625582018
vvEPA
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Technology Transfer
Environmental Pollution
Control Alternatives:
Sludge Handling,
Dewatering, and Disposal
Alternatives for the
Metal Finishing Industry
-------
Technology Transfer EPA 625/5-82-0?F
Environmental Pollution
Control Alternatives:
Sludge Handling,
Dewatering, and Disposal
Alternatives for the
Metal Finishing Industry
October 1982
U.S. Environmental Protection Agency
Region V. '/v-.ry
230 South Doaibcrn Street
Chicago, Illinois 60604
Technical content of this report was provided by the
Industrial Environmental Research Laboratory
Cincinnati OH 45268
-------
This alternatives report was prepared by Centec Corporation of Reston VA for
the Industrial Environmental Research Laboratory's Nonferrous Metals
and Minerals Branch in Cincinnati OH. The EPA Project Officer is
Alfred B. Craig, Jr.
EPA thanks the following companies and organizations for providing
information and assistance: Barrett Centrifugals, Chemical Waste Manage-
ment, Inc., Industrial Filter and Pump Manufacturing Company, Komline-
Sanderson, Lenser America, Inc., SCA Services Company, and William R.
Perrin Company.
Photographs were provided by Aqualogic® Inc., Industrial Filter and Pump
Manufacturing Company, Komline-Sanderson, and D. R. Sperry & Co.
The contact for further information is:
Nonferrous Metals and Minerals Branch
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati OH 45268
This report has been reviewed in accordance with the U.S. Environmental
Protection Agency's peer and administrative review policies and approved for
presentation and publication.
COVER PHOTOGRAPH: Recessed plate filter press with in-ground bin
to receive discharged cake
U,S. Environmental Protsction Agency
-------
Contents 1 • Overview 1
2. Resource Conservation and Recovery Act 4
Regulatory Framework 4
Exclusion from RCRA 6
Reducing the Waste Generation Rate 6
Seeking Nonhazardous Status 6
Regulatory Requirements 6
Transporters of Hazardous Wastes 6
Treatment, Storage, and Disposal Facilities 7
3. Hazardous Waste Disposal Sites 8
Landfill Design 8
Waste Identification Procedure 8
Disposal Cost Factors 8
Pretreatment Capabilities 10
4. Reducing Sludge Generation and Cost of Disposal 11
Reducing Sludge Generation 11
Waste Composition 11
Water Use 12
Treatment Processes 12
Sludge Dewatering 14
Sludge Segregation 16
5. Sludge Dewatering Equipment 18
Filter Presses 19
The Equipment 19
Determining Applicability 20
Costs 21
Vacuum Filters 22
The Equipment 22
Determining Applicability 24
Costs 25
Basket Centrifuges 26
The Equipment 26
Determining Applicability 27
Costs 27
Pressure Belt Filters 29
The Equipment 29
Determining Applicability 30
Costs 30
Evaluating the Cost for Sludge Dewatering Alternatives 33
References 35
in
-------
Illustrations Figures
1. Hazardous Wastes from Electroplating Operations 5
2. Influence of Wastewater Heavy Metal Concentration on Sludge
Volume 11
3. Sludge Generation Factors for Alternative Chromium Reduction
Processes 13
4. Sludge Volume versus Solids Concentration 14
5. Recessed Plate Filter Press: (a) Unit and Auxiliaries Needed for
Sludge Dewatering and (b) Annual Sludge Disposal Cost 15
6. Plate-and-Frame Filter Press 20
7. Recessed Plate Filter Presses: Unit Prices 20
8. Filter Press Dewatering Systems: Annual Sludge Disposal Costs ... 21
9. Rotary Vacuum Filter: (a) Basic Principle of Continuous Rotary
Filtration, (b) Filter Cake Capacity and Cake Dryness (from Filter
Leaf Test), and (c) Scale-Up Performance 23
10. Rotary Vacuum Filters: Unit Prices and Power Requirement. ... 24
11. Rotary Vacuum Filters: Annual Sludge Disposal Costs 25
12. Basket Centrifuge 26
13. Basket Centrifuges: (a) Large Unit Price and Hydraulic Drive Horse-
power and (b) Small Unit Price 27
14. Basket Centrifuge Systems: (a) Dewatering System with Auxiliary
Equipment and (b) Annual Sludge Disposal Costs 28
1 5. Pressure Belt Filter 29
16. Pressure Belt Filter: Unit Price and Power Requirements 30
1 7. Pressure Belt Filter System: Annual Sludge Disposal Cost 31
Tables
1. Structure of RCRA Subtitle C Regulations 4
2. Four Hazardous Waste Characteristics 5
3. Extraction Procedure Toxicity Limits 6
4. Typical Secure Chemical Landfill Sites: Summary of Costs 9
5. Sludge Generated and Sludge Disposal Volume for Electroplating
Waste Treatment System 12
6. Dewatering Equipment for Electroplating Sludge: Typical Per-
formance Characteristics 18
7. Performance of Dewatering Equipmentfor Electroplating Sludge ... 19
8. Comparative Total Investment and Annual Operating Costs for
Sludge Dewatering 32
9. Economic Evaluation of Precoat Rotary Vacuum Filter Sludge Dis-
posal Alternative 33
10. Sludge Disposal Under Four Dewatering Alternatives: Analysis of
Annual Costs 34
IV
-------
1. Overview
Under regulations implementing the
Clean Water Act of 1977 (Public
Law 95-217) metal finishing facil-
ities may be required to treat
spent process wastewaters to
remove regulated pollutants before
the wastewaters are discharged.
Treatment for heavy metal pollutants
generally consists of reducing the
solubility of the metals, then
separating the resulting precipitants
from the wastewater. Consequently,
the treatment yields a solid waste,
or sludge, containing a high con-
centration of potentially harm-
ful or toxic substances. This sludge
must be disposed of in a manner
that ensures that the pollutants,
once removed from the wastewater,
will not pose a threat to the
environment.
Recognizing the increased rate of
solid waste generation and the need
for environmentally safe disposal,
the U.S. Congress included provisions
for solid waste disposal in the Re-
source Conservation and Recovery
Act (RCRA) of 1976 (Public Law
94-580). Subtitle C of RCRA contains
provisions for hazardous waste
management. It directs the U.S. En-
vironmental Protection Agency
(EPA) to identify those wastes that
are hazardous, and to establish
national standards for generators and
transporters of hazardous wastes
and for operators of hazardous waste
management facilities involved
in the treatment, storage, and dis-
posal of these wastes.
The EPA has classified the following
metal finishing wastes as hazard-
ous materials:1
• Plating baths and the sludge
accumulated in these baths
• Stripping and cleaning solutions
• Sludge resulting from wastewater
treatment
Metal finishing shops disposing
of any of these wastes are regulated
by the RCRA standards. Under
RCRA, EPA holds waste generators
responsible for the ultimate safe
disposal of their wastes. Waste gen-
erators are also required to keep
records, use proper labels and
containers, and keep a manifest sys-
tem to document proper disposal.
The more stringent control of
hazardous waste disposal means
that plating shops may have difficulty
in finding licensed disposal facili-
ties, and may incur higher prices for
hauling and disposal than if their
wastes were nonhazardous.
Hauling costs depend on the distance
to the disposal site and the size of
the load. Haulers typically use
trucks designed for loads of 40,000
Ib (18,000 kg), or 5,000 gal (19,000 L);
they can transport liquids or solids.
A partial load would be charged
the same price as a full load. Prices
for long hauls are in the order
of $3 to $5 per loaded mile for the
5,000-gal (19,000-L) load, based on
the distance one way.3 A 300-mi
(480-km) trip, therefore, would cost
$0.18/gal to $0.30/gal of waste,
assuming the truck had a full load.
Because there are so few dis-
posal sites, long-distance hauling is
becoming the rule, not the exception.
Disposal facilities operating state-
of-the-art secure chemical landfills
charge according to volume, type
of waste, and type of container.
Disposal of drum quantities is by far
the most expensive. Fees at disposal
sites range from $25 to $50 for
each drum that requires burial in the
site. The equivalent cost per
gallon would be $0.60 to $1.20,
based on 42-gal (159-L) drum capac-
ity. Adding drum and hauling costs
could bring the total disposal
cost to $100 per drum.
Bulk liquids, which include dilute
sludges and spent process baths, are
less expensive to dispose of than
drum quantities; however, the
disposal cost includes the cost for
solidifying the waste before it
is placed in the landfill site. Costs
range from $0.25/gal to $0.75/gal.
"All costs in this report are in 1981 dollars.
-------
Elevated installation of recessed plate filter press
For dewatered sludge, which is
placed untreated in the landfill site,
disposal cost ranges from $0.20/gal
to $0.50/gal.
The hazardous waste generator
has two alternatives for reducing the
cost of disposal. One approach
is to seek relief from the RCRA regu-
latory requirements; the other is to
reduce the volume of waste.
The generator can avoid the regula-
tory requirements of RCRA by
having the waste classified as non-
hazardous. The EPA has established
a procedure that provides a means
of petitioning the Agency to ex-
clude from regulatory control a waste
that is generally classified as
hazardous.1'2 Obtaining such an
exclusion for a wastewater sludge
usually entails proving that the
sludge does not leach hazardous sub-
stances at harmful concentrations
into the ground water. If such
proof is to be established, the waste
must be subjected to the Extraction
Procedure, a test developed to
simulate the aggressive leaching
that occurs in a municipal codisposal
landfill.
An exclusion from many of the RCRA
requirements has been allowed
for generators producing less than
2,200 Ib/mo (1,000 kg/mo) of
hazardous waste.1 The waste must
still be disposed of safely, but many of
the associated record-keeping and
reporting responsibilities are not
required of generators of small
quantities of waste.
There are several means of lowering
the cost for waste hauling and
disposal. The generator can reduce
the amount of metals, chemical
compounds, and wastewater that
must be treated by the waste
treatment process. The solids can be
concentrated with dewatering
equipment to reduce the volume of
water contained in the sludge.3
Minimizing wastes, implementing
recycle and recovery modifications
where possible, and using processes
and reagents that generate less
sludge can significantly reduce the
amount of sludge solids requiring de-
watering. The remaining solids
can be dewatered for final off-site
disposal for a further reduction
in volume of more than 90 percent.
The high cost of sludge disposal
justifies purchase of dewatering equip-
ment for all but those plants gen-
erating very small volumes of sludge.
-------
The properties of individual sludges
vary widely, however, and some
form of pilot testing is needed to
determine whether a particular
type of dewatering equipment is
suitable.
Of the types of equipment available,
filter presses are usually the least
expensive to install. Filter presses
have further advantages in their
mechanical simplicity and in their
ability to achieve higher cake solids
concentrations than other de-
watering equipment types. Good
performance with a filter press
requires a sludge with good filtration
characteristics. Sludges that have
highly compressible, delicate
particles or that tend to blind the
media are not well suited for equip-
ment of this type.
Poor-filtering sludges can be
dewatered by centrifuges, pressure
belt filters, or vacuum filters that
use a precoat filter aid. These
devices are more mechanically
sophisticated than filter presses and
usually cost more. Their automa-
tion, however, often reduces the need
for operating labor.
This report is provided to aid the
metal finisher in assessing waste gen-
eration alternatives and developing
a cost-effective means of compliance
with the regulatory requirements.
The section that follows constitutes
an overview of the regulatory
framework developed for hazardous
waste disposal. Section 3 reviews
the disposal methods and associated
costs of commercially operated
secure chemical landfills. Section 4
reviews factors influencing waste
generation and describes what
can be done to reduce waste
volume and the cost of disposal. The
main emphasis of the report is the
final section, which evaluates
the types of dewatering equipment
available and their cost and
performance.
-------
2. Resource
Conservation and
Recovery Act
On May 19, 1980, EPA issued regu-
lations under RCRA as a basis for
a national hazardous waste manage-
ment program. The regulations
came as a result of 1976 Congres-
sional legislation that directed EPA to:
• Identify those wastes that are
hazardous
• Establish national standards for
generators and transporters of
hazardous wastes and for
operators of hazardous waste man-
agement facilities involved
in the storage or disposal of
these wastes
Hazardous wastes are regulated
from the time they are created to the
time of their disposal. This cradle-
to-grave monitoring is achieved
by a manifest system.4"6 Any
waste that is transported off site for
treatment, storage, or disposal
must be accompanied by a manifest
that:
• Identifies who generated the waste
• Provides a full description of the
contents and quantity of the waste
• Designates the facility to which
the waste must be shipped
Under RCRA, EPA holds the generator
of a waste responsible for the ulti-
mate safe disposal of that waste.
Strict civil penalties can be imposed
for any violations of the regulations.
In addition, regulations governing
the transportation of hazardous
materials over public roads were pub-
lished on May 22, 1980, by the
U.S. Department of Transportation
(DOT).7
Regulatory Framework
Table 1 gives the structure of
the RCRA regulations. Part 261 of
Subtitle C defines four characteristics
of wastes that would present an
environmental threat if disposed of
improperly (Table 2). A solid waste is
hazardous if it exhibits any of these
four characteristics, or if it is spe-
cifically listed in Part 261 as haz-
ardous.1 In the latter case, EPA
evaluated the hazard associated with
unregulated disposal of wastes
for which adequate information was
available. Then, if the findings so
warranted, the Agency made a
general determination that a given
waste is hazardous. Figure 1 shows
the four common plating shop
wastes that are generally classified
as hazardous. All four wastes
were determined to be toxic; plating
baths, sludge from plating baths,
and stripping and cleaning solutions
may exhibit reactive properties
as well.
RCRA standards for solid waste
generators such as plating shops are
defined in Part 262 of Subtitle C.4
They include provisions for record
keeping, reporting, implementing a
manifest system, and obtaining
an EPA identification number.
Table 1.
Structure of RCRA Subtitle C Regulations
Description
Part (40 CFR)
General provisions and definitions
Identification and listing of hazardous waste
Standards applicable to:
Generators storing wastes <90 d
Transporters
Permitted treatment, storage, and disposal facilities . . .
Interim status standards applicable to treatment, storage, and
disposal facilities
Permits for treatment, storage, and disposal facilities
Guidelines for State hazardous waste programs ,
260
261
262, Sec. 262.34
263 (and Pts. 171-179
of 49 CFR)
264
265
122, 124
123
SOURCE: U.S. Environmental Protection Agency, "Hazardous Waste Management System:
General," Federal Register 45(98).33067, May 19, 1980.
-------
Table 2.
Four Hazardous Waste Characteristics
Characteristic
Description
Ignitabihty. .
Corrosivity
Reactivity
The waste is capable of causing fires during routine transportation
to storage and disposal, or of burning so vigorously as to
create a hazard.
The waste is aqueous and has a pH <2 or >12.5 or corrodes
steel at a rate >0.25 m/yr.
The waste is extremely unstable and tends to react violently or
explode, thus posing a problem at all stages of waste
management.
When the waste is subjected to a specified leaching procedure,
the leachate fraction contains certain contaminants in a
concentration >100 times that specified in the National
Interim Drinking Water Standards.
SOURCE: U S Environmental Protection Agency, "Hazardous Waste Management System:
Identification and Listing of Hazardous Wastes," Federal Register 45(98):331 21 -33122, May 1 9,
1980.
Extraction Procedure
toxicity
Workflow
Product
Stripping and
cleaning solutions
(F009)
Wastewater treatment
EPA hazardous
waste no.
Hazardous waste
Hazard code
F006
F007
F008
F009
Wastewater treatment sludge Toxic
from electroplating operations
Spent plating solutions Reactive, toxic
Spent plating bath and sludges Reactive, toxic
from bottom of bath
Spent stripping and cleaning Reactive, toxic
solutions
SOURCE: U S. Environmental Protection Agency, "Hazardous Waste Management System:
Identification and Listing of Hazardous Waste," Federal Register 45(98):33123, May 19,
1980
Figure 1.
Hazardous Wastes from Electroplating Operations
If a hazardous waste is transported
off site for treatment, storage,
or disposal, the generator must pre-
pare a manifest.4'5 The manifest
designates the treatment, storage,
or disposal facility to which the
waste is being transported; in the
event an emergency prevents delivery
to this facility, an alternate receiv-
ing facility is designated. The
manifest must contain a full descrip-
tion of the waste being shipped
in terms of contents and quantity,
and it must identify the generator and
transporter. Sufficient copies of
the manifest are needed to provide
a copy for the records of the gen-
erator, each transporter, and
the receiving facility, as well as a
copy to be returned to the generator
after disposal of the waste.
The EPA considers that a generator
storing hazardous waste on site
for more than 90 days is an operator
of a storage facility, and therefore
must have applied for a facility
permit before November 19,1980, to
continue such operations.6 Storage
for less than 90 days does not
require a permit, but certain standards
must be met. Regulations specify:
• Provisions for inspection of
containers
• Precautions for ignitable or reac-
tive waste containers
• Personnel training
• Contingency plans and emergency
procedures for dealing with
the release of hazardous wastes
from their containers or tanks
The hazardous waste management
regulations issued on May 19, 1980,
specified that plants operating
a waste treatment system regulated
by the Clean Water Act still had to
comply with the RCRA standards for
hazardous waste treatment and
storage facilities. After reviewing
comments on the regulations,
EPA decided to award operators of
wastewater treatment and neutraliza-
tion units a permit-by-rule if they
comply with certain specified
standards. Accordingly, plants with
waste treatment facilities do not
have to apply for a treatment and stor-
-------
age facility permit or comply with
the interim status standards for
those facilities. (Some State agencies
administering the RCRA Program,
however, have not adopted the
permit-by-rule exclusion.) Details
of this Wastewater Treatment
Tank Exclusion were published on
November 17, 1980.8
Exclusion from RCRA
Costs related to RCRA compliance
can be lowered in one of two
ways. The rate at which hazardous
waste is generated can be reduced to
below 2,200 Ib/mo (1,000 kg/mo),
or the generator can have the
waste declared nonhazardous.1
Reducing the Waste Generation
Rate. An exclusion from many
of the RCRA requirements has been
allowed for generators producing
less than 2,200 Ib/mo (1,000 kg/mo)
of hazardous waste (small genera-
tors). This upper limit applies to
the total mass of waste and includes
water and other nonhazardous
constituents. The waste must still
be disposed of either in authorized
hazardous waste management
facilities or in facilities approved
by a State agency for municipal or
industrial waste disposal. The
associated record keeping, reporting,
and waste manifest are not required
of the small generator. Lacking an
EPA identification number and
waste manifest, however, the gen-
erator may have more difficulty in
finding a disposal facility that
will accept metal finishing wastes.
Moreover, within 2 to 5 yr, EPA
may initiate rules to include in the
RCRA requirements small gen-
erators producing more than 220
Ib/mo (100 kg/mo) of hazardous
waste.
Seeking Nonhazardous Status.
For some plating shops, having the
waste declared nonhazardous
(delisted) is effective in reducing
RCRA-related costs.1'2 The EPA has
established an appeals procedure
that provides a generator with a
means of petitioning the Agency to
exclude from the regulatory controls
a waste that is usually classified
as hazardous. The Agency has
authority to grant a temporary exclu-
sion on the grounds of significant
likelihood that the appeal will
be successful.
A plating shop seeking to have its
waste delisted must prove its waste
nonhazardous, which means
proving that the waste does not ex-
hibit toxic or reactive characteristics.
Proving the waste nonreactive
generally requires testing to verify
a low level of cyanide. Proving the
waste nontoxic requires subjecting
the waste to the Extraction Pro-
cedure and proving that the sludge
could not leach hazardous sub-
stances at harmful concentrations
into ground water. The Extraction
Procedure is designed to sim-
ulate the aggressive leaching that
occurs in municipal codisposal land-
fills. A sample of the waste is
extracted and analyzed to determine
whether it possesses any toxic
contaminants identified in the
National Interim Primary Drinking
Water Standards (NIPDWS) and, if so,
at what levels. The waste will be
considered hazardous if it contains
concentrations of contaminants
100 times greater than those speci-
fied in the NIPDWS (Table 3). In
addition, a complete chemical assay
of the waste must be included in
the delisting petition.
If a waste is to be delisted, it must be
tested for each characteristic that
is assumed to be present. Sufficient
tests (at least four) of each type
must be conducted to ensure repre-
sentative results. Costs to perform
the testing should range between
$300 and $1,000. The test results are
an essential step in the appeals
procedure. Every reason for a waste
being judged hazardous must be
refuted. Even if a waste passes
the Extraction Procedure, however,
EPA may rule that sufficient hazard
exists to warrant denying the appeal.
Table 3.
Extraction Procedure Toxicity Limits
Contaminant
Maximum
concentration
(mg/L)
Arsenic.
Barium
Cadmium
Chromium
Lead.
Mercury
Selenium
Silver
5
100
1
5
5
02
1
5
SOURCE U S Environmental Protection
Agency, "Hazardous Waste Management
System" Identification and Listing of
Hazardous Wastes," Federal Register 45(98).
33122, May 19, 1980
Cadmium, chromium, and lead are
the only common plating com-
pounds included in the Extraction
Procedure toxicity limits (Table 3).
These contaminants usually
result from only a few point sources
within a plating facility. Therefore,
segregating these wastes from
the rest of the plant's waste streams
can result in the major waste stream
exhibiting nontoxic character-
istics during the Extraction Proce-
dure testing.
Regulatory Requirements
Transporters of Hazardous Wastes.
Under RCRA, the role of the haz-
ardous waste transporter is simply
to supply transportation to the
generator. The transporter delivers
the waste to the hazardous waste
management facility that the genera-
tor designates on the manifest. Any
person transporting hazardous
waste within the United States must
obtain an EPA identification number,
and must comply with EPA and DOT
regulations for transporters of
hazardous wastes.5 7 The EPA regu-
lations are adopted from the regula-
tions developed by DOT. They include
vehicle specifications, requirements
for reporting hazardous material
incidents, and requirements
for handling, loading, unloading,
and segregating hazardous materials.
-------
The requirements for transporters
apply to both inter- and intrastate
transportation and are enforceable by
EPA or DOT.
A transporter may not accept a
hazardous waste from a generator
unless both parties have an EPA
identification number and the
waste is accompanied by a signed
manifest. The transporter must sign
the manifest and return a copy to
the generator before leaving the gen-
erator's property. When the ship-
ment is transferred to another
transporter or to the designated
hazardous waste management facil-
ity, the original transporter must
obtain the signature of the next party
on the manifest and keep one
copy. The second transporter re-
tains one copy of the manifest and
transmits the remaining copies
to the next party. The designated
treatment, storage, or disposal
facility is required to send one copy,
with all the signatures, back to the
generator. Special requirements
exist for bulk shipments by rail
or water.
Treatment, Storage, and Disposal
Facilities. Facilities that treat, store,
or dispose of hazardous wastes
are also regulated under RCRA.6 In-
terim operating permit status has
been granted to all such facilities pro-
vided they:
• Had been in operation or under
construction before November 19,
1980
• Had notified EPA of their haz-
ardous waste activities by
August 18, 1980
• Had applied for a permit by No-
vember 19, 1980
Requirements for disposal sites cover
• General facility standards
• Emergency precautions and actions
• The manifest system
• Record keeping and reporting
Requirements forfacilities on interim
status cover:
Reactive, ignitable, and incom-
patible wastes
Closure and postclosure care
Containers and tanks
Surface impoundment, waste piles,
land treatment, and landfills
Incinerators
Underground injection
Thermal, chemical, physical,
and biological treatment
Financial responsibility and
liability
For the foreseeable future, disposal
in a landfill is the only feasible method
of disposing of many hazardous
wastes. The regulations are intended
to provide long-term protection of
ground water and human health.
They specify monitoring require-
ments; failure to monitor the
land treatment facility is a viola-
tion of the regulations. They include
requirements for controlling and
monitoring water run-on and run-off,
as well as general requirements for
ignitable, reactive, and incompat-
ible wastes. Owners and operators
must consider specific factors
and methods in addressing closure
and postclosure requirements.
Also, record keeping and surveying
are required so that the exact loca-
tion and contents of each waste
cell will be known.
EPA has proposed financial require-
ments intended to ensure that
funds will be available for closure
of treatment, storage, and disposal
facilities, and for postclosure
monitoring and maintenance at
disposal facilities.9 The proposed
requirements also include liability
coverage for injuries resulting
from operation of a hazardous waste
management facility. These pro-
posals allow a number of ways
to provide financial insurance.
-------
3. Hazardous Waste
Disposal Sites
The secure chemical landfill is the
state of the art for disposal of metal
finishing waste treatment sludges.10'11
It is designed to preclude the risk
of ground water contamination
by toxic heavy metals that would
leach through the soil and into the
ground water unless prevented from
doing so. The secure chemical
landfill provides a means by which
toxic wastes can be buried in an
environmentally acceptable manner.
Landfill Design
There are basically two designs
for a secure chemical landfill.
The first takes advantage of natural
geological barriers created by imper-
meable clays. The second adds a
flexible elastomer liner as further pro-
tection against leaching of pollut-
ants into the ground water. In both
cases, disposal involves direct
burial of wastes in cells designed
to avoid contaminating the surround-
ing environment.
The wastes to be buried are classified
and segregated, and their positions
within a burial cell are recorded.
Bulk liquid wastes are solidified with
lime or cement dust before burial;
bulk solids are buried directly.
Drums of wastes are surrounded
by sufficient sorbent material to ab-
sorb the entire contents of the drum,
thereby eliminating the presence
of any free liquids in the cell. Only
compatible wastes are placed in
a given disposal cell. When a cell is
full, a compacted clay cover is placed
over the top to prevent precipitation
from filtering into the cell, thereby
minimizing the formation of leachate
and preventing its migration from
damaged drums.
A piping system for leachate collec-
tion is buried in a permeable bot-
tom layer at the center of each
cell. All leachate is recovered and
is periodicalty pumped out of the cell
through a standpipe connected to
the piping system. The recovered
leachate is solidified, then buried in
the landfill. A monitoring-well
system is placed outside the
landfill cells for early detection
of any leachate that may leak out
of the area. A properly operated
secure chemical landfill does not
usually experience leachate in
its monitoring wells.
Waste Identification Procedure
Before a hazardous waste disposal
site will accept a waste for disposal, it
will require the generator to submit
a completed waste identification
profile. The procedure includes an
analysis of a sample of the waste
to be landfilled. The analysis includes,
for example, pH, flash point, and
heavy metals content. From the pro-
file and analysis, the disposal
facility can determine whether
the waste is compatible with the
landfill disposal methods and
operating permits. (Many secure
chemical landfills do not accept re-
active wastes containing cyanides.)
If the waste is acceptable, the gen-
erator prepares a shipping manifest
that identifies the waste origin
(generator), destination (disposal
facility), hazard class and material
identification number, EPA haz-
ardous waste number, and weight.
The manifest is carried by all
transporters and is presented to
the disposal facility when the waste
is delivered. Appropriate copies of
the manifest are returned to the
transporter and the generator.
When the waste is received at
the landfill site, a representative
sample is taken and analyzed to en-
sure that the waste material received
is the same as that identified by
the waste profile documents. If the
waste is accepted, it can then be
landfilled. If rejected, it is usually re-
turned to the generator.
Disposal Cost Factors
The total cost for disposing of sludge
wastes consists of the costs for
hauling, for disposal site pre-
treatment, and for landfilling. These
costs depend on the physical
nature and chemical composition
of the wastes, and on the distance be-
-------
Table 4.
Typical Secure Chemical Landfill Sites: Summary of Costs
Company and site
Chemical Waste Management
Emelle AL
Rollins Environmental Services .
Baton Rouge LA
U.S. Pollution Control, Inc
Lone Mountain OK
SCA Services Inc
Pinewood SC
Nuclear Engineering Co
Louisville KY
Description
Clay base 500-700 ft thick, permeability
<10~8cm/s Liquid waste solidified
with lime or cement dust. Leachate
collection system in segregated
cells, sampling wells.
. . . Clay base, elastomer-lined cells
Leachate-monitoring wells.
Clay liner, permeability <10~8 cm/s.
Leachate-samplmg wells.
Fuller's earth base 10 ft thick, permeability
<10~8 cm/s, elastomer liner over
5 ft compacted clay, 2 ft clay over liner.
Leachate collection system, leachate-
monitonng wells, segregated cells
. . . Impermeable clay base Leachate-
monitoring system
d
Disposal
$25-$35/drum
$0.25/gal liquid
$0.025-$0.03/lb
for dewatered
sludge
$0.03/lb for de-
watered sludge
$44/drum
$003/lb bulk waste
$26-$31/drum
$0.065/lb bulk
liquid
$0.04/lb for de-
watered sludge
$0055-$0.10/lb
for dewatered
sludge
3Sta
Haulmgb
$2.96/loaded mi
NS
$0.42/1 00 Ib at 20 mi
$5/100 Ib at 700 mi)
$099/100 Ib bulk
liquid (<100 mi)
$320/100 Ib bulk
(>100 mi)
$2/loaded mi
a1981 dollars
b40,000-lb truckloads
Note —NS = data not supplied
SOURCE- Secure chemical landfill companies
tween generator and disposal site.
Table 4 summarizes information
on transportation and landfilling costs
for a number of hazardous waste
management facilities.15
The cost for burying a sludge waste
depends on whether the waste is
delivered as a bulk liquid, as a
bulk solid, or in drums. Sludge is
generally considered a liquid if it is
pumpable. Drum disposal is usu-
ally the most expensive—ranging
from $25 to $50 for a 42-gal (1 59-L)
drum—because considerably more
handling is needed. Moreover, the
value of the drum container is lost.
Sludge disposal in bulk liquid quan-
tities is the next most expensive,
ranging from $0.25/gal to $0.75/gal.
Although bulk liquid is more easily
handled than drums, the liquid waste
must be solidified before burial. In
bChemical Waste Management and SCA
Services, Inc., personal communica-
tions to Peter Crampton.
Dewatered sludge discharged from centrifuge
this step, the liquid is usually mixed Disposal of bulk solid quantities of
with lime, cement dust, or clay,
and the cost of the solidification
material becomes part of the total
disposal cost.
waste sludge is the least costly,
typically ranging from $0.025/lb to
$0.05/lb ($0.20/gal to $0.50/gal). A
bulk solid needs no special treat-
ment before burial if it contains
-------
no free liquid. Also, the overall cost
of disposal is lower per unit of dry
solids because of the smaller volume
of the water associated with the
waste. In general, the average
disposal costs for all bulk loads of
sludge are approximately $0.25/gal
(see Table 4). Unfortunately, some
disposal sites do not have the
material-handling capabilities to
dispose of nonpumpable wastes. It
is important, therefore, to determine
the local disposal conditions before
developing a waste disposal
strategy.
The cost for hauling sludge wastes
depends on three significant factors:
• Load size
• Distance hauled
• Fuel costs
A typical bulk load of sludge is hauled
by a truck with a 40,000-lb (18,000-
kg) capacity. A load that is less
than full will incur the same haul-
ing cost as a full load. From Table 4,
the average hauling cost is $3 per
loaded mile in bulk loads. Therefore,
if a bulk load of sludge is hauled
300 mi (480 km), the average hauling
cost is about $0.18/gal. When
sludges are hauled to distant
disposal sites, it is common for a
number of small generators to com-
bine and thus make full use of the
hauling capacity of a truck. To
maintain proper responsibility for
the individual waste volumes,
the wastes are segregated in sepa-
rate hoppers or drums.
Pretreatment Capabilities
In general, the hazardous waste
management facility does not
pretreat or process sludge wastes
on site except to mix liquid waste with
solidification materials. Physical or
chemical treatment—such as de-
watering, drying, orpH adjustment—
is not performed on site. Some of
the large chemical waste disposal
companies do have or are planning to
have facilities that offer a wide
range of treatment capabilities. For
example, RCRA does not allow land-
fill disposal of sludges containing
reactive materials such as cyanide.
The disposal facility would therefore
increase its potential market by
providing chemical treatment for
cyanide oxidation, making these
wastes compatible for landfill
disposal. As an alternative to
solidifying dilute sludges, sludge
dewatering could also be done at
the treatment facilities.
10
-------
4. Reducing Sludge
Generation and Cost of
Disposal
Sludge handling and disposal costs
normally depend heavily on sludge
volume. The high cost of disposal
provides a strong incentive for modi-
fying plating procedures to reduce
this volume. A program to mini-
mize chemical losses and water con-
sumption can reduce sludge gener-
ation significantly.12"14 After
wastewater treatment, the dilute
sludge can be dewatered mechani-
cally to reduce the volume by 90
to 95 percent.
Many factors contribute to the
formation of insoluble solids
during wastewater treament; major
factors are:
• Concentration of heavy metals
and other dissolved solids that pre-
cipitate during treatment
• Volume of water to be treated
• Reagents, conditioners, and unit
processes used in treatment
Incremental reductions in the amount
of hazardous waste generated will
lower disposal costs. Reducing
the waste generation rate to below
2,200 Ib/mo (1,000 kg/mo) will
exempt the plant from reporting re-
quirements defined in RCRA for
hazardous waste generators.
Eliminating toxic materials (cad-
mium, lead, chromium) from the
waste stream will result in a sludge
that would prove nonhazardous
if analyzed according to the Extrac-
tion Procedure.
Reducing Sludge Generation
Waste Composition. The concen-
tration of heavy metals in the
wastewater will influence the
amount of solids generated in the
neutralization-precipitation proc-
ess.14-15 Particularly with systems
using lime as the neutralizing
reagent, however, metal hydroxides
will usually constitute less than
25 percent of the solids. The rest
will be calcium salts (carbonates, phos-
phates, sulfates) and other insoluble
compounds that are formed by re-
actions with the lime.
Figure 2 shows the sludge volume
resulting from lime neutralization of
electroplating wastewater over a
o
Q
D
30 i-
20
10
O
O
o
o
I
I
I
I
100 200 300 400
HEAVY METAL CONCENTRATION (mg/L)
500
"Volume of sludge per volume of wastewater treated after 1 h settling Treatment consists of
lime neutralization.
SOURCE: Robinson, A K., and J. C. Sum. "Sulfide Precipitation of Heavy Metals," prepared
for U S. Environmental Protection Agency under EPA Grant No. 5805413, Seattle WA,
Boeing Corporation, undated
Figure 2.
Influence of Wastewater Heavy Metal Concentration on Sludge Volume
11
-------
range of heavy metal concentrations.
Based on the relationship shown,
lime neutralization of 1,000 gal
(3,800 L) of wastewater containing
100 mg/L of heavy metals would
yield, after 1 h of settling, 90 gal
(340 L) of sludge. Although the data
do not define the suspended solids
concentration of the sludge, metal hy-
droxide sludges will typically
gravity settle to between 1 and 5
percent solids by weight.
Reducing chemical losses, therefore,
will reduce sludge generation rates
as well as chemical replacement
and wastewater treatment costs.
The m-plant modifications that
can reduce chemical losses are
well-documented16"18 and include
such procedures as:
• Dragout recovery and recycle
• Maximum use of stripping and
cleaning solutions before they are
discarded
• Drip trays and splash guards
to direct losses back to the bath
• A good housekeeping and
maintenance program to permit
rapid finding and repair of
leaks in tanks, valves, and pump
seals
Table 5 gives the amount of metal
hydroxide solids precipitated
during treatment for various metals
in the raw wastewater, as well
as the associated sludge volume
at 3 percent and 25 percent solids by
weight. Loss of 1 Ib (0.45 kg) of
nickel into the wastewater will result
in 6.1 gal (23.1 L) of sludge at 3 per-
cent solids by weight. The cost of
disposing of this volume of sludge will
usually be greater than the original
cost of the nickel salt.
Water Use. The amount of sludge
generated is also affected by the
volume of water needing treatment.15
In areas of hard water, precipitation
of natural water contaminants,
such as carbonates and phosphates,
can generate a sludge volume
exceeding that associated with
chemicals discharged to the waste
stream. Moreover, consumption
of many treatment reagents and chem-
ical conditioners used in waste-
water treatment depends on the vol-
ume of water treated. These com-
pounds frequently end up in the
sludge and increase its volume.
Several steps can be taken to
reduce water use.16'18 The major
water requirement is for rinsing;
multiple stage counterflow rinse sys-
tems and adequate agitation in the
rinse tank will significantly reduce the
amount of rinse water needed. For
Table 5.
Sludge Generated and Sludge Disposal Volume for Electroplating Waste
Treatment System
Sludge (gal/lb metal
Waste metal components
Aluminum
Cadmium .
Chromium .
Copper . .
Iron . .
Nickel
Zinc . . . .
Dry solids
(Ib/lb metal
precipitated)3
2.89
1.3
1 98
1.53
1 61
1 58
1 52
precipit
Generated
(at 3% solids
by weight)
11 2
5
7.7
5.9
62
6 1
59
ated)
Dewatered
(to 25% solids
by weight)
1 14
0.51
079
0.6
063
0.62
0.6
'Using sodium hydroxide
SOURCE U S Environmental Protection Agency, Environmental Pollution Control Alternatives-
Economics of Wastewater Treatment Alternatives for the Electroplating Industry, EPA 625/5-79-
016, June 1979
automated plating lines, flow restric-
tors on the rinse water feed can be
used to control fresh water additions
at the minimum required for good
rinsing. Rinse tank conductivity
meters can do the same for inter-
mittent plating operations. Reusing
spent rinses or treated effluent
for less critical water requirements
will reduce water consumption.
The benefits of reducing water use
go far beyond decreasing the amount
of sludge generated, but the impact
on sludge disposal should not be
ignored in cost-benefit evaluation of
potential modifications.
Pretreating the water to reduce its
hardness level can reduce the
contribution of naturally occurring
water contaminants to sludge gen-
eration. Water softeners using ion
exchange resins or reverse osmosis
systems can remove calcium and
magnesium from water supplies.
Treatment Processes. The reagents,
conditioners, and unit processes
employed in wastewater treatment
should be evaluated in terms of
their effect on sludge generation
rates. For example, lime and caustic
soda are the two alkali neutraliz-
ing agents used most frequently.15
Lime has advantages over caustic
soda in that it costs less per unit
of neutralizing capacity, produces
sludge that settles and dewaters more
readily, and can reduce the solubil-
ity of metals to lower levels in
some applications (primarily
because of the complex-breaking
capabilities of the calcium ions).
Lime has disadvantages, however, in
that it requires a higher investment
in the reagent feed system, takes
longer to react in the wastewater, and,
depending on the chemical compo-
sition of the wastewater, can produce
as much as 10 times the dry weight
of sludge produced by caustic soda.
Coagulating agents commonly
used to improve floe formation before
clarification also can contribute to
sludge generation.12 Alum and ferric
12
-------
chloride are widely used, and ulti-
mately both are converted to
hydroxides and add to the amount
of sludge for disposal. Although
polyelectrolyte conditioners are more
expensive than inorganic coagu-
lants, they do not add to the quantity
of sludge and have provided effective
solids-settling rates. Their actual
cost may therefore be lower.
The significance of treatment
process selection can be appreciated
when the different systems used
to reduce chromium are consid-
ered.14'15 Three types have been
demonstrated:
• Chemical reduction using a sulfur
compound — sulfur dioxide (S02)
or sodium bisulfite (NaHS03)
• Electrochemical reduction using
sacrificial iron electrodes
• Reduction using a slurry of in-
soluble ferrous sulfide (FeS)
Using sulfur dioxide or sodium
bisulfite has an advantage because,
exceptfor the chromium, no insoluble
byproducts are formed in the re-
duction reaction:
3S02 + 2H2Cr04 + 3H20 -*
Cr2(S04)3 + 5H20
In electrochemical reduction units,
an electric current is used to gen-
erate ferrous ions that react with the
hexavalent chromium ions:
3Fe+3 + Cr+3 + 8Or-T
The ferric ions generated by the
reduction will precipitate at a neutral
pH and add to the sludge volume.
Similarly, using ferrous sulfide as
the reducing reagent will generate
ferric ions and sulfur, both of
which will add to the sludge volume:
H2Cr04 + FeS+4H20 —
Cr(OH)3 + Fe(OH)3 + S + 2H20
It would appear that the electro-
chemical and ferrous sulfide reduc-
tion processes would be unfavorable,
at least in terms of sludge generation
rates; however, there is an addi-
tional factor to consider. Reduction
using sulfur dioxide or sodium
bisulfite requires a wastewater pH
between 2 and 3. Consequently, a
significant amount of acid may be
needed to lower the pH, then a signif-
icant amount of base would be
needed to raise it back to neutral
to precipitate the chromium as
chromic hydroxide [Cr(OH)3].
Electrochemical and ferrous sulfide
reduction systems can operate at
neutral pH. Particularly in sulfur
dioxide and bisulfite reduction sys-
tems employing sulfuric acid and
lime, the amount of calcium salts pre-
cipitated can exceed the amount of
precipitants resulting from the
ferric ions and sulfur generated in
the alternative reduction processes.
Figure 3 shows the sludge generation
rates of the three reduction sys-
tems over a range of hexavalent
chromium concentrations.14-15 The
electrochemical and ferrous sulfide
processes actually generate less
sludge solids at chromium concen-
15
12
08
tn o
sl
Legend:
ferrous sulfide reduction
electrochemical reduction
sulfur dioxide reduction
50 100 150
HEXAVALENT CHROMIUM (mg/L)
200
SOURCES: U.S. Environmental Protection Agency, Control and Treatment Technology for
the Metal Finishing Industry: Sulfide Precipitation, EPA 625/8-80-003, Apr. 1980.
U.S. Environmental Protection Agency, Economics of Wastewater Treatment Alternatives
for the Electroplating Industry, EPA 626/5-79-016, June 1979.
Figure 3.
Sludge Generation Factors for Alternative Chromium Reduction Processes
13
-------
trations below 75 mg/L It is im-
portant to remember that the solids
generation rates are based on
assumptions regarding the initial pH
of the wastewater and the neutral-
izing reagent employed. Any
firm conducting a similar analysis
for its treatment system should test
to determine how much sludge is
generated by each of the differ-
ent treatment alternatives.
To reduce sludge disposal costs,
it is necessary to select the waste
treatment techniques that generate
the least amount of waste sludge.12-13
Although some of the newer treat-
ment techniques produce an effluent
of high quality, they generate much
more sludge than the conven-
tional approaches they replace.
The high cost of waste disposal re-
quires that the foregoing sludge
generation factors influence the selec-
tion of wastewater treatment
systems.
Sludge Dewatering
Although the volume of sludge can
be reduced significantly by modifica-
tions that reduce the pollutant and
wastewater loadings on the treatment
system, a sludge residue will
result from wastewater treatment.
The cost to dispose of this residue
will depend primarily on volume. The
volume of sludge can be reduced
significantly by mechanical de-
watering equipment. Figure 4 shows
the reductions possible when
sludge is dewatered from 1 percent
solids by weight to different solids
concentrations.
Normally the clarifier underflow
will contain between 0.5 and 3 per-
cent solids by weight. Allowing
the clarifier underflow to settle in a
thickener tank will increase the
solids content to between 2 and 5
percent by weight. Using the curve
in Figure 4, 1,000 gal (3,800 L)
1,000
500
_
o
100
50
10
I
I
I
I
5 10 15 20 25 30
SOLIDS CONCENTRATION (% by weight)
35
Note —Initial conditions, 1,000 gal at 1% by weight
Figure 4.
Sludge Volume versus Solids Concentration
of sludge at 1 percent solids by weight
would be reduced to 330 gal
(1,250 L) when thickened to 3 per-
cent solids by weight. A mechanical
dewatering device will achieve
anywhere from 10 to 50 percent
solids by weight, depending on
the type of equipment and the
dewatering properties of the sludge.
Assuming dewatering to 25 per-
cent solids by weight, the sludge vol-
ume would be reduced to 40 gal
(150 L)—4 percent of the original
clarifier underflow volume.
Vacuum filters, filter presses, pressure
leaf filters, belt filters, and centri-
fuges have been applied successfully
for mechanical dewatering of metal
hydroxide sludges. The properties
of individual sludges vary widely,
however, and some pilot evaluations
are necessary to determine whether
a particular type of dewatering
equipment is suitable. As a rule, equip-
ment vendors will provide testing
if supplied with a sample of the sludge.
14
-------
Four features are common to sludge
dewatering systems (Figure 5a):
• A solids collection sump
• One or more feed pumps
• Elevation of the dewatering device
• Filtrate return upstream
The solids collection sump receives
the dilute clarifier underflow and
provides a reservoir of feed solution
so that the mechanical dewatering
device can be fed continuously.
The feed pump delivers the sludge
to the dewatering device. Pump
type depends on the physical
properties and viscosity of the sludge,
and on the type of dewatering
device. Specially designed centrifu-
gal, diaphragm, and progressive
cavity pumps are suitable for
handling slurries.
Elevating the dewatering device
facilitates handling dewatered sludge.
Ideally, the dewatered sludge
should be discharged directly into
a hopper—the transport medium to
the disposal site. If this approach
is impractical, a straight run of
conveyors can be used to transport
the sludge to a point overthe hopper.
Filtrate is returned to the clarifier
or other upstream process vessel.
Usually the level of suspended
solids is too high to allow direct
discharge.
The basic premise in the design of
sludge-handling systems is to
prevent the flow path from becoming
plugged with sludge or debris.
Plugging is usually caused either by
buildup of debris behind an obstruc-
tion in the flow path, or by solids
settling in the pipes. To minimize the
chance that such occurrences
will interrupt operation, the system
should use valves, instrumenta-
tion, and so forth, that do not obstruct
flow through pipes, and should
include provisions for flushing out
clogged lines.
Filtrate to clanfiei
Filter press
Clarifier underflow i
SOURCE: U.S. Environmental Protection Agency, Environmental Regulations and
Technology: The Electroplating Industry, EPA 625/10-80-001, Aug. 1980.
(b)
8
03
O
o
O
a.
03
D
125 r
100 -
75 -
at 3% solids by weight
at 20% solids by weight
80 120
CLARIFIER UNDERFLOW (gal/h)
160
200
aAt $0.50/gal.
Figure 5.
Recessed Plate Filter Press: (a) Unit and Auxiliaries Needed for Sludge
Dewatering and (b) Annual Sludge Disposal Cost
15
-------
Basket centrifuge discharging centrate
Determining the capacity needed in
the dewatering system requires
testing. If a treatment system is
already in place, capacity can be de-
termined easily by measurement
of the clarifier underflow volume and
suspended solids concentration.
Lacking a treatment system, a
representative wastewater sample
should be treated in a manner
similar to the manner employed in
the proposed treatment system.
After treatment and settling, the vol-
ume of sludge generated per unit
volume of water treated can be
determined visually. A sample of the
settled sludge can be analyzed
for suspended solids content.
The high cost of sludge disposal will
justify purchase of dewatering
systems for all plants except those
generating very small volumes
of sludge. As a further incentive, if
dewatering reduces the mass to
below 2,200 Ib/mo (1,000 kg/mo),
the generator is excluded from many
of the regulatory requirements of
RCRA. Before evaluating the
benefits of sludge dewatering,
however, the capability of local dis-
posal sites to handle nonpumpable
sludges should be ascertained.
Consider, for example, the installation
of a recessed plate filter press to
dewater a dilute clarifier underflow
from 3 percent to 20 percent
solids by weight. Figure 5b com-
pares the annual cost of disposal, at
$0.50/gal of sludge, for the two
concentrations. At 20 percent
solids by weight, the figure shows
the disposal cost with as well
as without the annual cost to operate
the filter press. Even with its cost
included, the filter press reduces an-
nual disposal costs at underflow
rates exceeding 8 gal/h (30 L/h).
Thus, mechanical dewatering
is usually cost effective, except for
plants generating very small sludge
volumes. Under RCRA requirements,
sludge must be dewatered or
solidified before it is used in land
application. Modern disposal
facilities that accept industrial solid
waste are likely to have some
means of dewatering or solidifying
the waste. Plants generating
small sludge volumes may find it
more cost effective to use the
dewatering capabilities at a central
disposal facility.
Sludge Segregation
A mixture of hazardous and non-
hazardous waste is considered
a hazardous waste. Segregating
wastes will reduce the volume of haz-
ardous waste considerably and,
therefore, the cost of treatment and
disposal.2'19 The sources of toxic
contaminants in plating are usually
limited to a few operations. Cadmium,
16
-------
lead, and chromium are the only The segregated sludge containing Toxic substances can also be
common plating materials on toxic substances must be disposed of eliminated from a plant's waste by
the EPA list of toxic substances. If in a manner acceptable for haz- recovery and recycle of the toxic
toxic wastes are separated from ardous wastes; however, the volume contaminants. Dragout from
the rest of the waste stream, the treat- should be considerably less than cadmium, lead, and chromium
ment residue from the nontoxic the total amount of waste generated, plating operations has been recov-
waste streams should be able to pass The generator will avoid the report- ered by recovery systems. The
the Extraction Procedure and be ing and manifest requirements combined benefits of material recov-
judged nonhazardous by EPA. if the hazardous waste amounts to ery, waste treatment cost reduction,
Disposal of a waste that is judged less than 2,200 Ib/mo (1,000 kg/mo), and producing a nonhazardous
nonhazardous should be less costly. sludge can provide significant
returns on the investment in
recovery equipment.
17
-------
5. Sludge Dewatering
Equipment
Mechanical dewatering devices are
used to achieve a higher sludge
solids concentration than can be ob-
tained by gravity thickening. Weak
attractive forces bind much of
the water contained in a sludge to
the solid particles; when the bonds
are subjected to mechanical
force, much of the water remaining
in the sludge after gravity thickening
can be removed. The following
types of equipment can be used for
mechanical dewatering of electro-
plating sludges:
• Pressure filters
• Vacuum filters
• Centrifuges
• Compression filters
When pressure filters are employed,
the dilute sludge is pumped into
the filter; the solids are retained
on the filter membrane and the
filtrate is discharged. Recessed plate
filter presses use this method.
They can dewater sludge to high
solids content owing to the large pres-
sure gradient they can apply across
the sludge cake.
Vacuum filters dewater sludge
by applying a vacuum on one side
of a water permeable membrane that
has a sludge layer or suspension
on the other side. In response
to the pressure gradient, the water
passes through the membrane.
Rotary drum and vacuum belt
filters use this principle.
Centrifuges dewater sludge similarly
to gravity thickening, but by rapidly
rotating the sludge, they create a
centrifugal force thousands of
times more powerful than normal
gravity. The strong centrifugal
force greatly speeds up the settling
process and magnifies the com-
paction effect. This dewatering
mechanism makes centrifuges most
suitable for compressive sludges
that settle well.
Compression filters dewater
sludge by squeezing it between
water permeable membranes.
They have proven effective mainly
for dewatering highly compressive
sludges typical of those resulting
from polyelectrolyte conditioning.
Criteria for selecting one of the
foregoing devices for a specific ap-
plication include:
• Sludge properties (solids
concentration, particle size,
compressibility)
• Volume of sludge to be dewatered
• Local disposal requirements
Table 6 compares some of the
characteristics of the different equip-
ment types. Table 7 summarizes
the performance of different
dewatering systems for metal
finishing sludges in industrial appli-
cations.
The data in Table 7 indicate that
centrifugation will normally achieve
Table 6.
Dewatering Equipment for Electroplating Sludge: Typical Performance
Characteristics
Equipment
Filter press
Precoat vacuum filter
Pressure belt filter
Fee
Rate
(gal/mm)
2-250
1 -250
1-250
2-60
5-200
id
Solids
(% by
weight)
1-5
3-10
05-3
2-5
2-6
Solids
(%by
weight)3
95-99
50-99
95-99
50-95
90-95
concentration
(% by weight)
20-50
15-40
20-50
5-25
20-40
Installed
cost
($1,000)b
20-200
30-150
30-150
20-175
40-200
"Feed solids in sludge cake
b1981 dollars. Includes auxiliary equipment
18
-------
Table 7.
Performance of Dewatering Equipment for Electroplating Sludge
Equipment type and unit size
Recessed plate filter press.
8.5-ft3 sludge-holding capacity . . .
10-ft3 sludge-holding capacity ...
12.5-ft3 sludge-holding capacity . . .
21 -ft3 sludge-holding capacity
260-ft3 sludge-holding capacity. . . .
Vacuum filter3
Rotary precoat vacuum filter:
9.4-ft2 filtration area
37.7-ft2 filtration area
Basket centrifuge:
1-gal bowl capacity
4-gal bowl capacity
Pressure belt filter 9.8-ft-wide sludge-
holding capacity . .
Sludge
feed
rate
(gal/h)
95
300
NA
825
1 2,000
NA
100-150
330
120
120-150
800
Solids (% by
weight)
Feed
3
1-2
NA
5
3-6
3-5
NA
1-5
3
5
2-6
Cake
30
30
40
35-40
30
70-75
15
30
12-20
18
NA
Comment
NA
Plastic-plating lime
sludge
Chromium hydrox-
ide dewatermg
Operation attention
2-h shift, unit
cleaned every 6-8
wk with recircu-
lated 50% HCI
Heat-treated zinc
hydroxide sludge
Metal oxide waste
treatment sludge
Low maintenance
Cloth life 1 yr;
repairs <$200/yr
Satisfactory per-
formance; low
maintenance
High maintenance
Aluminum hydrox-
ide sludge
"Unit size not available
Note —NA — not available
solids concentration in the range
of 12 to 20 percent by weight.
Both precoat vacuum filtration and
pressure filtration can achieve
30 percent cake solids by weight
if the sludge has good filtration
properties.
The one data point given for vacuum
filtration (without precoat) shows
that the equipment achieved 70
to 75 percent cake solids by weight.
This high cake concentration resulted
because the sludge was composed
of metal oxides rather than metal
hydroxides. The metal oxide precipi-
tants can be dewatered to higher
solids content than can hydroxides
because, unlike hydroxides, they
do not have water chemically
bound to the metal oxide molecule.
Also, the metal oxide solids are
much less compressible and they
filter better. It is possible to generate
metal oxide sludge, but the waste-
water treatment is significantly
different from that of conventional
hydroxide precipitation systems.
Only one data point is given for
a belt filter press; this equipment has
been used to a limited degree for
metal finishing waste sludge.
Its higher cost usually restricts its
use to applications where other
equipment types cannot operate
satisfactorily.
Filter Presses
The Equipment. Filter presses come
in two basic types: recessed plate
and plate and frame. In both cases,
the press is a series of parallel
plates pressed together by a hydraulic
ram, with cavities between the
plates. The plates are recessed on
each side to form the cavities
in the recessed plate press. A frame
of equal dimension is placed between
flat plates in the plate-and-frame
press (Figure 6). The plates come in
a variety of materials; originally
they were fashioned from wood, later
from steel or ductile iron. Plates in
predominant use today are made
of light weight, chemically durable
polypropylene or fiber-reinforced
polyester.
A filter press is a batch unit. At
the start of the cycle, slurry is pumped
into the cavities through a port that
runs through the bank of plates.
When the cavities are full, the
pressure forces the filtrate through
the filter media, along the drain-
age surface of the plates, into
orifices that are located in the cor-
ners of the plates and connected
to the filtrate port. The process con-
tinues until the cake solids in the
cavities thicken to a degree
such that, at the pressure limit of
the press, only a small volume
of filtrate is being produced. The
pump is shut off at this point, the ram
is withdrawn, and each cavity
is emptied individually. The press
is then closed and the cycle begins
again. Usually the filter cakes are
dropped into a hopper under the
press or are transported to a hopper
by a screw conveyor, which also
breaks up the cakes.
The filter press has a number of
advantages over other filtration equip-
ment. Filter presses can operate
well at variable or low feed solids
conditions. They can produce a
very dry cake because of the high
pressure differential they can exert
on the sludge. Some commercial
units are designed with a pressure
limit of 225 Ib/in2 gauge (1,653
kPa), and produce sludge cakes with
solids content in the range of 50
to 70 percent by weight. Filter
presses are mechanically reliable;
the hydraulic ram and the plate-
shifting mechanism (which facilitates
cake discharge on the larger units)
19
-------
Plate
Frame
Fixed
Movable head
Closing device
Filtrate
Sludge fee
under pres:
• Filter cloth
Figure 6.
Plate-and-Frame Filter Press
are the only moving parts. Power
consumption is low; the only
significant power use is for feed
pump operation [2 to 20 hp (1.5 to
15 kW)].
The disadvantages of the filter
press include its batch operating
cycle, the labor associated with re-
moving the cakes from the press, and
the downtime associated with find-
ing and replacing worn or dam-
aged filter cloths. At the end of each
filtration cycle, about 30 min of
operator labor will normally be
needed to empty the press and start
a new cycle. A filter press is usually
sized to operate on a 4- to 8-h cycle.
Determining Applicability. The most
reliable way to test whether a fil-
ter press is applicable is to obtain a
small bench-scale unit from a
press manufacturer. Several design
specifications can be determined
by bench-scale testing:
• Press filter cake volume
• Press filtration area
• Pressure limit
• Optimum cycle time
cake solids concentration and fil-
tration cycle time.
Press filtration area actually relates
to the optimum cake thickness
between the plates in the press. Re-
cesses between plates in filter
presses range from 1 in (2.5 cm) to
as much as 3 in (7.6 cm). The
wider the spacing, the less expen-
sive the filter press per unit of
cake volume, but the lower the filtra-
tion capacity (gallons of filtrate
per hour) per unit of cake volume.
80
60
40
20
Legend:
MM**
automatic plate shift, hydraulic ram
manual plate shift, manual closure
10
20
30
40
50
60
70
80
FILTER CAKE VOLUME" (ft3
'Includes carbon steel frame, polypropylene plates, and filter cloths.
bBased on sludge cakes 1.25 in thick
SOURCE- Equipment vendors
Press filter cake volume relates to the Figure 7.
solids loading rate and the expected
Recessed Plate Filter Presses: Unit Prices
20
-------
Feed solids concentration and filter-
ability of the sludge affect the
choice of spacing.
Pressure limit relates to the solids
content of the sludge cake versus the
applied pressure.
Optimum cycle time is determined
by the press volume and filtration area
specified. The longer the cycle
time, the lower the labor require-
ments, because cake discharge and
restarting occur less frequently.
A longer cycle time will require a
larger unit, which of course will
cost more.
Testing with a bench-scale filter
press, however, can be costly
and time consuming. The applicabil-
ity of a filter press, or of any filtration
technique, can be determined
more simply by use of a filter leaf
test apparatus.20 Several factors can
be evaluated, for example, filter-
ability, medium fouling, and cake
release from the medium. Vendors
of filter press equipment can scale up
filter leaf test data to determine
the required size of a filter press.
As an alternative to testing, filter
press vendors need only a sludge
sample and the volumetric rate and
solids content of the press feed
to determine the required size and
cost of the unit.
Costs. Figure 7 shows the relation-
ship of filter press purchase price
to the volume of cake solids for
a press with a 1.25-in (3.2-cm) cake
recess and an operating pressure
of 100 Ib/in2 gauge (790 kPa). The
cost covers only the purchase price of
the press; auxiliary equipment
needed includes:
• High pressure feed pump or pumps
• Sludge feed storage
• Filtrate return to the clarifier
• Cake solids handling and discharge
The cost of auxiliaries can be
considerable, but fortunately
there are alternatives that can elim-
inate some of the expense. Sludge
feed piping, filter, and filtrate
discharge piping can be designed
as a closed hydraulic system. This ap-
proach enables the sludge feed
pump to provide the pressure
head needed to return the filtrate to
the clarifier, thus eliminating the
need for a filtrate receiving tank and
return pump in applications where
gravity flow return is not feasible.
Handling and disposal of the cake
solids can be simplified by elevation
of the press, enabling the filter
cake to be discharged directly into
the disposal hopper. This approach
will eliminate the need for solids
conveying systems and reduce
the amount of operator attention
associated with discharge"of
the cake solids.
Owing to the batch operation
of the filter press, storage volume
is needed for the sludge feed.
Normally, a sump to receive the
clarifier underflow will provide the
necessary storage volume. If the
solids retention time in a clarifier is
high, the clarifier can be used as
the storage chamber; otherwise a
holding tank will be required to
provide adequate sludge storage.
Operational costs associated with the
press are for power to operate the
feed pumps and labor to turn the
press around at the end of each cycle.
Minor operational costs are asso-
ciated with maintaining the filter
media in good condition. These
costs include replacement of dam-
aged or worn filter cloths and
periodic cleaning to control media
blinding.
In Figure 8, the cost associated
with using a filter press for sludge
140
120
100
I 80
o
CO
g 60
CO
Q
3
Z
<
40
20
Legend:
including annual cost of filter press
disposal only
20-ft3
filter capacity
10-ft3
filter capacity
5-ft3 filter
capaci
120 180 240 300
CLARIFIER UNDERFLOW6 (gal/h)
aAt $0.43/gal. Sludge dewatered to 25% solids by weight.
bAt 3% sohds by weight.
Note.—4,800-h/yr operation.
360
Figure 8.
Filter Press Dewatering Systems: Annual Sludge Disposal Costs
21
-------
Rotary vacuum filter with belt discharge for enhanced cake-medium separation
dewatering is shown as a function of
the clarifier underflow rate, including
costs for transporting and dispos-
ing of the sludge at a secure chemical
landfill as well as costs associated
with the filter press. It is assumed
that the clarifier underflow is
dewatered from 3 to 25 percent
solids by weight. Disposal is assumed
at $0.43/gal ($0.25 for disposal
and $0.18 for hauling), representing
the average cost for a bulk load of
40,000 Ib (18,000 kg) shipped
300 mi (480 km) to the landfill. Cost
for the filter assumes a unit sized
to operate on a 4-h cycle.
For example, with a clarifier (or
thickener) underflow rate of 100 gal/h
(380 L/h) at 3 percent solids by
weight, the annual disposal cost
would be $39,000: $21,000 for
transporting and disposing of the de-
watered sludge and $18,000 for
depreciation and operation of a press
with 10ft3 (0.3 m3) of filter capacity.
If the same volume of sludge were
disposed of without dewatering,
the annual disposal cost would
be $200,000. Based on the assump-
tions in Figure 8, the filter press
installation saves $161,000 peryear.
The economic benefits of installing
dewatering equipment are also
realized for plants generating small
volumes of sludge. The filter press
achieves a net disposal cost reduc-
tion where clarifier underflow
rates exceed 5 gal/h (19 L/h).
The rate of return on the investment
associated with a recessed plate
filter press is a function of the
volume of sludge and the cost of
sludge disposal. For a plant dispos-
ing of its sludge at $0.43/gal, the
investment in a filter press with a 5-ft3
(0.15-m3) filter capacity yields a 30-
percent after-tax return when the
feed rate exceeds 12 gal/h (45 L/h).
The cost of the press would be
$19,000; installation with the re-
quired auxiliaries should bring
the total to $29,000.
Vacuum Filters
The Equipment. The rotary drum (Fig-
ure 9a) is the most common type of
vacuum filter. The drum is positioned
horizontally and rotates partly
submerged in a vat filled with a slurry.
22
-------
Dewatenng zone
(cake drying)
Rotation
Discharged
filter cake
(b)
12
10
>
\-
(J
Q-
Cake
Filtering zone
30
25
in
20
15
0123
CYCLE (min/r)
Note —At 3% feed solids by weight
(C) 25
(fi
Q
_i
O
20
15
100 200 300 400
FEED RATE (gal/h)
500 600
Note —At 3% feed solids by weight
Figure 9.
Rotary Vacuum Filter: (a) Basic Principle of Continuous Rotary Filtration,
(b) Filter Cake Capacity and Cake Dryness (from Filter Leaf Test), and
(c) Scale-Up Performance
The surface of the drum, which is
covered by a filter medium, consists
of a series of horizontal panels.
Vacuum is applied independently
to each panel by pipes inside
the drum; the pipes connect to a
common vacuum source, usually pro-
vided by a vacuum pump.
The filter has three basic operating
zones: filtering, cake drying (de-
watering), and cake discharge.
In the first zone, vacuum is applied
as a section of the drum submerges in
the slurry. A cake forms on the filter
medium as the solids are captured,
and the filtrate is drawn to the
vacuum source. The vacuum is main-
tained as the drum section rotates
out of the slurry into the second
zone. The vacuum removes additional
water and draws air through the
cake to promote further drying. In
the last zone, the discharge of
the cake is accomplished when the
vacuum is replaced with a blast of
air that separates the cake from
the medium.
Other means have been developed
to facilitate discharge of the filter
cake. In one variation, a series
of parallel strings, tied around the
drum, separate from the drum in
a tangential plane at the discharge
point, lifting the filter cake from
the medium. The strings pass around
a roller and the cake separates from
the strings and is discharged. In
another variation, the medium is sep-
arated from the drum, passes over a
roller where the cake is discharged,
and is washed before being di-
rected back to the filter drum by an-
other roller. These variations
were developed to make the rotary
filter more versatile—able to handle
slurries forming gelatinous cakes
that are difficult to discharge and,
consequently, that foul the filter
medium.
A third variation of the rotary drum
filter uses a precoat, usually diatoma-
ceous earth, that acts as the filter
23
-------
medium. As the drum rotates past a
scraper, a thin portion of the precoat
cake is removed along with the
collected solids, resulting in a clean,
unfouled surface each time a sec-
tion of the drum enters the slurry.
Precoat filtration provides excellent
filtrate quality and can remove
slimy solids that are difficult to filter
and that would rapidly foul a per-
manent filter medium.
Precoat filtration is generally used to
dewater dilute sludges because it
offers a high filtration rate per
unit of filter area. Precoat consump-
tion usually ranges from 5 to 20 Ib
(5 to 20 kg) for each 100 Ib
(100 kg) of sludge solids ($0.50-
$2/100 Ib sludge solids). The pre-
coat does add to the quantity of
solids for disposal, but often precoat
filtration yields a cake with
higher solids content than does
standard vacuum filtration.
Determining Applicability. The filter
leaf test20 is the common proce-
dure for evaluating the applicability
of vacuum filtration and determin-
ing required unit size. The filter
leaf is a small disk with drainage
grids similar to production filters. Fil-
ter cloths of different materials
and weaves can be attached to
the disk for evaluation.
The leaf is submerged in the agitated
slurry for a specified time, then re-
moved. After the vacuum has
been allowed to dry the cake for
a set period of time, quantity of filter
cake, cake solids percentage, and
filtrate volume are determined.
The test is repeated with different fil-
tration and drying times to determine
the most effective operating con-
ditions. The two key factors are
dry solids capacity in pounds per
hour per square foot (kilograms
per hour per square meter) of filter
area, and cake solids concentration.
Varying the test cycle time is equiv-
o
o
o
70
60
50
LLJ 40
a
30
20
10
Legend'
- 20
- 15
a
DC
a:
o
Q-
10
40 60 80
FILTER AREA (ft2)
100
120
aSkid-mounted unit complete with vacuum pump, separator, filtrate removal pump, and
internal piping
blncludes power for vacuum and filtrate pumps and for belt drives
SOURCE Equipment vendors
Figure 10.
Rotary Vacuum Filters: Unit Prices and Power Requirement
alent to varying the revolution
speed on thefilterdrum. Aftera num-
ber of runs, curves similar to those
in Figure 9b can be developed.
The figure shows that slowing the
drum revolution speed increases cake
solids concentration and reduces
filter capacity.
The curves can be used to predict
the performance of a full size
filter as a function of filtration area. As
the solids loading rate (or feed rate)
for a particular application is set,
the cake solids concentration
will depend on filtration area (Fig-
ure 9c). A disposal cost analysis
is needed for different filter sizes to
determine the optimum filter size
for a given application. The analysis
will require defining the disposal
cost of the sludge as a function
of solids concentration and obtaining
equipment and operating costs for
the filters.
It is also important in pilot testing
to determine how easily the cake re-
leases from the medium at the end of
each run. To determine the poten-
tial effect of cloth blinding, the
filter leaf test can be repeated a num-
ber of times while the different
variables are held constant. If the
collected filtrate is measured
after each run, any deterioration
24
-------
120 r-
100 -
=- 80 -
U)
O
u
U)
O
Q.
60 -
40 -
20 -
includes annual cost of vacuum filter
disposal only
100 200 300
CLARIFIER UNDERFLOW11 (gal/h)
400
aAt $043/gal for bulk 40,000-lb truckload shipped 300 mi to landfill Sludge dewatered
to 25% solids by weight.
bAt 3% solids by weight.
Note —4,800-h/yr operation.
Figure 11.
Rotary Vacuum Filters: Annual Sludge Disposal Costs
in filtration rate caused by medium
fouling can be observed. Selecting a
suitable filter cloth fabric and
weave can often reduce cloth blinding.
As a rule, rotary drum vacuum filters
perform best with feed solids
ranging from 3 to 5 percent by weight.
A thickener should be installed to
concentrate dilute sludge from
a clarifier. Precoating the filter is
an effective means of dewatering
dilute sludges. Without precoat, good
filtration rates and trouble-free cake
release are usually realized with a
sludge having solids that are not
too sticky or compressible. Chemical
and thermal conditioning or pre-
coat body feeding will often improve
sludge filtration characteristics.
Such conditioning can become eco-
nomically attractive in reducing
equipment size and improving
performance.
Vendors of filters, filter cloths,
filter aids, and sludge conditioners
will provide guidance in test pro-
cedures and supply product samples
to aid the potential customer in
evaluating the applicability of vacuum
filtration to a given sludge disposal
situation.
Costs. Figure 10 shows the unit
cost and power requirement for a
rotary vacuum filter as a function of
filter area. The cost is for a prepiped,
skid-mounted unit that includes
the filter, the vacuum pump and asso-
ciated vacuum lines, a vapor-liquid
separator, and a filtrate removal
pump. No costs are included
for handling the discharged sludge
cake. The major cost of operating
a vacuum filter is associated with
the power supplied to the vacuum and
filtrate pumps and the drum agitator
drives. The unit operates continuously
and should only require operator
attention for maintenance and
repairs. As with most membrane
filters, the filter medium requires peri-
odic cleaning with an acid or alka-
line solution to remove fouling
agents. Regular filter cloth replace-
ment is also necessary, but should
not constitute a major expense.
Figure 11 shows the annual disposal
cost for sludge dewatered using
a rotary vacuum precoat filter
as a function of the clarifier underflow
rate. Total disposal cost includes
the cost of operating the filter
as well as that of hauling and land-
fill disposal. A typical precoat
filtration rate of 12 gal/h/ft2 (489
L7h/m2) was used to determine
25
-------
the necessary filtration area. It is
assumed that the filter dewaters the
underflow from 3 to 25 percent
solids by weight. Using the cost
components indicated, the vacuum
filter realizes a cost reduction
compared with sludge disposal at
3 percent solids by weight when
the underflow exceeds approximately
10 gal/h (38 L/h)—even though at
that feed rate the smallest commer-
cial unit available would have
considerable excess capacity.
The savings in hauling and disposal
cost justify investment in the
equipment. For a plant disposing
of its sludge at $0.43/gal, the
installation of a vacuum filter with a
19-ft2 (1.8-m2) filter area yields
a 30 percent after-tax return on in-
vestment when the feed rate exceeds
approximately 30 gal/h (113 L/h).
The investment required for installa-
tion of a 19-ft2 (1.8-m2) filter would
total $73,000.
Basket Centrifuges
The Equipment. Centrifuges dewater
sludge in a manner similar to gravity
thickening, but by rapidly rotating
the sludge they create an apparent
gravity thousands of times more
powerful than normal. The centrifugal
force thus created speeds up
settling and magnifies the compac-
tion effect, making centrifuges
most suitable for compressive sludges
that settle well. Several centrifuge
types are available commercially, in-
cluding basket, scroll, and disk
centrifuges. Only the basket centri-
Motor
Basket
Polymer
feed pipe
Sight glass
Ce'ntrate
discharge
Feed inlet
fuge is used to any degree to
dewater plating sludge.
The basket centrifuge (Figure 12)
is a vertical rotating bowl that
has a lip extending inward at the top.
Sludge is introduced into the
bottom of the unit and the solids,
owing to their greater density,
are thrown against the inner wall of
the basket. When the basket
becomes full, clarified liquid (or
centrate) is decanted over the inner
lip and removed from the unit.
The rotating basket comprises two
zones: against the outer wall of
the basket is the solids retention
zone, which contains the accumulated
sludge solids; the rest of the basket
constitutes the clarification zone,
which separates the solids from
the incoming feed. As the cycle con-
tinues, the volume of accumulated
solids increases and consequently re-
duces the capacity of the clarifica-
tion zone until the residence
time of the fresh feed in the clari-
fication zone is insufficient to settle
out the suspended solids. At this
point, the level of solids in the
centrate increases dramatically.
This change, or "breakover," is
detected by a monitor. The feed is
cut off and a skimmer is run into the
basket to remove excess water
from the cake surface. The basket
then decelerates from operating
speed (anywhere from 1,000
to 3,000 r/min) to approximately
75 r/min. A plow enters the basket
and pushes the cake out at the
bottom of the centrifuge. As the plow
retracts, the basket is accelerated
and the feed is resumed.
The time required for the phases
of the operating cycle when the unit
is not receiving feed usually varies
from 6 to 8 min. A unit of this
type has a feed rate up to 60 gal/min
(225 L/min), with solids recovery
of 50 to 95 percent. It can produce
a sludge cake ranging from 10
to 25 percent solids concentration.
Figure 12.
Basket Centrifuge
26
-------
Determining Applicability. Centri-
fuges are effective for dewater-
mg sludges that contain solids of an
apparent density greater than
water, that is to say, solids that want
to settle. Usually the feed is treated
with polymer conditioning agents
to improve the settling character-
istics of the solids. Centrifuges,
unlike some filters, are well suited
for dewatermg compressible sludges.
They are also attractive because
of their compact size and automated
operation.
Cake solid concentration obtainable
with a given sludge can be deter-
mined by testing the sludge on
a bench-scale centrifuge. Most ven-
dors have a number of these units
and will perform field tests or
do the work in their own laboratories
if a feed sample is supplied.
The pilot test procedure determines
the required size and performance
of a solid bowl centrifuge for a
particular sludge. Tests are per-
formed to determine whether polymer
conditioning is needed and, if so,
the optimum dose. The unit's
performance is evaluated at the
optimum dose with varying feed ca-
pacities. Normally, the cake solids
concentration decreases with
increasing feed rate, as does solids
recovery (the percentage of feed
solids contained in the filter cake).
There is a disadvantage, however,
in many centrifuge applications;
owing to high rotation speed,
if rotating elements are not well
maintained, frequent breakdown can
occur. This problem is particularly
likely with sludges containing
abrasive solids. Moreover, consider-
able power is required to operate
the unit. New low-speed units,
capable of operating with improved
power efficiency, are being marketed.
Costs. Figure 1 3 shows the unit
cost for the two available classes of
basket centrifuge. These units
typically use 75 percent of the avail-
able power during the feed-and-skim
stage of the cycle. Figure 13a
(a)
60
? 40
cc
cc
20
cc
Q
Legend/
-i 110
100 §
L>
CC
Q_
90
80
10
BASKET VOLUME (ft3)
15
20
SOURCE Equipment vendors
(b) ^ 15r
10
0
I
I
0123
BASKET VOLUME (gal)
SOURCE Equipment vendors
Figure 13.
Basket Centrifuges: (a) Large Unit Price and Hydraulic Drive Horsepower and
(b) Small Unit Price
includes the horsepower rating of
the unit drive for the large centrifuge.
Large units are available with basket
volumes between 8 and 16 ft3
(0.23 and 0.45 m3).
Small units, which are not automati-
cally cycled, are available with
basket volumes of 1 to 2 gal (3.8 to
7.5 L). Small units have minimum
installation requirements, and
their unit price (Figure 13b) is
markedly lower than that of the large
units. They are well suited for
small operations disposing of drum-
load quantities of sludge. The de-
watered sludge can be discharged
directly into the drum. The 2-gal
(7.5-L) unit can handle as much as
100 gal/h (378 L/h) of sludge
feed. Solids recovery, however.
27
-------
(a)
Sludge feed sump
(b)
150 i-
125 -
8
o
=. 100 -
o
Q_
55
Q
50 —
25 -
Note —PD = positive displacement.
Legend:
Includes annual cost of centrifuge
Hauling and disposal only
I
I
0 100 200 300 400
CLARIFIER UNDERFLOW* (gal/h)
"At $0.43/gal. Sludge dewatered to 20% solids
by weight.
bAt 3% solids by weight.
Note.—4,800-h/yr operation.
Figure 14.
Basket Centrifuge Systems: (a) Dewatenng System with Auxiliary Equipment and (b) Annual Sludge Disposal Costs
28
-------
tends to deteriorate at the higher
flow rates.
The unit costs in Figure 13 include
the centrifuge and drive system
and the control hardware needed to
cycle the unit properly. Other
associated costs are for site prepa-
ration, feed pump, centrate removal,
and piping. Operating cost will be
primarily for power to operate
the unit and for polymer conditioning
agents, which are commonly used
to improve the performance of
the unit and increase its processing
capacity. The small manual units
will also require operating labor to
cycle the unit. The large units
are automated and should require
minimum operator attention.
The small units are reasonable in
cost, but the operating labor required
is excessive for dewatering systems
with average feed rates over 15
gal/h (57 17h). As an example,
a 2-gal (7.5-L) basket centrifuge
dewatering sludge from 3 to 20 per-
cent solids by weight could proc-
ess approximately 13 gal (50 Lj
of sludge feed per cycle. Where the
sludge feed rate is 50 gal/h (190 L/h),
the unit would have to be cycled four
times per hour. Each cycle requires
operator control, so labor cost
would be excessive compared with
that of other equipment.
Figure 14a is a flow diagram of
a typical basket centrifuge sludge
dewatering system. Figure 14b shows
the annual cost, based on clarifier
underflow rate, of disposal of
sludge dewatered in a basket centri-
fuge. Based on the assumptions
in Figure 14b, at underflow rates
exceeding 1 8 gal/h (68 L/h) the cen-
trifuge system nets a reduction in
annual disposal cost compared
with disposal of sludge without de-
watering. At a disposal cost of
$0.43/gal, installing a 2-gal (7.5-L)
basket centrifuge will generate
a reasonable after-tax return on in-
vestment (over 30 percent) for a
clarifier underflow greater than 1 8
gal/h (68
Pressure Belt Filters
The Equipment. The pressure belt
filter (Figure 1 5) is finding increased
application because it offers
certain advantages over other
commonly used dewatering devices.
This filter is especially suitable for
dewatering the large, highly
compressible particle floe char-
Gravity drainage
stage
Sludge dumps
to new belt:
internal water
released
Dewatered
cake
Flocculated
sludge
Sludge dumps
to new belt:
internal water,
released
Low pressure
stage
High pressure
stage
Medium pressure
stage
Figure 15.
Pressure Belt Filter
29
-------
acteristic of polymer-treated
sludges. A common problem with
such sludges is that, when subjected
to a pressure gradient, the solid
particles collapse against the filter
medium and block the transport
of water through the medium.
The belt press eliminates this prob-
lem by using gravity to remove
most of the water. Then, as the belt
travels through successive regions, a
gradual increase in pressure forces
additional water from the sludge.
In the first stage of unit operation,
the polymer-dosed sludge is spread
over a slow-moving filter cloth
belt and any free water drains off.
To be suitable for further processing,
the sludge should form a cohesive,
continuous blanket in this region.
The sludge blanket leaves the
drainage section and enters the
mild compression zone. It is com-
pressed between water permeable
membranes, more water is forced out,
and the sludge layer becomes a
more nearly solid mass. The more
cohesive the sludge layer becomes,
the more compressive force it can
adsorb without extruding through the
filter medium or being forced from
between the belts. The compressive
force gradually increases as the
sludge layer travels through the unit—
some models have compressive
limits as high as 100 Ib/in2 (680
kPa). Sludge properties, cake
thickness, time under compression,
and the magnitude of the compres-
sive force all influence the cake
dryness.
The capacity of a belt press is
determined by belt width and belt
speed. Belt width depends on
the model selected, and ranges
from 1 to 10 ft (0.3 to 3 m). Belt speed
sets the time the sludge will travel
through the press. Unit capacity
can be increased by adjustment to
the belt speed to compensate for
a higher feed rate, but only to a
limited degree. The major criterion
for good filter operation is formation
of a cohesive, solid sludge blanket
in the gravity drainage zone.
When feed rate increases, the belt
speed will normally be lowered
to allow additional drainage time;
however, as with other filtration
equipment, cake dryness will usually
fall off as feed rate increases. For
greater flexibility in meeting chang-
ing feed conditions, some units
have separate filter belts and
speed controls for the gravity de-
watering and compression zones.
This design also permits the use in
each zone of a filter medium designed
specifically for that zone.
Determining Applicability. Pressure
belt filters are suitable for polymer-
treated sludge that drains well
and forms a cohesive, compressible
sludge cake when dewatered.
The best way to determine their
performance on a specific sludge is
to have a press manufacturer perform
pilot tests with a bench-scale unit.
The pilot testing will determine
the performance of the filter in terms
of cake dryness and solids capture
efficiency at varying polymer
dose rates, thus establishing the
optimum polymer dose. Once
the optimum dose is known, the
performance of the unit as a function
of feed rate can be determined.
From these relationships, the size
and performance of full-size units can
be estimated.
Costs. Figure 16 shows the cost
and power requirement of a high-
pressure belt filter package unit
as a function of belt width. The pack-
age unit comes complete with a
Legend
o
o
o
140 r~
120
100
80
60
T 50
40
30
20
f-
O
O
Q.
10
4 6
BELT WIDTH (ft)
10
12
''Skid-mounted unit complete with polymer feed pump and dilution tank, flash mix chamber,
sludge feed, and wash water pump
^Includes power for press drives, conditioning tank drive, polymer feed pump, polymer tank
mixer, and belt wash pump (if needed)
SOURCE Equipment vendors
Figure 16.
Pressure Belt Filter: Unit Price and Power Requirement
30
-------
polymer-mixing and feed system,
and includes belt-washing auxiliaries.
The necessary piping and valves
are preassembled and the unit
is skid mounted. The cost range
shown is representative for high-
pressure units; however, prices vary
among the different systems
marketed, mainly because of varia-
tion in belt configuration and degree
of compression achieved.
Belt presses have a major advantage
over centrifuges and vacuum filters
in that they consume consider-
ably less power. Depending on the
size and manufacturer of the belt
press system, total power consump-
tion ranges from 5 to 30 hp (4 to
22 kW). Centrifuges and vacuum
filters, sized to accomplish the same
service, consume between 15
and 100 hp (11 and 75 kW).
Other associated operating costs
include chemicals for polymer
conditioning and wash water for
continuous cleaning of the filter be It.
Polymer cost can be significant in
the operation of belt filters; for
some sludges, optimum polymer dose
per ton (megagram) of dry solids
can be as high as 200 Ib (100 kg).
Polymer dose rates generally
range from 10 to 50 Ib/ton (5 to 25
kg/Mg), at a cost of $30/ton to
$150/ton of dry solids. Rates for
wash water to clean the filter belt of
fouling agents vary from 10 gal/
min (38 L/min) for small units to 100
gal/min (380 L/min) for larger
models. The wastewater load asso-
ciated with the belt wash water
has been reduced significantly by
use of the filtrate from the gravity de-
watering zone of the press to
supply part, if not all, of the wash
water.
In Figure 17, the annual cost of
sludge disposal associated with
pressure belt filtration is shown as
a function of clarifier underflow
rate, assuming dewatering to 20 per-
cent solids by weight. The pressure
Legend:
includes annual cost of belt filter
disposal only
o
Q_
5
_j
D
Z
<
120
100
80
60
40
20
J_
0 50 100 150 200
CLARIFIER UNDERFLOW" (gal/h)
dAt $0 43/gal Sludge dewatered to 20% solids by weight
bAt 3% solids by weight
Note —4,800-h/yr operation
250
Figure 17.
Pressure Belt System: Annual Sludge Disposal Cost
belt unit size used in the cost
analysis was determined by assum-
ing a maximum hourly feed rate of
approximately 300 gal/ft (37 L/cm) of
belt width. At this feed rate, a unit
with a belt 1 ft (30 cm) wide (the
smallest size available) could
process about 75 Ib (34 kg) of dry
solids per hour. Based on the
disposal cost formula used in Figure
17, the pressure belt filter yields
a cost reduction compared with dis-
posal at 3 percent solids when
the underflow rate exceeds 10 gal/h
(38 L/h)—despite the unit's high
initial cost and its low utilization at
that feed rate.
For a plant disposing of its sludge
at $0.43/gal, the pressure belt
31
-------
Pressure belt filter
Table 8.
Comparative Total Investment and Annual Operating Costs for Sludge Dewatering
Costs ($)
Feed
sludge
volume3
(gal/h|
50
100
1 50
200 . . .
250
300
Filter
press
Installed c
h Annual
investment
. . 29,900
. . 39,100
.. . 39,100
. . 51,700
51,700
. .. 51,700
23,000
37,000
51,000
63,000
76,000
89,000
Precoat
vacuum
Installed
investment6
73,000
73,000
73,000
78,000
78,000
78,000
rotary
filter
Annual0
24,000
39,000
55,000
68,000
80,000
93,000
Basket
Installed
investment
34,000
34,000
48,000
1 74,000
1 74,000
1 74,000
centrifuge
b Annual0
37,000
70,000
85,000
89,000
102,000
115,000
Pressure
Installed
investment11
1 04,400
1 04,400
1 04,400
1 04,400
104,400
125,000
belt filter
Annual0
31,000
50,000
69,400
88,000
107,000
1 26,000
aAssumed at 3% solids by weight.
blncludes all system auxiliaries.
"Includes equipment operating, fixed, and sludge disposal costs
Note.—1981 dollars.
32
-------
filter installation with a belt 1 ft (30
cm) wide would have a reasonable
rate of return (30 percent return
on investment after taxes) at feed
rates exceeding 50 gal/h (190 L/h).
Table 9.
Economic Evaluation of Precoat Rotary Vacuum Filter Sludge Disposal
Alternative
Item
Cost
Evaluating the Cost for Sludge Installation of modifications ($)•"
Dewatering Alternatives E—^ 45000
Auxiliary equipment 11,500
The foregoing sections have de-
scribed the operation, cost, and per- Total equipment cost 56,500
formance of equipment types installation 4-500
commonly used for sludge dewater- „ „„„
.. ... ... Total cost including installation 61,000
ing. Vacuum filters, filter presses. Contingency, 20% of total mcludmg installation 12,200
centrifuges, and belt presses
have all found application for de- Total installed cost 73,200
watering sludge from metal finishing "
waste treatment. It is not usually Annual costs ($/yr).b
necessary, however, to evaluate each Flxed costs
alternative before selecting a de- Depreciation on equipment 7,300
° Taxes and insurance 1,600
watering system. Some general Cost of capltal NA
guidelines follow.
Total fixed costs 8,900
If disposal costs are less than
$15,000/yr, it is unlikely that de- ^"^^
watering equipment would be justi- Power at $o.o5/kWh (io hp at 0.75 kWh/hp) 500
fied economically. Many landfill Precoat chemicals at $0.1 o/ib 700
Sites can solidify or dewater dilute Operating labor, at $10/h (0.5 h/8 h operation) 700
sludge, and their capabilities Maintenance 1.000
should be used. Total equipment opera,,ng cos, 2,90o
Sludge disposal fee, at 25% solids and $043/gal 12,380
The lowest cost alternatives in —
terms of capital investment are filter Total annual operating cost 15,280
presses and Small manual basket Total annual cost including fixed cost 24,180
centrifuges. Minimum size versions
of both systems can be installed , .
A (tin r\nr> Investment justification:
for under $3O,000. Current disposal cost at 3% solids ($/yr) 103,000
Reduction from current cost ($/yr) 78,820
The Small filter press System, Average return on investment (%)d 59
although equal in COSt to the Centri- Investment payback (yr)e K5
fuge. will usually have more capacity. aElevated Insta,,atlon of precoat rotary vacuum ,,,ter wlth 19.ft2 fllter area.
At low feed rates, the cost per unit
of capacity is lowest for the filter Based on 1 -050 h/vr of f"ter °Pera"on-
press. The lOW capacity per cycle Of C4,800 h/yr and 22% operating factor.
the basket centrifuge will require d($78,820X0.55)/$73,200 (0.55 based on a 45% tax rate).
significant operating labor at =$73,2oo/[($78,820 x 0.55) + 7,300],
flow rates above 10 to 15 gal/h
(38 to 57 L/h) Note.—1981 dollars. Dewatering 50 gal/h from 3% to 20% solids by weight. NA = not applicable.
Poor-filtering sludges can usually
be dewatered by precoat vacuum fil-
tration. With polyelectrolyte con-
ditioning, most sludges can be
dewatered effectively with a centri-
fuge or belt filter.
Table 8 compares the investment
and annual operating costs of
the four equipment alternatives for
flows ranging from 50 to 300
gal/h (190 to 1,135 L/h). At all levels,
the filter press was least costly in
general; however, at the higher
range of flows the cost advantage
was less significant. Table 9 gives the
cost factors included in the analysis,
using the example of a precoat
vacuum filter dewatering 50 gal/h
(190 L/h) of clarifier underflow. The
investment had an excellent return,
with payback after only 1.5 yr.
33
-------
Table 10.
Sludge Disposal Under Four Dewatering Alternatives: Analysis of Annual Costs
With installed modifications
Item
Disposal solids concentration (% by weight) ....
Cost of modifications ($)
Annual cost of modifications ($}.
Fixed
Operating
Total annual cost ($)
Annual savings ($)
Average return on investment (%)' . .
Investment payback (yr)9
Present
conditions Filter
press8
... 3 25
.... — 29 900
— 3 100
. . — 7,600
1 03 000 1 2 300
103,000 23,000
— 80,000
— 147
— 06
Precoat
rotary
vacuum
filterb
25
73 200
8 900
2,900
12 300
24,100
78,900
59
1.5
Basket
centrifuge0
20
34 000
4 000
1 6,400e
16 600
37.000
60,200
97
09
Pressure
belt
filterd
20
104 400
12 500
2,500
16 000
31,000
68,200
36
22
"5-ft filter capacity, 4-h cycle time
b19-ft2 filter area
°Batch solid bowl centrifuge with 2-gal basket
d1-ft-wide belt
eDoes not include cost of polymer treatment, which may be required.
'(Annual savings X 0 55)/total investment (0 55 based on a 45% tax rate).
9Total mvestment/[(annual savings X 0.55) + depreciation].
Note—1981 dollars 50 gal/h clanfier underflow.
Comparing the vacuum filter with
the other equipment types (Table 10),
however, shows that equipment
payback ranges from 0.6 yr for a filter
press to 2.2 yr for a belt filter press.
The filter press proves the best
choice, mainly because of the low
investment and manpower require-
ments. The 5-ft3 (0.14-m3) cake
volume of the press would only need
dumping every 6 h. A larger press
could be selected to reduce labor, but
the investment would be greater.
Of course, if pilot testing indicated a
sludge with poor filtration proper-
ties, either the properties would
have to be modified or different equip-
ment would have to be selected.
34
-------
References
'U.S. Environmental Protection
Agency. "Hazardous Waste Man-
agement System: Identification
and Listing of Hazardous Wastes."
Federal Register 45(98):33084-
33133, May 19, 1980.
2National Association of Manufac-
turers. Hazardous Waste Man-
agement Under RCRA, A Primer for
Small Business. Washington DC,
NAM, Oct. 1980.
3Richard W. Grain. "Solids Removal
and Concentration." In U.S.
Environmental Protection Agency
and American Electroplaters'
Society (cosponsors). Third Con-
ference on Advanced Pollution
Control for the Metal Finishing In-
dustry. EPA 600/2-81-028.
Feb. 1981.
4U.S. Environmental Protection
Agency. "Hazardous Waste Man-
agement System: Standards for
Generators of Hazardous Waste."
Federal Register 45(98):33140-
33148, May 19, 1980.
5U.S. Environmental Protection
Agency. "Hazardous Waste Man-
agement System: Standards for
Transporters of Hazardous Waste."
Federal Register 45(98):331 50-
33152, May 19, 1980.
6U.S. Environmental Protection
Agency. "Hazardous Waste Man-
agement System: Standards for
Owners and Operators of Hazardous
Waste Treatment, Storage, and
Disposal Facilities." Federal Reg-
ister 45(98):331 54-33258,
May 1 9, 1 980.
7U.S. Department of Transportation.
"Transport of Hazardous Wastes
and Hazardous Substances."
Federal Register 45(101 ):34560-
34705, May 22, 1980.
8U.S. Environmental Protection
Agency. "Hazardous Waste Man-
agement System: Suspension of
Rules and Proposal of Special
Standards for Wastewater Treat-
ment Tanks and Neutralization
Tanks." Federal Register 45(223):
76074-76083, Nov. 17, 1980.
9U.S. Environmental Protection
Agency. "Hazardous Waste Man-
agement System: Proposal to
Modify 40CFR Part 265—Sub-
part H—Financial Requirements."
Federal Register 45(98):33260-
33278, May 19, 1980.
10National Paint and Coatings
Association. Hazardous Waste
Treatment and Disposal Facilities.
Environmental Bulletin. Wash-
ington DC, NPCA, Feb. 1980.
11 U.S. Environmental Protection
Agency. Information About Haz-
ardous Waste Management
Facilities. NTIS No. PB 274881.
July 1975.
12Roy, Clarence. "Methods and
Technologies for Reducing the
Generation of Electroplating
Sludges." In U.S. Environmental
Protection Agency and American
Electroplaters' Society (co-
sponsors). Second Conference on
Advanced Pollution Control for
the Metal Finishing Industry. EPA
600/8-79-014. NTIS No. PB
297453. Feb. 1979.
13Steward, F. A., and Leslie E. Lancy.
"Minimizing the Generation of
Metal-Containing Waste Sludges."
In U.S. Environmental Protection
Agency and American Electro-
platers' Society (cosponsors). First
Annual Conference on Advanced
Pollution Control for the Metal
Finishing Industry. NTIS No.
PB 282443. Jan. 1978.
14U.S. Environmental Protection
Agency. Environmental Pollution
Control Alternatives: Economics of
Wastewater Treatment Alter-
natives for the Electroplating Indus-
try. EPA 625/5-79-01 6.
June 1979.
15U.S. Environmental Protection
Agency. Control and Treatment
Technology for the Metal Finishing
Industry: Sulfide Precipitation.
EPA 625/8-80-003. Apr. 1980.
35
-------
16U.S. Environmental Protection 18U.S. Environmental Protection 19Crampton, Peter. "Sludge Handling
Agency. Control and Treatment Agency. In-Process Pollution and Disposal." Paper presented at
Technology for the Metal Finishing Abatement: Upgrading Metal- U.S. Environmental Agency
Industry: In-Plant Changes. Finishing Facilities to Reduce Pol- ' Seminar on Handling Electro-
EPA 625/8-82-008. Jan. 1982. lution. EPA 625/3-73-002. NTIS plating Wastes, Philadelphia PA,
No. PB 260546. July 1973. Sept. 30, 1980.
17Kushner, Joseph B. Water and
Waste Control for the Plating Shop. 20Liptak, B. G. (ed.) Environmental
Cincinnati OH, Gardner Publi- Engineers Handbook. Vol 1.
cations, 1976. Radnor PA, Chilton Book Co.,
1974. P. 905.
U.S. Environmental Protection Agency
Region v 'JV^'V
230 3o i-cn Do-vbo-n Street -'
Chicago, Illinois 60504
36
Tfr U S G PO -1982 - 561- 644
------- |