A central water tank is used in a multi unit development to supply water to common area uses (such as landscaping, car washing bays or cooling towers) or to more than one dwelling, for private garden watering, toilet flushing or laundry. A central tank can collect rainwater and/or stormwater.
A central water tank may collect water from:
- the roof– of any number of unit blocks or dwellings in the development;
- impervious areas– surfaces that do not allow natural infiltration of rainfall to the underlying soil and include concrete, gravel and timber surfaces.;
- garden/lawn areas; and
- planter boxes.
BASIX assesses central water tanks based on:
- the area of water collection;
- the tank size– the volume in litres; and
- whether the overflow is diverted to another tank;
- the uses to which the water is allocated.
Water from central water tanks may be allocated to:
- the area of common landscape served– in square metres;
- the number of car washing bays; and
- central cooling systems with cooling towers;
- private uses for individual dwellings within the development.
There are additional limits to alternative water use from central rainwater tanks, for example it cannot be used for topping up swimming pools or spas. As with individual tanks, untreated water from central stormwater tanks can only be used for sub-surface garden irrigation.
Where a central stormwater/rainwater tank is being designed to service not only the residential development subject to the current development application, but future residential development; or where the central stormwater/rainwater tank is being designed to service both residential and commercial/retail development, the details of the central system must be entered as an "alternative water supply". A hydraulic engineer will need to determine the amount of stormwater and/or rainwater available to the residential component of the current development application (in litres/day).