Living Green # 9 Feeding GREEN with GREY


We all know that 75% of the Earth’s surface is water. But do you know that of the 500,000 cubic kilometres of rainfall each year, only 20% falls over the land, and only 1% of that is stored and useable for us?

If you think water is renewable – yes, you are right. But do not think there is a never-ending supply.

Have you ever thought about how much water goes down the drain each time we take a shower, do the laundry, water our garden, wash our car, or just wash our hands quickly? In a typical household, more than 1,000 litres of water is used in a day, and at least 60% of that can be reclaimed for use elsewhere because it is not totally “black”, it is just “grey”. Let me explain.

In construction terminology, the water coming into our houses is thought of in terms of clean, “white” water, and that going out from our toilet is known as “black” water. So, “grey” water is somewhere between. By most definitions, grey water is tap water soiled by our use in washing machines, bath tubs, showers and hand-basins. It’s not sanitary, but it’s also not toxic and generally disease free. Black water is produced after flushing the toilet as it contains harmful bacteria and disease-causing pathogens which cannot be used for domestic irrigation like on a lawn.

What separates white from grey are the types and concentration of additives caused by our washing, bathing, cooking and cleaning. Therefore, grey water may contain soap particles, fat and oil from cooking, hair and even flakes of human skin.

A typical house has at least three water systems. The first one is used for white water distribution, the second for grey, the last for black. Only the first comes into the house and the last two go to the sewer, but with special treatments.

Grey water normally passes filtration for solid waste then goes directly to the city sewer. Black water must go through the water treatment system (i.e. septic tank) to ensure it contains no pathogens before being discharged to the city sewer.

However, if the grey water coming from your kitchen contains too much fat and oil, it is advisable to send it to the oil filtration and treatment system also.

From all this, you can find an opportunity to reduce the consumption of white water, not only to reduce your water bill, but also to help conserve the environment. One good example is to not use drinking water to flush toilets or on your garden. Grey water can be used to feed plants in your garden as it contains nutrients such as nitrogen, phosphorous, and potassium, the basic ingredients in fertiliser, which can sustain plant life and recharge topsoil. However, some substances found in soap such as sodium, chlorine and boron will cause problems for plant life. If you are going to use grey water for your garden, start looking at what you send down the drain.

You can keep chemical contamination to a minimum by using environmentally friendly, biodegradable soaps and detergents whenever possible.

Also, for soap, using high-phosphate brands will harm your plants as they can only live within a limited level of phosphorous concentration.

Some may wonder how to collect grey water? And how can you feed your plants? First of all, your house must have separated waste water pipes; one for grey and the other for black. The grey water pipe is commonly known as the waste pipe, whereas the black water pipe is called the soil pipe. Second, if grey water is in good condition for plants, you need a water tank equipped with a filter. Try to estimate the quantity of grey water use daily so that you do not have it left in the tank.

If you have more grey water than you need for your garden, simply let it go to the city sewer. To prevent contamination, do not store it for too long as it will turn into black water within 24 hours after bacteria in the tank start to grow.

You might face some problems with clogging in the filters and pumps due to bits of hair, skin and food. Remember that chemical clog removers are harsh and can seriously harm not only your plants but also anybody in contact with the garden such as pets and kids. Natural solutions, such as boiling water or vinegar and baking soda treatments might be less damaging to plant life.

Lastly, what kind of plants can be fed with grey water?

Fruit and vegetable plots are best irrigated with white water or rain water, so it is recommended that you prevent grey water from coming into contact with such plants due to the contamination risk, especially if the produce will be consumed raw. Consider using grey water for plants with some distance between them and the food.

All these “Feeding Green with Grey” suggestions might be difficult at first. But if we realise that we have the potential to reduce a large amount of our household water consumption by recycling all this grey water, it is quite a sound measure to reduce the impact on the environment. There are still plenty of ways to live “green” without having to grow more trees.

ผู้ช่วยศาสตราจารย์ ดร.อรรจน์ เศรษฐบุตร

Asst. Professor Dr. Atch Sreshthaputra

Faculty of Architecture, Chulalongkorn University

Chairman of Green Building Program. The Association of Siamese Architects.


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