A range of filtration and storage tanks already exist. One such tank is called a StormBank. This is probably the most advanced and most efficient. system.

How a typical storage / filtration tank works:

Grey water or green water enters the tank via the gutter and drain pipe. As it enters the tank it passes through a filtration system that removes debris. The debris is allowed to run into the soakaway system. The filtered water is stored in the tank ready for use in showers and WCs.

In the event of a storm in which there is excess water entering the tank. The over flow system ensures that excess water flows into the soakaway.

A soakaway system is essential when storing water. Usually the amount of rainfall collected will not exceed the volume of the tank. However, if a storm occurs or there is a period of heavy rainfall the storage tank will soon fill. The excess rainwater cannot be allowed to simply run onto the ground surface outside a building. If this happens the ground will soon become waterlogged and the soil structure will become unstable. In this case the ground soil will wash away and it is possible that buildings close by will suffer subsidence. Subsidence can seriously damage walls and entire buildings making them dangerous to inhabit.
A soakaway is usually composed of sand and gravel. Water flowing into a soakaway is slowly allowed to disperse into the surrounding ground. This ensures the soil remains stable and that water does not collect on the surface causing a hazard.