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6.4 CHAPTER SIX

When one is designing for coagulant application, as much flexibility as possible should
be allowed, to accommodate changing conditions. Several points of addition for coagu-
lant chemicals, particularly polymers, should be provided in the rapid mixing and floc-
culation processes. The order of chemical addition is also important in almost all waters.
Sludge quantity and disposal are important considerations in selecting the coagulant
to be used. Metal-ion coagulants produce considerably larger volumes of sludge than poly-
mers. The ability to predict the exact reaction and quantity of sludge that will be produced
solely by the reaction formulas is limited. For this reason, predictions of treatability, chem-
ical dosages, and sludge quantities must generally be determined by laboratory and pilot
plant tests.
The coagulation process may, in some cases, be improved by preozonation. Ozone
may significantly reduce coagulant requirements to the point where low residual solids
(or filtration efficiency) make direct filtration feasible. However, due to the increasing
power costs, in an effort to reduce ozone requirements, many recently constructed ozone
facilities incorporate ozonation after clarification or filtration.
Oxidation with air and chemical oxidants such as chlorine and potassium permanganate
may also aid coagulation by oxidizing iron and manganese, which can aid floc formation.
Carbon addition, typically in the form of powdered activated carbon (PAC), may also im-
prove coagulation, as it would remove a fair amount of organic matter prior to the coag-
ulation process, thereby, reducing coagulant demand and the associated levels of sludge
production as well as improving overall turbidity and organics removal. Similarly, new,
specialty adsorbants/resins are actively being considered in the drinking water treatment
community. One such adsorbant, a magnetic ion exchange (MIEX) resin by ORICA Wa-
tercare (Melbourne, Australia, and Englewood, Colorado), is specifically designed to re-
move low-molecular-weight organics, which are a primary contributor to many DBP pre-
cursors. As the DBP rules become more and more stringent in future years, there is little
question more specialty-type coagulant aids will continue to be developed to further im-
prove the removal of these precursors.

Adjustment of pH
Control of pH and alkalinity is an essential aspect of coagulation. The optimum pH for
coagulation varies but is generally within the following ranges for turbidity removal:
• Alum: pH 5.5 to 7.5; typical pH 7.0
• Ferric salts: pH 5.0 to 8.5; typical pH 7.5
It can be necessary to adjust the pH of some source waters to achieve optimum coag-
ulation. The pH is often lowered by adding carbon dioxide or an acid. Alum and ferric
chloride consume alkalinity and can lower pH; however, reducing pH by adding more
chemical than is required for coagulation should be avoided as it increases overall chem-
ical costs and sludge production/costs. In some source waters with low pH or low alka-
linity, it may be necessary to add caustic soda or lime to raise pH and to offset the acid-
ity of metal-ion coagulants, even in an enhanced coagulation mode of operation. A thorough
discussion of the effects of pH on coagulation appears in Water Quality and Treatment.
For waters that require enhanced coagulation to remove organic matter, the pH of co-
agulation should be lowered as compared to coagulation for turbidity removal only. Typ-
ically, the optimum pH for organics removal with alum is between 6.0 and 6.5, and be-
tween 5.5 and 6.0 for ferric coagulants. Often, polyaluminum chloride can provide organics
removal without as significant a decrease in pH.
There are a number of secondary impacts of utilizing the higher coagulant dosages and
lower pH values for enhanced coagulation. A few of these impacts include the following: