Thorough Mixing Crucial to Healthy Potable Water Tanks
By Ed Sullivan
Increasingly stringent regulatory standards for potable water quality are causing suppliers to review their reservoir/tank treatment, mixing and cycling operations for both chlorine and chloramine-based systems. Of interest to anyone who manages potable water storage facilities, recent tests and analyses have confirmed a breakthrough in water circulation technology that enables not only meeting higher standards, but also the improvement of overall health and quality of potable water treated through chlorination or chloramination.1
For the most part, potable water in North America is still treated by chlorination. However, much of that water comes from lakes, which often contain a substantial amount of organic matter. Organic matter translates to chlorine demand; so in order to maintain the desired residual chlorine level throughout the area supplied by the tank/reservoir, a higher level of chlorine is often injected into potable water that originates from lakes and other surface waters. This is particularly the case during warmer months.
Chlorine applied to organic matter can produce unhealthful disinfection byproducts (DBPs) including trihalo-methanes (THMs) and haloacetic acids (HAAs). Because of its substantial organic content, the potable water of many communities using a lake as a source is often approaching or exceeding the chlorine limit of 80 µg/L during peak periods.
Chloramines in storage tanks
Chloramines, composed of chlorine and ammonia, are an effective treatment that do not produce DBPs. However, when autodecomposition of chloramines occurs in water storage reservoirs/tanks, some of the ammonia component is set free, creating the potential for nitrification. Nitrification can lead to methemoglobinemia, a blood disorder caused when nitrite interacts with the hemoglobin in red blood cells. (Sometimes called Blue Baby Syndrome, the methemoglobin formed in this interaction cannot carry sufficient oxygen to the body’s cells and tissues.)
A practical remedy for such ammonia problems in potable water is breakpoint chlorination, whereby chlorine is injected until the ratio of chlorine to ammonia is 7.7 to one. With that proportion of chlorine, any free ammonia is oxidized. However, it is virtually impossible to accomplish breakpoint chlorination in a large storage tank unless a very effective water mixing system is used. Otherwise, the injected chlorine does not disperse sufficiently to destroy all of the free ammonia.
Cycling, or turnover, is another problem common to stored potable water. The temperature differential between water flowing into the tank and that already present causes stratification, which becomes compounded over time. With chlorine systems, this results in a loss of residual chlorine in layers of water that are old from the lack of turnover, and can lead to propagation of deadly organisms such as giardia. With chloramines systems, the problem is more serious; stratification makes it highly likely that autodecomposition will create free ammonia in old water and, for the same reason, breakpoint chlorination cannot be performed effectively.
The solution is mixing
The common denominator to these problems with chlorine- and chloramines-treated potable water storage is the need for effective mixing. Until recently, mixing was done using one of two popular technologies: water check valve systems or propeller-driven water mixer systems. Unfortunately, both technologies have drawbacks.
Check valve systems attempt to mix water by positioning the water inlet pipe near the top of the tank and the water outlet near the bottom. This separation of inlet and outlet ensures a certain amount of turnover. However, check valve systems also create backpressure that may adversely affect pumps and can waste energy. Also, because many tanks are built to support future population growth, the present day inflow/outflow may be so insignificant that turnover is negligible. If it should become necessary to isolate a tank, then there is no way to continue any amount of mixing.
The other popular mixing technology, propeller-driven turbulent mixers, involves electric motors powering six-foot mixing blades on heads that are lowered into the water. While this does achieve a certain amount of mixing, it is somewhat uneven and does not reach the corners and sides of water tanks. Typically, effective water circulation is limited to a radius of 15 meters.
A new alternative
”We looked into check valve systems, but the average cost of those was exorbitant,” says Jeff Graham, Senior Production Operator at the Santa Clarita Water Division, Santa Clarita, Calif., which uses both chlorination and chloramination injection in its 64-million-gallon potable water storage facilities. With 42 tanks in their system, they use chloraminate in 60 percent of their water (the amount coming from the state water project). However, the tanks are over-sized for future growth, allowing them to circulate the water through the tanks pretty quickly, Graham said. So they’re currently using only 36 of our storage tanks.
”From all of our research on chlora-mines, we found that the majority of problems occur in the tank. The fact that the water sits there…doesn’t really circulate in your tanks. So we were looking for a technology that would improve turnover in our tanks,” he said.
In order to get the most thorough and efficient mixing of water in potable water tanks, Santa Clarita decided to test a solar-powered circulation system that is in wide use for aerating lakes and reservoirs. These systems are self-contained floating units that can draw up to 10,000 gallons of water per minute and spread it gently across the surface for continuous aeration and mixing.
The system can accommodate chlorine and chloramines injection systems, and facilitate thorough breakpoint chlorination whenever necessary. The system is powered by solar panels that may be detached for use on top of enclosed tanks.
”We did an initial pilot with one unit in February 2005,” he says. “The pilot only took a couple of weeks. We put the system in and did some testing with different temperatures throughout the tank. Then we injected some chlorine, and almost within a month, we were very satisfied with the circulation results we were getting.”
The ability of this technology to effectively mix large water tanks has also been well documented in a recent installation at San Francisco Public Utilities Commission (SFPUC). At the Sunset Reservoir South Basin the patented technology achieved remarkable successes with both chlorine and chloramines systems.1
Two tests were performed (with and without the system) in 2002 when chlorine was used, and two additional tests in 2003/2004 when the reservoir was supported by chloramines.1 In summary, the study found that three major objectives could be accomplished by using the unit’s mixers in city reservoirs to:
- Eliminate short-circuiting and minimize water age in oversized distribution system reservoirs;
- Minimize residual loss in reservoirs; and
- Facilitate emergency disinfection or breakpoint chlorination in a reservoir.
Unlike nozzle devices and check-valve, inflow-outflow piping, this circulation causes no adverse effect on system flow rate capability, no loss of energy at the nozzle, no losses in pump efficiency and no changes to other distribution system characteristics.
Compared to turbulent mixers the unit mixes the entire reservoir and has far lower operational and maintenance costs. The system can be equipped with SCADA output signals (a chlorine injection system) and with various solar and 24-hour power kits as needed, depending on reservoir characteristics. The system’s flotation system, together with the variable length intake hose, self adjusts at all times for peak performance regardless of water depth in the reservoir.
- SolarBee Mixer Study for the Sunset Reservoir South Basin prepared by the San Francisco Public Utilities Commission, Water Quality Bureau, Aug. 2004.
About the author
Ed Sullivan is a Hermosa Beach, Calif.-based writer. He has researched and written about healthcare, finance, real estate and high technologies for over 25 years. The solar-powered mixing discussed here is patented technology from SolarBee. For more information, contact SolarBee headquarters at 530 25th Ave. E., Dickinson, ND 58601; phone (866) 437-8076 or (701) 225-4494; fax (701) 225-0002; e-mail email@example.com; or visit the web site www.solarbee.co