AN INNOVATIVE COPRECIPITATION TECHNIQUE FOR
REMOVING HEAVY METALS
Ju-Chang Huang, Professor
Department of Civil Engineering
University of Missouri-Rolla
Rolla, Missouri 65401
Patrick M. McCole, Sanitary Engineer
John Carollo Engineers
Fountain Valley, California 92708
Rodney K. Breuer, Environmental Engineer
Department of Natural Resources
State of Missouri
Jefferson City, Missouri 65101
One of the major problems confronting today's wastewater treatment operation is the
disposal of the treatment plant sludge in a manner which is economically acceptable and
environmentally safe. This is especially true when the sludge contains some toxic materials
like heavy metals. Taking the municipal treatment plant sludge as an example, if the sludge
contains no toxic substances, it can be disposed of by land application. This practice is, in
most cases, cost effective and ecologically sound because it benefits agricultural operation.
However, if there are toxic metals present in the sludge at sufficient concentrations, the
land application practice would not be feasible because some toxic metals in sludge can
be taken up by food crops after the sludge is applied to farm land [ 1 ]. Therefore, in dealing
with heavy metal pollution problems, it is not only necessary to remove toxic metals from
wastewater, but there is also a genuine need to avoid accumulating these metals in the sludge
phase.
Unfortunately, the most common method being used today to remove heavy metals is
by chemical precipitation. This method is able to remove much of the toxic metal content
of the liquid waste stream. But most of the removed metals actually become part of the
wastewater sludge because this treatment approach merely induces the metals to precipitate
by reducing their solubilities and then enhancing their attachment to settleable solids
through adsorption and/or entrapment. As a consequence, this approach only solves part
of the toxic metal problem-removing the toxic metals from the liquid waste stream. At
the same time, it creates another problem by increasing the metal accumulations in the
sludge phase, which makes the sludge less acceptable for land disposal. Thus it is obvious
that a new technology needs to be developed that will not only remove metals from the
liquid waste, but will also not accumulate them in the wastewater treatment sludge.
The objective of this study was to develop such a novel technology capable of precipitating and removing toxic metals from wastewater without simultaneously removing the
organic suspended and settleable solids. To achieve this, an upflow-expanded sand bed was
used with lime feed to foster coprecipitation of metals, calcium carbonate, and calcium
hydroxyapatite on the sand grains. This new process was intended to remove toxic metals
ahead of conventional wastewater treatment, thus leaving the primary and secondary wastewater treatment sludges with lower metal contents.
The experimental work of this study consisted of evaluating the feasibility of the proposed technology for removing toxic heavy metals at four different pH's: 10.5, 10.0, 9.5,
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