Encyclopedia of Environmental Science and Engineering, Volume I and II

PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS 975

The precise zeta potential that yields optimum coagu-

lation must be determined for a given wastewater by actual

correlation with jar test or plant performance. The control

point has been reported to be in the range of 0 to −10 mV

when raw sewage is coagulated by alum. It is important that

coagulants contribute polyvalent ions of charge opposite to

the zeta potential of the dispersion. On a molar basis, biva-

lent ions seem to be about 10 to 50 times and trivalent ions

about 300 to 700 times as effective as monovalent ions for

destabilization of dispersion in wastewater (Rich, 1963). The

zeta potential is unaffected by pH in the range of 5.5 to 9.5

(Eckenfelder, 2000).

Since most dispersions encountered in wastewaters

are stabilized by negative charges, coagulants required are

polyvalent cations such as aluminum, ferric, ferrous, or cal-

cium. Organic polyelectrolytes are also effective coagulants.

Dispersions stabilized principally by electrostatic force are

in general amenable to coagulation inasmuch as addition of

small doses of suitable electrolytes may effect a significant

change in zeta potential of the particles. The most widely

used chemicals for coagulation of wastewater are the salts

of aluminum and iron. Lime alone has also been used for

precipitation of phosphates.

Aluminum Sulfate In order to form flocs, aluminum sulfate

requires the presence of alkalinity, which, if naturally present

in wastewater in the form of bicarbonate, would lead to the

following reaction:

Al 2 (SO 4 ) 3 .xH 2 O  3Ca(HCO 3 ) 2 →

2Al(OH) 3 ↓  3CaSO 4  xH 2 O  6CO 2 (1)

In case of insufficient alkalinity in the wastewater, lime is

generally added, and the reaction with alum becomes:

Al 2 (SO 4 ) 3 .xH 2 O  3Ca(OH) 2 →

2Al(OH) 3 ↓  3CaSO 4  xH 2 O (2)

In the presence of phosphate, the following reaction also

occurs:

Al 2 (SO 4 ) 3 .xH 2 O  2PO 4 -2 →

2AlPO 4 ↓  3SO 4 -2  xH 2 O (3)

Aluminum hydroxide flocs are least soluble at a pH of

approximately 7.0. The floc charge is positive below pH 7.6

and negative above pH 8.2 (Eckenfelder, 1966). The solubil-

ity of AlPO 4 is related to the pH and the equilibrium con-

stant for the salt. Stumm and Morgan (1970) state that the

solubility of aluminum phosphate is pH-dependent, and the

optimum pH for phosphorus removal lies in the range of 5.5

to 6.5. Generally, at pH above 6.3, the phosphate removal

occurs either by incorporation in a complex with aluminum

or adsorption on the aluminum hydroxide flocs. According to

Yuan and Hsu (1970), the reaction mechanism for precipita-

tion of phosphates by aluminum hydroxide is very complex.

They have proposed that the positively charged hydroxy-

aluminum polymers are the species that accounts for the

precipitation of phosphates and that effective phosphate pre-

cipitation can occur only when the positive charges on the

polymers are completely neutralized. It is also reported that

the effectiveness of aluminum is related to the nature and

concentration of the foreign components present and to the

ratio of phosphate to aluminum.

Alum has been used extensively for phosphate removal

in raw wastewaters. Bench-scale tests of alum addition were

conducted at Springfield, Ohio, and Two Rivers, Wisconsin

(Harriger and Hoffman, 1971 and 1970, respectively). Raw

wastewater at Springfield required an average Al:P mass ratio

of 1.9:1 to achieve 80% removal, while at Two Rivers the

average mass ratio was 0.93:1 to obtain phosphate removal

of 85%. The stoichiometric equation (3) indicates that each

kilogram of phosphorus requires 0.87 kg of aluminum for

complete precipitation.

Ferrous Sulfate and Lime If ferrous salts are used for

wastewater coagulation, addition of a small amount of base,

usually sodium hydroxide or lime, is essential. The required

dosage is related to the alkalinity of water. Ferrous sulfate

reacts with calcium bicarbonate in water, but this reaction

is much delayed and therefore cannot be relied on (Steel,

1960). Caustic alkalinity, due to the addition of lime to the

wastewater, produces a speedy reaction. The lime is added

first, and the following reaction takes place:

FeSO 4 .7H 2 O  Ca(OH) 2 → Fe(OH) 2  CaSO 4  7H 2 O (4)

The ferrous hydroxide is not an efficient floc, but it can soon

be oxidized by the dissolved oxygen in wastewater as ferric

hydroxide:

4Fe(OH) 2  O 2  2H 2 O → 4Fe(OH) 3 ↓ (5)

An insoluble hydrous ferric oxide is produced over a pH

range of 3 to 13. The floc charge is positive in the acid range

and negative in the alkaline range, with a mixed charge over

a pH range of 6.5 to 8.0. This process is usually cheaper than

the use of alum but needs greater skill to dose with the two

chemicals.

Ferric Chloride Ferric chloride has been used successfully

for wastewater coagulation because it works well in a wide

pH range (Steel, 1960; Wuhrmann, 1968). The reactions

of ferric chloride with bicarbonate alkalinity and lime are,

respectively:

2FeCl 3  3Ca(HCO 3 ) 2 → 2Fe(OH) 3 ↓  3CaCl 2

 6CO 2 (6)

2FeCl 3  3Ca(OH) 2 → 2Fe(OH) 3 ↓  3CaCl 2 (7)

Wuhrmann (1968) was successful in removing phosphates

from sewage effluent by precipitation with a mixture of

ferric salt and lime. The ferric dosages varied between 10

and 20 mg/l and the lime dosages from 300 to 350 mg/l

in order to raise the pH to values between 8 and 8.3. The

actual lime dosage required is related to the alkalinity of

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