Our main motivation for building a passive house was a desire to live more sustainably. So we installed a rainwater harvester to reduce our demand for mains water. We plan to use rainwater in the garden (we have a polytunnel and two greenhouses), for the washing machine and for flushing the loo in the bathroom. (We only have a single, family bathroom and no en-suites).
We bought a large (7500 litre) tank from www.rainwaterharvesting.co.uk. It is a direct system - water is pumped from the tank on demand when a connected tap is opened.
Our first issue was the complexity of installation. Our harvester is supplied with a unit called "RainBackup in a Box" to top up the tank with mains water during droughts so we can continue to use rainwater for the washing machine and loo. The unit needs a power supply, 15mm mains water connection, a 32mm pipe supplying water to the tank and a 32 mm overflow. (We have fed the top-up supply into one of the downpipes rather than running it all the way to the tank). There is also a control cable running to a level sensor inside the rainwater tank.
RainBackup in a Box is WRAS approved and seems to be a reliable way of managing the system.
(The large stop valve in this picture is the 50 mm mains water supply for our fire sprinkler system).
Between the house and the tank, we buried the control cable, power supply for the pump and the rainwater supply pipe inside this blue conduit. Our trenches were 600 mm deep to prevent the rainwater pipe freezing in winter.
Rainwater collected from our flat roof drops into the upper 110 mm connection. The lower connection leads off to a soakaway.
Soakaway? Despite collecting all of the rainwater from its roof, the house still needs a soakaway as an overspill when the rainwater harvester tank is full. You cannot replace a soakaway with a rainwater harvester.
Our first reality check was the size of the hole needed to bury the tank. The tank's dimensions are 3340 x 2310 x 1615 mm and it weighs 310 kg. Burying the tank is not a DIY task and we had to call on our groundworker to excavate the hole, remove the spoil by grab lorry and lower in the tank using his digger.
And our biggest mistake was to bury the tank too deep. We excavated down to 1600 mm for the base of the tank but now recognise that we could have made a shallow dig to 1200 mm. This increased the cost and has also caused us problems with connecting to the soakaway. The soakaway outlet on this tank is located beneath the rainwater inlet so the drainage pipe is a whole metre below ground level before it starts to drop into the soakaway. In our part of Norfolk, we have about 6 m of clay under our topsoil so the soakway has been excavated in largely impermeable clay and (so far) appears to be largely ineffective.
Last weekend, we had sustained heavy rain (40 mm in one day) and our tank and soakway were swamped. Once we have moved in, we should be able to manage surplus water (after loo flushing and clothes washing) by running it off the garden tap. For now, we are regularly pumping out the tank to manage its capacity in heavy rainfall.
So much water fell so quickly last weekend that the water level rose up the turret of the tank, spilling over the collar and saturating the backfilled earth around the buried tank.
In fact, the tank seems to fill remarkably quickly. We are collecting water from both the house roof (153 sq m) and the garage roof (42 sq m). It will be interesting to find out whether we always have a surplus in winter (not much garden watering required) but also whether we have sufficient rainwater in summer.
We have also been surprised by the additional water regulations imposed by the supplier (Anglian Water) when you have a rainwater harvester as well as a mains supply. We had understood that the extent of the regulations was simply clear labelling of the rainwater pipe; the pipe itself is distinctively coloured black and green. We found out a very late stage that a number of check valves and drainage points are required to safeguard against contamination of the mains water by rainwater from the tank which has not been treated and may contain atmospheric and environmental (e.g. bird poop) pollutants and maybe pathogens that have bred in the tank.
Having a rainwater harvester also means you need an additional inspection and the water connection will not be made until all plumbing is complete to second fix - so that the installation of all the valves and labels can be checked. This makes the new connection process slower and more cumbersome. And we will be subject to a spot-check sometime in the next 2 years to check that we have not modified our installation.
So we have found that although rainwater harvesting is sustainable, it is neither cheap nor easy. Our system cost £2.6k but the associated groundworks cost > £1k. The installation is complicated and (in our experience) water suppliers are not used to working with them.
Hopefully, a plentiful supply of rainwater next year will make it all worthwhile.