tank, poor mixing in the middle of the tank, and the water entering the tank would mix slightly with the bottom of the tank and exit quickly during the next draw down."
The Tideflex outlet valves.
The solution was the installation of a custom-designed mixing system. The mixing system eliminates stagnation and dead spots by preventing hydraulic short-circuiting, improving mixing and circulation and optimizing the turnover rate and detention time. The duckbill design of the check valve has no moving parts and operates solely on the differential pressure that is already present to drain/fill the tank.
In the Old Bass River tank, a new inlet/outlet system was installed that utilized a manifold of patented duck bill check valves as the outlet and a check valve at the top of the pipe located about 45 feet from the bottom of the tank. This duck bill was angled 45 degrees upward. This configuration dramatically improved the mixing of the inlet and stored water. Larkowski and his staff installed the new mixing system without any problems.
The Tideflex inlet valve at a 45-degree angle.
The Denis Water District staff monitored the temperature in the tank before and after installation of the mixing system. By adding a 100-foot wire to a digital temperature probe, it was possible to collect temperature readings in 10-foot increments.
The temperature stratification over a 30-minute period for incoming finished water is shown by this 3D Computation Fluid Dynamics model.
After the installation of the mixing system, the same collection method was used to compare tank temperatures. According to Larkowski, "The entire tank from top to bottom had a more uniform temperature. At the top of the tank during a period of filling, you could visibly see movement in the top of the water. There was no observation of this kind before the system was installed."
The Coliform and HPC bacteria were also monitored before and after installation. Before the installation, the samples taken near the bottom of the tank shows good results. Larkowski and his staff were always concerned about the water in the upper portion of the tank that was not being sampled. They were also concerned that with the mixing system, changes in the bacteria level would increase due to the slight rise in temperature from the more complete mixing.
Samples after the installation of the mixing system confirmed what they suspected. The HPC samples began to rise into the 3000, 4000 and 6000 count ranges. "Samples in another tank located a half-mile away continued to be free of HPC," according to Larkowski.
To combat the HPC, chlorine was added to the tank during a filling cycle and mixed by the mixing system. This reduced the HPC counts. During the summer of 2002, two pounds of chlorine were added each week to maintain good sample results.
"In all, we find the system to work well at its intended goal, which was to mix the water to eliminate stagnation and improve water quality," Larkowski added.
Because of the success of the Old Bass River tank retrofit, it has been decided to install a similar system in the 2.7 million-gallon Hokum Rock tank.
This custom-designed mixing system was manufactured by Red Valve Co., Inc., 700 Bell Avenue, P O Box 548, Carnegie, PA, 15106, 412/279-0044, Fax: 412/279-7878, or www.redvalve.com.
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