United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268 '/
Research and Development
EPA-600/S2-82-053 August 1982
Project Summary
An Innovative Coprecipitation
Technique for Removing Heavy
Metals from Wastewater
Ju-Chang Huang
This project attempts to develop
innovative technology to remove toxic
heavy metals from raw municipal waste-
water without simultaneously removing
the largely organic suspended and settle-
able solids. The object was to reduce the
metal concentrations In the sludge pro-
duced by subsequent wastewatar treat-
ment. An upflow expanded sand bed was
used with lime feed to cause copreclpita-
tion of metals and calcium carbonate on
the surfaces of sand grains. Through
continuous plating of the precipitates,
the sand grains increased In size and
eventually formed large pebbles a few
millimeters in diameter that could be
easily handled in the ultimate disposal
process.
Six heavy metals were selected for
this study: Cd. Cr, Cu, Ni, Pb, and Zn.
Metal removal efficiency was evaluated
at four pH levels from 9.0 to 10.5. Other
operational parameters such as reduc-
tions of chemical oxygen demand (COD),
suspended solids (SS), volatile suspended
solids (VSS), and total phosphorus
(total-P) were also Included.
The minimum pH for achieving reason-
ably effective coprecipitation was 9.5,
but the highest efficiency was obtained
at pH 10.0 to 10.5. At pH 10.5, the
metal precipitator was able to remove
70% to 92% of the soluble Cd, Cu, Ni,
and Pb from a spiked tap water. In muni-
cipal wastewater (where less than 10%
of the metals occurred In a spluble state),
the removal efficiencies were substan-
tially reduced. For a municipal waste-
water with a typically heavy metal con-
tent, the experimental precipitator was
able to reduce Cd 38%, Cr 36%, Cu
44%, NI 33%, Pb 41 %, and Zn 42% at
an operating pH of 10.5. The accompa-
nying reductions for COD, SS, VSS, and
total-P were 21%, 38%, 43%, and
64%, respectively.
Since the percentages of metals and
SS removed were approximately the
same, the hoped-for reductions in metal
concentrations in sludge are unlikely to
be achieved.
This Project Summary was developed
by EPA's Municipal environmental Re-
search Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a sep-
arate report of the same title fsee Project
Report ordering information at back).
Introduction
One of the major problems confronting
today's wastewater treatment operation
is the disposal of the treatment plant
sludge in a manner that is economically
acceptable and environmentally safe. If
the sludge contains low levels of toxic
substances, it can be disposed of by land
application. In most cases, this practice
is cost effective and ecologically sound
because it benefits agricultural operation.
But if toxic metals are present in the
sludge in sufficient concentrations, the
land application practice is not feasible
because some toxic metals in sludge can
be taken up by food crops after the
sludge is applied to farmland. Thus to
deal with heavy metal pollution, toxic
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metals must be removed from waste-
water, and accumulations of these
metals must also be avoided in the
sludge phase.
Chemical precipitation is currently the
most common method for removing
heavy metals. Though this method can
remove much of the toxic metal content
of the liquid waste stream, most of the
removed metals become part of the
wastewater sludge. Thus this approach
solves only part of the toxic metal prob-
lem and creates an additional problem by
increasing the metal accumulations in
the sludge phase. A new technology is
needed to remove metals from the liquid
waste, and to prevent their accumulation
in the wastewater treatment sludge.
The object of this study was to develop
such a novel technology for precipitating
and removing toxic metals from waste-
water without simultaneously removing
the organic suspended and settleable
solids. This new process was intended
to remove toxic metals ahead of conven-
tional wastewater treatment, thus leaving
the primary and secondary wastewater
treatment sludges with lower metal
contents.
Experimental Approach
Six vertical plexiglas columns 5.7 cm
in diameter and 2.1m high with tapered
conical bottoms were used as metal pre-
cipitators. The columns were divided into
two parallel sets, each of which consisted
of three columns connected in series for
upflow operation. Each column con-
tained 0.9 m of 40- to 100-mesh filter
sand. A variable-speed pump was used
to pump wastewater into the bottom of
the first column in each set. The flow
rate was controlled at 650 ml/min to
achieve a 48% sand bed expansion.
Another variable-speed pump was used
to inject lime into the wastewater. In the
initial process evaluation, lime was in-
jected before the wastewater entered
the bottom of the first column. But much
more efficient metal removal was ob-
tained by injection of lime directly into
the fluidized sand bed, so the systems
were modified accordingly. One set of
columns was dosed with alkaline tap
water at pH 10.5 to precoat the sand
grains with calcium carbonate before
metal removal studies began.
The two systems were first operated
at pH 10.5 to evaluate the potential for
removing soluble metals from spiked
tapwater. The systems were then oper-
ated at pH 10.5 and pH 10.0 to remove
largely insoluble metals from spiked, de-
gritted municipal wastewater with lime
injection ahead of the first sand columns.
After the lime feed was relocated within
the sand beds, operation was re-evaluated
on the spiked municipal wastewater at
pH 10.5 and 10.0 and further evaluated
at pH 9.5 and 9.0. Removals of six
metals were evaluated: Cd, Cr (VI), Cu,
Ni, Pb, and Zn. For wastewaters, remov-
als of COD, SS, VSS, and total-P were
also measured.
Experimental Results
The two systems were able to remove
about 70% to 90% of the soluble Cd,
Cu, Ni, and Pb from the spiked tapwater.
Removals of Cr and Zn were quite erratic
and averaged < 30%. No consistent dif-
ferences were observed between the
precoated and the other system.
When lime was injected ahead of the
sand beds treating wastewater, metal
removals were relatively poor and quite
erratic. On occasion, negative removals
attributed to washout of SS occurred.
After relocation of the lime feed point,
metal removals were greatly improved,
and negative removals no longer occurred.
The data for pH 10.5 are as follows:
Parameter
Cd
Cr
Cu
Ni
Pb
Zn
COD ....
SS
VSS
% Removal
38
36
44
33
41
42
21
38
43
Total-P 64
Operation at the lower pH values gen-
erally resulted in decreased removals.
Conclusions
This feasibility study showed that the
coprecipitation process removed ap-
proximately the same percentages of
both metals and SS. The desired reduc-
tions of metal concentrations in the
sludge are therefore unlikely to be
achieved.
The full report was submitted in fulfill-
ment of Grant No. R-806467-01 by the
University of Missouri-Rolla under the
sponsorship of the U.S. Environmental
Protection Agency.
Ju-Chang Huang is with the Environmental Research Center. University of
Missouri-Rolla. Rolla. MO 65401.
Sidney A. Hannah is the EPA Project Officer (see below).
The complete report, entitled "An Innovative Coprecipitation Technique for
Removing Heavy Metals from Wastewater," (Order No. PB 82-227 240;
Cost: $10.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
t, U.S OOVERNMENTPBINTINa OFFICE-1982-559-017/0769
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