Save thousands of liters of water per year by installing a rainwater harvesting system. A real-life guide to retrofitting your home.

The whole of Europe is in a frenzy about how to save energy. Gas, electricity, and diesel are in short supply due to the war in Ukraine. All fine, but what about saving water?

Saving water for the sake of it is certainly worthwhile, especially when seeing the pace at which glaciers melt. Glaciers are water reservoirs, feeding rivers and irrigating whole continents in the end.

If you want to save water and increase your resilience at home at the same time, I suggest using a rainwater tank to feed your toilets. It is fed with rainfall from your roof, and available sizes vary from 1,000 liters to 15,000 liters.

Where to Start?

Where is most of the freshwater used in a typical household? Showers and toilets. Putting showers on the rainwater supply is not that easy, because shower water should be hot. You would need two separate boilers for that.

However, putting toilets on rainwater supply is easy, and makes sense. Look at a rough calculation from our family of five:

• During working days (Mon-Fri), every person flushes the toilet 4 times
• During weekend days (Sat-Sun), every person flushes the toilet 7 times
• Flushing the toilet once requires 7 liters of freshwater

This means we are using 52 x 5 x (5 x 4 + 2 x 7) x 7 = 61,880 liters of fresh water to flush the toilet. Crazy, even when the price of water is (still) negligible.

I suggest you start eliminating this huge waste of freshwater before looking any further. And if you bother with installing a rainwater system for your toilets, you can also connect your outdoor taps to irrigate your garden.

The Installation Process

I recently installed a rainwater system for my toilets and outdoor taps. I used a Graf water tank as outlined in the picture below:

What a beautiful installation in that picture taken from a catalog.

It looks slightly messier when you install this thing in real life, especially when you retrofit it to an existing house (which is what I did).

Here are some real-life experiences to consider.

The Dig

Digging a hole for a rainwater tank sounds straightforward. Consider that for a tank of 7’500 liters, excavation of around 15 m3 is required — this is because the tank needs to be buried around 1m below the surface to allow for the minimum slope between the downpipes and the tank.

Here is how it began:

When the dig was complete, this was what it looked like:

Higher up than we thought, groundwater broke in. So we had to pump the water away before we could place the rainwater tank.

The rainwater tank was filled around halfway before covering it. Then the builders took their weekend break and promised to return the week after to finish up the installation.

Over that April weekend, there were heavy rainfalls. And because of the clay at the bottom of the hole, the entire rainwater tank started to float, even when filled halfway.

Fortunately, the pipes were not connected yet, so there was no damage done.

Unfortunately, the manufacturer of the rainwater tank suggests weighing down the tank with 8 tons of concrete.

So that’s what we did, and here is what the tank looked like after weighing it down and installing the pipes:

The rest is history. The rainwater tank is now safely buried underneath the grass.

The Plumbing

A rainwater tank needs a pump to get the rainwater into your house’s plumbing. Furthermore, you need to separate freshwater plumbing and rainwater plumbing. And last but not least, you need a cross-feed between rainwater and freshwater plumbing, to assure your toilet still flushes in dry periods when the rainwater tank is empty.

Easy, right? Graf delivers the rainwater tank together with a compact pump and cross-feed tank, dimensions are around 70x30x20 cm.

Super easy when you plan the installation in a new house, and the technical facilities room is large enough.

Super easy when you plan the installation in an existing house, and you replace the oil heating including the tank at the same time — you will have ample space when removing the oil tank in your technical facilities room.

Not so easy when you retrofit the installation in an existing house with a heat pump, as the technical facilities room in such a house is usually small and cramped.

Here is what it looks like in a cramped technical facilities room:

The Costs

The cost for material and installation was around 25k CHF. At today’s freshwater prices, there is no business case for installing a rainwater tank.

However, think 10 years ahead.

Before 24th February 2022, I was often laughed at for my “power anxiety”, my “Armageddon fantasies” and my apparent stupidity to spend extra money on building technology since the 2010s.

Looking at things from today’s perspective, I feel satisfaction to have taken the right decisions back then. Since 24th February 2022, I regularly get calls from friends and acquaintances, asking for my advice on photovoltaics, energy storage, etc. Even from those people who laughed at me some time ago.

Think about what climate change will do to water supplies, also in moderate climates.

I’d rather be able to flush my toilets on rainwater in the 2030s and invest some 25k CHF for it now, rather than paying the 2030s freshwater prices for a complete waste of freshwater.