Environmental Engineering

1. COAGULATION
CIVIL ENGINEERING BATCH 2013

2. COAGULATION
 The very fine suspended particles and the colloidal matter present in water can not be settled in
plain sedimentation tank. So they need special treatment to remove, for this purpose certain
chemicals compound called coagulants are added to water, which on thorough mixing form a
gelatinous precipitated called folc.
 The process of addition and mixing of chemicals is called coagulation.
 In addition to removing turbidity from water , coagulation and flocculation is beneficial. In other
ways the process to remove many bacteria which are suspended in the water and can be used to
remove color from water.
 The coagulation before sedimentation is almost universally adapted in all the major water
treatment pant and is followed by rapid sand filtration.

4. COMMON COAGULANTS
 The coagulants like aluminum sulphate (Alum), ferrous sulphate etc carry positive charge and attract
negatively charged colloidal particles of clay, turbidity, and color. Thus helping in the removal of these
impurities from water.
 Following are common coagulants:
1. Aluminum sulphate
2. Ferrous sulphate
3. Chlorinated copperas
4. Magnesium carbonate
5. Poly electrolytes
6. Ferric chloride
7. Sodium aluminate etc

5. ALUMINUM SULPHATE
 This is most common and universal coagulant used in water works.
 Its chemical composition is Al2(SO4)3.18H2O. It requires the presence of alkalinity in water to
form flux. Many water have bicarbonate alkalinity naturally in them. When dissolved in water
aluminum sulphate tends to hydrolyze into aluminum hydrolyze as is evident from following
reactions
 Al2(SO4)3 18H2O+3Ca (HCO3)= 2Al( OH3)+3CaSO4+18H2O+6CO2
 If Natural alkalinity in water is insufficient to react with alum , lime is also added to water to form
calcium hydrodoide Ca(OH)2 Or hydrated lime which reacts with Alum as follows
 Al2(SO4)3 18H2O+3Ca(OH)2=2Al( OH3)+3CaSO4+18 H2O

6. ALUMINUM SULPHATE
 Sometimes sodium carbonate known as soda ash is added to form alkalinity the resulting reactions are
 Al2(So4)3 18H2o+3Na2co2 = 2Al(OH)2+3NaSo4+3cO2+18H2O
 though soda ash does not cause hardness , it is expensive than lime and is found to be effective when
ph value of water varies from 6.5 t0 8.5.
 Its dose depends upon turbidity , color , taste, ph value and temperature of water . The dose of alum
varies between 10 t0 30mg per dm3 of water . Alum is preferred over other coagulants because it
reduces taste and odour etc effectively. It is cheap and floc formed is quite tough to be broken easily.
 It produce crystal clear water. The main disadvantage of alum is that it is difficult to dewater sludge
formed.

8. FERROUS SULPHATE
 Ferrous or copperas can react with naturally calcium carbonate and alkalinity in water but this
is very much delayed one , hence lime is used with ferrous sulphate and the following reactions
take place.
 Fe(SO4) 7H2O+ Ca (OH)2= Fe(OH)2+ Caso4+ 7H2O
 The ferrous hydroxide is satisfactory gelatinous floc , which is heavier than one formed by Alum.
 The effective Ph range for this is 8.5 and above. Ferrous sulphate is not suitable for soft colored
water.

9. CHLORINATED COPPERAS
 Hydrated Ferrous Sulphate FeSO4.7H2O is traditionally referred to as Copperas. It has too high
solubility to act as satisfactory coagulant . It is therefore, first oxidized to Ferric Sulphate
[Fe2(SO4)3] and Ferric Chloride (FeCl3) by mixing its solution with a feed from chlorinator the
.6FeSO4.7H2O + 3Cl22Fe (SO4)3+ 2 FeCl3+ 7H2OFe2(SO4)3and FeCl3, produced instantaneously
are called chlorinated Copperas .
 .Fe2(SO4)3effective pH 4 to 7 and >9FeCl3. it is effective over pH value 3.5 to 6.5 and >8.5The
combination has therefore proved to be very effective coagulant for treating low pH water

10.  Polyelectrolytes:
 These are mostly used to assist the flocculation process and are often called
flocculation aids. They are polymeric organic compounds of long polymer chains that
act to enmesh particles in the water. Polyelectrolytes can be cationic (carrying a positive
charge), anionic (carrying a negative charge) or non-ionic (carrying no net charge).
 This coagulant removes both temporary and permanent hardness and is effective for Ph
range of 6 t0 8.5 naturally available in water.
 Ferric chloride:
 (FeCl3) is commonly used as a coagulant. When added to water, the ion precipitates as
ferric hydroxide (Fe(OH)3) and the hydroxide flocs enmesh the colloidal particles in the
same way as the aluminium hydroxide flocs do. The optimum pH for precipitation of iron
is not as critical as with aluminium and pH values of between 5 and 8 give good
precipitation.

11.  POLYMERS :
 Polymers long-chained, high-molecular-weight, organic chemicals--are becoming more
widely used, especially as coagulant aids together with the regular inorganic
coagulants. Anionic (negatively charged) polymers are often used with metal
coagulants.
 Low-to-medium weight, positively charged (cationic) polymers may be used alone or in
combination with the aluminum and iron type coagulants to attract the suspended solids
and neutralize their surface charge. The manufacturer can produce a wide range of
products that meet a variety of source-water conditions by controlling the amount and
type of charge and relative molecular weight of the polymer. Polymers are effective over
a wider pH range than inorganic coagulants. They can be applied at lower doses, and
they do not consume alkalinity.

12. CHAMBERS OF COAGULANT BASIN
 Inlet chamber: water is allowed to enter coagulant tank through it.
 Feeding device: the selected coagulant is added to water by feeding device.
 Mixing device: there are many types of Mixing device. A suitable one is adapted for mixing
operation. Generally the device consist of paddles , which are rotated by deriving unit. The
paddles go on rotating continuously and coagulant are mixed in this way.
 Flocculation chamber: in this chamber the mixture of water and coagulant is detained for some
period or it is allowed to flow in very low velocity so that the floc may be formed and get settled
down by arresting the suspended particles . Thus sludge is deposited at bottom of coagulant
chambers.

13. CHAMBERS OF COAGULANT BASIN
 Sludge removal: at the time of removal the
sludge is agitated by scrappers fixed with
rotating arms which are operated by deriving
unit. The sludge is taken off through the
sludge removal pipe by opening the valve.
 Collection of clear water: the clear water
from top is collected in a draw off chamber
from where it is taken to the next unit
through the outlet pipe.