DISTRIBUTION OF CADMIUM AND NICKEL
IN ACTIVATED SLUDGE SYSTEMS
Prasad S. Kodukula, Instructor
James W. Patterson, Professor
Department of Environmental Engineering
Illinois Institute of Technology
Chicago, Illinois 60616
INTRODUCTION
In recent years, there has been considerable interest in the topic of heavy metals in sewage treatment plants with numerous studies being reported in the literature on their source, fate, effect, and
control [1-8]. Whereas several investigators have attempted to characterize adsorption of metals to
well-defined oxide surfaces [9-14] and, more recently, to estuarine particulate matter [15], there seems
to be a relative lack of information on fundamental aspects of adsorption processes controlling the
removals of heavy metals in activated sludge systems. An understanding of the fate of heavy metals in
the activated sludge treatment system requires characterization of metal distribution in terms of the
various physical, chemical, and biological interactions during each stage of the treatment. The extent
and affinity of such interactions is a complex function of intrinsic variables, such as, the individual
metal, its concentration, and presence of other metals; the physical-chemical characteristics of the
aqueous medium, such as, temperature, solids content, pH, alkalinity and complexing ligands; and
operational factors, including solids retention time.
Among the mechanisms which are reported to affect the metal distribution between the soluble
and solid phases in an activated sludge system are: adsorption, complexation, ion-exchange, precipitation, and biological uptake. Several investigators [16,17] have concluded that biological uptake metals
is rather slow and insignificant compared to the physical-chemical mechanisms influencing the metals
distribution.
Whereas Jenkins et al. [18,19] attributed the observed metals removals in raw sewage and digested
sludge primarily to precipitation, most investigators in recent years have assumed that precipitation is
insignificant in their studies involving adsorption in activated sludge mixed liquor [17,20]. However,
Brown and Lester [21] recently suggested that both precipitation and adsorption of metals have taken
place in their batch cultures of K. aerogenes. an organism commonly found in activated sludge.
A thorough evaluation of experimental procedures and results reported in the literature pertaining
to the characterization of metal adsorption processes and description of these processes by isotherm
models raises several interesting questions, which need to be answered for a better understanding of
the adsorption process in activated sludge systems: Is the assumption that the test liquid is in equilibrium (forward adsorption rate = backward desorption rate) with metal after a few hours of contact
time valid? How do pH and concentration of total metal and suspended solids affect the adsorption
behavior of metals? Is precipitation a significant factor affecting the distribution of metals? This investigation makes an attempt to answer these questions.
CONCEPTUAL MODEL
In order to be able to characterize the adsorption process and its relation with precipitation in
solid-liquid suspensions, a conceptual model is proposed here, as described below.
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