Practical considerations

Now I am very lucky as I already have a perfectly suitable water-butt on the downpipe at the front of my garage. To reduce my environmental impact as much as possible I'm going to use the one I've already got, but if I had to buy one specifically for this purpose I'd be looking for one that is:

• Large: for maximum rainwater storage;
• Tall: for maximum hydraulic head to minimise the effort that the pump has to put in to raise water to the loft tank;
• Plain: textured "wood effect" faces can make it harder to add bulkheads if you need to add an outlet, as the ridges can prevent them from sealing property;
• Attractive: or at least, not unattractive; and
• Cleanable: having a threaded outlet at the very bottom of the tank makes it much easier to clean out the butt-sludge (there go all my readers with Safe Search on!) that otherwise can build up and clog up your pump or discolour your water.

I quite like the water-butts that Graf / Garantia make - they tend to be high volume and reasonably practical. That being said they are intended for the European market, so the taps provided with them are much bigger than you see on the UK market, which makes it quite hard to find compatible hoses. The threads are 3/4" BSP female though, which suits us fine. Here are some examples of theirs:

From left to right the above water-butts are 400L, 1300L, and 500L. Which brings me onto...

Choosing the volume

Obviously the bigger the water-butt the less likely it is to run out, but at some point your water-butt will be so large that it'll be very unlikely to ever fill, at which point that extra volume is just an eyesore and not actually giving you any meaningful functionality. So how do we work out where this point is?

Empirical methods

BS 8515 (Rainwater harvesting systems: Code of practice) has an easy and boring method called the "intermediate approach" which says:

storage capacity should be [...] the lesser of 5% of the annual rainwater yield or 5% of the annual non-potable water demand

How much water runs off your roof?

The amount of water that ends up in your water butt when it rains is:

\[ \text{runoff volume} = (\text{catchment area} \times \text{rainfall depth}) \times (1 - \text{loss factor})\]

This is heavily simplified - for example rain doesn't fall down vertically, it is carried by the wind, so a pitched roof facing the prevailing wind will capture a larger volume of rainfall than one facing facing away from it, and downpipe diverters become less efficient at higher downpipe discharge rates - but for our purposes it'll do.

We'll assume that you have an impermeable roof, not a grass roof, and that your guttering isn't horribly leaky, so we'll say that \(\text{loss factor} = 5 \%\). The downpipe I'm working with is connected to about a \(12 \text{m}^{2}\) (plan) roof, so our equation becomes:

\[ \text{runoff volume} = (12 \text{m}^{2} \times \text{rainfall depth}) \times 0.95\]

The "intermediate approach" wanted the annual yield, which you can either look up using the maps in the above standard (which you really really musn't download from Anna's Archive rather than spend £322 on), or guesstimate using Met Office long term climate averages. For my location the annual rainfall depth is suspiciously close to 1000mm, giving us:

\[ \text{runoff volume} = (12 \text{m}^{2} \times 1000 \text{mm} \times 0.95 = 11400 \text{l}\]

How much water will I need?

The Energy Saving Trust estimates that flushing the toilet accounts for ~22% of an individuals water consumption, or a little under \(32 \text{l}\) per day, or \(11680 \text{l}\) annually.

This is an astonishingly similar figure to how much rain will run off our roof annually, but we'll behave ourselves and choose the smallest (the annual rainwater yield) and multiply it by \(5\%\) to get a suggested storage volume of \(570 \text{l}\).

The problem with this approach is that the suggested volume doesn't change when I remember that I'm married, and that my wife has a propensity to use the toilet too sometimes. I also don't like empirical methods because 🤮.

Quantitative methods

There's a nice open-access paper, "Optimal sizing of rainwater harvesting systems for domestic water usages: A systematic literature review, Semaan et al. - Resources, Conservation & Recycling: X - 2020", which gives loads of approaches for how to work out the perfect size for your water-butt. The most popular is a very simple simulation, introduced by Jenkins and Pearson in 1978, referred to as the mass-balance method.

The idea is that, at each time step, some rainfall enters the system and some water leaves the system (is used), and you can just step through this on a day by day basis and work out how often it runs dry. Run enough of these with different storage volumes and you'll get a good idea of how much storage you feasibly need.

As luck would have it the EA make it very easy to download daily rainfall records from their gauges using their Hydrology Data Explorer, which is what I did for my nearest gauge.

I won't share any more information than this, as my work probably owns the actual implementation of this method as they employ me to do this sort of thing. The end result, however, is that in my case there's very little meaningful difference between a 500L water-butt and a 1000L water-butt, because it just doesn't rain heavily enough often enough to make much of a difference over such a small catchment area.

What next?

So we've worked out what our system needs to do to be compliant, designed it, and decided we want a ~500L water-butt, but we still need to work out what we need to buy to build it, and how much that will cost. Tune in next time, and we'll do just that!