the drinking water treatment process

The Drinking The Drinking Water Water

Treatment Treatment ProcessProcess

Why do we need to treat water?Why do we need to treat water? There are many impurities in the raw water There are many impurities in the raw water These impurities can be grouped into three categories:These impurities can be grouped into three categories:

Physical:Physical:

– materials that do not dissolve in water and make the water materials that do not dissolve in water and make the water appear "dirty" appear "dirty"

Chemical: substances dissolved in the water from both natural Chemical: substances dissolved in the water from both natural and man-made processes and man-made processes

Biological: Biological: viruses, bacteria, algae, and other small living viruses, bacteria, algae, and other small living organisms.organisms.

Is the drinking water that comes out of Is the drinking water that comes out of our tap "pure"?our tap "pure"?

No "Chemically pure" water, entirely free from any other No "Chemically pure" water, entirely free from any other materials, does not exist in nature.materials, does not exist in nature.

Distilled water, is usually flat and tasteless and few Distilled water, is usually flat and tasteless and few people enjoy drinking it. people enjoy drinking it.

It would be prohibitively expensive and possibly It would be prohibitively expensive and possibly unhealthy to purify our entire water supply to that level.unhealthy to purify our entire water supply to that level.

"Natural water", free from any man-made additives (if it "Natural water", free from any man-made additives (if it still exists) contains concentrations of minerals such as still exists) contains concentrations of minerals such as calcium, magnesium, and iron which are beneficial to calcium, magnesium, and iron which are beneficial to human health in small quantitieshuman health in small quantities ..

Drinking water treatmentDrinking water treatment

Large particles and debris are removed from the raw Large particles and debris are removed from the raw water by travelling screens just as the water enters the water by travelling screens just as the water enters the

treatment plants.treatment plants.There are There are five five different processes our water goes different processes our water goes

throughthrough

1. 1. Coagulation, Flocculation, and Coagulation, Flocculation, and SedimentationSedimentation

Rapid mixing of chemicals known as Rapid mixing of chemicals known as coagulants coagulants

Coagulant make the small physical Coagulant make the small physical particles in the water clump together particles in the water clump together (coagulation),(coagulation),

gentle mixing to form larger groups gentle mixing to form larger groups of particles known as floc of particles known as floc (flocculation).(flocculation).

Thicker, denser floc floats down and Thicker, denser floc floats down and settles out of the water in large tanks settles out of the water in large tanks (sedimentation) or is removed during (sedimentation) or is removed during the next stage, filtrationthe next stage, filtration

CoagulantsCoagulants Alum (aluminum Alum (aluminum

sulphate), polyaluminum sulphate), polyaluminum chloride and a group of chloride and a group of chemicals known as chemicals known as polyelectrolytes polyelectrolytes

.Large +ve Charge attracts .Large +ve Charge attracts -ve charged clay particles-ve charged clay particles

Zeta potential Zeta potential

– Large charge on small ion Large charge on small ion Al+++ Fe +++Al+++ Fe +++

SettlementSettlement

2. Filtration2. Filtration

removal of the removal of the remaining floc, remaining floc,

other chemical and other chemical and physical impurities,physical impurities,

and most of the and most of the biological impurities biological impurities (bacteria, etc.) (bacteria, etc.)

. Dual media filters are . Dual media filters are layers of sand and layers of sand and anthracite,anthracite,

3. 3. DisinfectionDisinfection

The addition of the chemical chlorine,The addition of the chemical chlorine, The chemical is added to our water at different points in The chemical is added to our water at different points in

the treatment process.the treatment process. When chlorine is added at beginning pre-chlorination. When chlorine is added at beginning pre-chlorination. After the filtration stage it is known as post-chlorination. After the filtration stage it is known as post-chlorination.

Superchlorination when the levels of bacteria are high. Superchlorination when the levels of bacteria are high. Sulphur dioxide is then added to remove excess Sulphur dioxide is then added to remove excess

chlorine chlorine

Break Point ChlorinationBreak Point Chlorination

4. 4. FluoridationFluoridation

Add additional fluoride after the filtration Add additional fluoride after the filtration stage to raise the level to 1.2 mg/l.stage to raise the level to 1.2 mg/l.

5. 5. AmmoniationAmmoniation

Ammonia is added at the end of the treatment Ammonia is added at the end of the treatment process and combines with the remaining process and combines with the remaining chlorine. chlorine.

This stabilizes the chlorine so that it remains This stabilizes the chlorine so that it remains dissolved in the treated water for longer dissolved in the treated water for longer Ammoniation also prevents chlorine from Ammoniation also prevents chlorine from evaporating out of your drinking water evaporating out of your drinking water causing smells and associated tastes.causing smells and associated tastes.

Ground Water TreatmentGround Water Treatment

groundwater is significantly easier to treat groundwater is significantly easier to treat than surface water. than surface water.

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THE TREATMENT PROCESSTHE TREATMENT PROCESS

AerationAeration. Raw water pumped from the well is . Raw water pumped from the well is mixed with air. mixed with air.

The mixing releases carbon dioxide and The mixing releases carbon dioxide and hydrogen sulfide gases present in the water. hydrogen sulfide gases present in the water.

Aeration also oxidizes any iron, Aeration also oxidizes any iron, – causing it to "precipitate" (or settle out) causing it to "precipitate" (or settle out)

– removed by precipitation and filtration.removed by precipitation and filtration.

Lime (calcium hydroxide)Lime (calcium hydroxide) Added to remove the calcium and magnesium salts.Added to remove the calcium and magnesium salts. The pH of the water is raised from approximately 7.6 to a The pH of the water is raised from approximately 7.6 to a

range of 10.4 to 10.6. range of 10.4 to 10.6. Converts the calcium and magnesium from a soluble to Converts the calcium and magnesium from a soluble to

insoluble form, insoluble form,

CaCOCaCO33 + Ca (OH) + Ca (OH)22 --> Ca (HCO --> Ca (HCO33 ) )22 + H + H2200

causing the insoluble material to precipitate out. causing the insoluble material to precipitate out. Lime "sludge" on the bottom of the basins , re-use in Lime "sludge" on the bottom of the basins , re-use in

agriculture.agriculture.

Recarbonation.Recarbonation.

Liquid carbon dioxide is mixed with the water. Liquid carbon dioxide is mixed with the water. The liquid carbon dioxide converts insoluble The liquid carbon dioxide converts insoluble

salts back to soluble salts. salts back to soluble salts.

Ca (HCOCa (HCO33 ) )22 + CO2 --> Ca CO + CO2 --> Ca CO33 + H + H220 0

FiltersFilters

Composed of layers of filter sand and graded Composed of layers of filter sand and graded gravel.gravel.

These are washed cleaned from the filter bed These are washed cleaned from the filter bed approximately every 50 hours through a approximately every 50 hours through a process known as "backwashing"..process known as "backwashing"..