Rainwater harvesting

What the water is for

Harvested water is not drinking water, and it does not need to be. The demand it usually serves is the demand that does not require potable quality: irrigation, vehicle washing, toilet flushing, and wash-down.

That matters to the sizing. A harvesting tank is sized by the demand it has to meet between rainfalls, not by the roof area alone. A tank sized only on the roof will fill and overflow during most storms, and the volume that actually gets reused will be small.

Why the material matters more here

An attenuation tank holds water for hours. A harvesting tank holds it for weeks, sitting against the module the whole time, and then the water is used.

The polypropylene we mold does not precipitate anything into the water after immersion, has no odor, and resists strong acid and alkali. Those are the properties that matter when water is held for a long period and then drawn off.

What has to be designed in

A harvesting tank needs more built into it than a tank that only holds water. A pump position has to be left clear so the water can be drawn off. An inspection well has to reach the tank so the level and the condition can be checked. A pre-tank filter is usually needed as well, because roof runoff carries grit and leaf litter that will settle in the tank.

All of it goes into the module layout before assembly starts. A buried tank cannot be altered afterward, and the access that was not built is the access that will never exist.

The honest limitation

Guidance is clear that a geocellular system generally provides no water quality treatment of its own, and that a buried system can be difficult to monitor and to maintain.

On a harvesting job that means two things. The filtration has to happen before the water reaches the tank, not inside it. And whatever the filter catches has to be reachable, because it will need clearing.

Neither is a reason to avoid the system. Both are reasons to design the access properly, which is cheaper than fixing it later.