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Water quality requirements
The degree to which water needs to be purified depend upon its intended use. What may be very
important for one use may not be so important for another use. Public water supply is the most
important large-scale water use where quality is very important and supplies may require alot of
treatment. Many physical, chemical and biological criteria are used to access the water quality for
public supply. Potable (drinkable) water does not have to be completely pure, but it must not contain
unacceptable levels of hazardous materials nor look, taste or smell unpleasant.
Suspended material must be removed because they give water an unpleasant appearance and taste and
the settling of suspended solids would block pipes. The concentration is expressed as TSS (Total
suspended solids) in mg 1-1.
If water is too acidic it can dissolve copper pipes. If the pH is too high or low it can make the water taste
unpleasant. The ideal range is pH 6.5-8.5.
Dissolved calcium ions make the water 'harder'. Hard water is good for health as it can help to reduce
tooth decay, osteoporosis and heart disease. But it can react with soap to produce scum and produces
solid limescale if the water is heated.
All pesticides are toxic but not necessarily toxic to humans. Water should be treated to remove virtually
Heavy metal concentrations
Heavy metals such as lead and mercury are neurotoxins and damage the nervous system, but low
concentrations cause no detectable damage.
Low dissolved oxygen levels can make water smell musty or of hydrogen sulphide (`bad eggs' smell).
Some toxic metals are more soluble in water with a low dissolved oxygen content.
Chlorine is added to water to keep it sterile. Other materials present in the water and the pipe's wall
itself may react with the chlorine and reduce its concentration. It may become necessary to add more
chlorine or use chloramines, which gradually release more chlorine.
Sewage contamination of water could cause the spread of many serious disease caused by pathogens
such as cholera and typhoid, but the bacteria that cause these are uncommon, so testing for them would
not prove there was no sewage present. E.coli is a very common gut bacterium and is always present if
sewage contamination has occurred, whether or not there are disease-causing pathogens present.
The `coliform count' is the measure of the number of bacteria similar to E.coli that are present per
litre of water. For potable water the coliform count should be 0.