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Growing more food with less water - Improving water usage in agriculture



Expectations for the population to grow by 40 per cent to more than 9 billion by the year 2050 have raised the global question of how to grow more food with less water. With agriculture responsible for 70 per cent of all freshwater withdrawals, efficient and sustainable water use is needed for our own generation and future generations.

With our global water crisis in mind, we have created this resource to provide factual water news and information.


Growing more food with less water - Grey Water

Grey water is wash water, all waste water except toilet wastes and food wastes - that is, bath, dish, and laundry water.

None of these sources carries water which is likely to contain disease organisms of anywhere the same magnitude as those in toilet wastes. By far the greatest source of pathogens in waste water is excrement – or black water.

The most significant difference between black water and grey water lies in the rate of decay of the pollutants in each. Black water consists largely of organic compounds that have already been exposed to one of nature's most efficient "treatment plants": the digestive tract of the human body and the by-products from this process do not rapidly further decompose when placed in water.

The significance of the differences in the rates of decomposition between grey and black water are evident in terms of their relative impacts on ground water where treatment of black water and grey water is separated. Because of its rapid decomposition rate, grey water discharged into a stream or a lake will have a more immediate impact on the water at the point of discharge than combined waste water. However, grey water will also decompose faster in soils after infiltration and does not travel to pollute nearby drinking water as combined waste water or black water discharge would do.

Unlike black water, grey water is not malodorous immediately after discharge, but, if it is collected in a tank, it will very quickly use up its oxygen and will become anaerobic. Once it reaches the septic state, grey water forms sludge that either sinks or floats depending on its gas content and density. Septic grey water can be as foul-smelling as black waste and will also contain anaerobic bacteria, some of which can be human pathogens.

Grey water contains far less nitrogen than black water

Nine-tenths of the nitrogen contained in combined waste water comes from from toilet wastes - the black water and nitrogen is one of the most serious pollutants affecting a potential drinking water supply. It is also very difficult to remove.

Grey water contains far fewer pathogens than black water

Medical and public health bodies regard faeces as the most significant source of human pathogens. Keeping toilet wastes out of the waste water stream dramatically reduces the danger of spreading such organisms via water.

Grey water decomposes much faster than black water

Because pollutants in grey water decompose more rapidly the solution becomes stable more quickly and the water is less likely to be polluted.

A key to successful grey water treatment lies in its immediate processing before it turns anaerobic.

The simplest, most appropriate treatment technique consists of directly introducing freshly generated grey water into an active, live topsoil environment.

When properly managed, grey water can be a valuable resource, which horticultural and agricultural growers as well as home gardeners can benefit from.

It can also be valuable to landscape planners, builders, developers and contractors because of the design and landscaping advantages of on-site grey water treatment or management.

Containing phosphorous, potassium and nitrogen, grey water is a source of pollution for lakes, rivers and ground water, but they are also excellent nutrient sources for vegetation, when this particular form of wastewater is made available for irrigation. Grey water irrigation has long been practiced in areas where water is in short supply.

However, proper precautions for its use have not always been observed. This has posed a problem for health officials, who contend that there is no good management method for grey water, which both balances user needs with public safety considerations.

The options for making safe use of grey water as a source for irrigation are many and diverse and this use makes sense from both the environmental and "waste" management points of view.