New Kapiti houses lead the way

This Issue This is a part of the Water use feature

By - , Build 105

Residential growth on the Kapiti Coast in the last 20 years has put severe pressure on the local water supply. To meet the challenge, the council now requires new houses to include water saving technology.

Figure 1: Water consumption for Raumati, Paraparaumu and Waikanae from 1998 to 2007.
Figure 2: Water savings possible from using rainwater or greywater.
Figure 3: Possible reductions in peak demand from using rainwater or greywater.

The Kapiti Coast is renowned for its climate and lifestyle. Increasing settlement over the last two decades has resulted in one of the highest residential growth rates in the Wellington region. The district has been transformed from a string of ‘weekender’ coastal communities to a group of higher density, more urban centres.

New houses and subdivisions have been dependent on town supply water. Each water supply has a consented daily allowance to ensure enough water is available to support the surrounding ecology. High summer demand, especially for outdoor irrigation in Waikanae, has been approaching the carrying capacity of the existing water supply. Base demand has also increased as the Raumati, Paraparaumu and Waikanae communities have grown (see Figure 1).

By 2050, Beacon Pathway believes homes built since 2007 will be up to one-third of the district’s building stock. Without any intervention, these homes will continue to be dependent on town supply water and be vulnerable to climate shock and water supply constraints. The council had to look at how it would provide secure water supplies for the long term.

Water – a finite resource

In 2004, a supplementary supply from a borewater system was completed. This provides greater resilience but does not increase the overall volume of water available. Hence the council began work on alternative tank and other on-site non-potable household systems.

Supported by the community during the community planning process, council began investigating long-term options for providing a quality water resource to growing communities, without exhausting the natural systems the local water supplies depend on. A key theme was acknowledging that water is a finite resource and that each water supply has a limited carrying capacity for future development. Demand management to maximise the water resources available was identified as an important issue.

Kapiti Coast residents supported the new direction, and council has been working towards creating strong resilient water supplies for the area.

Looking at the options

The council identified pressure on water supply as an important environmental issue, believing each new home needs to incorporate water saving measures.

In 2006, the BRANZ Water End-use and Efficiency Project (WEEP) monitored water use in 13 homes on the Kapiti Coast (see pages 47–48). Results showed that public water use could be reduced by almost 60% if a household used other water sources for the outdoors, toilet and washing machine.

The council saw scope in increasing the minimum requirements for how homes use public water and other water sources. By making new homes more water efficient, local water supplies will last longer, pressure on natural systems will be reduced and costly upgrades to infrastructure and increasing supply can be deferred.

During research into the options, the council used the assessment criteria of:

  • reducing peak demand by 30%
  • no outside taps connected to town supply
  • providing an acceptable level of service to each new home
  • protecting the household and public health, while allowing people lifestyle choices such as gardening
  • using other water resources for toilet flushing, laundry and outdoors
  • not adding significant costs to each new home
  • ensuring any option could be maintained by residents and serviced by local plumbers.

Modelling rainwater tanks and greywater

The council selected rainwater to supply water to toilets, outdoors and possibly laundry. Any rainwater tank specified would be topped up with 600 litres of town supplied water a day when the water remaining in the tank fell below 1,000 litres. Greywater diversion would harvest water from the laundry and bathroom to be used in subsurface irrigation.

The council modelled different sized rainwater tanks with greywater diversion using the PURRS (probabilistic urban rainwater and wastewater reuse simulator) software developed by Peter Coombes from Newcastle University. PURRS enabled the council to assess how effective using rainwater or greywater would be in reducing peak and overall demand during typical and dry years.

The council has used the following Kapiti Coast data to paint a clear picture of residential water use:

  • demand profile of a typical house (from WEEP study)
  • 6-minute step rainfall data for Paraparaumu Airport (from NIWA’s Cliflo database)
  • age and income data (from 2001 Census).

Rainwater alone not enough

Even small rainwater tanks were effective at reducing demand (see Figure 2), but rainwater collection alone did not perform well during the peak summer demand period. A 12,000 litre tank had little effect in reducing demand, as low summer rainfall could not recharge the rainwater tank sufficiently to meet outdoor irrigation demand (see Figure 3).

On the other hand, greywater diversion reduced overall and outdoor demand significantly.

Rainwater and greywater for new buildings

Council selected 4,500 litre rainwater storage plus greywater diversion as its preferred option. Storage of up to 10,000 litres of rainwater was included as an alternative for those not wanting greywater irrigation.

The council notified plan change 75 on 1 February 2008. All building consents submitted after this date must incorporate the plan change requirements. Although it is a requirement of the district plan to incorporate water saving devices, any device installed must comply with the New Zealand Building Code or associated standards.

The council is optimistic that the plan change will enable each water supply to continue to provide a quality level of service by reducing the demand profile of each new connection.

Retrofits may be next

Later in 2008, the council will consult with the community on the Long Term Council Community Plan and will discuss ways to encourage water savings for existing households, possibly through incentive packages for retrofitting with rainwater or greywater systems.

Although Kapiti Coast is the first local authority to require water efficient technology in new homes, other areas facing increasing competition for scarce water resources may consider this a possible regulatory option in the future.

Figure 1: Water consumption for Raumati, Paraparaumu and Waikanae from 1998 to 2007.
Figure 2: Water savings possible from using rainwater or greywater.
Figure 3: Possible reductions in peak demand from using rainwater or greywater.

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Articles are correct at the time of publication but may have since become outdated.

Figure 1: Water consumption for Raumati, Paraparaumu and Waikanae from 1998 to 2007.
Figure 2: Water savings possible from using rainwater or greywater.
Figure 3: Possible reductions in peak demand from using rainwater or greywater.

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