28    ENHANCED METAL REMOVAL FROM
WASTEWATER BY COAGULANT ADDITION
K.G. Karthikeyan, Graduate Research Assistant
Department of Agricultural & Biological Engineering
Herschel A. Elliott, Professor of Agricultural Engineering
Chairman, Environmental Pollution Control Program
Fred S. Cannon, Assistant Professor
Department of Civil & Environmental Engineering
The Pennsylvania State University
University Park, Pennsylvania 16802
INTRODUCTION
Besides metallurgical industries, metal-containing wastewaters are generated in the manufacturing/processing of batteries, petroleum, photographic materials, paints, inks, leather, and wood.
The heavy metals of concern are cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel
(Ni), silver (Ag), and zinc (Zn). The volume of metal-laden sludges, wastewaters, and treatment
residues generated in 1984 was about 5.8 billion gallons.1 A large number of publicly owned
treatment works (POTWs) also accept metal-containing wastewater from industries.2 The toxic
nature of the heavy metals has resulted in the promulgation of standards requiring very low concentration of metals in the treated effluent. To comply with the strict regulatory requirements, it
is necessary to treat the wastewaters (both industrial and municipal) before discharging them into
natural water bodies.
Simple precipitative removal of metals as insoluble hydroxides, carbonates, or sulfides is used
by about 75% of the electroplating facilities to treat the wastewater. Removal as the hydroxide,
accomplished by increasing the pH using lime, caustic, or soda ash, is the most common precipitation method.3 Hydroxide precipitation, however, requires a high operating pH and produces voluminous sludge that can be resolubilized if the pH is reduced. Also, the presence of soluble and
insoluble complexing agents can reduce the concentration of free metal ions in solution thereby
reducing the efficiency of the precipitation process.3 In an activated sludge basin about 50% of
the metals present in the wastewater are adsorbed by the biomass.4 Some POTWs are facing
strict effluent standards that will require greater metal removal than normally achieved by adsorption onto the biomass. Satisfactory operation of a biological wastewater treatment system
also requires low concentration of heavy metals in the activated sludge basin.
Adsorption of divalent metal ions onto a colloidal hydrous oxide surface occurs at a lower pH
than simple precipitation.5 The adsorption process involves the formation of the colloidal hydrous oxide surface prior to contact with the heavy metals to be removed. The pH-dependent removal of metals from aqueous solution by precipitation and adsorption follows a sigmoidal profile.6 A schematic illustration of the enhancement of metal removal by adsorption as compared to
that achieved by simple precipitation is shown in Figure 1, which is representative of the metal
removal profiles obtained in this study. This enhancement is possible even at low hydrous oxide
dosages. As the hydrous oxide dosage is increased, the adsorption profile is shifted in the direction of lower pH. Numerous studies on trace metal adsorption by hydrous oxides of Al and Fe
exist in the literature.5'" However, this study is different as metal removal by adsorption and co-
precipitation onto these hydrous oxides is compared.
In the present context, coprecipitation is the formation of a mixed solid phase by incorporation
of a heavy metal ion into the crystal lattice of another precipitating solid phase [e.g., Cd2+ incor-
50th Purdue Industrial Waste Conference Proceedings. 1995, Ann Arbor Press, Inc.. Chelsea, Michigan 48118.
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