Encyclopedia of Environmental Science and Engineering, Volume I and II

PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS 979

membranes and membrane reactors in green technology and

water reuse was published by Howell (2004). In it, water

problems in different regions of the world are discussed,

different membrane systems are presented, and different

approaches for new research are introduced.

Reverse Osmosis There are several reverse-osmosis units cur-

rently in use to produce freshwater from seawater. With recent

improvements in membranes, this process is also being used

for purification of wastewater. Substantial removal of BOD,

COD, total dissolved solids, phosphate, and ammonia by this

process has been reported (Robinson and Maltson, 1967).

In a reverse-osmosis process, wastewater containing dis-

solved materials is placed in contact with a suitable semi-

permeable membrane in one of the two compartments of

the tank. The pressure on this compartment is increased to

exceed the osmotic pressure for that particular waste in order

to cause the water to penetrate the membrane, carrying with

it only a small amount of dissolved materials. Therefore, the

dissolved material in the wastewater gets concentrated con-

tinuously, while highly purified water collects in the other

compartment.

The performance of the reverse-osmosis process depends

mainly on (1) the membrane semipermeability or its effi-

ciency to separate dissolved material from the wastewater,

and (2) the membrane permeability or the total amount of

water that can be produced with appropriate efficiency for

the removal of dissolved materials. It has been reported that

the conventional cellulose-acetate membranes give adequate

separation efficiency, but the flow rate of water is too small

to be of practical interest. However, cellulose-acetate mem-

branes allow a much higher flow rate of product water, at

the same separation efficiency, which makes it applicable in

wastewater-treatment practices (Goff and Gloyne, 1970).

The operating pressure, as well as the rejection perfor-

mance of the membrane, is dependent on the membrane

porosity. Rejection performances of three graded mem-

branes with secondary sewage effluent were investigated by

Bray et al. (1969).

Merten et al. (1968) evaluated the performance of an

18.9 m^3 /day pilot reverse-osmosis unit in removing small

amounts of organic material found in the effluent of carbon

columns treating secondary effluent. With a feed pressure

of 2760 kPa and water recovery of 80 to 85%, 84% removal

of COD present in the carbon column effluent, averaging

10.8 mg/l, was achieved. Problems of clogging have occurred

when operating with waters containing high concentrations

of bicarbonate, and as such, adjustment of pH to prevent

calcium-carbonate precipitation is normally required.

Sadr Ghayeni et al. (1996) discuss issues such as flux

control and transmission in microfiltration membranes and

biofouling in reverse-osmosis membranes in their use for

the reclamation of secondary effluents. The process used

for the study consisted of a microfiltration membrane fol-

lowed by a reverse-osmosis membrane. The performance

of this combined system was evaluated by Sadr Ghayeni et

al. (1998b), as was the study of the adhesion of bacteria to

reverse-osmosis membranes (1998a).

Effluent

Transfer pipe

Storage

compartment

Filter

compartment

Collection

chamber

Air Underdrain nozzles

Recycle

Backwash

Equalization tank

Drain

Sump

Coal

Sand

Filter

backwash

Polymer

Three-way

valve

Water level

Alum

Influent

Filter

Backwash

FIGURE 2 Typical dual-media filter (From Eckenfelder, 2000. Reprinted with permission

from McGraw-Hill.)

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