REMOVAL OF CADMIUM AT HIGH AND LOW DOSAGES
BY AN EXTENDED AERATION PROCESS
William L. Lowe, Research Fellow
Anthony F. Gaudy, Jr., Professor
Department of Civil Engineering
University of Delaware
Newark, Delaware 19716
INTRODUCTION
The search for ways to minimize the production of excess sludge and safely dispose of that which
is produced continues to be of great interest in the pollution control field. Although fraught with
many problems, disposal on land is attractive in many instances and offers a means to approach the
ideal situation, i.e., direct recycling of organic matter for fertilizer value. However, the biomass
generated in the treatment process has been shown to possess the capability of accumulating conservative toxic pollutants such as heavy metals and, for obvious reasons, this property tends to negate
the practice of land disposal. The heavy metals may be routed to the animal food chain or, under
acidic conditions, they can be solubilized and become active contaminants of surface and groundwater [1]. In fact, municipal sludge has been shown to release a significant portion of its heavy metal
loading upon acidification [2]. The fact that biomass can remove and concentrate heavy metals from
solution and/or colloidal suspension can be helpful because the sludge is, in effect, a gratuitously-
manufactured sorbent for ridding the waste, at least partially, of its heavy metal load. Thus, there
are possibilities of developing biological processes for removal of heavy metals from wastes and such
processes have been suggested by several workers [3,4,5]. An extended aeration process, with modifications based on our laboratory studies, seems particularly effective for heavy metal removal.
For over a decade, a portion of the research effort in our laboratories has been concerned with
research germane to minimization of excess sludge production via concurrent wastewater purification
and autodigestion of biomass, i.e., the extended aeration process [6,7,8,9]. One of the process modifications arising from this work was the periodic acid hydrolysis of small portions of the sludge to
assist the autodigestion process and to control suspended solids levels in the system. Also, recently,
we have found that modification of the flow scheme of an extended aeration plant, so that the return
sludge is taken from a separate aerobic digester rather than directly from clarifier underflow, permits
closer control and more stable operation even without the benefits of periodic acid hydrolysis [10],
In Figure 1, this flow scheme is compared to the more common modes (A and B) of operation
for extended aeration plants. It is apparent that the suggested modification permits one to operate
an extended aeration plant with low overall specific growth rate (high mean cell retention time, 9C)
but also allows the option of using a faster growth rate (lower sludge age) in the purification reactor.
A bench scale pilot plant study using this flow scheme has shown it to have promise for very
stable operation with no sludge wasting and with some savings in volume of reaction fluid under
aeration [10]. Also, preliminary cost estimates show a decided savings in initial and O/M costs over
conventional plants [11]. In addition, we have recently reported results of studies in which the
hydrolytically-assisted extended aeration process was employed for removal and recovery of heavy
metals concurrently with wastewater purification [12]. Button and Gaudy employed an internal recycle system, like that shown in the top portion of Figure 1, and dosed it with increasing amounts
of cadmium (0.05 mg/1 -» 1.0 mg/1) added as Cd(NO,)2. It was found that the system could operate
for a significant period of time without leakage of the dosed cadmium and that the accumulation of
cadmium-laden biomass had little adverse effect on wastewater purification. It was also shown that
acid hydrolysis of a portion of the cadmium-laden sludge solubilized both the cadmium and the
biomass. Addition of sodium sulfide caused precipitation of the cadmium but not the solubilized
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