A builder,Phillip Stavropoulos, is turning his pool into a rain tank and emailed me this question:
“Hi,
I’m a builder. When I studied I was brought to your house in 1997 for a field trip. You inspired me deeply! Thank you.
I’m now building my own home over a pool which is being used as a 45,000 litre rainwater tank. In my research for filters, I can’t find a filtration system that doesn’t need replaceable cartridges. Is there a direction you could steer me in? I remember you used a straw bail filtration system for the drinking and sewerage water. This doesn’t seem to be commercially accessible. What should I do?
The rainwater will be used for everything. Drinking, washing, flushing and the garden. I would like to do the sewerage composting and use the water from that for the garden but until that happens I’m not sure if I’ll have enough water to live off. Capacity is about 45,000 litres. Roof area is roughly 180m2.
I can’t wait to hear from you. Even wondering if I could come and see you again to check out the developments at your place.
Thanks heaps,
Philip Stavropoulos
BLUE ROCK CONSTRUCTION PTY LTD”
My answer:
For sure. This is a walk in the park, or, perhaps more accurately, a dip in the pool.
If you have one square metre of roof and, say, 1000 mm of rain a year your one metre of roof will harvest over 1000 litres a year. Applying this figure to a 180 m2 roof with that rainfall then your pool can harvest over 180,000 litres of water each year.
After you’ve built the concrete roof over the old pool, just give your pool a couple of doses of chlorine liquid (follow the instructions on the container) over a two week period.
Take a sample of water each week and put it directly into a 500 ml sterile container (eg an emptied bottle of newly-purchased mineral water).
Within a few hours of taking the sample, leave it at a medical centre or other place such as a chemist where there there are regular sample pick ups during the day.
Put a sheet of paper with the sample with: your name, address, phone number, email, and date of the sample and ask for a lab test for each of: faecal coliforms, total suspended solids, lead, mercury, zinc, cadmium.
Within a week you’ll get lab tests and if you get any asterisked results add some more chlorine to kill any bacteria.
If you get heavy metals results you’ll need to coat the pool with a coat of protective paint. Choose the paint by checking out the Materials Safety Data Sheet (MSDS sheet) but beware; this information is mostly limited to protecting workers using the product and often does not extend to listing all the ingredients of the paint. Don’t trust assertions by the manufacturer that the paint is safe for drinking – test it first.
Before you apply the paint coat a bucket with it and when dry put rainwater or mains water into it and test that water – you want to double check the paint is safe before painting the pool.
If you put a first flush device in before the rainwater enters the new rain tank, and the lab tests show the water is clean, you won't need a filter; see the data in my book, Sustainable House.
May the raindrops be with you, go you good thing,
Michael
PS: you're very welcome to join any of the tours of my house; latest item is R2Me, my new off-grid battery cabinet that's sitting on my balcony. Last night it charged my laptop I'm typing on now; sweet to use that energy that leaves the sun and arrives here on Earth 7 minutes later, every second.