Make a Rainwater Toilet

Here in the South East of England, our water companies seem to run out of water pretty much annually. This year is no exception. Despite the wettest April for a hundred years, we're still in drought and still have usage restrictions. Whatever the future holds, it's very unlikely water bills are going to get any cheaper. So after all this, there's a way many of us could save up to 24% of our water bills - fill your toillets with rainwater.

This whole thing isn't as hard as it seems, although because it involves a bit of plumbing it does make it difficult to retro fit this into your beautifully finished bathroom. We managed to get the necessary plumbing in when we refitted our bathroom a couple of years ago. If that's not an option for you, there may still be some ways to do this, although perhaps not quite as I did here. For any plumbing questions, I recommend having a look around on Ultimate Handman's forums.

We learned a lot about rainwater harvesting and use for toilets specifically from an article at REUK (and from Neil at REUK who's been very helpful), but the general jist is shown here (click the picture to enlarge):

Description

The water butt collects water that flows down the pipe from the roofing gutters (using a rainwater diverter). Inside that water butt is a pump which pushes water up to the tank in the loft. The toilets are plumbed into the water from the tank using standard plumbing pipes.

When you flush the toilet, the cistern empties and is refilled with water from the loft tank. When the loft tank level drops below full, a sensor in the tank causes the switching circuit to trigger the pump in the water butt to run, which pumps water into the loft tank (assuming there's water in the butt).When the loft tank is full again, the pump stops. Likewise, if the water in the butt empties completely, then this also stops the pump so that it doesn't run endlessly when there's no water.

The loft tank has an optional mains water backup, which fills the tank to a couple of inches (~5cm) deep if there's no rainwater, which means you'll never run out of water for the toilet. This backup is built from a modified version of the original ballcock valve. Essentially, the valve arm is lengthened so that the float is only a couple of inches from the bottom of the tank. Since standard floats are very boyant, we've changed it for a smaller float so that it doesn't break the arm of the valve when the rainwater fills the tank to the top.

The pump and switching circuit in our installation is powered by the mains (via a 12V power supply). For extra "green points", it's possible to run this whole setup from a solar panel. That means that it'll only fill the loft tank during the day though, but since the loft tank is pretty big, there should always be plenty of water for a night's worth of toilet visits.

A note about regulations: Regulations almost certainly exist for any projects like this wherever you happen to live. Check to see what your specific regulations are. For us in the UK, we have three specific rules. The first is that the mains water backup inlet to the tank has to be 15cm (150mm) above the overflow level of the tank. This is called an air gap, and is mandated so that it's impossible for rainwater (also known as grey water) to enter the mains water system.

Secondly, all loft water tanks need to have a fully covering lid that doesn't create an airtight seal. If your tank has been seen by a plumber sometime in the last 20 years then there's a good chance you've already got a lid. If not, then you should buy one. While you're at it, consider putting some insulation around your loft tank, and insulate the mains pipes too ;-)

The third regulation is that all non-potable water (ie. anything that isn't safe for drinking - which is pretty much anything that doesn't come from your water company) has to be marked as "non potable". Some simple stickers or labels are sufficient (possibly just writing on the side of the tank with a suitable pen might be enough). Apparently DIYers need to make sure that thirsty people don't climb up to the loft or drink out of the toilet(!).

Construction

Constructing this consists of the following parts:

• Water butt (we used this 136 litre one)
• Rainwater diverter (we bought one from B&Q, along with some drainpipe joints so we could fiddle with the downpipe a bit)
• Pump (we bought a Whale GP1002)
• Hosepipe (I recommend a reinforced hosepipe so that it's not likely to split if it freezes in winter)
• Two float switches (we tried liquid sensors, but they caused a few problems)
• A relay with a 12V coil (and contact ratings in the orders of 2-3 amps)
• 2 x general purpose NPN transistors (we used 2n3704s, but BC108s or 2222As or any number of others would be fine too)
• A handful of resistors and capacitors
• A plastic box, a few bits of cable and some other assorted bits and bobs
• Optional: A 3-4A 12V solar panel (for maximum green credentials) - we're leaving this one for later as they're quite expensive

(We don't really stock any of these parts, but we can get them if needed - contact us for details)

I won't go into too much detail about how to get all this fitted to your house. However, suffice to say it'll involve some clambering about in the loft, some very awkward hole drilling and some funky cable and pipe routing. Ultimately, you'll need to get power to the pump, and a signal from the loft tank down to the pump. You'll also need to get water from the pump into the loft, which will mean you'll need that hosepipe to go between them too.

The diameter of the hosepipe, the height you need to push the water up and how quickly you want to move the water essentially give you the specifications of the pump you need. The Whale pump that we used will lift water by about 5 metres, and will move 10 litres per minute. The actual lift we need is about three metres, and honestly, even one litre per minute would probably be enough. To work out the specifications you need, try an online pump calculator (in which the term "head" is the height you need to lift the water). This should give you a pressure rating (usually measured in Bar - our Whale pump is 0.6Bar).

So assuming you've managed to get the pump-to-tank linkage in place (which took me about a day to do, by the way), then we need to get it all connected up. We need to put a float switch in with the pump (which we'll use to stop the pump if the rainwater tank empties), and we'll put a float switch in the loft tank (to turn the pump on if the tank isn't full yet). Also, the pump needs a relay to turn it on and off, so we'll need a circuit that switches the relay on if one liquid sensor detects water, and turns it off if the other doesn't.

Since we've got access to a 3D printer, we designed and made a stand to keep the pump upright and hold a float switch in the right place. This is optional though - we could have stuck the switch to the pump with a couple of cable ties and just dropped the pump into the tank. Either way, the pump and a sensor need to go in the bottom of the water butt.

Likewise, a switch needs to go into the loft tank. You won't want to drill holes in the tank to hold the switch in place, and neither did we! Instead, we used a bit of plastic "tile trim" which we found in the clearance section of the DIY shop. We actually went in there looking for a piece of plastic pipe - any length of plastic will do really, even maybe an old ruler or something. Just cable-tie the sensor onto the plastic stick and then clamp the stick to the top of the tank (above the water line!). Obviously, the switch needs to go just below the overflow level of the tank so that it stops the pump from filling the tank at that level. Lastly, connect the liquid sensor to the signal cable that goes back down to the water butt.

Fitting the hosepipe to the loft tank can be a bit of a challenge because it's not standard plumbing so no fittings you can buy really apply to it. We found some plumber's pipe clips, some bits of wood and some cable ties seemed to do the best job. Either way, make sure everything is really well fitted and not wobbly in any way - if the pipe works loose, then you could end up squirting water into the loft which will create a horrible mess (and in fact, it will empty the rainwater butt into your loft because there'll be nothing to turn the pump off!). One last thing - I'd advise you don't submerge the end of the hosepipe in the tank - many pumps don't have one-way valves in them, so when the pump switches off, all the water left in the hose will fall back down the hosepipe into the rainwater butt. If the pipe is submerged in the loft tank, it'll actually suck all the water out of the tank as well!

The mains water backup for the loft tank is optional, and specific to your tank and your plumbing system. If you're not sure what to do here, either just turn the water off to the tank entirely (so you have no backup), or get some advice from a plumber. Mains water is generally quite high pressure, so if you make a mistake with it you can make a mess really fast, and possibly cause damage to your house.

To sum up, so far we have:

• A float switch attached to the water pump, both submerged at the bottom of the rainwater butt outside
• A float switch at the right height in the loft water tank
• Optionally: a mains water backup that will fill the loft tank to a couple of inches deep
• A signal cable from the loft tank liquid sensor to the same area as the other liquid sensor and pump connections
• A hosepipe from the pump, securely fitted and going into the loft tank
• 12V power near the pump (either from the mains, or from a solar panel)

To finish this off, we need a simple switching circuit that can turn the pump on and off at the right times. We'll describe that below.

Once you've made the circuit, connecting it all up is pretty easy. Testing it out can either be a rather messy experience or else hard to test all possible scenarios. I recommend trying it out by dunking the pump in a bucket of water before getting it all connected up for real. Either way, once you've got it all connected up, switch it on and watch it filling the loft tank with water from the rainwater butt. Obviously make sure that the switching circuit stops the pump when the tank is full (before it starts overflowing), and maybe also try to test what happens if the rainwater butt empties while the loft tank is still filling.

The Switching Circuit

(You knew there was some real electronics in this project somewhere, didn't you!?)

The purpose of the circuit is to turn the pump on when the level of the water drops in the loft tank, and turn the pump off again when the loft tank is full. The circuit also needs to disable this behaviour whenever there's no water available to pump up to the loft.

The circuit we designed to do this is below (click to enlarge):

The float switches are arranged as "normally open", which means that they switch on when water goes above the switch. We use them to turn transistors on and off; one transistors turns on when it's switch is wet, and the other turns on when it's switch is dry. By putting the pump relay between the two, we achieve the right behaviour that we want. If either sensor goes out of it's normal state, then it's corresponding transistor turns off, and so does the relay, and so does the pump.

We added a capacitor to the tank switch to overcome a problem that happens when the rainwater butt empties. When this happens, the pump stops which causes all the water in the pipe to fall back down to the butt, which fills it up just enough to trigger the switch, so the pump starts again and then stops again, and so on. The capacitor means that the pump stays on for a monite or two after the switch tells the circuit to stop. This means that we pump just enough extra water to stop the pump switching on again when the water comes out of the pipe.

Enhancements

There are lots of ways to improve on this design, depending what you want to achieve. Here are some ideas:

• Use a 5V relay instead of the 12V one. If you do this, you'll need a resistor in series with it to drop the voltage down sufficiently. However, this arrangement means the relay switches on at a lower voltage, which means the supply can be lower - this is especially useful if you're using a solar panel.
• Monitoring! It's easy to monitor the two sensors, but adding a couple more can give you lots of extra information. I'm adding one half way down the rainwater butt, and another just above the level of the mains water backup. With this, an entirely separate project can work out how many litres of water we've used, when the waterbutt gets to half full, and if we ever use the mains water backup. That's the subject of a whole different article though ;-)
• Use a demand pump - this could be a way to make this arrangement work if you don't have a loft. The waterbutt and the toilet can be at the same level; whenever the toilet "demands" water (when it needs filling) the pump detects this, automatically turns on and pumps the water into the toilet. When the toilet is full, the demand stops, and so does the pump. This arrangement needs permanent power, but doesn't need a loft tank (or a loft!), and may simplify the plumbing required - depending on your circumstances.
• Water filters or purification.Any unfurified water has living things in it that grow into algae and smelly things. It's possible some careful filtering or exposure to ultraviolet light, or something else might be beneficial. Even just putting some household bleach into the water might be enough.

Conclusions

Saving a bit of water by harvesting rainwater is a pretty simple idea. Actuallly fitting it into your house is quite involved and definitely not for everyone. However, if you can put up with a bit of sweat and tears getting all of this actually fitted, the building of it really isn't that hard, and actually it's pretty cheap. We estimate we'll have saved the materials cost of this project in the first year (of course, getting some green credentials is another reward).

In terms of the day-to-day experience, so far the whole thing has been very successful. The toilet itself looks exactly the same as it used to - rainwater really is very clear and relatively clean, so there are no concerns with clenliness. The pump makes almost no noise at all, and once it's flowing, the water makes less noise than the mains used to. There is a bit of a gurgling sound when the pump starts and stops as the water and air move around in the pipe, although you can't really hear it once the lid is on the tank.