Transcript The Drinking Water Treatment Process

The Drinking
Water
Treatment
Process
Why do we need to treat water?

There are many impurities in the raw water

These impurities can be grouped into three
categories:

Physical:
– materials that do not dissolve in water and make
the water appear "dirty"

Chemical: substances dissolved in the water from
both natural and man-made processes

Biological: viruses, bacteria, algae, and other small
living organisms.
Is the drinking water that comes out of
our tap "pure"?

No "Chemically pure" water, entirely free from any
other materials, does not exist in nature.

Distilled water, is usually flat and tasteless and few
people enjoy drinking it.

It would be prohibitively expensive and possibly
unhealthy to purify our entire water supply to that
level.

"Natural water", free from any man-made additives
(if it still exists) contains concentrations of minerals
such as calcium, magnesium, and iron which are
beneficial to human health in small quantities.
Drinking water treatment
Large particles and debris are removed from the raw
water by travelling screens just as the water enters the
treatment plants.
There are five different processes our water goes
through
1. Coagulation,
Flocculation, and
Sedimentation

Rapid mixing of chemicals
known as coagulants

Coagulant make the small
physical particles in the water
clump together (coagulation),

gentle mixing to form larger
groups of particles known as
floc (flocculation).

Thicker, denser floc floats down
and settles out of the water in
large tanks (sedimentation) or
is removed during the next
stage, filtration
Coagulants

Alum (aluminum
sulphate),
polyaluminum chloride
and a group of
chemicals known as
polyelectrolytes

.Large +ve Charge
attracts -ve charged
clay particles

Zeta potential
– Large charge on small
ion Al+++ Fe +++
Settlement
2. Filtration

removal of the
remaining floc,

other chemical and
physical impurities,

and most of the
biological impurities
(bacteria, etc.)

. Dual media filters are
layers of sand and
anthracite,
3. Disinfection

The addition of the chemical chlorine,

The chemical is added to our water at
different points in the treatment process.

When chlorine is added at beginning prechlorination.

After the filtration stage it is known as postchlorination.

Superchlorination when the levels of bacteria
are high.

Sulphur dioxide is then added to remove
excess chlorine
Break Point Chlorination
4. Fluoridation

Add additional fluoride after the filtration
stage to raise the level to 1.2 mg/l.
5. Ammoniation

Ammonia is added at the end of the treatment
process and combines with the remaining
chlorine.

This stabilizes the chlorine so that it remains
dissolved in the treated water for longer
Ammoniation also prevents chlorine from
evaporating out of your drinking water
causing smells and associated tastes.
Ground Water Treatment

groundwater is significantly easier to treat
than surface water.
.
THE TREATMENT PROCESS

Aeration. Raw water pumped from the well
is mixed with air.

The mixing releases carbon dioxide and
hydrogen sulfide gases present in the water.

Aeration also oxidizes any iron,
– causing it to "precipitate" (or settle out)
– removed by precipitation and filtration.
Lime (calcium hydroxide)

Added to remove the calcium and
magnesium salts.

The pH of the water is raised from
approximately 7.6 to a range of 10.4 to 10.6.

Converts the calcium and magnesium from a
soluble to insoluble form,

CaCO3 + Ca (OH)2 --> Ca (HCO3 )2 + H20

causing the insoluble material to precipitate
out.

Lime "sludge" on the bottom of the basins ,
re-use in agriculture.
Recarbonation.

Liquid carbon dioxide is mixed with the water.

The liquid carbon dioxide converts insoluble
salts back to soluble salts.

Ca (HCO3 )2 + CO2 --> Ca CO3 + H20
Filters

Composed of layers of filter sand and graded
gravel.

These are washed cleaned from the filter
bed approximately every 50 hours through a
process known as "backwashing"..