Grey Water Harvesting

-Ar. Alex Shrestha

In most areas of Kathmandu where available water supplies are limited, there is a strong need to adopt alternative water technologies. Grey water harvesting is one such technology.

The average person uses a whopping 150 litres of water per day and one way of cutting down is by using grey water. Grey water recycling represents a major contribution to ecological sustainability and also does not diminish our quality of life but can provide numerous benefits. It’s estimated that just over half of a household’s total waste water could be recycled as grey water, saving potentially hundreds of litres per day.

In most areas of Kathmandu where available water supplies are limited, especially in view of rapidly growing urbanisation, there is a strong need to adopt alternative water technologies. The apparent madness of using fresh drinking water to flush the toilet or to water the garden somewhat explains the appeal of reusing bath and shower water or rain from the roof for these purposes.

What is grey water?

Wastewater from all sources in a property other than toilets is known as grey water. Grey water gets its name from its cloudy appearance mainly from soap and from its status as being between fresh, potable, and sewage water. Grey water differs from water from the toilets, which is designated sewage or black water to indicate that it contains human waste. Water from dishwashers and kitchen sinks is often referred to as dark grey water, because it has a higher load of chemicals, fats, and other organic matter.
Uses and benefits of grey water

Installing a grey water treatment system will give you safer water to reuse for your laundry and flushing the toilet, and can also be immediately used to water your garden. The system will reduce the need for fresh water and can significantly reduce your water bills. Grey water recycling for irrigation also replenishes groundwater, helping the natural hydro-cycle to keep functioning and support plant growth in areas that might otherwise not have enough water. The nutrients in the grey water are broken down by bacteria in the soil and made available to plants, which help to maintain soil fertility.

Grey water systems
A typical grey water system collects waste water from baths, showers, and washbasins, excluding the more contaminated water from washing machines, kitchen sinks, and dishwashers. The water is then treated and pumped to a tank for storage until it is required. It?is, in effect, a small wastewater treatment plant and can be installed in new or existing buildings. In the simpler direct diversion systems, devices allow you to divert your grey water away from the sewer system to your garden, and you have control over when you want to divert your grey water to your garden or to the sewer. The more complex grey water treatment system collects, treats, and stores grey water to a high standard for reuse, and varies greatly in style and cost. These can include components such as wetlands, sand filters, soil filters, and aerated wastewater treatment systems.

Precautions to take
Most grey water is easier to treat and recycle than black water because of lower levels of contaminants. However, all grey water must be assumed to have some black water-type components, including pathogens of various sorts. Recycled grey water of this kind is never safe to drink, but a number of stages of filtration and microbial digestion can be used to make it visually cleaner and safer. Grey water should also be applied below the surface wherever possible, and not sprayed, as there is a danger of inhaling the water as an aerosol.

In any grey water system, it is essential to put nothing toxic down the drain: bleaches, bath salts, artificial dyes, chlorine-based cleansers, strong acids/alkali, solvents, and products containing boron, which is toxic to plants at high levels. Most cleaning agents contain sodium salts, which can cause excessive soil alkalinity, inhibit seed germination, and destroy the structure of soils by dispersing clay.

Environmental impact
A correctly installed, well-maintained grey water system could reduce water taken from a domestic supply system. Widespread adoption of such schemes could have a significant impact on the environment, reducing pressure on water resources and the quantity of sewerage requiring treatment. However, grey water systems also use energy which works against some of the environmental benefits of saving water. Cheaper, less energy-intensive ways of saving water include low-flush toilets, low-flow taps, and showers, and changing our behaviour, such as turning off the tap when brushing our teeth.

Freshly generated grey water is not as nasty as black water, but if it’s not handled properly, it can soon become so. Grey water decomposes at a much faster rate than black water and if stored for as little as 24 hours, the bacteria in it use up all the oxygen, and the grey water becomes anaerobic and turns septic. After this point, it is more like black water—smelly, and a health hazard.
Is grey water suitable for you?

Installing a grey water system may not be suitable if you do not have enough land area, since you need space to build tanks and have enough soil to process the grey water, and enough plants to filter and use it. Also, if your soil is either too permeable or not permeable enough, you may not be able to recycle your grey water, or you may need a system with some modifications.

You should also consider the cost/benefit ratio and calculate if the grey water system you are considering is more expensive and requires more maintenance than a properly functioning septic or sewer system. To recycle grey water safely, you must also understand the nature of the grey water itself as well as the natural cycles and processes involved in its purification. Each set of circumstance requires its own unique recycling system for optimum results. For most residential purposes, low-tech, homemade grey water systems tend to outperform and outlast expensive pre-made systems.

Challenges for grey water systems
The use of grey water technologies in Nepal is still in its infancy, but some companies based in Kathmandu, such as Smart Paani, are pioneering in this field. The cost of fitting such systems is relatively high, especially if you want to put the system in an existing property. Before implementing any grey water system, be sure you have taken all possible measures to conserve water, for example: low-flow shower heads, low-flush/composting toilets, aerators on faucets, efficient front loading washing machine, natural landscaping, and rainwater harvesting. Choose the simplest design that meets your needs and build it as well as you can.

Ar. Alex Shrestha is a professional architect and urban planner who has over 15 years experience working in this industry in Nepal, South East Asia, and Europe. He can be reached at or

Our Sister Sites

Copyright © 2012 ECS Media Pvt Ltd. All rights reserved.
The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of ECS Media Pvt Ltd.