It’s been quite dry recently (relative term – after all this is the UK in summer we are talking about) and having replanted a clematis at the back of our garden, we were concerned about whether it would survive the change. It wasn’t the most auspicious of sites as it was in the rain shadow of a lilac tree that grows out of a bed that’s about 800mm square. It also has a buddleia there. You know buddleias. Grow to tree size out of a crack in a wall… Well not this one. It was in its second year and was still one scrawny shoot about 300mm high. We did plant the clematis in a chimney pot full of earth to give it a head start, but didn’t hold out much hope for its survival.

Image of PlantAnd then I had a brainwave and this is the result…pretty impressive huh? And the secret? Well, it’s courtesy of everyone who showers round at ours. Our shower flow rate is approximately 7 litres/minute (thermostatically controlled shower fed from a combi boiler). I average just under two minutes per shower and Suhith (who is not impressed by the Australian method of stop start showering) averages about four minutes. Although all our visitors are constrained by showering chez Cath and nip in and out as fast as possible, I still reckon that over the past three weeks the shower has supplied 1008 litres to that bed.

No wonder the plants are looking so ridiculously happy! Every day they are guaranteed 42 litres of water with a bonus when friends are staying. That will of course suddenly reduce to zero water for two weeks when we go on holiday, but as it’s not a hydroponic system I am hoping they will cope and not collapse with shock.

So are there any downsides to using greywater outside? Well, we have to pump the greywater across to the back of the garden and that of course uses power. Those of you are avid readers of this column and just love maths, will remember that in the UK it takes on average 1.2kWh of electricity to supply 1m3 of water into our buildings and to take away the corresponding 1m3 of sewage for cleaning. So, as long as we use less than 1.2 kWh to pump the greywater to the back of our garden then not only are we saving water we are saving power too (this assumes we would have used mains water in place of the greywater) . The pump in our greywater recycling system (the Banbuster from Watermatic) is very efficient and we are pumping against a small head (about 1m or 0.1 bar) and little frictional resistance (open ended half inch hose); I measured the power consumption and it takes 0.4 kWh to pump 1m3 of water to the back of our garden and up into the chimney pot so it is two thirds more energy efficient than using mains water. (Lyds, you still with me?)

So that’s all good. Anything else? Well, yes. Greywater is pretty yucky stuff after it has hung around for 24 hours as it holds bit of skin, hair and soap residues in solution. The pump is controlled by a float switch and the greywater is pumped onto the garden almost immediately. But it is coarsely filtered before it enters the sump chamber and cleaning the accumulated gunk that collects in the filter has to be done every week and is not a pleasant job at all.

In fact I reckon cleaning out sewers blocked with pooh is a preferable job to cleaning our greywater filter – but that’s another story… for a different blog… in a different dimension…

July 2013 – Showers and greywater