Encyclopedia of Environmental Science and Engineering, Volume I and II

980 PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS

A study on the processing of composite industrial effluent

by reverse osmosis was published by Sridhar et al. in 2003.

The effluent used in the study was from combined bulk drug

and pharmaceutical companies, obtaining a removal of 88%

of dissolved solids, COD, and BOD, with reasonable water

recovery. They also present a comparison between aerobic

and reverse-osmosis treatment for this effluent.

A physical-chemical process for the treatment of chemi-

cal mechanical polishing process wastewater is presented by

Lin and Yang (2004). In it the authors used chemical coagula-

tion using different coagulants followed by reverse osmosis,

obtaining water capable of being reused in the process due to

its characteristics.

Electrodialysis Electrodialysis involves the removal of

inorganic ions from water by creating an electrical potential

across two electrodes dipped in water. One of the two strips

serves as a cathode and the other as an anode. The treatments

that can be achieved by electrodialysis include:

1. Removal of inorganic ions: Under the effect of
   applied potential, cations and anions migrate to
   the cathode and anode, respectively. By alternat-
   ing membranes, a series of concentrating and
   diluting compartments can be created. For a long
   run and better efficiency, it is essential that turbid-
   ity, SS, colloids, and trace organics are removed
   from the wastewater before it enters the electrodi-
   alysis unit.


2. Effective bacteria reduction in wastewater: Most
   of the municipal wastewaters contain a high con-
   centration of chloride ions. Oxidation of chloride
   at the anode produces chlorine, hypochlorite,
   or chloramines, depending on the nature of the
   wastewater. Chlorine in these forms is a good dis-
   infectant and also provides an effective means of
   reducing soluble BOD.

In order to reduce the operating cost of the electrodialysis pro-

cess, the eroding anodes made of aluminum or iron are now

being replaced by nonconsumable noble anodes, which appear

to have more potential in wastewater treatment (Culp and Culp,

1971). The cost of disinfection by electrodialysis is reported to

be 0.053 $/m^3 of wastewater as compared to 0.095 $/m^3 for

the conventional chlorination (unpublished proposal, 1970).

However, some other sources have reported that the cost

of electrolytic treatment of wastewater was too high for the

removal of a large percentage of secondary effluent COD.

Grimm et al. (1998) present a review of electro-assisted

methods for water purification, including electrodialysis.

Fukumoto and Haga (2004) applied this technique for the

treatment of swine wastewater with removal rates for NO 3 − and

PO 4 −3 ions of 99% and an average color reduction of 58%.

Gas Stripping

In domestic wastewaters, most of the nitrogen that gets

converted to ammonia during biological degradation is

present either as ammonia or in organic form. When the

carbon concentration in wastewater becomes low and the

nitrifying bacteria are populous, this ammonia can be oxi-

dized by bacteria to nitrites and nitrates in the presence of

dissolved oxygen. The stripping process can be employed

either before or after secondary treatment for removing

high levels of nitrogen that is present as ammonia. If it

is to be used as pretreatment prior to a biological system,

enough nitrogen, N:BOD  5:150, must be left in the efflu-

ent to satisfy the nutritional requirement (Eckenfelder and

Barnhart, 1963).

In wastewater, ammonium ions exist in equilibrium with

ammonia and hydrogen ions:

NH 4 ↔ NH 3 ↑  H (10)

At pH levels of 6 to 8, ammonia nitrogen is mostly present

in the ionized form NH 4 . Increasing the pH to above 10

changes all the nitrogen to ammonia gas, which is remov-

able by agitation. The stripping of ammonia from wastewater

is carried out with air. In this operation, wastewater is agi-

tated vigorously in a forced-draft countercurrent air-stripping

tower when the ammonia is driven out from the solution and

leaves with the air exhausted from the tower. The efficiency

of ammonia removal in the stripping process depends upon

the pH, airflow rate, tower depth, and hydraulic loading to

the tower.

Slechta and Culp (1967) have shown experimentally that

the efficiency of the ammonia-stripping process is dependent

on the pH of the wastewater for pH values up to 10.8. However,

no significant increase in ammonia removals was achieved

by elevating the pH above 10. Kuhn (1956) had come to the

same conclusion. It has also been reported that the efficiency

of the ammonia-stripping process depends on maximizing the

air–water contact within the stripping tower. Higher ammo-

nia removals and lower air requirements were obtained with

a 40  50-mm packing than with a 100  100-mm packing.

Increased tower depth, which provides additional air–water

contact, results in greater ammonia removals and lower air

requirements. Ammonia removals of 90%, 95%, and 98%

were obtained at airflow rates of 1875, 3000, and 6000 m^3 per

cubic meter of wastewater, respectively.

Gas stripping is also used for removal of H 2 S and VOCs

from wastewater.

Ion Exchange

The ion-exchange process has been adopted successfully

in wastewater-treatment practice for removing most of the

inorganic dissolved salts. However, the cost of this method

for wastewater treatment cannot be justified unless the efflu-

ent water is required for multiple industrial municipal reuse.

One of the major applications of this technique is the treat-

ment of plating-industry wastewater, where the recovery of

chrome and the reuse of water make it an attractive choice

(Eckenfelder, 2000).

Gaffney et al. (1970) have reported that the modified

DESAL process, developed for treating acid mine drainage

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