Rainwater harvesting

11 Jan 2013
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United Kingdom
Morning all,

Has anyone had a go at this? Specifically, is there an reason why I can't use one or 2 of those cheap 1000 litre liquid cubes (going on Ebay for about £40) instead of a fairly pricey purpose-built tank?

Plan would be to bury the thing(s) and use the top access hole for all purposes (water in from gutters, power and pipe from submersible pump out) except overflow- that would be via a compression fitting cut into the side of the thing with a pipe running up a bit (so the overflow doesn't start flowing till tank level is very near full-sort of a reverse U bend) then down to soakaway.

Output pipe then up to small (50 litre or so) tank in the loft with a float switch to control the pump. If I feel flash I'll run mains cold water up to the header tank as well (with a standard ball valve set to a lower level than the float switch and a valve somewhere easily accessible so that during the long hot summers we get up here the thing doesn't run dry)

Any hepful hints and tips?
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Generally yes.

But why not just use a plastic dustbin to start off?

Those tanks are expensive although useful.

Sometimes you can find an old plastic oil tank being sold cheaply too.

You can also fit a mains supplied ball valve low down to ensure the loft tank does not run dry. But you MUST have an in line double check valve in the supply to the valve if you do that!

Ta- re the bin idea, aye not a bad plan but it'll look a bit shabby sat outside & (250 litres in a bin?) it'd struggle to cope with 3 toilets and a washing machine. Besides which we're on wheelie bins here and I already have an alternate purpose for mine (if you cut them down to about 700 height they make a superb trolley for wheeling essentials to pit lane at the Isle of Man)

And yes good point re the double check valve- of course the mains ball valve will be submerged most of the time! Will I get into issues with a standard ball valve if it is submerged (to do with excessive force being applied by the ball to the valve) ?
You can't use a standard float operated valve with a DC valve on a rainwater harvesting set up. The water regulations require a type AA air gap as the only method to prevent contamination of the water main as rainwater is classed as a fluid category 5 risk. A type AA air gap is an air gap with unrestricted discharge above the spillover level of the storage cistern. This means that the outlet of the float operated valve supplying mains water should be higher than the edge of the cistern containing the rainwater.

Special cisterns are available that have a housing above the spillover level that has a special cranked arm float operated valve fitted.

Any pipes conveying the rainwater to and from the cistern should also be specially marked to prevent accidental connection. The water regs specify that the pipe should have black and green markings and have the words RECLAIMED WATER printed on the pipe in black text on a green background.

Google water regulations schedule 2: paragraphs 14 and 15.
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Yes, that is quite correct to fully comply with the Water Regs.

It might be possible to use a standard ball valve mounted above the lip of the tank but with the arm extended down to a low level.

Cheers both- especially about the pipe markings, hadn't even considered that! Special cisterns eh, lets see if Google is my friend, along with pipe marking tape (not sure green PVC tape and a sharpie will do the job! )
Black/green MDPE is now available for RW harvesting.

Cheers chap, yes I found some last night (Google was my friend), not a bad price either. No joy on the special cistern/header tank yet but no big hurry, need to get the roof finished before I start messing on with internals :D
I'm thinking a flat brass bar with a hole drilled either end. One end on the arm of the valve with a nut holding it in place, other end with a bolt through attaching the float to it. Solder or braize the top end to stop it pivoting and you've probably achieved it for about £20.
That sounds about right- the other thing I'd probably do is put some sort of hard stop above either the ball or the horizontal bit of the rod (so that when tank water level is at rainwater level- which will be at least 30 litres above ball valve opening level- the ball buoyancy isn't putting excessive pressure on the ball valve).
Do you think the ball pushes upwards harder when its 500 mm down compared with just 100 mm down?

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