‘Grey water’ is the water that comes from showers and taps in a house. This is different to ‘black water’ which comes from toilets.
Grey water isn’t 100% clean. It will contain detergents and particles of food, soil and other things and left for 24 hours, or less in a hot climate, it will go foul. However, it isn’t particularly dangerous if you use it straight away on your garden, so long as it is not used on vegetables. Grass and trees are very unlikely to be harmed by grey water irrigation. You can take precautions as well, such as disconnecting from the grey water system when bleach or other strong chemicals are used, and filtering the water before it goes on the garden.
A typical 3 bedroom house, fully occupied, will produce in excess of 500 litres of grey water a day. In most houses all this precious water is simply washed down the drain. This is a waste as grey water can be used for a variety of purposes, reducing overall household consumption. With the changes caused by climate change and the resulting water shortages it is crucial that all properties make the best use of grey water where ever possible. There has been a particularly bad drought in Thailand this year which has made it difficult to water the garden at Sunrise Villa. Grey water is our environmentally (and really cheap) way of solving the problem.
Step 1 – Plan Your System
Before you start to build make sure you plan all the parts. You will need to know:
1. Which downpipes are carrying the grey water
2. Where to place the surge tank, and how big it needs to be
3. Whether a pump is required
4. How to filter the water without ever having the risk of the filter being blocked up and the water backing up in your pipes
5. What connections you need to and from the tank and pump, and what connections you need to join the pipework
6. Where to place the pipework and whether to irrigate with sprinklers or perforated pipes
7. What volume of grey water will the system produce at any one time
Step 2 – Set up Your Surge Tank
A surge tank is where the water first goes when it leaves the down pipe. At the bottom of the tank there will be one or more outlets to the pipework of the irrigation system.
The reason you need a surge tank is that the water may enter the system faster than it can run away. This can happen for instance if a bath is drained. The size of the surge tank can be estimated by considering the likely maximum amount of water to flow through the downpipe at any one time in comparison to the rate at which it flows away.
In the picture we used a 40 litre plastic bin (300 Thai Baht or 10 USD), with two 1 inch outlets right at the bottom. With 2 showers running and the kitchen tap the surge tank never gets more than 10% full. If we had a bath then the surge tank would need to be around 100 litres.
Step 3 – Installing the Surge Tank
We have sited our surge tank under one of Sunrise Villa’s two grey water downpipes. The surge tank was placed in a pre-dug hole and we partially buried it after fitting the connections to the pipes of the irrigation system. The existing downpipe was then cut and the end going into the cistern tank capped. On top of the surge tank we created a filter by cutting a large hole in the existing bin lid and using some netting which we attached using wire fed through holes drilled into the edge on the plastic. Note that the downpipe is venting about 30 cm above the filter to avoid the grey water ever backing up into the house if the filer get clogged.
Step 4 – Design your Irrigation Method
Because we are trying to irrigate the grass, which has short roots we opted for a sprinkler system with sprinklers rising above the surface of the lawn from buried pipework. The pipework doesn’t have to be terribly good quality, small leaks don’t matter, although the connections have to be good. For our system we recycled discarded 1 inch plastic pipework (free) but used brand new connections and sprinklers to avoid excessive leakage (cost of 2,500 THB or 83 USD).
What you will need to do to install some pipework and sprinklers is to work out how many sprinklers you need to cover the whole garden. Our system is gravity feed, no pump, and areas each sprinkler will irrigate is about a 1 metre radius. This means to get full coverage we need to position sprinklers with valves about 2 to 3 metres apart.
Step 5 – Managing the System
The pressure in our grey water system is sufficient to run two sprinklers at any one time. This means the valves on the sprinklers need to be opened and closed in rotation every day. You can judge from the colour of the grass which parts of the garden are most in need of water at any time.
Step 6 – Being Green means Going Grey
Every time you run a tap or shower your garden benefits directly, and more water is left in the world for its inhabitants to use for drinking, washing, transport, irrigation etc. Larger areas on the map every year are becoming classified as ‘water stressed’ or even in drought. The pressures of burdening population, pollution and climate change means that we can no longer view water as an ‘unlimited’ and ‘free’ natural resource. It needs to be husbanded to guarantee the health of future generations. We are making our small contribution by recycling our grey water.