WATER TREATMENT 1319
The equilibrium of the first of these equations is purely
physical, since the solubility of gas and water is determined
by the pressure of that gas and the temperature and a number
of other physical parameters.CoagulationThe principle function of chemical coagulation is known as
destabilization, aggregation, and binding together of col-
loids. Alum, or aluminum sulphate, (Al 2 (SO 4 ) 3 · 18H 2 O) is
one of the most common coagulants which may be added
to a water system. Such a coagulant possesses tiny positive
charges and therefore has the ability to link together with
negatively charged color or turbidity particles by mutual
coagulation. Alum also reacts with the natural alkalinity
(carbonate- bicarbonate system) of the water to produce a
precipitate which is usually thought to be aluminum hydrox-
ide. If the reaction takes place with natural alkalinity, it may
be expressed as follows:Al 2 (SO 4 ) 3 · X H 2 O 3Ca(HCO 32 ) → 2Al(OH) 3 3CaSO 4
X H 2 O 6CO 2.In the event that there is insufficient natural alkalinity for
this to occur, then calcium oxide (lime) may be added to
create the same effect. Because this system is very poorly
understood, the optimum dosage required in practice has to
be done by trial and error through a series of tests known as
jar tests.
In these jar tests, the flash mixing and flocculation steps
described previously are stimulated at various concentra-
tions of alum and the clarification which takes place and the
reduction of turbidity and the rate at which the floc settles
are all observed in order to determine the optimum dosage
of coagulant. If too much coagulant is added, then the col-
loidal system which is primarily negatively charged will
become supersaturated by the aluminum system which is
primarily positively charged and the suspension will become
restabilized and this can be observed by conducting jar tests
over a wide range of concentrations of coagulant.
The reason why alum is so generally used is that it is
highly effective over a wide pH range in waters of vastly
different chemical make-up. Other materials such as ferrous
sulphate are occasionally used to increase the settling rate of
plankton and thus increase the time of the filter run, making
the filter process more efficient.PrecipitationThere are two important processes which are associated with
precipitation in the treatment of water. One is the reduction
of hardness (calcium and magnesium) and the other is the
reduction of iron and manganese.Water Softening The lime-soda-ash process involves the
addition of Ca(OH) 2 and Na 2 CO 3 to water. The reactions
which occur are as follows:(^)
Ca(HCO ) 32 Ca(OH) 2 2CaCO 2H O
Lime
3
- → 2
(1)
Mg(HCO )^32 Ca(OH)^2 →MgCO^3 CaCO^3 2H O^2 (2)
MgCO^32 Ca(OH) →Mg(OH)^23 CaCO (3)
(^)
CaSO 42 Na CO CaCO Na SO
Soda Ash
3 → 324. (4)
In this reaction it can be seen that the lime is added to precipitate
the carbonate hardness, while the soda ash provides the car-
bonate ion to precipitate the non-carbonate hardness.
Precipitation of Iron and Manganese Normally, iron and
manganese are only highly soluble if they are in their ferrous
(Fe^2 ^ ) and manganous (Mn^2 ^ ) forms. Normally, these two
metals will only occur in this form if there is an absence of
dissolved oxygen. However, on occasions when the water is
particularly acid, such as might occur in mine drainage areas,
the metals may remain in solution even though a very high dis-
solved oxygen is present. Under these circumstances, aeration
is frequently sufficient to drive off the surplus carbon diox-
ide, increase the pH and bring about a natural precipitation
of these materials in their ferric and manganic form. In order
to catalyze or accelerate this reaction, the water is frequently
caused to trickle over coke or crushed stone, or to flow upward
through some contact material. This allows deposits of iron
and manganese to accumulate on the surfaces and catalyze the
further precipitation of ferric and manganic oxides.
If the pH of the system is forced to values higher than
7.1, the positively charged ferric hydroxide particles may be
Drive
Motor
Control
Stirrer
FIGURE 13 Jar test equipment—coagulant dosage varied in each jar to deter-
mine optimum concentration.
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