Water

By designing our system to share several large tanks between clusters of homes and supplementing this supply with city supply over the dry months we arrived at what we felt was a practical, cost effective solution. So often a sustainable solution is a creative one, rather than an 'ideal' one.

We future-proofed by designating small pockets of land between house rows as common "land to allow later installation of extra water tanks, to increase the available storage and therefore the percentage of water supplied from on site.

A recent study of water supply data collected over 4 years, described later, shows that on average we are providing nearly 60% of our own water needs from rainwater in a typical year, and over 70% for the wetter half of the year. This is lower than our original expectation of 80%, but still represents a significant saving in both the environmental and financial costs of water supply.

While wanting to avoid PVC downpipes as unhealthy to people and the environment, we did need sealed pipes for the underground line between the houses and the tanks. We chose polypropylene pipes, known as a benign "food-grade" plastic. They don't cope well with the strong UV light in New Zealand, however, and do need to be protected with paint to avoid becoming brittle. We used copper for all internal water pipes in Stage I for maximum durability, though made a switch to polypropylene in Stage II for greater insulation of the hot water pipes.

All houses have a dual water supply. Those taps used for drinking water (kitchen sink and bathroom basin cold taps) are supplied directly from the city mains, and all other taps (hot water, shower and bath, WC, laundry and outside tap) are supplied from the tank water line.

The 6 tanks and mains supply, in addition to supplying the 32 houses, also supply communal garden taps and the common house, a large community building shared by all the residents. While a few houses have their own laundries, most houses choose not to have their own washing machines but use a communal laundry in the common house.

Unfortunately by the end of 2009, a total of 8 pumps had needed replacing (3 of those still under warranty) at a cost of around $1500 each, along with sundry mascontrols, ball floats and split pipes. In many cases the bearings of the pumps were found to be shredded. We installed float switches to all the pumps in 2007 to avoid them running when the tank water was low, but the problems have persisted.

The pumps are designed for up to 40 starts per hour, which should have been adequate for 6 or 7 houses each running a tap or flushing the WC up to 6 or 7 times an hour. However it appears that this has been more than the pumps could handle and put too much pressure on the system. On the advice of the pump manufacturer we are trialing going "back" to the traditional pressure tank system. This is similar to a header tank system, where the pump fills a smaller tank which stores enough water for several uses and delivers it under pressure to the houses, avoiding the need for the pump to start every time someone turns a tap on.

The first of these new systems has recently been installed, with a 450 litre Challenger pressure tank now sitting beside one storage tank. As water is used in the homes, the pressure drops in the pressure tank until it reaches a pre-set level (30 psi), when a pressure gauge and switch activate the pump. The pump continues feeding the taps and then recharges the pressure tank once enough taps are turned off.

This system should have several advantages over the original system. The number of times the pump has to start should be significantly reduced, saving wear and tear on the pump. Only one pump should be needed per tank, as the pressure tank stores enough water at a high enough pressure to supply several houses at once. The mascontrols are no longer needed, replaced by the pressure gauge and switch which are simpler devices and give readable information to understand what the system is doing at any time. We are hoping this gives us a simple, reliable system that will prove its value over time.

In order to learn from our system, the joint venture monitoring project with EcoWater has included the monthly testing of our tank water for some years for e coli, pH and turpidity. The results have been quite variable from month to month, depending on the frequency and volume of rainfall in the preceding month, the amount of water in the tank at the time of testing (and therefore whether the water was collected at the top or close to the bottom of the tank), and whether the gutters had been cleaned or the downpipe lines flushed. Proper analysis of the data has yet to be completed so conclusive results are unfortunately not yet available.

Water Charges

There is one bulk meter for the city mains water that is delivered to the site, which is paid by the Body Corporate. The Body Corporate then charges individual households according to their use of 'tank water' (or the water that comes down the tank line, some of which will be city water in dry periods), measured by the check meter on the tank water line to each house.

In order to minimize up-front costs, meters were not installed on the city water line to each house at the time of development. The assumption was that the volumes of potable water consumed by each household would be generally proportional to the amount of all other water used by that household, and therefore the appropriate rate charged for the 'tank water' could be calculated to include the city water use. In practice this has rather muddled our records of how much water people are actually using, and contributed to some confusion when debating water charges.

The Body Corporate, being an active group of all of the households, made the joint decision from the start to promote water conservation by setting a two-stage tariff for water, that varies for season and volume used. In summer, the first 3 cubic meters per household per month are charged at a lower rate than subsequent water use per month. In winter, when there is expected to be a water surplus, the first 8 cubic meters are charged at the lower rate. This tariff reflects the fact that, in summer, a much higher proportion of the water comes from the city mains, and is therefore a higher cost to the Body Corporate.

This two-stage tariff has proved controversial as a water-conservation strategy, as it doesn't make allowance for different numbers of people in a house. A one-person household has little trouble keeping within the cheap rate even with profligate water habits, while a family of four, though they may be more careful in their water use, will much more often fall into the higher rate. The charging system may well undergo review soon to more fairly reflect this, though there will always have to be a balance between implementing a system that fairly reflects use and promotes water conservation, and simplicity of accounting.

As stated above, ongoing problems with our tank system have meant huge maintenance costs to date, and combined with the capital costs of the tanks, pumps, float switches, flushpoints etc of the system, our tank water system could not be justified on cost/benefit alone!! It is clear that our system was either not well designed for our situation or we have had extraordinarily bad luck. We are hopeful that the re-design of the system will prove simple and reliable in the future.

However even with these extraordinary costs we are still saving over 25% of the cost of the water that we use than we would pay if it all came from the city mains supply. Given that Earthsong's water use per person is only 72% of the Auckland regional average, an average Earthsong household's water costs are only 54% of the equivalent Auckland household.