Civil Engineering Reference
In-Depth Information
Because polymers do not affect the pH of water, their use offers a clear advantage
for treating low-alkalinity waters. This is particularly true of the low-alkalinity waters
that are high in turbidity. Such waters would require considerable qualities of alum,
which would require the addition of soda ash or lime to replenish the buffering capacity
of the water and maintain desirable pH.
Coagulation with Magnesium
Magnesium precipitated as magnesium hydroxide (Mg(OH)
2
) has been shown to be
an effective coagulant for the removal of color and turbidity from waters.
28,29
Coagu-
lation is achieved by enmeshing the colloidal particles in the gelatinous hydroxide
precipitate. Similar findings have been shown in lime treatment of wastewaters.
Waters that are high in naturally occurring magnesium can be coagulated by raising
the pH to the point at which Mg(OH)
2
is precipitated. Normally, pH elevation is
accomplished through the addition of lime. For waters that are low in magnesium, it
is necessary to add a suitable magnesium salt. In using lime to raise the pH, one
advantage is that both lime and magnesium can be recovered from the sludge and
reused. The recovery process takes the form of recalcination in a furnace, typically a
multiple hearth furnace. The sludge disposal problems are reduced by the lime recov-
ery. However, the sludge-handling processes are complex. The economics of treatment
must be carefully evaluated before recalcination is implemented.
The chemistry of magnesium coagulation with lime addition is based on water
softening and coagulation theories. The reactions involved in Mg(OH)
2
precipitation
are:
CO
Ca(OH)
→
CaCO
H O
(10-11)
2
2
3
2
Ca(HCO )
Ca(OH)
→
2CaCO
2H O
(10-12)
32
2
3
2
Mg(HCO )
Ca(OH)
→
MgCO
CaCO
2H O
(10-13)
32
2
3
3
2
MgCO
Ca(OH)
→
Mg(OH)
CaCO
(10-14)
3
2
2
3
The solubility diagram for magnesium (Fig. 10-5) shows that precipitation is en-
hanced at high pH values. In practice, sufficient lime is added to raise and maintain
pH at least 10.7 through the clarification process.
Recovery of magnesium is achieved by recarbonation to lower the pH to the point
at which Mg(HCO
3
)
2
is formed and removed. The magnesium can then be returned
to the coagulation process. Alternatively, the magnesium can be held with the lime
sludge for recalcination for lime recovery. The recovered lime is then used to raise the
pH as previously explained. Lime-soda softening and recarbonation are described in
Chapter 21.
Coagulant Aids
Ideally, flocculated colloidal particles should settle rapidly and be strong enough to
resist shearing forces. Often, the flocs do not possess these characteristics, and a co-
