Page 110 - Plant design and economics for chemical engineers

86 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

solute such as hydrogen-bonding and valency affect the membrane selectivity. In
contrast to these two membrane processes, electrodialysis employs the removal
of the solute (with some small amount of accompanying water) from solution
rather than the removal of the solvent. The other major distinction is that only
ionic species are removed. The advantages due to electrodialysis are primarily
due to these distinctions.

CHEMICAL TREATMENT. In wastewater treatment, chemical methods are gen-
erally used to remove colloidal matter, color, odors, acids, alkalies, heavy
metals, and oil. Such treatment is considered as a means of stream upgrading by
coagulation, emulsion breaking, precipitation, or neutralization.
Coagulation is a process that removes colloids from water by the addition
of chemicals. The chemicals upset the stability of the system by neutralizing the
colloid charge. Additives commonly used introduce a large multivalent cation
such as Al+3 (from alum) or Fe+3 (from ferric chloride). Emulsion breaking is
similar to coagulation. The emulsions are generally broken with combination of
acidic reagents and polyelectrolytes. The common ion effect can also be useful
in wastewater treatment. In this case an unwanted salt is removed from solution
by adding a second soluble salt to increase one of the ion concentrations.
Coagulant aids may then be needed to remove the precipitate.
One method for treating acid and alkaline waste products is by neutraliza-
tion with lime or sulfuric acid (other available materials may also be suitable).
Even though this treatment method may change the pH of the waste stream to
the desired level, it does not remove the sulfate, chloride, or other ions.
Therefore, the possibility of recovering the acid or alkali by distillation, concen-
tration, or in the form of a useful salt should always be considered before
neutralization or dilution methods are adopted.
Chemical oxidation is frequently another tool in wastewater treatment.
Chemical oxidants in wide use today are chlorine, ozone, and hydrogen perox-
ide. The historical use of chlorine and ozone has been in the disinfection of
water and wastewater. All three oxidizers are, however, receiving increased
attention for removing organic materials from wastewaters that are resistant to
biological or other treatment processes. The destruction of cyanide and phenols
by chlorine oxidation is well known in waste-treatment technology. However,
the use of chlorine for such applications has come under intense scrutiny
because of the uncertainty in establishing and predicting the products of the
chlorine oxidation reactions and their relative toxicity. Ozone, on the other
hand, with only a short half-life is found to be effective in many applications for
color removal, disinfection, taste and odor removal, iron and manganese re-
moval, and in the oxidation of many complex inorganics, including lindane,
aldrin, surfactants, cyanides, phenols, and organo-metal complexes. With the
latter, the metal ion is released and can be removed by precipitation.
The need for chemical reduction of wastewaters occurs less often. The
most common reducing agents are ferrous chloride or sulfate which may be
obtained from a variety of sources.