The high demands on water
resources and landfills resulting from increased industrialization
and population growth are problems facing the global community.
As industrial discharges continue to grow and the cost of treating
such discharges continues to rise, there is a growing need for new
technologies and products that are efficient, cost effective and
protective of the environment and public health. The EPA’s
new proposed Metal Products and Machinery (MP&M) effluent limits,
published January 3, 2001 (66 FR 423), have placed an extra burden
on industries and presented wastewater treatment professionals with
new challenges, namely, to come up with economically feasible and
innovative technologies to meet the limits.
The
wastewater treatment program established within a facility is influenced
by factors such as the nature and volume of the waste streams, discharge
limits, available space and the generated waste. The nature and
volume of the waste stream and discharge limits will influence the
choice of equipment and treatment chemicals. The overall cost of
treatment involves operational costs such as energy, labor, treatment
chemicals, maintenance and waste disposal. The Resource Conservation
and Recovery Act (RCRA) prohibits disposal of sludge/solid waste
that does not pass Toxicity Characteristics Leaching Procedure (TCLP).
This type of waste is hauled away and treated at waste treatment
facilities prior to being landfilled.
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1 - Treatment Setup
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Industries
that discharge metal-bearing effluent most likely will not be able
to meet the proposed limits using conventional hydroxide precipitation
alone, which is considered the best available technology (BAT).
The industries will have to introduce ion exchange, reverse osmosis
or implement a secondary step using a polishing agent such as dithiocarbamate
(DTC), in order to achieve the proposed limits. Recently, it has
been reported that metal-DTC waste (cadmium, copper, lead and mercury)
is unstable, and under the conditions of TCLP testing, the leachate
showed levels exceeding RCRA limits. DTC is toxic, and it has deleterious
effects on the environment, e.g., the case of the dithiocarbamates
spills into the White River in Anderson, Indiana in 1999 that killed
more than 80,000 fish along a 50 mile stretch.
Aquasil®
is a single-step treatment with enhanced efficiency in removing
contaminants such as heavy metals, oil and grease, organic matters
and suspended solids simultaneously. It employs proprietary advanced
materials that are made to fit the chemistry of the particular waste
stream. The products are used in both batch and continuous processes,
and effluent can be treated at low, neutral or moderately high pH.
It overcomes problems associated with hardness and, in most cases,
the presence of complexing or chelating agents, surfactants and
detergents. Figure 1 illustrates a continuous setup of the treatment.
Case
Studies
Case 1: Circuit Board
A circuit board manufacturer in the Midwest generates about 28,400
liters (7,500 gal) of wastewater daily. The waste stream is acidic
(pH = 1.8) and contains mainly copper and lead, with tin and nickel
as minor metallic components. Additionally, the waste stream also
contains several cleaners that have various chelating agents. The
treatment employs hydroxide precipitation along with dithiocarbamate
(DTC), due to the presence of chelates. The facility is looking
for an alternative technology that can eliminate the use of DTC
and bring about compliance with the EPA’ s proposed MP&M
effluent limits.
The
acidic waste stream was treated with the new treatment without any
pH adjustment. Analytical data shown in Table
1 indicate that the treatment brought the levels of copper and
lead below the new MP&M limits. The treatment produced large,
dense floc that settled efficiently. Moreover, the treatment eliminated
DTC, caustic and polymer. The treated water can be reused. The treatment
also generates less waste, which passes the TCLP test.
Case
2: Metal Finishing
A plating facility on the East Coast has an acidic waste stream
(pH about 2), that comes from two sources, cyanide and non-cyanide
rinses. The waste contains mainly nickel and copper along with some
lead and silver. Cyanide is destroyed with calcium hypochlorite
at a pH between 9-11. The non-cyanide rinses are neutralized with
Mg(OH)2 (magnesium hydroxide) to maintain a pH of about 5.25-8.
Both cyanide and non-cyanide rinses are collected in two 2,500-gal
equalization tanks. The wastewater is pumped from equalization tanks
to a reaction tank such that a ratio of cyanide to non-cyanide of
1:3 is maintained.
The
pH is adjusted to 9.9-10.4, and a coagulant is added. The wastewater
then flows to a flash-mixing tank where a flocculent is added and
then to a clarifier. Sludge from the clarifier is pumped to a sludge
holding tank and dewatered in a filter press. Treated water flows
to the final tank where pH is adjusted to between 6.5-9.5 prior
to discharge.
The
treatability study was done on the wastewater composite (pH = 2-3)
without Mg(OH)2 neutralization. The wastewater was treated directly.
Large, dense floc formed within a few minutes and settled quite
efficiently. The treated effluent was clear and free of pin floc.
Filtration was easy and filter cake dried well. Final pH was about
9.5-10. Treatment eliminated the use of magnesium hydroxide, caustic,
coagulant and flocculent.
Metal
concentrations in the effluent are far below the proposed MP&M
limits. Table 2 shows analytical results of
raw and treated effluent. Moreover, the treatment generated a much
drier cake.
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| Figure
2 - Results at Finishing Facility
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Case
3: Aircraft Maintenance Facility
An
aircraft maintenance and service facility in the Southeast generates
an oily waste that contains high levels of heavy metals, emulsified
oil and heavy petroleum oil. Through an environmental initiative,
the facility was required to implement an innovative technology
into its operation. The proposed technology was evaluated for environmental
benefits, labor and cost savings, and the ability to interface with
the various routine operations.
A
treatability study was conducted on a sample of the oily waste.
As a result, one treatment was chosen as the most cost effective
technology, and a fully automated batch system was installed to
treat the oily waste.
Table
3 shows that the system delivered results far below discharge
requirements for all parameters. The system achieved almost 100%
percent removal of heavy metals and oil and grease and more than
99% of suspended solids. The system requires very little maintenance
and only minimal operator attention.
Before
and after treatment analytical data for the above cases were provided
by the respective facilities.
All
conventional techniques used in the treatment of waste streams produce
sludge/waste that can be hazardous due high concentrations of contaminants.
Safe disposal of such waste is troublesome and the problem is aggravated
by the continued increase in the number of wastewater treatment
plants. Currently, waste is transported to treatment plants where
it undergoes further treatment to render it suitable for disposal.
This adds to the overall cost of treatment.
Waste
generated by this particular treatment is already stabilized and
does not require any further treatment to render it suitable for
disposal in a landfill. Characteristic waste passes the TCLP test
and meets regulatory requirements for disposal as nonhazardous material.
Listed waste also passes the TCLP test and can be disposed of likewise
once exclusion has been granted. Table 4 shows
TCLP test results for the aircraft maintenance waste (see case 3).
This
technology is a fast process with enhanced efficiency in removing
contaminants such as heavy metals, oil and grease, organic matters
and TSS, simultaneously. The treatment lowers the levels of chromium
(VI), arsenic, selenium, phosphate, fluoride, TDS, COD and phenol.
The treatment is easy to implement and operate, effective, requires
less energy and maintenance, delivers high-quality effluent, achieves
zero-discharge through the recycle/reuse, and generates nonhazardous
waste that does not need further treatment prior to disposal. Moreover,
it brings about compliance.
References
- Manahan
S.E., Environmental Chemistry, 6th Edition, Lewis Publishers,
Boca Raton, FL, 1994.
- Wall
E., Henke K., Matlock M., and Atwood D., “Aqueous Leaching
Properties and Environmental Implications of Cadmium, Copper,
Lead, and Mercury Dimethyl-dithiocarbamate Compounds,”
Symposium Paper Presented Before the Division of Environmental
Chemistry, American Chemical Society, April 2001.
- Hering,
J.G. and F.M. Morel, “Kinetics of Trace Metal Complexation:
Ligand-Exchange Reactions,” Environ. Sci. Techn., Vol.
24, pp242-252 (1990).
- Huang,
Y. C. and S. S. Koseoglu, “Separation of Heavy Metals
from Industrial Waste Streams by Membrane Separation Technology,”
Waste Management, Vol. 13, p481 (1993).
- Amer,
S.I., “Shortcut to Success,” Environmental Protection,
September 1998.
- “Identification
and Listing of Hazardous Waste,” (1986) Code of Federal
Regulations, 40, July 1, Part 261, U.S. Government Printing
Office, Washington, DC, pp. 359-408.
- “Toxicity
Characteristic Leaching Procedure,” (1986) Test method
1311 in Test Methods for Evaluation of Solid Waste, Physical/Chemical
Methods, EPA Publication SW-846, 3rd ed., (November), as
amended by Updates I,II,IIA, U.S. Government Printing Office,
Washington, DC.
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