972PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS
INTRODUCTIONThe substances in domestic and industrial wastewater having
significance in water-pollution control, disposal, and reuse
are (1) dissolved decomposable organic substances result-
ing in dissolved oxygen depletion in streams and estuaries
and/or causing taste and odor; (2) suspended organic solids
resulting in dissolved oxygen depletion; (3) inert suspended
solids (SS) causing turbidity and resulting in bottom sedi-
ment deposits; (4) toxic synthetic organic substances and
heavy metals; (5) oil, grease, and floating materials; (6) acids
and alkalis; and (7) dissolved salts, including nutrients like
phosphorus and nitrogen.
Conventional wastewater-treatment practices have been
oriented to the removal of grit and floating matter followed
by the removal of suspended and dissolved organic matter.
The removal of suspended matter has been achieved by
sedimentation, and the bulk of the soluble organic matter
is removed by biological oxidation and flocculation.
These processes, when carried out in combination, have
proved to be economical and effective means for remov-
ing organic matter from wastewaters. However, there are
certain disadvantages associated with them. These include
the following:- Biological process require considerable operating
control and often generate operating problems of
a complex nature. - Biological processes are easily upset by shock loads
and require time to regain efficient operation. - Biological processes are unable to remove certain
nutrients, heavy metals, and inorganic salts, when-
ever there is a requirement for their removal. - Many waste streams contain certain compounds
that do not respond to biological treatment or,
alternatively, require extensive pretreatment.
In the last decades, physical-chemical treatment of waste-
water has been studied both on laboratory and pilot-plant
scales with important industrial and municipal wastewater-
treatment applications. This type of treatment is used either as
a pretreatment, tertiary treatment, or advanced treatment given
to the effluent from secondary treatment, or as a substitute for
conventional biological treatment. In the latter case, it is found
to produce effluent of a quality at least equal to that produced
by conventional biological treatment.The first study on the treatability of raw wastewater
by physical-chemical processes was reported by Rudolfs
and Trubnick in 1935. In this study, solids were removed
by chemical coagulation with ferric chloride followed by
absorption of dissolved impurities with activated carbon.
Stander and Van Vuuren (1969) investigated the treat-
ment of raw wastewater in a pilot plant where solids removal
was achieved by primary sedimentation and chemical coagu-
lation with lime, and adsorption with activated carbon. Rizzo
and Schade (1969) have also reported results on the pilot-
plant treatment of raw wastewater with chemical coagulation
and anionic polymer and adsorption with activated carbon.
Zuckerman and Molof (1970) studied the efficiency of a
treatment system in which raw wastewater was lime-clarified
at high pH and then activated-carbon-treated: their results
showed that the chemical oxygen demand (COD) values of
the final effluent were significantly lower than those associ-
ated with good conventional treatment. Moreover, they con-
cluded that the removal of soluble organics with activated
carbon was enhanced because of the hydrolytic breakdown
of high-molecular-weight organic compounds, at a higher
pH value, which are absorbed more readily by activated
carbon.
Weber et al. (1970) investigated the chemical clarifi-
cation of primary effluent with ferric chloride followed
by activated-carbon adsorption. Their results showed that
65% of the organic matter present in primary effluent
was removed by chemical treatment with ferric chloride.
Overall removal of biochemical oxygen demand (BOD)
was reported as being consistently in the range of 95 to
97%. Final effluent from the system contained approxi-
mately 5 mg/l BOD as compared to 30 mg/l for the same
wastewater treated conventionally.
In another study, Villiers et al. (1971) showed that the
treatment of primary effluent by lime clarification and acti-
vated carbon, in a steady flow system, produced an efflu-
ent with total organic carbon (TOC) averaging 11 mg/l and
turbidity averaging less than 2. Phosphates and SS removal
were consistently 90% or better. These product characteris-
tics are comparable to those associated with products from
well-operated conventional treatment plants.
Shuckrow (1971) had developed a sewage-treatment
process involving chemical coagulation for SS removal, fol-
lowed by adsorption of soluble organics on powdered carbon.
The advantages cited for this process were (1) a total treat-
ment time of less than one hour, (2) a high-quality effluent,C016_005_r03.indd 972 11/22/2005 11:25:15 AM