The project recognised the importance of water as an especially critical issue for the 21st century. Considering that no shortage of water falls from the sky each year, the key to meeting our water needs on-site was storage.
Our initial advice was based on a equivalent degree of water self reliance to rural houses, ie 5000 gal (25,000l) tank per house--which we clearly would not have room for. What was proposed instead was three large 3 325,000 litre underground tanks. These would measure 8.7m across by 5.5m high, and be constructed on site with a strong enough top to withstand complete burial. This option would have provided approximately 85 days storage, based on 1.25m annual rainfall, and 140l/day usage by 33 units.
This turned out to be a very expensive solution, in the vicinity of $350,000, and we still needed a fire hydrant on site. Council also required full filtration systems, including UV and/or ozone sterilisation.
We sought advice on a more practical solution. As it happened council had done quite a lot of research on the subject as a result of the water crisis, and Waitakere's objection to the Waikato pipeline. Some of the reports they produced showed us how as little as 3000-6000 litres of storage per house could actually meet 80% of our water needs. By redesigning our system sharing one 30,000l tank between 6 homes and supplementing this supply with city supply over the dry months we arrived at a practical cost effective solution. So often a sustainable solution is a creative one, rather than an 'ideal' one.
So the total storage volume is around 190,000 litres or 5900 l per house. We chose 7000 gal pre manufactured Hynds concrete tanks, and they generally sit with about 500mm in the ground, on a gap6 bedding. These tanks are about 4m wide, and need a very big truck and wide enough access. One tank in an area of especially high water table needed some extra concrete in the base to prevent the tank popping out of the ground. The tanks are located strategically at the head of each dwelling cluster, and provide about 20 days storage.
Many people have concerns about the proliferation of chemicals such as chlorine, fluorine, aluminium sulphate and asbestos in the city's water. Having a predominantly rain supplied water system means residents are free of this fear for most of the year. We expect to set up a central reverse osmosis filter water for drinking purposes, and this will save residents having to purchase filters individually. Also many people prefer the softer water for showering.
While studies do show that urban rainwater does contain a level of industrial contaminants in many cases it is nonetheless still found to be cleaner than city supplies.(eg Mobb's Sydney study). In any event the first flush of rain contains the contaminants and this is reasonably readily removed. As a compromise the Earthsong kitchen and bathroom cold taps are connected directly to the city supply. These two taps represent about 6% of water use. All other water is from the tank supply.
All roof water, including the carports and common house flows direct to the water tanks, via 100mm underground plumbing. Any overflow from these tanks is directed to the swale stormwater system.
The water from the tanks is pumped using dual 3 phase pressure pumps and presscontrols to each home, and in the event of the tank becoming empty will automatically (or manual changeover valve switch over to the city supply. Having these tanks scattering around the neighbourhood is also comforting should the city supply ever fail. In fact we have also talked about building a water tower, as a new symbol of the importance of water to humanity, and an additional water reserve at head.
By diverting most of the rain that falls on our roofs we also substantially reduce the impact of stormwater on streams and harbours. When the tanks do overflow, the runoff will gently be absorbed into swales before reaching the pond. In all cases the water is being used for something instead of being seen as a problem to piped off the site at great cost.
With some creative budgeting we managed to avoid the use of doxin containing PVC for the rain collection and reticulation system. Polypropylene was the material of choice as it is a relatively benign plastic while allowing the sealed underground plumbing we required. All internal plumbing is copper for maximum durability.
A cleaning eye is located on each pipe at the inlet to each tank. This can be used to remove accumulated sediment, and can also be used in the traditional "Waikato method" to divert the first flush after long dry periods. In time specially designed first flush diverters will be installed, however at around $100 each currently available proprietary products were too expensive. Each house has 5 downpipes, that's $16,000. First flush devices should be located as close as possible to the gutter, so as to avoid long underground piping runs storing large volumes of first flush water.
Tank supply pipe is 63mm from the tank to houses, as is the mains supply pipe. The mains line meets at the fire hydrant, and becomes 100mm diameter at that point. A testable double backflow device, one bulk meter, and main gate valve is located their in a rather large cabinet.
As with all sustainable supply systems it is important to simultaneously look at ways to reduce consumption. In many cases household water use can easily be reduced by half just be using low flow appliances and fitting. Using both these approaches we expect a total reduction in water use of 50% from standard regional averages, whilst only using around 15% of city supply water from the average. The graph below shows this over the 12 months of the year.
This is a very simple water model, and the monthly water balance method shows even better results.
With respect to billing, Council invoices for the one bulk account for mains water to the body corporate. Dwelling water use is sub metered, and on-charged as part of Body Corp levies. This is to encourage conservation of the precious and limited resource. An experimental fee scale we have adopted varies by season and volume used. Water generally will be cheaper in the winter and more expensive in the summer. The initial tariff is as below:
| SUMMER incl gst (nov-apr) | WINTER incl gst (may-oct) | |
| Basic use /mth | First 3 cubes @ $1.20/m3 | First 8 cubes @ $1.20/m3 |
| Extended use /mth | Balance at $2.10 | Balance at $2.10 |
Estimated average household use is around 7.5 cubes per month. The tariff above reflects the fact that we have a water surplus in the winter, and to encourage its use. The lower tariff allowance will likely be varied by house size, in the near future. Given that the only running costs are electricity, and pump maintenance, it is likely the water will cost the project significantly less than the revenue from these tariffs. Any profit the project makes on water, will simply serve to offset body corp levies. In this way conservation oriented tariff's are not punitive on the whole. A discount for early payment will help ensure timely payment.
Meters are located internally, and will be read by residents themselves who will email the office or drop off a note bimonthly, with the body corp conducting annual checks to ensure accuracy.
The houses each have a Maric flow control valve on the tank line, which limits the flow to any one house at 25l/min. Standard 3/6 l flush toilets throughout, and AAA Methven 9 l/min adjustable tap and shower fittings.
Most houses will not have laundries, instead using the common laundry provided. This further helps distribute the available water. The 2 tanks and large roof areas of the common house and carports serve the common laundry.
It can and has been readily demonstrated to residents that it is their best interest to replace old water hungry appliances. One example is the classic F&P washer. At 200 litre per wash the machine uses around $60 of water per year. At say 3 wash/wk x 52 weeks x $1.85/1000 litres that amounts to, over ten years, $480 in today's money (NPV). It makes sense to spend the $480 to upgrade to a frontloader that uses only 45 litres per wash.
The pump setup was designed by the project's Plumber CF Reece. 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.
The proposed control valve to switch over to mains supply, is a Dorit electric model EL series DN40. This is a 40mm 3 way normally closed solenoid controlled valve capable of switching 200 l/min. It is activated by a low water level sensor in the tank. These valves are still to be installed.
You don't get much change out of $1000 per pump/and controller, but they are built like a BMW, and you can take them apart and change parts as needed. In Europe they are the standard for the shallow wells common there. Here they are commonly used for dairy effluent and a range of heavy duty uses. They are quiet, almost disconcertingly so.
As with a number of other services on site, the scale of the project proved to work in its favour for water supply. The economies have been achieved by the clustered approach. Expensive hardware has been shared between 5-7 houses. The use of one tank where 5-7 would normally occur has resulted in a large saving rather out of proportion to the difference in stored water volume. The fact that most tank systems are sized for the driest period means they are totally over spec'ed for the majority of the year. Earthsong has chosen to accept requiring mains water through the brief months of January and February particularly. With 20% of the storage we achieve rainwater supply 80% of the time. Its a great result, we think, for under $1500 per house all up.