Rain water harvesting
UPDATE 13 April: I’ve had to correct some of my figures the the “Grange” isn’t the same size as house at all (sorry i rushed my calcs) so this has reduced the figures by at least 1/3rd however everything is still impressive and we’ll be pushing ahead.
It seems no matter if you’re in England or France then Good Friday weather is it the same. RAIN and rain of some possibly appropriate biblical proportions right now.
Hiding in the house i didn’t think much until looking out of the window i noticed the sheer amount of water running off of the porch roof (the photos don’t do it justice) and then there was the flash (that’s the lighting) and it struck me (not the lightning) that we’ previously been thinking about rain water harvesting but didn’t know how successful it would really be, and looking at this small demonstration from nature we could currently fill a bath with what has come off the roof.
Some quick maths says our house was a total area (front and back) of 126Sqm of roof and thus rain capture. The Grange has a less at around 46Sqm together. That’s a combined area of around 172Sqm which seems a total waste to not do anything with.
Doing the math
Here are some figures for the maths :
- Average rainfall in this area of France, 76cm (0.76m)
- Area of roof on the main house 126m square
- Area of roof on the grange 46.5m square
House Roof Only
So 126m2 x .76 = 95.76m3 of water
138m3 of water = 95,7000 litres of water for possible harvesting for the house alone.
Adding the Grange roof @ 46.5sqm
172.5m2 x .76 = 131.1m3 of water
131m3 of water = 131,000 litres of water for possible harvesting total.
These are crazy figures surely everyone should be doing this???
To give you some idea of how much water this is here is a comparison for you.
- Average pool = 20,000 litres of water. We could fill 6.55 pools (the pool we are looking at is 11,000 UK gallons or 41,639 litres so 3 times)
- Average shower use (8 minute shower) is 120 litres. We could have 1091 showers
Now of course I’m not expecting to be able to recover every last drop of rain but even if we were only to recover 50% it is still a vast amount of water.
Some very interesting figures
A 2006 UN Human Development study (available HERE) found that the “average” UK person uses 149 litres of water a day (France comes in at 287 litres per person per day!!! A French habit we WILL NOT be picking up) .
According to a BBC Water calculator my personal usage is around 111 litres (click here to get yours), these figures take into account all sorts of toilet flushing, showering, bathing & washing machine use. Using these figures i want to show how self sustainable we could be using this “grey” water and I’m going to reduce the rain capture by 50% to help bring the figures what i personally think to be a more “real world” scenario .
Rain water capture @ 50% = 65,500 litre
1 person @ 149 litres per day could last 439 days before buying in extra water
2 people @ 149 litres per day each could last 219 days (7 months) before buying in extra water.
Now if we use the BBC figures we get different picture
1 person @ 111 litres per day could last 590 days before buying in extra water
2 people @ 111 litres per day each could last 295 days (10 months) before buying in extra water
For those who aren’t sure of what I’m on about I’ll post more in the tech section shortly but here’s an info graphic for you to give you a quick idea.
PS : This is the reference pool we are using for our calculations since this is the one we are look at, it’s from SunSoka
You’ve got a great site here. I’ll be keeping up on it regularly. I did some research into rain harvesting on my site as well – though on a much smaller scale. It’s some pretty intriguing stuff. Your calculations are quite convincing, though I’d be interested in seeing the ROI for a rain harvesting system of this size. Best of luck with your green renovations. Look forward to reading more.
Hi Jason,
the good news on ROI is that with the Grand Designs show coming up shortly we’ll be meeting with a company that can supply the whole kit. Also on Tuesday morning we have a meeting with the local mayor (the Marie in France) to find out out cost per cubic meter of water, so i should be able to provide you some rough ROI figures shortly.
Many thanks for participating in the blog.
Lee
PS: Don’t forget to click the Google ads that way i can actually afford to buy the rain water system;)
So are you planning to use a pool to collect the water rather than a water storage tank like what you can find at plastic-mart.com? They have a 10,000 gallon tank for $4400. I realize you’re not in the U.S. so those figures are just for comparison.
As for ROI, if you’re looking at money saved by installing a rainwater system that provides non-potable water for your entire home, it would take years to get your money back in savings. A $4,000 investment would take roughly 15 years for me to break even. Not sure how things would add up in your neck of the woods, but money saved isn’t a determining factor where I’m at. Unfortunately, water is so cheap around here it’s taken for granted.
I think you do rain harvesting either out of necessity – you live in a region that receives very little rainfall per year or simply because you care a great deal about the environment. Either way, I think you’re doing a good thing. Cheers!
No no no the pool is going to be a pool. The reference to the pool was a bit of justification in our heads that we aren’t just using 11000 litres of water and that we can justify filling the pool as we have captured more than we have used.
Looking at a storage systems from someone like http://www.rainwaterharvesting.co.uk/ they have an excellent excel spreadsheet to give you some calculations and the 3750 litre tank they recommend is around £2,500 so $4,000 but this is for a full kit and not just the tank.
I’ll go through the ROI in a follow up post shortly but at about €1 per m3 of water at present in France this isn’t going to pay for itself any time soon. This is more about being responsible users of the resource.