2009 February

As promised in my previous post about drinking water, here are my thoughts on our rainwater storage.  In our current home town of Brisbane, Australia, the dams that supply our drinking water recently bottomed out at around 14% capacity. The severe water restrictions that we’ve had for years have meant that even in our inner-city suburb about half the houses have rainwater tanks, as without them you are effectively not allowed to water your garden. These aren’t your British-style 200 litre water-butt either. Hardly any would be less than 3000 litres, most would be 5000 litres, and quite a few have 10,000 litres. We’ve currently got two 3000 litre tanks, plumbed into to an underground weeper hose that runs through all the garden beds, controlled by a programmable timer that runs the two watering circuits on alternate days. This has ensured that we have kept our lush tropical garden alive through a seven year drought.

What it doesn’t do is supply any water to the inside of the house. For a while we did use it for our washing machine (a separate tap that we connected the cold water hose to) but as this required a manual changeover it only happened rarely.

To peak-oil-proof our water supply I want our UK system to be fully plumbed in to the house and able to supply our basic, emergency  requirements, and ideally all our requirements. I’m going to try to do this relatively scientifically – based on the Tank Size Calculator from RainWater Harvesting. Manchester provides a very different scenario to our Brisbane rainfall – there’s 400mm less a year for a start, and it is more evenly spread thoughout the year, minimising the need for a huge tank to cope with peaks and troughs.

With our roof area (about 100m2) and Manchester’s annual rainfall (806mm) we can expect to collect around 64,500 litres/year (at 80% efficiency). That sounds like a huge amount, but when you consider that our current water consumption is around 250 litres a day, we currently use 91,250 litres/year! So to be self sufficient in water with our current roof size we’ll need to reduce our in-home water consumption by around 30% – that looks like the subject of another post!

So what size tank do we need to make sure that we collect as much water as we can, without wasting any? I concocted my own monthly water use chart, based on meeting a target of 170 litres/day (then found out that there’s a very good one in the RanWater Harvesting spreadsheet!).  From looking at this (below) I’ll have a shortfall of 3000 litres in the first year – so in this table I’ve pre-filled the tanks with 3000 litres to ensure that they don’t drop below zero. In reality this “top up” would happen in stages during the year depending on our actual daily consumption to give us the chance to be extra-economical as the levels drop. 

In this our water storage peaks at around 5,500 litres, so unless we manage to keep at or below our tight 170l/day target for several years there is limited benefit to installing a tank any larger than 6,000 litres. 

Tank Suppliers Rainwater Harvesting – 6500l Graf Carat Kit for £2461 Combined Harvesters – 6500l  Graf Carat Kit for £3410 Freewater – 6500L GET tank Sun, Wind and Rain – 6500l Freerain domestic system for £3100 Wirral Landscaping – Graf Carat suppliers Ecosure – 7000l tank for £1470 (no pump or fittings) Ecosure – 8400l full kit for £3160 Ecosure – 5600l kits for £2450 Resources RainWater Harvesting – Tank Size Calculator RainWater Harvesting – A great range of underground systems Combined Harvesters – 6500l underground system Freewater – Underground System Rainwater Economy Systems – FAQs about rainwater tanks Making rainwater drinkable Met Office – Annual Rainfall at Manchester Airport Freerain water tanks Installation of Graf Carat tanks British Eco – tank systems up to 5000 l Water-Tanks.net  - underground tanks up to 7000l