Thursday, 24 April 2014

Harvesting rainfall

There seems to be a lot to write about water, whether suspended in the air or precipitated.

I've been thinking about harvesting rainfall for a while, with the usual constraints of elegant design, low cost, and aesthetic beauty. In fact it was on the list of things to incorporate into the building of the house, but it soon went onto the back burner along with all the others. It's not a good idea to leave water on a back burner for too long as it will evaporate and you'll burn the kettle.

We need water for the garden and rather than switching on the tap and pouring rainwater down the drain, it would probably save us a fair bit of money to collect rainwater and use it for the plants. 

Just to deconstruct the problem, we'd mostly be looking at collecting water from spring, and using it mostly in the summer. If we could collect enough then, we wouldn't need to worry about the winter, and could use that time to empty the system, which may be preferable to having a collection of ice. 

Looking at the supply side, there's roughly 100 mm rainfall a month, a little bit more from May to July, and we have a total roof area of around 100 square metres. Technically speaking, the roof area is bigger than that, since it's at an angle, but that's the area as far as the rain is concerned, which I guess we have to assume is coming down vertically. 

Anyway, down the four drainpipes we'll get a total of 10 cubic metres of rain water per month, or 10,000 litres. So we'd have no trouble filling a 500 litre tank from one of the drainpipes. 

We could get a big tank somewhere, but would probably want to put it underground to save space, in which case we'd need a pump to get the water out. What would be more useful is something tall and thin that could stand against the wall, right next to the drainpipe. 

A 500 litre tank could be 80 cm in diameter and a metre tall. Or if you had two 250 litre tanks, they could each be 60 cm in diameter and 90 cm tall. At least those are the internal dimensions. You'd probably want to add a bit for the thickness of the walls. 

Since we don't really want water tanks in front of the house, and the gap between the side of the house and the fence is only a little over a metre, we wouldn't really want 80 cm tanks along there. We could make them taller though. 

A 5 metre tall tank of 25 cm diameter would hold 250 litres. We could tip it onto it's side, and run it along the whole width of the house, north to south along the west wall, so that it could fill from two of the drainpipes. Then one tank could be 8 metres long and 20 cm in diameter.

As usual, somebody has already thought of tall this. Rainwater hog make a chunky modular system, and there's Jackson in the UK.

But going back to the tank, as it gets longer it's starting to turn more into a pipe than a tank, so if it is going to be a pipe, then perhaps it could run through the garden towards the plants where it is going to be used. Then it could be 15 metres long and only 15 cm wide. Of course we'd be back to needing a pump again. 

So the best ideas are probably a couple of thin horizontal tanks, or thick horizontal pipes, running along the side of the house, or a few vertical ones, possibly cascading like pan pipes.

So it's time to start looking around for parts and construction techniques. The question is, do they have them in a nice colour? I don't want grey pipes on the side of my house.