Washington state facility uses mixer to combat thermal stratification
Standpipes are among the most problematic tank geometries to mix. Inlet velocities are typically small in magnitude and horizontal in direction. The substantial majority of water in the standpipe must remain in the tank to produce and maintain pressure in the distribution system, so there is often a hard limit (typically 70% to 90% of capacity) below which the operators cannot draw. This takes away the default (yet energy- and labor-intensive) method of mixing—the forced drawdown and refilling of tanks.
The Spanaway Water Co. in Washington state discovered the problem of low-turnover standpipes firsthand when an operator noticed a layer of condensation on the outside of one of its standpipes. He surmised that the cold water inside the tank was causing this condensation, but worried why the condensation was only visible 20 ft up the side of the tank and did not cover its full height. An investigation of the temperature inside the tank validated his theory—the tank had substantial thermal stratification. The volume and velocity of the incoming water during a regular fill cycle was not sufficient to overcome the thermal loading the tank received on its large, exposed surface area, even in a state not known for its thermal extremes. Testing at the top of the tank during normal operation showed almost zero chlorine residual. The warm, low-residual water remains trapped in the top of the tank until a period of high demand lowers the water level enough to potentially allow this poor quality water [often with unpleasant taste, temperature and odor, as well as possible high disinfection byproduct (DBP) levels] to enter the distribution system.
Sudden loss of water quality in standpipes can also occur at the beginning of winter due to “inversion,” which occurs when the old, stagnant water at the top of the tank is no longer warmed by the summer sun. Instead, the cold winter air chills the water. This colder water suddenly sinks to the bottom of the standpipe, which causes the standpipe to rapidly invert. After inversion, the first water to be flushed directly into the distribution system to supply customers is the oldest, lowest-quality water from the top of the tank, while the good water remains trapped at the top of the tank.
Standpipes present a particular challenge for most mixing systems because of their height. The PAX mixer has been demonstrated to restore water quality, reduce the need for onsite chemical dosing and improve reliability and homogeneity in ground storage tanks, but those tanks tend to be wider than they are tall. PAX’s analysis suggested that, despite the unfavorable ratio of height to diameter of this standpipe, the PAX mixer would be able to penetrate an established temperature gradient and completely circulate all 120 ft of the water column.
The installation of the PAX mixer took less than a day and required no crane or heavy equipment. In contrast, older-style mixing systems previously used in standpipes required impractical 120-ft-long draft tubes or tree-like piping structures built into the tank and a long, expensive construction and installation process. These processes also required draining the tank to weld and recoat the inside surface. For the PAX mixer, the only preparation was that the reservoir water level was lowered to 100 ft and isolated until divers could complete the installation. The customer saw an immediate benefit.
Approximately every hour, another vertical foot of the stratified water column was blended with the cool, fresh, chlorine-residual-rich water. After seven days, the entire water column was completely blended and the temperature of the upper layer had fallen from 19°C (approximately 66°F) down to below 16°C (61°F). Over the next several weeks the average temperature of the reservoir dropped even further as new water was added. The reduction in temperature allows the residual to last longer in the upper layers and lowers the rates of reaction and DBP formation. Grab samples taken after one week of mixer operation showed a dramatic increase in residual at the top of the tank—from 0 to 0.31 mg/L.
With the active mixing provided by the PAX mixer, water quality throughout the tank has been improved and the entire volume of water is now available for regular or emergency use. There is no chance of an inversion while the mixer is operating in the tank. The reliability and quality of Spanaway’s distribution system has been significantly improved by this simple infrastructure upgrade, and minimal interruption of tank use was required.
“Consistency in the quality of water provided is the key to customer satisfaction,” said Tim Tayne, water programs manager for Spanaway. “This also ensures the water always meets the drinking water standards, even during high demand such as fighting fires or abnormally hot weather. The volume of water in this reservoir with high water quality went from 0.25 million gal to almost 1.5 million gal by installing the PAX mixer. We will be evaluating our other reservoirs in the near future.”