SR384 Calculating potential network savings through employing rainwater and greywater systems (January 2020)

Strain on New Zealand's water networks is forecast into the future as populations continue to increase alongside rates of urbanisation and per capita water use. Compounded by climate change, the stability of the freshwater resource in New Zealand in future years is an issue of growing concern. Adoption of rainwater and greywater technologies could help to alleviate that burden to water networks around the country.

Eight commercial buildings were monitored during the 2015/16 period. This found rainwater use ranged from 45 to 1,147 kL per month in summer, 22 to 1,039 kL per month in winter and an annual water use of 309 to 23,525 kL/year. The average proportion of total water use that comprised of non-potable, non-contact end uses (i.e. toilets and urinals) was 23%. Assuming that this non-potable water could all be sourced from rainwater, this indicates a potential optimistic saving of 23% of total water from the water network under optimistic conditions. This also equates to a financial saving for both the building owner and the water service provider. Rainfall used for non-potable purposes is not required to be treated, thus saving the water service provider the energy and financial cost of treating and transporting the water. Similarly, this results in financial savings for the building owner by reducing their volumetric water charge. However, when aggregated regionally, the proportion of non-potable supply actually met by the buildings ranged from 4% in the Bay of Plenty region to 38% in the Canterbury region. These observed supply rates provided the basis for extrapolation of current impacts to the water networks in 50 years, alongside the optimistic scenario.

Non-potable uses were extrapolated for the building stock and projected to the year 2066 across the four regions (Canterbury, Auckland, Wellington and Bay of Plenty). This enables an indication of the potential volumetric savings to the network in a range of uptake scenarios in 50 years’ time.

See the BRANZ Facts factsheet for a short and accessible summary of this report