Water treatment is the process of
removing all biological, chemical, or physical substances that are potentially
harmful to the water supply for human and domestic use.
The essence of water treatment is
to produce water that is safe, palatable, clear, colorless, and odorless. Water
also needs to be non-corrosive, meaning it will not cause damage to pipework.
Below
are seven major steps involved in the large-scale water treatment:
SCREENING
Screening is the first unit
operation used in water treatment. It removes objects such as rags, paper,
plastics, and metals to prevent damage and clogging of downstream equipment,
piping, and appurtenances.
There are coarse and fine
screens.
Coarse screens are made of
corrosion-resistant steel bars spaced 5–15 cm apart, which are used to exclude
coarse materials (such as logs and fish) from entering the treatment plant.
Fine screens, which come after
the coarse screens, keep out material that can block pipework at the plant.
They consist of steel bars that are spaced 5–20 mm apart.
AERATION
After screening, the water is
aerated (supplied with air) by passing it over a series of steps to take in
oxygen from the air. This process helps in expelling soluble gases such as
carbon dioxide and hydrogen sulfide which are acidic, so this process makes the
water less corrosive.
It equally expels any gaseous
organic compounds an undesirable taste to the water. Aeration also removes iron
or manganese by oxidation of these substances to their insoluble form. Iron and
manganese can cause peculiar tastes and can stain clothing. Once in their
insoluble forms, these substances can be removed by filtration.
COAGULATION AND FLOCCULATION
After aeration, coagulation occurs
to remove the fine particles, which are less than 1 µm in size, suspended in
the water. In this process, a chemical called a coagulant, with a positive electrical
charge, is added to the water which
neutralizes the fine particles' negative electrical charge. The coagulant's
addition takes place in a rapid mix tank where a high-speed impeller rapidly
disperses the coagulant.
Since their charges are now
neutralized, the fine particles come together, forming soft, fluffy particles
called 'flocs.' Two coagulants commonly used in the treatment of water are
aluminum sulfate and ferric chloride.
The next step is flocculation.
Here the water is gently stirred by paddles in a flocculation basin, and the
flocs come into contact with each other to form larger flocs.
The flocculation basin often has
a number of compartments with decreasing mixing speeds as the water advances
through the basin. This compartmentalized chamber allows increasingly large
flocs to form without being broken apart by the mixing blades.
SEDIMENTATION
Once large flocs are formed, they
need to be settled out, and this takes place in a process called sedimentation
(when the particles fall to the floor of a settling tank). The water, after
coagulation and flocculation, is kept in the tank for several hours for
sedimentation to take place. The material accumulated at the bottom of the tank
is called sludge; this is removed for disposal.
FILTRATION
Filtration is the process where
solids are separated from a liquid. In water treatment, the solids that are not
separated in the sedimentation tank are removed by passing the water through
sand and gravel beds. With a flow rate of 4–8 cubic meters per square meter of
filter surface per hour, rapid gravity filters are often used.
When the filters are full of
trapped solids, they are back-washed. In this process, clean water and air are
pumped back up the filter to dislodge the trapped impurities, and the water
carrying the dirt (referred to as backwash) is pumped into the sewerage system
if there is one. Alternatively, it may be discharged back into the source river
after a settlement stage in a sedimentation tank to remove solids.
CHLORINATION
After sedimentation, the water is
disinfected to eliminate any remaining pathogenic micro-organisms. The most
commonly used disinfectant (the chemical used for disinfection) is chlorine, a
liquid (such as sodium hypochlorite, NaOCl), or a gas. It is relatively cheap
and simple to use. When chlorine is added to water, it reacts with any
pollutants present, including micro-organisms, over a given period of time,
referred to as the contact time. The amount of chlorine left after this is
called residual chlorine. This stays in the water through the distribution
system, protecting it from any micro-organisms that might enter it until the
water reaches the consumers.
SUPPLEMENTARY TREATMENT
Supplementary treatment may be
needed for the benefit of the population. One such instance is the fluoridation
of water, where fluoride is added to water. It has been stated by the World
Health Organization that ‘fluoridation of water supplies, where possible, is
the most effective public health measure for the prevention of dental decay.
The optimum fluoride level is around 1 mg per liter of water (1 mg l–1).
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