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i) Plain sedimentation
- Sedimentation is a simple, physical pre-treatment of water prior to application of other purification treatments such as filtration and disinfection. It removes undesirable small particulate suspended matter (sand, silt and clay) and some biological contaminants from water under the influence of gravity. The longer the water is held undisturbed, the more the suspended solids and pathogens will settle
to the bottom of the container. Adding coagulants can accelerate the sedimentation
process. Three common chemicals used for this purpose are aluminium sulphate,
polyaluminium chloride (also known as PAC or liquid alum) and ferric sulphate.
ii) Coagulation
- The coagulation process involves the addition of the chemical (e.g. alum) and then
a rapid mixing to dissolve the chemical and distribute it evenly throughout the
water. Alum produces positive charges to neutralize the negative charges on the
particles. Then the particles can stick together, forming larger particles which are
more easily removed.
iii) Filtration
- Water flows through a filter designed to remove particles in the water. The filters
are made of layers of sand and gravel, and in some cases, crushed anthracite.
Filtration collects the suspended impurities in water and enhances the
effectiveness of disinfection. The filters are routinely cleaned by backwashing.
iv) Disinfection
- Water is disinfected before it enters the distribution system to ensure that any
disease-causing bacteria, viruses, and parasites are destroyed. Chlorine is used
because it is a very effective disinfectant, and residual concentrations can be
maintained to guard against possible biological contamination in the water
distribution system.
Kavungya answered the question on March 22, 2019 at 09:07
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