Disinfection of water by chlorination

• It means the chlorination of water to such an extent that living organisms as well as other
organic impurities in water are destroyed.
• Chlorine is cheap, reliable and easy to handle. Moreover it is capable of providing residual
disinfecting effects for long periods and thus prevent future recontamination of water.
• It involves addition of sufficient amount of chlorine to oxidise organic matter, reducing
substances, and free ammonia in raw water, leaving behind mainly free chlorine, which
possesses disinfecting properties against pathogenic bacteria.
• It is also known as free residual chlorine.
• When chlorine is added to water, it forms hypochlorous acid or hypochlorite ions which
have immediate disastrous effect on most forms of microscopic organisms.
Cl2 + H2O pH < 5 HOCl + HCl

• The hypochlorous acid is unstable and dissociates as

<
• All the three forms HOCl, -OCl, Cl2 existing in a sample of water are termed as
free chlorine. HOCl is found to be most destructive according to enzymatic
hypothesis given by Gleen and Stumpt. For this reason the pH value of water
during chlorination is generally maintained slightly less than 7 so as to prevent
the dissociation of HOCl. The HOCl also reacts with ammonia likely to be
present in water to form various chloramines as

• In the usual chlorination process, the pH is kept slightly less than 7 around 6.5
and hence dichloramine is the predominant species.
• These chloramines so formed are stable and are found to possess disinfecting
properties. They act as chlorine reserves. The chlorine in this form is called
combined chlorine. As compared to the free chlorine, combined chlorine is
less effective.
 Break point chlorination
• The amount of chlorine required for disinfecting water depends upon the
inorganic and organic impurities present in water.
• When chlorine is added to water and its amount is estimated after a few
minutes, it is found that the available chlorine is not equal to the amount of
chlorine added.
• A relation ship between the amount of chlorine added to water and the free
residual chlorine is shown in figure. The curve shown in figure can be divided
into four stages.
Stage I: Initially for lower doses of chlorine, there is no free residual chlorine since all the
added chlorine gets consumed for complete oxidation of reducing substances present in water.
This is due to the fact that initially Cl2 reacts with the inorganic impurities present in water
forming chlorides. These chlorides do not have any residual oxidizing power.
Stage II: As the amount of chlorine dosage is increased, amount of residual chlorine also show
steady increase. This stage corresponds to the formation of chloramines. But chloramines
respond to the test for estimation of chlorine in the same way as free chlorine and hence curve
represents the chlorine available from combined residuals. Chloramines remain in water body
for longer periods which help to kill the micro organisms even in a distribution system.
Simultaneously this stage also corresponds to the formation of chloro-organic compounds
without oxidizing them. These are responsible for bad odour and unpleasant taste in water.
Stage III: At still higher dose of applied chlorine, the complete oxidation of chloro-organic
compounds and partial oxidation of chloramines takes place and accordingly free residual
chlorine also decreases and reaches a dip when the oxidative destruction is complete. This dip
is known as break point. The chlorination upto breakpoint ensures complete destruction of
organic compounds which give unpleasant taste and bad odour.
Stage IV: After break point, the added chlorine is not used in any reaction and the residual
chlorine agrees fairly well with the quantity of chlorine added. Thus free chlorine is available
only if the demand for chlorine by other reacting substances present in water is met with.
Hence, for effectively killing the micro organisms as well as bad tastes and odours, sufficient
chlorine has to be added. Addition of chlorine in such doses is known as break point
chlorination.
• Chlorination after break point increases the free residual chlorine (Cl2 , HOCl,
-OCl).

• The free chlorine as well as the combined chlorine will cause germicidal
action on micro organisms.
• Free chlorine will instantaneously kill the pathogens, while the combined
chlorine will provide long term germicidal effect.
• Hence to use chlorine as an effective disinfectant, the chlorine dosage has
to be slightly more than the break point.
• In general, water is satisfactorily disinfected if the free chlorine residual is
about 0.2 ppm i.e. the chlorine added in the dosage upto break point + 0.2
ppm.
• Amount greater than 0.2 ppm of free residual chlorine causes bad taste in
water and its harmful as it irritates the membrane of intestine.
WASTE WATER TREATMENT
 Waste water treatment process
Primary treatment -- Removes suspended solids and floating matter using
screens, a grit chamber and sedimentation tank, from which the sludge
goes to a digester.
Secondary treatment -- Wastewater moves into the aeration tank where air
is pumped in and aerobic bacteria break down organic material. The water
goes to a final sedimentation tank which allows more sludge to settle out.
Digester uses anaerobic bacteria to digest organic compounds left in the
sedimentation tanks and produces methane in the process. The methane
can be used to run equipment or to cool/heat the processing plant.
Tertiary treatment: Physical, chemical or biological processes depending
upon the impurities to be removed.
Disinfecting the water, usually with chlorine, is the final step in the tertiary
process.
 I. Primary Treatment

Pretreatment Sedimentation

Screening Grit removal

One way to deal with coarse material in waste water is to use a device
called comminuter, which grinds the coarse material in to small pieces
which flow along with water and handled in Sedimentation tank.
 Screening Grit removal
Removes large floating objects After screening, the waste water passes
into a grit chamber

Such as rags, sticks, wood and other large Velocity of water is reduced or it is
floating and suspended solids detained for a few minutes

A typical screen consists of a parallel Grit settling chambers are periodically

steel bars disconnected from the main system to
remove grit manually

Bars spaced anywhere from 2 to 7 cm For possible use in landfilling, road making
apart. and on sludge drying beds.

Followed by a wire mesh screen Grit also is a food manure for growing
crops
 Sedimentation

• From the grit chamber, the sewage passes to a primary settling tank
known as sedimentation basin.
• Here the velocity of the water is reduced considerably to allow most of
the suspended solids to settle out by gravity.
• The most common equipment used include horizontal flow
sedimentation tanks.
• The water is detained in the horizontal flow tanks for 2-3 hours resulting
in removal of 50% of the suspended solid matter.
• An efficient sedimentation tank or clarifier removes about 80-90% of
the suspended solids and 40% of organic matter. The solids that settle
are called primary sludge or raw sludge.
 Sedimentation aids
For the removal of finely divided solids mechanical flocculation or chemical
coagulation is employed.
Mechanical Flocculation Chemical Coagulation

Waste water is passed through a The coagulants react with colloidal matter
sedimentation tank which is fitted with in the sewage to form Floc. The floc
rotating paddles moving slowly at a entraps the smaller particles and
speed of 0.4-0.5 m/s. eventually settles down as sludge.

This slow mechanical stirring allows the Common coagulants used in sewage
finely divided solid particles to coalesce treatment are: alum, copperas, hydrated
into larger particles and settle out. lime, ferric chloride, and chlorinated
copperas.
In industrial waste water, sometimes the primary treatment also includes
equalization and neutralization:
Equalization Neutralization
Sometimes different types of wastes Acidic wastes are neutralized with
are produced by some industries. To lime stone and alkaline wastes are
apply uniform treatment, different neutralized by treatment with
effluents are held in big tanks for sulphuric acid or CO2 or waste
certain periods and are mixed boiler flue gas.
thoroughly to produce
homogeneous equalized effluent.

If both acid and alkaline wastes are produced in the nearby plants then mutual
neutralization by mixing them is the cheapest method of neutralization.
 II. Secondary Treatment
In waste water much of the organic material is dissolved or in colloidal form which
is not removed by primary treatment. Thus removed by secondary treatment.
secondary treatment is achieved through biological processes:
• Coagulation of the finely divided or colloidal matter.
• Oxidation of organic matter to CO2
• Conversion of nitrogenous organic matter to ammonia, which is eventually
converted into nitrite and nitrate.
• Anaerobic digestion of the sludge so obtained.

Three commonly used approaches are:

(i) Trickling filters
(ii) Activated Sludge Process
(iii) Oxidation Ponds (Lagoons)
 (i) Trickling Filters (Aerobic filteration)
A trickling filter consists of a rotating distribution arm that sprays the liquid over a circular
bed of rocks or other coarse material. Individual rocks get coated with layer of biological
slime (aerobic microorganisms, zooglea-bacteria, algae, protozoa etc.) that absorbs and
consumes wastes through the bed. Biological towers made with plastic media are prevalent.
Advantages
1. Simple to operate and can produce BOD removal to the extent of 65 to
85%.
2. Constant monitoring is not required.
3. Effluents so produced are of better quality.
Limitations
1. Microbial film formed is sensitive to temperature changes.
2. Efficiency of the filter is dependent upon the composition of waste, pH,
size uniformity of the filtering medium & supply of air.
3. Cost of construction is high.
4. Trickling filters are used for treating industrial waste water from dairy,
brewery, food processing, pulp and paper mills, pharmaceuticals,
petrochemicals etc.
 (ii) Activated Sludge Process
Most versatile biological oxidation method, employed for the treatment of waste
water containing organic matter. Mixture of waste water and activated sludge is
agitated and aerated.
The activated sludge is the sludge obtained by settling the sewage in presence
of excess of oxygen.
The activated sludge is biologically active because it is heavily laden with
microorganisms which are in active state of growth.

Activated sludge process

Advantages
1. The primary advantage is good effluent quality. The effluent after going
through activated sludge has little BOD (< 20mg/L)
2. It takes less area as compared to trickling water filters.
3. The activated sludge process equipment is less expensive.

Limitations
1. For the process to be efficient, at least 0.5ppm oxygen must be present.
2. The optimum pH 6.5 to 9.0 has to be maintained throughout.
3. The presence of detergents (which are not biodegradable) lead to the
formation of foam, making the process difficult.
4. The disadvantage of this process is production of a huge amount of
sludge, which should be digested and disposed off.
 (iii) Oxidation Ponds
• Shallow ponds, typically 1-2 m deep
• Organic matter is oxidized by microorganisms present in the pond
• Waste water enter the pond at one end and treated waste water is collected at
the other end
• Decomposition of the organic matter near the surface is aerobic (algal
photosynthesis), anaerobic near the bottom, hence, called facultative ponds
• deeper ponds (lagoons )are mechanically aerated.
Advantages
1. The process is simple and cheap.
2. Can be used for all types of waste waters
3. Due to the high pH of waste water in the pond, the heavy metal ions present
in waste water are precipitated as hydroxides which settle as sludge.

Limitations
1. The oxidation ponds require larger space.
2. Anaerobic conditions may lead to release of bad odours.
3. The main drawback of the above secondary treatment processes is the
formation of sludge.
4. The collection, processing and disposal of sludge can be the most costly and
complex aspect of waste water treatment.
 Sludge Treatment and Disposal

• Sludge is the watery residue from the primary sedimentation tank

and humus tank from secondary treatment. Quantity of sludge
produced may be as high as 2% of the original volume of waste water,
depending upon the treatment process used.
• The traditional method of sludge digestion is anaerobic digestion.
• It involves the microorganisms that thrive in absence of oxygen. The
organic material in sludge is digested by these microorganisms under
Anaerobic conditions to give carbon dioxide and methane gas. The
components of the sewage which can be converted into gases are
called volatile solids.
Sludge digestion in digester:

1 2

Sludge is maintained at 35°C for 30 days at pH 7.0 to 8.0.

CH4, CO2 and NH3 are liberated as the end products. Digested sludge is removed
from the anaerobic digester. This sludge contains 90 to 93% water and is
dewatered.
• Dewatering is accomplished by mechanical methods, the most common being
centrifugation and filtration, which includes pressure filtration and vacuum
filtration. Drying beds are also commonly used.
• The dewatered sludge is sent for ultimate disposal. Wet sludge is sprayed on to
crop land where it functions as fertilizer.
• Dried sludge may be used as a landfill or a soil conditioner.
 III. Tertiary Treatment
• The emphasis on recovery of valuables from industrial wastewaters have created
the need for tertiary treatment.
• Tertiary treatment improves the quality of the effluent further.
• The effluent after secondary treatment plant still contains suspended solids (20-
40mg/L) which may settle on the stream or river bed and inhibit certain forms of
aquatic life.
• Some amount of BOD, significant amount of nutrients, dissolved solids, traces of
organic chemicals and other contaminants are also present.
Type of tertiary treatment depends upon the specific goal which include removal of:
1. suspended solids
2. bacteria
3. dissolved organic solids
4. toxic substances
5. nutrients (phosphorus and nitrogen)
 (i) Removal of suspended solids (Micro-straining):
•This can be achieved by micro-straining.
•The filter media consists of finely woven stainless steel fabric.
•The treated waste is allowed to pass through it.
•The solids retained on the fabric are washed into a trough, which
recycles the solids to the sedimentation tank.

(ii) Removal of dissolved solids

a) Adsorption: Dissolved solids can be organics or inorganics which are
removed by adsorption on activated carbon. Special adsorbents are
commercially available for the removal of toxic heavy metals from
industrial waste water.
b) Solvent Extraction: Used to recover phenolic materials from waste
waters of refineries and coke plants. waste water is intimately brought in
contact with a solvent having high affinity for the solute.
c) Ion Exchange: Used to remove hardness and iron and manganese salts
from drinking water. This technique has been extended to waste water
treatment for the removal and recovery of waste during water treatment.
d) Reverse Osmosis: When waste water containing dissolved solids is
allowed to pass through a semi-permeable membrane at a pressure,
which is more than osmotic pressure, the water from the waste passes
through the membrane. Hence a highly concentrated solution containing
dissolved salts is left behind.
e) Chemical precipitation: The precipitating agents like lime etc. remove
heavy metal ions by precipitating these as hydroxides. Precipitating
agents include FeSO4 , alum and ferric chloride.
 (iii) Removal of Nutrients
(a)Nitrogen Removal: All forms of nitrogen in wastewater are harmful because
plants can utilize the inorganic forms as nutrients, NH3 can be utilized by bacteria
resulting in reduced oxygen in water.
• Ammonia stripping: Ammonia is present in natural water as ammonium ion. This
NH4 is changed to ammonia gas by raising the pH (the OH– concentration) of the
waste water by adding quick lime. The ammonia gas is liberated.
NH4+ + OH NH3 + H2O

• Another approach of nitrogen removal is nitrification i.e. to convert NH4+ to NO3– ,

followed by anaerobic stage in which microorganisms convert nitrates to nitrogen
gas (N2).
bacteria
NH4+ + 2O2 NO3- + 2H+ + H2O
2NO3- + organic matter N2 + CO2 + H2O
(b) Phosphorus Removal (Chemical precipitation): Phosphorus is present
in the form of orthophosphates (H2PO4–, HPO42- and PO43-). Phosphates are
removed by adding coagulants usually alum [Al2(SO4)3] or lime [Ca(OH)2].

Al2 (SO4)3 + 2PO43- 2AlPO4 + 3SO42-

3Ca(OH)2 + 2PO43- Ca3(PO 4)2 + 3H2O + O2

(iv) Removal of bacteria

Chlorination (already discussed in Ist Semester)
Bacteria are removed by retaining the effluents in maturation ponds or
lagoons for specified period of times.
 Raw waste water BOD - 200ppm
(primary treatment) NH4+ - 30ppm
PO4- - 25ppm
Undissolved & settleable solids,
grease and sugar removed

Primary sludge Effluent for secondary treatment

Air CO2
Sludge
thickening/ Activated sludge

Return
sludge
drying

Sedimentation
Anaerobic
digestion/ Biological
incineration towers

BOD - 25ppm
Solid disposal Effluent to
NH4+ - 20ppm
receiving water
PO4- - 25ppm

Primary & secondary treatment of municipal waste water