Potable rainwater harvesting tanks

The potable rainwater harvesting tank node serves to implement a representation of lot-scale rainwater capture and reuse for parcels within the modelled project area. The node functions by first assigning a number between 0 and 100 to the parcels contained within the GIS boundary layer used with the node – if this number is less than the value set for the potable rainwater harvesting tank percentage parameter, then the parcel is assumed as eligible for potable rainwater harvesting uptake.

A parcel’s actual utilisation of rainwater tanks for potable reuse is dependent on both its eligibility to harness other alternative water sources, and its current uptake of such initiatives. To begin with, harvesting tanks for non-potable reuse, stormwater harvesting and water recycling are given precedence, and will always be implemented over potable rainwater harvesting if available to a parcel. In addition, a lot that was assigned an alternative water source in a previous simulation also cannot employ potable rainwater reuse.

If a parcel has been specified as available to harness potable rainwater harvesting, and it has not been earmarked for non-potable rainwater reuse, stormwater harvesting or water recycling, it is designated as possessing harvesting tanks for potable reuse, and its corresponding water demands are aggregated and used as the demand stream for a conceptual storage body that serves to characterise the rainwater tank positioned on the lot; concurrently, the runoff generated via the roof area accommodated by the parcel is directed to the said storage body.

The storage body itself operates as a simple bucket or conceptual model, with the daily roof runoff and aggregated water demands as the inflow and drawdown, respectively. Any surplus in the former is taken as stored volume for that time step, minus the spillage that accounts for the storage body volume (which is set to 2kL for potable rainwater harvesting systems); and the daily demand satisfied by the inflow/stored volume is summed to the annual level – this serves to quantify the amount of water from the water supply network that can be substituted with harvested rainwater (owning to the amount of households that participate in the potable rainwater harvesting scheme) on an annual basis.

Lastly, the catchment runoff is adjusted to take into consideration the water volume internalised by the rainwater harvesting system. Using the total inflow (the annual roof runoff) and the spillage from the storage body, the combined amount of water servicing the demand stream and stored at the end of the simulation period is established, and this figure is subtracted from the overall runoff.

Parameter

Name

Type

Description

Default

Values

Potable RWHT uptake (%)

DOUBLE

Percentage of buildings with potable rainwater

50

0 - 100