When passive mixing is insufficient, potable water tanks can stratify, stranding warmer water near the top of the tank where residual disinfectants begin to dissipate. |
Thermal Stratification
Thermal stratification promotes deterioration of water quality, especially in the upper part of the tank. When water warms and stratifies, newly treated cold water coming into the tank may not mix well with the existing water, and tends to stay near the bottom. This cold water near the bottom is also the first to exit the tank, essentially short-circuiting or stranding the bulk of the warmer, stratified water above.
Thermal stratification can start to occur with temperature gradients as small as 0.1°C. If stratification persists for as little as five days, disinfection chemicals may begin to dissipate in the warmer, upper layers, allowing bacterial growth and an overall decrease in water quality.
Stratification also introduces sampling inconsistencies that can lead to over- or under-boosting of chemical disinfectant. Since water quality testing is based on sampling from selected locations within a tank, these testing points may not reflect the real health of the entire tank if it is stratified. However, having a fully mixed storage system provides a means for true sampling of water temperature and residual disinfectant from any sampling point.
Continuous Mixing
New solar-powered, near-laminar-flow continuous, active mixers were developed for solving water quality problems in lakes and raw water reservoirs and have the ability to circulate water out to a radius of 800 feet and to depths of more than 100 feet. They are also widely used to maintain water quality in potable storage tanks.
Instead of creating turbulence in the water, the near-laminar-flow mixer floats on top of the water and gently draws the water up from below. Exiting the mixer without turbulence, the surface water travels a long distance because it is essentially powered by gravity and experiences little resistance. This feature makes it possible to mix the water throughout the entire water column, from the bottom of the tank to the outer walls and everywhere in between.
While these mixers can circulate up to 10,000 gallons per minute, they require very little power due to the minimal head, or lift, in the up-flow pump design. This feature allows them to be driven by an electronically controlled DC motor that is significantly more efficient than induction motors typically used on turbulent mixers. The low-power requirement allows them to be powered by 80- to 300-watt solar photovoltaic arrays combined with a deep-cycle storage battery to allow 24/7 operation.
With the mixer floating in the water tank and a solar power source installed on the tank roof, the equipment can operate around the clock, completely mixing the tank to provide consistent disinfectant residual, and eliminate thermal stratification and deterioration.
Chloraminated Systems
While approximately 70 percent of municipal water systems use chlorine as their primary disinfectant, a growing number of municipal water agencies have switched from chlorine to chloramines as their disinfectant of choice. Continuous, active mixing is beneficial with both types of disinfectants, but is especially important in chloraminated systems.
If chloraminated water is allowed to age too long due to poor mixing, ammonia-oxidizing bacteria (AOB) may convert free ammonia to nitrites. As more and more water treatment facilities convert from chlorine to chloramines, the American Water Works Association (AWWA) has noted that nearly two-thirds or more of tanks with chloraminated water experience unwanted nitrification and a loss of residual disinfectant, especially when water temperatures rise during warm weather.
When the temperature of chloraminated water rises above 15°C (59°F), the AOB, which are 13 times more resistant to chlorine than most bacteria, begin to grow rapidly especially on the floor and walls of the tank. So it is critical that the mixing scheme circulates water along the walls and bottom of the tank and exposes AOB to disinfection chemicals.