Page 245 - Water and Wastewater Engineering Design Principles and Practice

6-22 WATER AND WASTEWATER ENGINEERING

2. Because of the diversity of species that occur when alum and/or ferric chloride are hy-
drolyzed, and because natural waters will contain ions that will react with the base, it is
not practical to calculate the dose. In actual practice, the dose is determined by titration
of a water sample.

6-4 COAGULATION PRACTICE
The selection of the coagulant and the coagulant dose is a function of the characteristics of the
coagulant (including its price), the concentration and type of particles, characteristics of NOM,
water temperature, and other constituents of the raw water such as alkalinity and phosphorus.
There is no formal approach to incorporate this collection of variables in the selection process.
Jar test experiments and experience play a large role in the selection process. Some of the factors
to be considered in the decision process are discussed in the following paragraphs.

Overview
High turbidity, high alkalinity water is the easiest to coagulate. Alum, ferric chloride, and high
molecular weight polymers have been used successfully for these waters.
Control of the pH is of utmost importance in coagulating high turbidity, low alkalinity water.
Polymers function well. Addition of a base may be required for alum and ferric chloride.
Alum and ferric chloride at high doses can coagulate low turbidity, high alkalinity waters.
A combination of alum followed by polymer often works well. For this system, that is, low tur-
bidity and high alkalinity, polymers cannot work alone. Coagulant aids may be required.
Low turbidity, low alkalinity waters are the most difficult to coagulate. Neither polymers
nor alum/ferric chloride work alone when the turbidity and alkalinity are low. pH adjustment is
required. Direct filtration should be considered for this type of water.
Coagulation of color is very pH dependent. Alum, ferric chloride, and cationic polymers are effec-
tive at pH values in the range of 4 to 5. The floc that are formed in coagulating color are very fragile.

Coagulant Selection

Metal Salts. As noted previously, the most common coagulants are alum, ferric chloride, and
ferric sulfate. The predominant choice of coagulant is alum, followed by ferric chloride and fer-
ric sulfate, respectively. While cost may be the overriding factor, the operating region, as noted
in Figure 6-9 , plays a significant role in coagulant selection. Ferric chloride is effective over a
broader range. Polyaluminum chloride (PACl) is less sensitive to pH and can be used over a pH
range from 4.5 to 9.5 (MWH, 2005).
2 3
The metal salt hydrolysis products react with SO 4 , NOM, F , and PO to form both
4
soluble and insoluble products. This will result in a requirement for increased dosage to achieve
the desired destabilization.
Typical dosages of alum range from 10 to 150 mg/L. Ferric chloride and ferric sulfate dos-
ages range from 5 to 150 mg/L and from 10 to 250 mg/L respectively (MWH, 2005).
NOM removal is a means of reducing disinfection byproducts. In the regulatory language of
the U.S. EPA, enhanced coagulation is a recommended technique for removing NOM. Because
NOM binds with metal ion coagulants, this is a consideration in selecting a coagulant and the
dose to be applied. Of the metal salts and prehydrolyzed metal salts, the most effective for the
removal of NOM, in order of increasing effectiveness, are iron, alum and PACl (MWH, 2005).