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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 - Black Water


Black Water is a term which dates back to at least the 1970 used to describe waste water containing faecal matter and urine. It is also known as brown water, foul water, or sewage. It is distinct from grey water or sullage, which contain the residues of washing processes.

The key difference between the grey water and black water is that black water has come into contact with faecal matter.

Faecal matter is a haven for harmful bacteria and dangerous pathogens that can cause disease.

This waste does not break down and decompose in water quickly or effectively enough for use in domestic irrigation without the risk of contamination.

Grey water, on the other hand, has not come into contact with solid human waste. This greatly decreases the risk of disease and increases the speed at which it can be broken down and safely reabsorbed into an active garden or lawn.

Black water can be used for irrigation and other uses but only after extensive and careful treatment to remove or nullify the harmful pathogens and bacteria.

Treatment methods make use of physical, biological, and chemical methods to treat the solid and liquid organic and inorganic waste, commonly known as sewage, produced by humans via bodily excretion and domestic activities, as well as agricultural, industrial and commercial activities.

The goals are to remove solids, break down organic compounds, eliminate microorganisms that cause disease, remove harmful chemical substances and prevent or eliminate offensive and harmful odours and soil discolouration.

The different types of black water treatment include septic tanks, cess pools, soil drain fields, chemicals, biodigesters, composting toilets and black water recycling systems. How effective a black water treatment, or grey water treatment, method is in treating the biological portion of the waste stream is the most important factor when considering the different types of black water treatment.

This portion contains bacteria, viruses and protozoans that cause diseases in humans.

Black water might also contain harmful inorganic chemical elements, such as nitrogen and phosphorous, which are commonly found in fertilisers, as well as potentially toxic heavy metals, such as cadmium and mercury.

Septic tanks and soil drain fields are the most common type of localised black water treatment systems which use both a physical and biological means of treatment. The physical form includes percolation and filtration — through gravel, sand and activated charcoal.

The biological treatments take advantage of anaerobic and aerobic digestion by naturally occurring bacteria for black water treatment.

Black water treatment can also include adding chemicals, such as chlorine, to disinfect the black water and chemicals can be added that cause reactions that result in the removal of harmful organic and inorganic substances.

Cess pools, or cess pits, which have been used for a long time as a cheaper alternative, are simply sealed tanks designed to hold the sewage until it is pumped out and transported elsewhere for treatment.

Alternatives that strive to mimic nature's way of treating black water are also being used because they are considered less ecologically harmful. These include biodigesters, man-made wetlands and reed beds, composting toilets and black water recycling systems.

Biodigesters typically seek to make more efficient, effective use of anaerobic and aerobic digestion to treat black water.

Plants that absorb nitrogen, phosphorous and even metals might be planted in a man-made wetland or reed bed.

Black water recycling systems make up another category of black water treatment systems. In general, these also use the various types of physical, biological and chemical methods for black water treatment. The difference between this and other types of treatment is that the treated water is then recycled back into the black water system loop rather than being discharged.1