Rainwater harvesting

Here in Northern Ireland we are fortunate to have a water supply that is generally reliable and clean. Our own water isn’t fluoridated, but it has been chlorinated to some extent, to kill bacteria, and it is reliant on the national grid to keep pumps (and thus the supply) running. Plus there are plenty of instances where human error has seen a variety of chemicals added to supplies with disasterous consequences.

This is why we installed our own rainwater harvesting system, which collects water from the cottage roofs, and stores it in 5000 litres worth of IBC tanks that form the back wall of our strawbale barn. It is a relatively cheap system, and easily put together. The important thing to remember when storing water is that the tanks need to be opaque, to slow the growth of algae. It seems that most IBC tanks that are classed as having contained food grade materials are white, so we took ours out of their metal cages, wrapped them in black plastic, and put them back again. You could also paint them black.

Water coming from roofs is going to contain whatever has washed off those roofs, which might include leaves, bird shit, paint, and material from the roof itself, depending on what it is, so filtration is a must before use. Even if you’re just going to water the garden with it, you don’t want your tank filling up with sludge.

Before: A scrubby patch in front of the rainwater tanks

The first and simplest thing to do is ensure that your gutters are kept clean. Easier done in single storey homes, and if you can’t easily reach them, consider using a downpipe filter to take out the biggest bits.

Our rainwater tanks

You could also use a first flush diverter, that basically dumps the first water to come through it, which in theory will contain the worst of the dirt. There are a variety of designs that you can buy or make, and they’ve been covered extensively elsewhere so I won’t go into the detail here.

What we do is use a coarse ‘sock filter’ – a fabric mesh, over the end of the downpipe, before a bottle trap, which allows debris to fall into a sump rather than flow directly into the tank. This has to be cleaned regularly to prevent it clogging but seems to work well.

The water then flows into the tanks, which are simply connected together at their outlets so that they fill as one tank. A pump then delivers the water into our irrigation system, and house if we need it. We use a couple of gate valves to switch between the public water supply and our rainwater. The pump has another filter on its inlet to reduce wear.

Filter housing – we’ve used 2 of these with different filters to take out smaller partices.

To date we’ve just been using it for irrigation, but now we’ve added a 2 stage fiter, using carbon filters on the outlet side of the pump, and we’ll also be adding a ceramic filter to remove bacteria. We chose common ten inch filter housings, so that it is easy and cheap to get the filter cartridges, although the ones we have are (in theory at least) cleanable and reusable. Be careful because there are a variety of filter systems that rely on you replacing filter cartridges fairly often which is costly and wasteful.

Our system means that we can supply both cottages if necessary,  independent of the mains supply, assuming we have enough in the tanks. Another step towards resilience and independence.


Posted on

January 8, 2019

1 Comment

  1. Hi guys

    Thanks for the info. I’m in the process of trying to put together a off grid market garden that sells to the public so I’m looking closely at all these things because I know it will be scrutinized with me selling to the public. For instance, I didn’t know that you can’t use harvested rainwater for watering vegetables. I have just emailed WRAS because I think my only option will be to install a UV unit that has alarms etc. My biggest concern is that I intended to do the same set up as you with the IBC’s because the alternative volume (I was going with 10,000 litres) WRAS approved storage is outrageous. Below is an extract which I found useful. Finally when/if I get the system into place I will have to have it tested which is my next project, to find out how much and how often.
    Anyway, just thought I’d share, good luck with everything


    Ps I’m still off grid living in my converted 7.5 ton Tuffnells delivery van 5 years on 😀

    Bacteria – the most common pollutant of rainwater off the roof of a building is bird droppings and the bacteria that these carry. Ultra-violet filtration kills bacteria, but remember that the filter must be turned on 24/7 and there must be a way of warning the users that a bulb is failing or has failed. Double bulb UV filters are available, as are ones with suitable alarms. A cartridge filter prior to the UV filter ensures that bacteria do not get through the UV filter hiding behind a particle; that’s what authorities might be concerned about. If the rainwater has been filtered and stored in an underground tank with rodent protection, calming device at the input and floating intake to the pump, then a second filter in line with the UV filter is not required because the water will be particle free (as with the Carat systems).

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