Water Treatment

Surface Water Quality

Contaminants:

–Suspended solids, soil (turbidity)

–Pathogens (coliform indicator)

–Color (decaying vegetation, algae)

–Taste & odor

–Other SDWA contaminants

Water Treatment

Flocculation

Sedimentation

Filtration

Chlorination

Water

Fluoridation

pH Adjustment

SedimentationThe water is

pumped into the

bottom of the

sedimentation

tanks, so as not to

disturb the clearer

water at the top.

The suspended

particles settle to

the bottom.

Filtration

Remaining suspended

particles are removed by

filtration. The water is

allowed to pass through

beds of graded sand and

gravel. The sand in the

filter bed acts as a filter

and removes the tiny

particles from the water

ChlorinationThis is the addition of

chlorine or chlorine

compounds to kill

microorganisms in the

water and to prevent

reinfection. Both

chlorine and sodium

hypochlorite are added

to the water to form an

active disinfecting

agent in the water

Fluoridation

This is the addition of

fluoride compounds to

drinking water to

prevent tooth decay.

Sodium fluorosilicate is

the usually fluoridating

agent.

pH adjustmentIt may be necessary to adjust

the pH of the water before

it leaves the treatment

plant. Tap water should

have a pH in the range of 6

– 8. If the water is too

acidic, lime is added to raise

the pH and if the pH is too

high, sulfuric acid is added.

Sewage Treatment

Primary Treatment

Physical Process

Secondary Treatment

Biological Process

Tertiary Treatment

Chemical Process

Primary Treatment

Solids and large floating debris are

screened from the waste water

Remaining solids are removed by allowing

the waste to settle in sedimentation tanks

Secondary Treatment

Aerobic Microorganisms break down

organic waste in the sewage by oxidation in

air to carbon dioxide and water

Activated Sludge Method

The activated sludge method operates

aerobically. The sewage is fed continuously into

aerated tanks that is kept oxygenated by

mechanical agitators.

Tertiary Treatment

Removal of nitrates and phosphates

Phosphates are removed by precipitation

with Aluminium Phosphate

Nitrates are removed by biological

nitrification.

Tertiary treatment is a costly process

Eutrophication of WaterEutrophication is caused

by the overenrichment of

water by nutrients such as

phosphates and nitrates.

Eutrophication of Water In effect, the nutrients

behave as fertilisers that

increase the growth of

plants such as algae in

lakes and rivers

The algae are short lived.

As they decay, much of

the dissolved oxygen in

the water is used up,

leading to the death of

many forms of animal life.

Nitrate Fertilisers

Waterways can also be polluted by

the run-off of excess fertiliser from

farmland. Excess nitrate in the

fertiliser may be washed into rivers

or lakes by rainwater.

Eutrophication will result.

Pollution of Heavy MetalsMetals with high relative atomic masses

such as mercury, cadmium and lead are

known as heavy metals.

When recycling is inadequate, quantities

of these elements are dumped e.g. Car

batteries containing lead

or dry batteries containing

cadmium.

Pollution of Heavy MetalsDipositive ions of these metals

e.g. Hg2+ , Cd2+ , and Pb2+ sometimes get into

waterways from industrial effluent and

consequently into drinking water.

These elements are cumulative poisons in

that frequent exposure causes build up in the

body, resulting in serious health damage.

Lead ions can be removed from the

effluent by precipitation

EU Legislation on Water Quality

There are limits to the quantities of

Hg2+ , Cd2+ , and Pb2+ ions that can be

tolerated in waterways because of their

toxic effects.

Limits on phosphates and nitrates help

to reduce the occurrence of

eutrophication in waterways.

Limits are also set for chemical species

dissolve in drinking water.