Renovating for water-efficiency

We bought this place and moved in almost two years ago with the intention of staying for at least twenty. Location, size and price were the key criteria. Water and energy efficiency were not even on the list - if they were we might have chosen not to buy this one.

This post is a bit of a look at the physical attributes of our house in relation to water efficiency and the improvements we're in the process of installing.

The building is basically rectangular with the top edge in this image facing just slightly east of north. It's about 22m long with no eaves at the ends, and about 9m wide including the eaves along each side. Were it a complete rectangle it'd be almost exactly 200 square meters of catchment area.

The roof has a single centre line running the length of the building and falling away to the east and west. There are five downpipes for stormwater drainage - one at each of the corners along the long sides. It doesn't make sense to try and get the water to flow just to one end of the house so two tanks are needed if all the run-off is to be captured. Since I don't have room to install tanks half way up each side of the house, the only alternative is to put them at each end and install pipes to bring the water around the corners.

Siting the tanks was made more difficult by the 1m concrete apron which runs completely around the house. Early ideas involved a new concrete slab which partly overlapped the apron but my more recent preference towards using crushed rock for the base pushed them that meter away. That in turn had an impact on designs for the stormwater pipes which need to be high enough for people to walk safely underneath. In the end they'll be positioned as per the image: it's a compromise between the practical and aesthetic challenges of running the pipes as well as the limits on available space.

The tanks we chose are 5000L each, calculated to hold approximately one month's worth of rainfall in an "average" year. They're made of plastic with a ten year guarantee and I hope to extend their life as much as possible by shading them from direct sunlight. At 2.2m diameter they're only 1.6m tall in the centre and about 1.3m at the inlet. An internal first-flush diverter (it has a dedicated drainage outlet at the side of the tank) will reduce contamination from dust and debris and the inlet filter is designed to take the weight of a child without breaking.

Up until recently (when I was writing up a comment on another blog) I had envisaged using one tank solely for gardening and the other for washing clothes and flushing the toilet. But after realising that Julie's tank, half the size of mine and with comparable inflow expectations, would make available something like 160L/day just for the garden I had to reconsider that approach. Our block is flat so I'm now looking at installing a permanent hose between ground-level outlets on each tank: this will allow the two tanks to function as a single 10kL reservoir.

In order to supply the toilet (thankfully there is just one, dual-flush) and the washing machine the plan is to install a small electric pump and two new taps - one in the toilet, one in the laundry. We'll manually attach the hoses to those taps when there's water available in the tanks, and manually switch them over to the town water supply when the tanks are running low. Doing this should avoid any problems with backflow or pressure and makes a top-up valve unnecessary. Both rooms are on the western side roughly two-thirds up.

If the tanks do end up linked together it would probably make sense to install an external pump-powered tap at the northern end near the gardens. And a tap over the laundry sink might be smart as a way to access tank water for other internal uses like filling our gravity-fed water filter or even just mopping the floor.

So after all of that, our two remaining consumers of town water will be the kitchen with its dishwasher and the bath and shower. We'll still be drawing some water from the river systems but hopefully the rain that does fall on our heads will be put to some use instead of being channelled straight out to sea.

Finally I ought to mention cost. A combination of Brisbane City Council and Queensland Government rebates worth $1,850 are applicable to this little project. The only expense so far has been the tanks themselves at about $2,600. I have another thousand dollars set aside to pay for materials, a pump, an electrician and a plumber... though I'm not entirely confident that's going to be enough.