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At Earthsong the water supply comes from both rain water and the city water supply. This is for two main reasons: lack of land area for enough tanks to be totally reliant on rainwater, and to meet public health requirements for treated potable water.
In New Zealand, many single family homes collect and drink their own rainwater without problems. However, a group of houses collecting and distributing drinking water to each other is considered a public, rather than private, health issue, and different requirements for treatment come into play. The main alternatives to chlorination were either ozone or UV treatment, and both came with considerable cost.
The other issue was that, to be totally self-sufficient in water, we would need enough storage to last through the couple or so months of dry weather over summer. Our advice was that 23,000 litres of water storage (i.e. one 5000 gallon tank) per house would be needed to achieve an equivalent degree of water self-reliance to rural houses - which we clearly didn't have room for. However, reports by Waitakere City at the time we were designing our system showed that as little as 3000-6000 litres of storage per house should actually meet 80% of our water needs, due to the frequent and regular rainfall throughout all but the driest of months.
We therefore opted to build 6 x 32,000 litre concrete tanks with each tank serving a cluster of 6 or 7 households, to give a total storage volume of 192,000 litres or 6,000 litres per dwelling.
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.
The roof water from all buildings is collected and piped to the 6 tanks placed throughout the neighbourhood. Each tank collects water from and supplies water back to 6 or 7 households, for all except potable use. When a tank runs dry after a period of little rain, a manual valve is opened at the tank to let city water down the 'tank water' pipe to houses. Thus the 'tank water' line is not exclusively for tank water, but is supplied by city water when the tanks are empty.
By collecting the rain that falls on our roofs we not only reduce the amount of city water we need, but also substantially reduce the amount of clean water that would otherwise be piped off the site to impact on streams and harbours.
When the tanks do overflow, the water flows into the overland stormwater system and thence into the pond. Rainwater is a valuable resource and enhances the biodiversity and beauty of our site; we do not see it simply as an engineering problem as in most developments.
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.
As with all sustainable supply systems it is important to simultaneously look at ways to reduce consumption. In many cases household water use can be significantly reduced just by using low flow appliances and fittings. Each house has a flow control valve on the tank line which limits the flow into the house to 25 litres/minute. Houses are fitted with standard 3/6 litre dual flush WCs, and all other taps are pre-set to limit the flow rate: showers to 9 litres/minute and other taps to 7 litres/min.
A water use monitoring project (a joint-venture project with EcoWater, the water unit of Waitakere City Council) collected detailed water use data at Earthsong monthly for the 4 years 2005 - 2008. Extra meters were installed to measure the total city and tank water supplied to a cluster of 7 houses, the city and tank water supplied to each of the 7 houses, and within one house, the water used by the hot and cold tap to each fitting. Read the WCC study.
The results of this study show that while total (tank and city) Earthsong water use per person was only 58% of the regional average in 2005, this increased over time to 74% by 2008. This result is disappointing. While the regional average was creeping down, Earthsong's use was creeping up.
There are several reasons why this might be so. Three of the houses within the 7-house cluster were completed just before the beginning of the study and full establishment of gardens followed over the next couple of years, with the accompanying need for watering in summer. Four of the seven houses have fitted dishwashers over that period. It is possible that the flow reducers on taps need adjusting.
And there is also the behaviour factor, which is usually more significant in the end than the technology. Perhaps people have relaxed the more idealistic careful habits they moved in with and are just generally using more water. The feedback to residents from this study will be a timely reminder.
Ongoing problems with our pumps have made this the least satisfactory aspect of the system. We had high hopes for the system, as described on our website in 2002:
"The pumps used are dual Italian Tesla Diver 100 and 150 HF submersibles. Each tank has two pumps suspended inside the tank. In essence the first does all the light work, and the second only starts when there is a heavy load. The first pump delivers 50 l/m at 30m, and the second delivers 110 l/m at 30m. The combined total of 160l/min means each house gets a 30l/min average peak allowance. This is quite adequate. Although they will deliver almost twice that volume under load, the diversity of 5-6 houses means it will not be necessary. The pumps use 0.75kW and 1.5kW respectively, but are only on for short periods as controlled dynamically by twin pressure control units. Three phase power is used, as single phase pumps of this size tend to create excessive power spikes on the electricity grid, without additional soft start controllers. The pumps are operated by Tesla Press/MasControls (electronic diaphragm switching device). These eliminate completely the traditional pressure tank scenario. The first mascontrol is set to start when the pressure drops below 25m, and the second to start at a pressure of 15m. These are also Italian made products, and come with a 2 year warranty."
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.
Tank Water Quality
While all drinking water at Earthsong in principle comes from the city water main supply, in reality small amounts of tank water mix with the potable water in the mixer taps, and water quality is still a potential concern.
Most of the contaminants from urban roofs can be avoided using automatic "first flush" diverters on each downpipe; however with 140 downpipes in the neighbourhood, that is a lot of diverters!! Instead we have opted for a manual system of flush points with removable caps installed at the lowest point of each downpipe line. With one downpipe line feeding water from each side of each building to the nearest tank, we have 20 flush points. Twice a year, generally at the first major rain after a period of dry weather, these caps are removed and the accumulated sediment is washed out with the flow of water stored in the downpipe.
We also invested in a siphon tank cleaning system called TankVac. This consists of a grid of pipes in the bottom of each tank that, when heavy rain occurs and the tank becomes full, sucks water from the bottom of the tank to discharge into the vegetated stormwater channels, rather than spilling the cleanest water from the top.
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.
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.