The following options for improving water management were explored:
- 4.5 star WELS dishwasher and washing machine
- 3 star or better WELS showerhead
- 5 star WELS kitchen and bathroom tapware
- 4 star or better WELS toilets
- Greywater recycling for WCs and garden
- Rainwater tanks
4.5 star WELS dishwasher and washing machine
There is no relationship between cost of appliance (whether dishwasher or washing machine) and standardised water use, i.e. the Water Efficiency Labelling Scheme (WELS) rating. There are only a few 2 and 3 WELS star rated washing machines available, with most being 4 or 5 stars, on average. A 4.5 star will use about 69 litres. The minimum WELS rating for dishwashers is 3. Together, a water savings of about 19,000 litres per year could be expected, the majority of savings coming from the washing machine upgrade.
Approach
Assuming one 8 kg load per day, a 3 star washing machine will use about 43,000 litres while a 4.5 star will use about 25,000 litres over a year. Thus, about 18,000 litres is saved a year on this washing regime.
For the dishwasher, assuming 14 settings, a commonly available 3 star WELS dishwasher uses just under 17 litres per wash while the 4.5 star uses just under 13 litres per wash. Assuming one wash per day (following AS/NZS 6400:2005 Water efficient products - Rating and labelling), this equates to 1,460 litres per year saved.
Harvey Norman and Noel Leeming online were used to establish prices and water WELS ratings.
3 star or better WELS showerhead
The ordinary showerhead typically makes up the largest percentage of indoor water use, accounting for about 30%. A number of showerheads are available that provide a very comfortable flow, although their water usage is modest at around 8-9 litres per minute. It is unknown how efficient the average showerhead is in new New Zealand homes. Given their significant contribution to water use in the home and the ease with which they can be substituted for something considerably more efficient, it makes sense to specify a high (3 or higher) WELS rated showerhead if possible in all bathrooms. Note that the showerhead must be matched to the home's water pressure/plumbing - check with the plumber.
Approach
A best estimate of the common flow rate of showers being installed is about 14 litres per minute. A 3 star WELS rated showerhead will provide a flow of 8.5 litres per minute. Based on three showers per household per day for an average time of 7.8 minutes, this equates to a savings of around 47,000 litres per year - enough water to fill the average sized home over seven times!
Bunnings and Plumbing World NZ online were used to acquire retail prices in February 2014. Prices for the extra stars are hard to determine because of the mix in quality but probably marginal extra costs.
5 star WELS kitchen and bathroom tapware
There are good water savings to be made from reducing the flow rates of tapware, mainly due to lower water wastage. There doesn't seem to be significant purchase price increases for the more efficient tapware.
Approach
The method is based on information obtained from the WELS site www.waterrating.gov.au. A tap is turned on full three times a day for 3 minutes by each household occupant, and half the water used is heated to 23°C. It is assumed that the tapware is upgraded from an inefficient 2 star WELS to a more efficient 5 star WELS. The energy saving is from the resulting reduction in necessary heating water, knowing that it takes about 5.5 Wh per litre of heated water (starting from 10°C). Note that 6 star WELS tapware is also available.
The purchase cost information was provided by a Jasmax study carried out in 2013 as part of an investigation into the cost implications of having new houses environmentally assessed.
4 star or better WELS toilets
It is estimated that toilets use about 18% of total house water (including that used outside for gardening), with the average person flushing the toilet about five times daily. This water is treated and pumped - both before and after use - requiring lots of resource use. Toilet water efficiency is standardised using the WELS system. Although dual-flush toilets are becoming the norm in new houses, most seem to go for the standard 6 litre/3 litre flush (i.e. 3 star WELS). These can be simply substituted with the considerably more efficient 4.5 litre/3 litre flush (i.e. 4 star WELS) toilet, with no loss in utility.
Approach
The yearly savings from the upgrade are based on the following calculation process. For dual-flush toilets, daily flow rates are calculated from the average of four half flushes and one full flush per person. Thus, the flow rate for the standard toilet is 3.6 litres while the more water-efficient toilet requires only 3.3 litres, effectively equating to a reduction of 8%. For a three-person household, assuming five uses per person per day, the original water usage is 19,710 litres versus efficient water use of 18,067 litres per year. This equates to a savings of 1,642 litres per year.
No obvious differences in purchase costs between the more commonly specified 6/3 toilets and the more water-efficient 4.5/3 toilets was found, using a variety of bathroom and plumbing retailers.
Greywater recycling systems
Greywater systems can take advantage of the wastewater from the shower, bath and some taps, which typically makes up about half of all the water used in the house. This water can be reused either for flushing the toilet or watering the garden. Greywater cannot be used for cooking, bathing, swimming or drinking. The system must be properly installed and maintained and kept from direct human contact if used in the garden. Also, untreated water should be used within 24 hours. Various types of greywater systems are available. Before investigating the options, make sure that your local/regional council permits their use.
Approach
Supply and wastewater charges were sourced (using Water New Zealand figures from their 2015/16 National Performance Review online resource. Where there was an option for both metered and unmetered water, it was assumed that all new connections in metered areas would be metered.
The table below shows 2015/16 local authority charges associated with supply and waste water based on 200 m³/yr.
| Supply water | Wastewater | |
|---|---|---|
| Auckland | $1.41 per kL | $2.88 per kL |
| Hamilton | $1.15 per kL | $0.985 per kL |
| Tauranga | $1.97 per kL | $1.9 per kL |
| Napier | $1.23 per kL | $1.23 per kL |
| Wellington | ||
| Nelson | $3.045 per kL | $1.95 per kL |
| Christchurch | ||
| Dunedin | ||
| Invercargill | $306.07 | $201.11 |
Based on a whole household of 600 litres a day water use, a 50% savings equates to saving 300 litres a day or some 109,000 litres a year. Using a popular wastewater system model, the costs work out as: base unit ($3,400), bracing ($120), irrigation system for garden ($300), freight ($150) and extra plumbing work ($200) - $4,200 in total. Cost-benefit calculations were based on these figures.
Rainwater tanks
The options explored were:
- 1,000 litre water tank, with feed pump to garden
- 5,000 litre water tank, with feed pump to laundry and toilets
- 25,000 litre water tank, with feed pump to whole house.
Rainwater tanks are becoming increasingly popular to supplement reticulated supplies as people are becoming more aware of resource use, independence and increasing council charges and requirements. A good deal of use can be gained from even a tank of smaller capacity - say 1,000 litres - if used well. About 11% of water use in homes is for the outside, changing with season. The usefulness of a rainwater tank is dependent on the size of the rainfall resource, the collection (usually roof) area, the size of the tank and the actual demand. There are decreasing returns by increasing storage tank size.
Approach
In terms of determining the water saved, the following approach was used. It was assumed that the roof collection area equates to 140 m² with the home occupied by three people year round who use an average 600 litres a day combined. The nine locations were divided into three rainfall climates: less than 800 mm/yr, 800-1,100 mm/yr and 1,100-1,400 mm/yr of rainfall.
Applying a rainwater calculator, these variables provided an estimation of the amount average yearly percentages of water supplied, by climate (location).
- The 1,000 litre tanks provide between 63,000 and 93,000 litres, depending on climate, for garden-only purposes.
- The 5,000 litre tanks provide between 76,000 and 120,000 litres, depending on climate, for laundry and toilet-only purposes.
- The 23,000 litre tanks provide between 83,000 and 134,000 litres, depending on climate, for laundry and toilet-only purposes.
These figures are indicative only, being based on average rainfall events, standardised occupant behaviour and so on.
Polyester (plastic) tanks were used for each solution, and costs included the tank, pump with protective housing, delivery, installation, trade labour and all plumbing. No earthwork was assumed to be required. Individual city cost differences were not explored, as they were expected to be much the same. In only those councils that currently meter water by usage (instead of a charging a flat rate) were savings calculated.
More information
Updated: 05 April 2024