Cistern

A cistern is a tank that holds water between the time it becomes available and the time somebody wants it. Everything upstream of it works only while rain is falling, so the tank turns an on-and-off supply into a steady one, and it is usually what a gravity-fed system draws from. Its size is set by the gap rather than by the yearly total, so a tank is sized against the longest dry spell on record. A cistern makes no water of its own, and filtering and disinfection stand between it and a drinking tap.

Five fittings are not optional: an inlet, an outlet, an overflow, a vent, and a port large enough to climb in through. A calming inlet slows arriving water so it does not stir up the sediment on the floor. The outlet sits at least 10 cm (4 in) above that floor so it does not draw sediment out. The overflow leaves from the top, slightly below the inlet, and the vent lets air in as water is drawn. Both are screened against mosquitoes. Opaque plastic starves algae of the light a translucent tank lets through, and concrete raises the pH of the water it holds, offsetting the natural acidity of rain.

The failures that destroy a tank are structural. A buried tank left empty in wet ground floats, so it has to be anchored. Its base must be solid, never mulch or anything organic, which settles and takes the tank out of level. The overflow needs somewhere to go, because loose discharge cuts the ground beside the tank and undermines that base. Concrete and ferro-cement crack if new work is not kept moist for seven days while it cures. The slow failure is water quality. Water left standing loses its dissolved oxygen to bacteria, sulfur-reducing bacteria take over, and the hydrogen sulfide they produce smells musty at about half a part per million and of rotten eggs above one. The cure is turnover, which is why a tank sized far beyond real demand is a problem rather than a margin.