Encyclopedia of Environmental Science and Engineering, Volume I and II

974 PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS

cost of the coagulants and the difficulty of disposing of the

larger amount of the sludge produced by this process caused

it to be abandoned. The revival of chemical treatment can be

attributed to a number of factors that accumulated as a result

of continuous investigations and reevaluation of the process.

These are (1) the decrease in cost of chemicals; (2) better

understanding of floc formation and the factors affecting

it; (3) the development of methods of sludge filtration and

processing that overcome, in part, the difficulty of greater

sludge bulk; and (4) the establishment of the relationship

between eutrophication in streams and nutrients, particularly

phosphorus, nitrogen oxides, and organic matter. This rela-

tion establishes the need of final effluent wastes free of such

pollutants regardless of the cost of additional treatment.

The settling velocities of finely divided and colloidal

particles in wastewaters are so small that removing them

in a settling tank under ordinary conditions is impossible

unless very long detention periods are provided. Therefore,

it has been necessary to devise means to coagulate these

very small particles into larger ones that will have higher

settling velocities. The aggregation of dispersed particles in

wastewater is induced by addition of chemical coagulants to

decrease the effects of stabilizing factors such as hydration

and zeta potential, and by agitation of the medium to encour-

age collisions between particles.

Because of the greater amount of suspended matter in

sewage, the doses for chemical coagulants are generally con-

siderably greater. Therefore, in order to keep costs down, it

is important that the chemical reaction involved with each

coagulant should be known and enhanced and that optimum

pH values be obtained by adjustment with acid or base to get

more efficient coagulation and clarification with least sludge

production.

Coagulation Coagulation is a process in which chemicals

are added to an aqueous system for the purpose of creat-

ing rapid-settling aggregates out of finely divided, dispersed

matter with slow or negligible setting velocities. The poten-

tial applications of this process in treating wastewater are:

(1) direct coagulation of organic matter present mostly as

colloidal particles in wastewater; (2) the removal of colloi-

dal substances prior to such tertiary treatment processes as

ion exchange, carbon adsorption, and sand filtration; (3) the

removal of colloidal precipitates formed in phosphate pre-

cipitation processes; and (4) the removal of dispersed micro-

organisms after a brief biooxidation process.

The majority of colloids in domestic wastewater or in

organic wastes are of a hydrophilic nature; that is, they have

an affinity for water. The affinity of hydrophilic particles for

water results from the presence of certain polar groups such

as −COOH and −NH 2 on the surface of the particles. These

groups are water-soluble and, as such, attract and hold a sheath

of water firmly around the particle. The primary charge on

hydrophilic colloidal particles may arise from ionization of the

chemical groups present at the surface of the particles, e.g., car-

boxyl, amino, sulfate, and hydroxyl. This charge is dependent

upon the extent to which these surface groups ionize, and thus

the particle charge depends upon the pH.

Equalization

Raw

wastewater

Spill pond

Filtration

Precipi-

tation

Oxidation/

reduction

Heavy

metals

Process

wastewaters

Organic

chemicals

Organics,

ammonia

In-plant treatment

Centrifugation

Drying

Sludge disposal Incineration

Lagooning

Land disposal

Filtration Sludge digestion

Gravity

thickening

Dissolved-air

flotation

Sludgedewatering

Air or

steam

stripping

To discharge or POTW

GAC

adsorption

Oxi-

dation

Neutralization

Coagulation

Filtration

Anaerobic

treatment Activated

sludge

Ozonation

PAC coagulant

PACT

Nitrification/

denitrification

RBC

TricklingAerated lagoon

filter

To publicly owned

treatment works (POTW)

Primary treatment

Acid or

alkali ChemicalsFlotation

mentationSedi-

Filtration

GAC

adsorption

Discharge to

receiving water

Secondary

treatment

Biological Tertiary treatment

roughing

Wastewater

Return flows

Sludge

GAC and PAC: granular and powdered activated carbon

RBC: rotating biological contractor

FIGURE 1 Alternative technologies for wastewater treatment (From Eckenfelder, 2000. Reprinted with

permission from McGraw-Hill.)

C016_005_r03.indd 974 11/22/2005 11:25:16 AM