Things You Will Need
Concrete cisterns, which typically deliver water via a pump system, need to be drained, scrubbed and flushed annually to deter mold, algae and bacteria. A full cleaning should include all grates, pipes and hoses in the cistern.
Draining
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Attach a drain hose to the cistern tank and direct the other end to an area that will not be harmed by a large release of water. Close any valves carrying water to the house. Open the drain valve by twisting the handle to the "Open" position.
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Manually turn the electric pump to the "On" position and drain the tank completely. Turn it to "Off" once the tank is empty.
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Close the drain valve by twisting the lever to the "Close" position. Open the lid to the cistern tank to make sure it has emptied.
Scrubbing & Flushing
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Locate the collection grating, normally located a few feet uphill of the actual tank. Pour 3 to 4 cups of chlorine bleach into the grating. Follow it with about 20 gallons of water from the garden hose. The bleach water will clean and disinfect as it moves down to the cistern tank.
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Place a ladder down into the open tank and take the long-handled scrub brush down into the tank. Wear safety glasses and gloves to scrub the ceiling, walls and floor of the cistern.
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Climb out and pull the ladder up and out of the tank. Close the lid and move the drain valve to "Open" and start the electric pump. Shut the pump off and close drain valve once the tank has emptied.
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Place the water hose over the collection grate and proceed to fill the cistern. This could take a while, depending on the size of the tank.
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Open the drain valve and turn on the pump to drain the tank after it fills all the way. Close the drain valve and wipe down all of the gratings with bleach water.