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Before waters is treated, its quality must be assessed to ensure that the appropriate
methods are used to purify it. The particular methods that are employed will depend
on the intended use and the quality of the water source.
Metal grills or meshes are used to remove floating vegetation, plastic and paper that
would clog later processes.
The water is allowed to remain static to let suspended solids such as silt to settle.
Bubbles of air or water sprays are used to aerate the water and ensure a high
dissolved oxygen content. Water sources that are anaerobic may contain hydrogen
sulphide from the decay of organic matter, which makes the water smell of bad eggs.
Some dissolved metals that are toxic or give the water a bad taste are removed by
aeration as they become soluble.
Flocculation / Coagulation and clarification -
Clay particles do not settle out in the sedimentation lagoons because electrostatic
charges on their surfaces cause them to repel each other. These can be neutralised by
adding flocculants such as aluminium sulphate (alum) or polyelectrolyte's. They are
quickly mixed with the water then passed into the clarifier tank where the particles
are allowed to settle.
Filters such as sand filters are used to remove any remaining suspended solids and
Activated carbon filters -
Particles of activated carbon are used to remove organic chemicals such as pesticides
and some substances that cause bad tastes.
The addition of chlorine, ozone, or exposure to ultraviolet light is used to sterilise the
water and kill pathogens. Chlorine is the most method of sterilisation but if the water
is from peaty sources then organic matter may be present, which could react with the
chlorine and produce toxic substances such as trihalomethanes that are carcinogenic.
Ozone also helps to break down pesticides.
The addition of chlorine should keep the water sterile during distribution but it is
gradually lost. Using chloramine keeps the water sterile for longer as it breaks down
gadually and releases chlorine.
Fluorides are added to the water in some areas to improve the dental health of people
that drink it.
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Seawater is the most abundant source of waterbut the high salt content makes it
unusable except where its composition is unimportant, such as condenser cooling
water in power stations.
Removing salt from sea water is expensive and is only carried out if other sources of
water are unavailable.
Reverse osmosis -
During reverse osmosis the saline water is filtered at very high pressure through a
partially permeable membrane of very small polyamide tubes.…read more