Thorough Mixing Crucial for Potable Water Tanks
Stringent regulatory standards for potable water quality are causing suppliers to review their reservoir/tank treatment, mixing and cycling operations for chlorine- and chloramine-based systems.
Mixing is usually done using one of two technologies: water check valve systems or propeller-driven water mixer systems. Unfortunately, both technologies have limitations.
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 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 6-ft 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 tanks. Typically, effective water circulation is limited to a radius of 15 meters.
"We looked into check valve systems, but the average cost of those was exorbitant," said Jeff Graham, senior production operator at the Santa Clarita Water Division, Calif., which uses chlorination and chloramination injection in its 64-million-gal potable water storage facilities. "Also, we found that with chloramines, we had too much storage, and we could circulate the water through the tanks pretty quickly. So we're currently using only 36 of our 42 storage tanks."
"From all of our research on chloramines, we found that the majority of problems occur in the tank, so we were looking for a technology that would improve turnover," Graham said.
In order to get the most thorough and efficient mixing in their potable water tanks, Santa Clarita decided to test a solar-powered circulation system that is widely used for aerating lakes and reservoirs. Manufactured by Pump Systems, Inc., SolarBee systems are self-contained floating units that can draw up to 10,000 gpm and spread it gently across the surface for 24/7 continuous aeration and mixing.
The SolarBee system can accommodate chlorine and chloramine 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.
In February 2005, Santa Clarita did a pilot study testing different temperatures throughout the tank. Chlorine was injected and the results were satisfactory within a month.