Eco-friendly flushing

Whilst the wet summer of 2008 hasn't brought water conservation to the forefront of many people's minds, we should still be looking carefully at our water usage as part of a holistic approach to minimising our resource usage. Even when the reservoirs are full to the brim and sunny days are but a distant memory, water usage comes with an environmental cost. Mains tap water is a heavily processed commodity: fresh water has to be purified and pumped around a massive distribution network, and the waste water after we have finished with it also needs collecting, transporting and cleaning. All of this requires energy use - every drip from your kitchen tap has a carbon cost!

Starting at the easy, no-plumbing-involved end of the spectrum, one obvious saving is to not flush unnecessarily. Urine is fairly sterile for most people and so it's relatively safe to not flush a household toilet if it has only been widdled in and is likely to be used again very soon. With a family of four doing this in the 'morning rush' you immediately save three or four cisterns' worth of water for no cost! Of course you can flush when guests are coming round or if you know that it will be some time before the loo is next used. Many people are already using this technique and it has picked up a little mnemonic rhyme: ‘If it's yellow, let it mellow. If it's brown, flush it down!'

Another simple trick for reducing the water used by flushing is to reduce the volume of water in the cistern. Modern slimline cisterns are already quite frugal with water but many of us have cisterns that are older and rather larger than they really need to be. Many water companies distribute ‘Hippos' which are plastic bags that you drop in your toilet tank and then quickly swell up to leave less room for flushable water. You can achieve similar results by filling a half-litre plastic drinks bottle with water, screwing the lid on and then putting that in the cistern. In fact that gives you the advantage of being able to tailor the amount of water displaced to your needs by altering how full it is. Don't put bricks or stones in the cistern though: these can degrade over time and clog up the flushing mechanism.

OK, those two tricks reduce the amount of tap water being flushed down the loo, but why are we still pouring drinking water down the pan when we've got other resources going to waste? These other resources are the various forms of ‘grey water' we have around the home. Grey water is non-drinking water that isn't actually foul sewage - things like used bath water, washing up bowl contents, shower pan waste water or even collected rain water. Grey water often just goes down into the sewers without a second thought, but it's a handy resource we can often (re)use.

The simplest way of using grey water to flush the loo is simply to save some in a bowl or bucket and keep it near the toilet. If your toilet is right next to the bath as mine is, the bucket can be used to scoop water straight out of the bath and down the loo. If you use perfumed toiletries in the bath you'll also make the toilet bowl smell nice as an added bonus.

For folk who are a bit more adventurous, or who are embarking on an eco-friendly build or renovation, grey water can be utilised in a more user friendly way by automatically collecting it and then using it to fill the toilet cisterns with, rather than mains water. The basic idea is to redirect grey water sources such as bath, shower or basin waste pipes into a large holding tank and then use this to feed the toilet cistern(s) using pipework unconnected to the mains.

If the toilets are at a lower level than the grey water sources and the storage tank then this system can simply use gravity to feed the grey water into the tank and from there to the toilet(s). The grey water storage tank will need an overflow to account for the rare occasions when a lot of grey water is generated with little toilet flushing to use it up. See figure 1 for the basic system layout.

Figure 1: Gravity-fed grey water supply

Unfortunately life is rarely convenient enough to arrange a home plumbing layout so that gravity alone can be used to allow grey water reuse. Some form of pumping is often required to either take the grey water up from its source to a holding tank in a roof space, or pump the water from a low-level store up to the toilet cisterns. This pump can be powered by solar, wind or mains supplied electricity or even done manually, although the latter loses some of the 21st century convenience the plumbing is supposed to provide. A system diagram involving a pump is shown in figure 2.

Figure 2: Pumped grey water supply

In an ideal world we would use roughly the same amount of grey water for flushing as we collect from our various sources. Again life can conspire to ruin that ideal: sometimes more flushing may be done than grey water collection. For example our family of four might all have come down with a tummy bug which has seen the loo in near constant use but little washing up or bathing done. Alternatively if the main grey water source is actually rainwater harvesting from the roof, a prolonged dry spell can deplete the storage tanks.

To contend with this situation, provision is needed to top up the storage tank with mains water. The cheap and cheerful way is to pour tap water into the tank using a bucket, hose or specifically provided mains water supply line. This gets done after someone suddenly notices that there's no water coming into the toilet bowl!

For folk who want to plan ahead there is the option of providing an automatic system. The basic idea is to have two float valves controlling water feeds into the upstairs header tank. The first float valve is arranged high up and controls the flow of grey water into the tank as normal. The second valve is much lower down and controls a mains water feed. The second float is set so that the header tank always has a minimum amount of water in it - usually just over the volume of a toilet cistern. When grey water is plentiful, the second float valve is always shut and all the water in the tank is supplied from the grey water source controlled by the first high-water-level float valve. In times of grey water shortage, the tank level falls until it activates the second low-water-level float valve, which opens and lets the mains water in. As soon as the mains water has filled the tank up to the minimum water level again, the second float valve cuts off the supply. This continues until the grey water supply is restored and the tank fills back up to the high water level with grey water. Figure 3 shows the layout for this sort of system.

Figure 3: Mains backed up grey water supply

There is one stage beyond using grey water to flush your lavatories: don't use any water at all. The use of water to wash away human waste into sewers or cesspits is not the only option. Composting toilets for example split the waste stream into solids and liquids. A high carbon content cover material such as sawdust is dropped over the faeces in layers and turns solid human manure (or ‘humanure' as some call it) into an earth-like material that can be used on flower beds and around fruit trees instead of animal manure or normal compost. The urine on the other hand can be used on a normal compost heap as an accelerator and diluted as a high nitrogen fertiliser. Composting loos come in all shapes and sizes, from home made ‘bucket and chuck it' lash ups to swish commerical systems for off grid homes. But that's a story for another time...

By Jim'll

photo (c) The Woolly Shepherd 2008