Grey water on the garden

No, not vague! See if you agree with my figures (would be good to have someone else check that I've not got an order-of-magnitude error somewhere!)

Lets start with rain water (rather than greywater - which might give better figures)

500 litre tank, probably would be filled twice a month in Summer? Lets say 4 times.

So for 5 or 6 months I will use it to water the garden.

6 months x 4 refils = 24 @ 500L = 12,000L. I can buy that as drinking water from the water board for £12. Long time to payback :(

Household water usage (grey water calculation). In winter (based on weekly meter readings) we average 385 L / day. (I work from home, there are often 5 or 6 adults here on working days)

Lets assume that 50% of that goes down the pan! and I could replumb to capture the other 50%

So I would get 200L-ish per day. Taking the same 6 Summer months that's 36,500L - £36 quid - plus the £12 from rainwater (provided that the tank is empty when it rains ), so about £50 a year saved

* I think it would be easy to have the tank "empty" when it rains. I reckon it the tank has water it should put it on the garden (each night, for example), so there would need to be a bit of organisation to make sure it was used each night.

Trouble is I would want more than 500L I think. Our Summer usage averages 1,000L/day - so take away the winter usage 1,000 - 400 = 600L. Currently we water the garden once or twice a week (leaky hose on hedges and hand watering of "young plants" from hose etc.) so I need 2,100L or 4,200L per watering session (although, as said, I should be able to organise the leaky hose to run in sections each night and spread that watering load).

@ 600L / day through the Summer - 6 months x 600L/day = 110,000L of garden watering (its less than that because we don't water in the early / late part of the summer to the same extent). If I got 100% of that from greywater I would save £110 a year - and, realistically, I'm never going to do that with a 500L tank, so allowing for a bigger tank the payback will be more than 5 years, probably more than 10 years, and then I will be hard to convince to spend the money :(

Changing the house plumbing to capture greywater is quite expensive - my easiest route would be to replace the sceptic tank with a digester producing water suitable for irrigation. Cost of that, and a reasonable sized tank, is what? £5,000? to save £100 a year :( That won't happen until the cesspit breaks ...

Rainwater is the other source. Requires running a new pipe all around the house - currently each downpipe goes to its own soakaway. So a fair amount of plumbing, but all through lawn with just a little through the drive.

I reckon if we collected the water from all useable roof area (including outbuildings) we would get about 15,000L from an inch of rain. Rain does not come every day! so I reckon I would need to store at least an inch rainfall, possibly two inches (and even more if I want to store some from Winter to use in Summer). That's an expensive tank - a pond would be easier (but that will evaporate at 1/2" per week in Summer I think - so deep and narrow would be better!)

Probably cheaper just to put a borehole in ...

All very depressing as I would love to do this, but I just can't afford something with a 10 year payback

Wish we'd built a new house - much easier to build-in all this than to retro-fit.

 

Thanks Loofah. My problem is that the sources of grey water are all four corners of the building, so it would be a lot of plumbing to collect it at all, and all of it is under drive or patio :( Hopefully may work for others though, I think my only realistic route is to let both the grey and foul water go to the sceptic tank, and have a better digester that gives me irrigation water out the back of it ... no new plumbing required huge cost involved :(

 

Blimey, you have done the maths there. No, it seems in your case, its not really worth the hassle. Hose pipe to drain the bath & a couple of water butts.

Like your idea of pumping a tank out at night, could apply that to a water butt. Simple enough to see up a pump & a timer.

That would come under part P of building regulations though with regard to external electricity, unless you powered it from 12volt solar, thats exempt from part P.

While we are on the subject, when it comes to hose pipe bans, some water authorities will do you just for using the hose to syphon the bath out. It doesn't matter to them that you are using grey water.

When the Gardeners Corner Party are sitting in the houses of parliament, things are going to change.