Preventing contamination in water storage tanks



Water storage tanks must be inspected and cleaned regularly to prevent contamination issues. The American Water Works Association states that, “tanks should be washed out and inspected at least once every three years, and where water supplies have sediment problems, annual washouts are recommended.”

Water tanks can be taken out-of-service to perform a dry inspection and cleanout. Alternately, an in-service remotely operated underwater vehicle (ROV) can perform an inspection and cleanout. Equipped with lights and a colour camera, ROVs are controlled by qualified inspection personnel trained to use the equipment. Thrusters allow for vertical and horizontal movement, and a ground monitor provides live viewing of the inspection. Cleanout robots equipped with brushes and a suction hose are able to clean and remove sedimentation.

An in-service ROV inspection and cleanout does not require draining the tank, so no water is lost. Lockout procedures and confined space permits are not needed, because no one enters the tank. ROV units are disinfected with a chlorine solution before being placed into the tank.

Inspections are able to cover structural, safety and coating conditions in accordance with industry and government standards. Vents, screens, accessways, overflows, ladders and drain valves are also inspected to ensure no unauthorized access. The floor and foundations should be checked for settling, stability and leaks. Several tests are able to determine the condition of the steel and coating system. Water temperature can also be checked every 1.5 metres during an inspection, to determine whether stratification is a problem.

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ROV inspections are able to provide a video recording of the inspection and cleanout, as well as a written report, including a detailed evaluation, photographs, recommended repairs, code updates and cost estimates.
An inspection alone is not enough to prevent water tank contamination. If the tank needs to be cleaned, clean it. If damaged screens and tank openings are found, fix them. If the water temperature indicates possible stratification, then take the necessary steps to eliminate it.

Erika Henderson is director of research with Pittsburg Tank & Tower. This article appeared in ES&E’s July/August 2014 issue.

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