United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S8-87/006 May 1987
SEPA Project Summary
A Handbook on Treatment of
Hazardous Waste Leachate
Judy L McArdle, Michael M. Arozarena, and William E. Gallagher
Twenty unit processes were re-
viewed for their applicability to the
treatment of hazardous waste leachate.
These processes are classified into four
categories as follows: pretreatment op-
erations, including equalization, sedi-
mentation, granular-media filtration,
and oil/water separation; physical/
chemical treatment operations, includ-
ing neutralization, precipitation/floccu-
lation/sedimentation, oxidation/
reduction, carbon adsorption, air strip-
ping, steam stripping, reverse osmosis,
uftrafiltration, ion exchange, and wet-
air oxidation; biological treatment op-
erations, including activated sludge, se-
quencing batch reactor, powdered
activated carbon treatment (PACT), ro-
tating biological contactor, and trick-
ling filter; and post-treatment opera-
tions, including chlorination. Typical
treatment process trains (i.e., combina-
tions of the above unit processes) are
presented for leachate containing or-
ganic and/or inorganic contaminants.
Management of treatment process
residuals (chemical/biological sludges,
air emissions of volatile organic com-
pounds, concentrated liquid waste
streams, spent carbon) is also ad-
dressed.
This Project Summary was devel-
oped by EPA's Hazardous Waste Engi-
neering Research Laboratory, Cincin-
nati, OH, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
The U.S. Environmental Protection
Agency (EPA) hazardous waste site
cleanup program, referred to as Super-
fund, was authorized and established in
1980 by the enactment of the Compre-
hensive Environmental Response, Com-
pensation, and Liability Act (CERCLA),
Public Law (PL) 96-510. This legislation
allows the Federal government (and co-
operating State governments) to re-
spond directly to releases and the threat
of releases of hazardous substances
and pollutants or contaminants that
could endanger public health or welfare
or the environment. Prior to the passage
of PL 96-510, Federal authority with re-
gard to hazardous substances was
mostly regulatory in nature through the
Resource Conservation and Recovery
Act (RCRA) and the Clean Water Act and
its predecessors.
Public Law 96-510 and the regulations
based on it not only govern accidental
releases that may occur from time to
time, but also releases that already have
taken place and continue to take place
from uncontrolled waste-disposal sites.
Leachate is one type of release covered
by this law. It is formed when water per-
colates through a waste-disposal site,
and if not properly contained and col-
lected, it can threaten the local hydroge-
ologic environment. The objective of
this handbook is to provide guidance in
the treatment of hazardous waste leach-
ate.
Leachate Generation
Leachate is generated by the move-
ment of water through a waste disposal
site. Precipitation falling on the land sur-
face will either infiltrate the cover soil or
leave the site as surface runoff, depend-
ing on surface conditions. Infiltrated
water that is not subsequently lost by
evapotranspiration or retained as soil
moisture will percolate down through
the waste deposit. Initially, this liquid
will be absorbed by the waste material.
-------
When the field capacity (moisture-
retention capacity) of the waste is ex-
ceeded (which may take from several
months to several years), leachate will
be produced. At waste-disposal sites
with no provisions for collection, this
leachate can contaminate underlying
ground-water aquifers or nearby sur-
face streams.
Leachate generation (flow) varies
greatly from site to site and over time at
the same site. Among the many factors
contributing to this variability are the
local climate and meteorology, site to-
pography, cover soil and vegetation,
and site hydrogeology.
On the average, leachate is generated
in low to moderate flows (less than
100,000 gal/day); however, seasonal
and day-to-day fluctuations in leachate
volume can have a significant impact on
the design of a leachate treatment plant.
With continuous treatment operations,
some form of flow equalization is nor-
mally required to handle peak flows and
to optimize plant performance. Proc-
esses that can be operated intermit-
tently have the advantage of being able
to meet increased or decreased treat-
ment demands over the life of the plant.
Leachate Characteristics
As water percolates through a waste
deposit, it solubilizes (leaches) various
components of the waste and becomes
polluted. This leachate typically exhibits
high concentrations of dissolved organ-
ics (BOD5, COD, TOO, toxics (TOX), and
metals; high color, odor, and turbidity;
and low pH.
The characteristics of leachate vary
widely from site to site as well as from
one site over a long period of time. The
factors having the greatest effect on
leachate composition are those that in-
fluence the degradation of the waste
and those that affect the mobilization of
waste components and degradation
products.
The chemical and physical character-
istics of leachate are the primary consid-
erations in the design of a treatment
system. The technologies applicable to
hazardous waste leachate treatment are
essentially the same as those applied to
municipal wastewater and contami-
nated ground-water treatment; how-
ever, hazardous waste leachate is typi-
cally more concentrated and contains a
wider range of organic and inorganic
contaminants than municipal waste-
water or ground water, and multistage
treatment is often required. The proper
combination of pretreatment, physical/
chemical treatment, biological treat-
ment, and post-treatment operation?
must be determined during the design
phase to optimize the cost-effectiveness
of treatment.
Treatability of Leachate
Constituents
Leachate characterization studies are
designed to ascertain the type and con-
centration of constituents in the waste
stream as well as the magnitude of vari-
ations in leachate flow rate and
strength. Data from leachate characteri-
zation studies are useful in the screen-
ing of potentially applicable treatment
technologies and as a baseline for eval-
uating the effectiveness of selected
technologies.
When the characteristics of a particu-
lar leachate stream have been ascer-
tained, potentially applicable processes
for conversion or removal of target con-
taminants can be identified from the
matrix in Figure 1. Each block of the ma-
trix contains a "+", an "o", or a "-".
Reading down a column for a contami-
nant of interest indicates which proc-
esses are effective in removing that con-
taminant (+). Reading across the row
for a technology indicates the con-
stituents that must be removed by pre-
treatment (-) to assure satisfactory per-
formance of that technology. For
example, volatile organics can be re-
moved from leachate by air stripping;
however, suspended solids and oil and
grease (which cause plugging of the
packed bed) should be removed by pre-
treatment. Constituents that are neither
removed by the technology nor require
removal by pretreatment prior to appli-
cation of the technology are indicated
by an "o".
The process applicability matrix can
be used to screen potential treatment
technologies for their applicability to
leachates whose compositions are
known. Treatability studies should then
be performed to guide in the selection
of the most cost-effective treatment al-
ternative from among the potentially
applicable technologies for a combina-
tion of leachate constituents. These
studies examine the actual effective-
ness of alternative methods as well as
define design and operating standards.
Treatability studies can be divided
into two groups—bench-scale and pilot-
scale—which differ in purpose, scale,
cost, time, and leachate volume re-
quired. Although the distinction is not
always clear, bench-scale studies are
generally used for the preliminary eval-
uation and selection or rejection of the
most promising treatment technolo-
gies, whereas pilot-scale studies are
generally used to develop and optimize
design and operating parameters of the
selected process(es).
Leachate Treatment Process
Train Selection
Treatment of hazardous waste leach-
ate is complicated by the diversity of
organic and inorganic constituents that
it contains. To effect a high degree of
treatment efficiency requires several
unit operations with specific applica-
tions and limitations. Because the char-
acteristics of hazardous waste leachate
vary considerably from one site to the
next, selection and integration of unit
treatment processes are highly site-
specific. Among the factors that influ-
ence selection are effluent discharge
alternatives/limitations, treatment proc-
ess residuals, permit requirements, and
cost-effectiveness of treatment.
Leachate containing primarily inor-
ganic contaminants can be treated by a
combination of physical/chemical proc-
esses. A typical process train might in-
clude equalization, oxidation/reduction,
precipitation/flocculation/sedimenta-
tion, neutralization, and granular-media
filtration. This process train is effective
for removing most metals, including
hexavalent chromium and soluble
metal-cyanide complexes.
Leachate containing primarily organic
contaminants can be treated effectively
by stripping, adsorption, and/or biologi-
cal treatment processes. Biological
treatment processes are typically pre-
ceded by equalization and neutralize
tion for protection of the microorgan
isms from toxic or inhibitory condition:
and followed by sedimentation and/o
filtration for separation of biologica
solids. For high-strength leachate, tw<
biological units can be used in sequenci
(e.g., a trickling filter followed by ai
activated-sludge system), with the firs
serving as a roughing unit for partia
degradation of the organics. Strippim
and adsorption processes, on the othe
hand, are typically preceded by sedi
mentation and/or filtration for prever
tion of plugging of the packing materu
or granular activated carbon. The mos
cost-effective treatment of leachate cor
taining biodegradable and refractory o
ganics includes a combination of bic
logical and adsorption processes
Normally, biological treatment pr<
cedes carbon adsorption in the proces
train. With this arrangement, the biolot
-------
Technology
Sedimentation
Granular-media filtration
Oil/ water separation
Neutralization
Precipitation/flocculation
sedimentation
Oxidation/ reduction
Carbon adsorption
Air Stripping
Steam stripping
Reverse osmosis
Ultrafiltration
Ion exchange
Wet-air oxidation
Activated sludge
Sequencing batch reactor
Powdered activated carbon
treatment (PACT)
Rotating biological contactor
Trickling filter
Chlorination
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not applicable for
applicable unless the leachate is
pretreated
for removal of the contaminant.
Figure 1. Process ano/icability matrix.
ical operation substantially
reduces the
downstream organic loading on the car-
bon adsorbers. Biological
and carbon adsorption can
degradation
also be per-
formed in a
single
operation.
as in the
patented PACT process.
In most
cases.
leachate contains
hazardous waste
both
inorganic and
organic contaminants, and the treat-
ment trains required to treat these
waste streams involve combinations of
the process schemes described previ-
ously. The best overall treatment effi-
ciencies generally can be achieved by
removing the inorganic constituents
first and then the organic constituents.
This approach protects the biological,
adsorption, and stripping processes
from problems caused by metals toxic-
ity, corrosion, and scaling.
The Stringfellow leachate pretreat-
ment plant in Glen Avon, California (Fig-
ure 2) illustrates a typical process train
for leachate containing both inorganic
and organic contaminants. Metals are
removed from Stream A by precipita-
tion/flocculation/sedimentation. The
clarifier overflow is filtered and then
mixed with Stream B. Organics are re-
moved from the combined leachate
stream by carbon adsorption, and the
effluent is discharged to a POTW.
Table 1 summarizes the process
trains that have been selected or pro-
posed for treatment of hazardous waste
leachate at Superfund sites. This table
also contains case-study examples of
process trains that incorporate innova-
tive treatment technologies [e.g., se-
quencing batch reactor, powdered acti-
vated carbon treatment (PACT), and
wet-air oxidation]. The information in
this table was compiled from a review
of approximately 130 Records of Deci-
sion available as of June 1986 and from
responses to inquiries in each of the
EPA Regions. A limited number of site
visits were conducted to gather operat-
ing and performance data; these data
are reported in the technology profiles.
Leachate Treatment Unit
Processes
Twenty unit processes were reviewed
for their applicability to the treatment of
hazardous waste leachate. The tech-
nologies are classified as pretreatment
operations, physical/chemical treat-
ment operations, biological treatment
operations, and post-treatment opera-
tions. The order in which the technolo-
gies within each category are presented
reflects the reliability of the processes
for leachate treatment applications (i.e..
technologies that have been widely
demonstrated are presented first; inno-
vative technologies or technologies that
have not been demonstrated with haz-
ardous waste leachate are presented
last).
-------
Stream A
Notes:
Lime/
Caustic Polymer
1 1 Clarifier
Mixed
Media
Filtration
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ualization
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Holding
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Stream B
Pressure
Filter
Dewatered
Sludge to
Class 1
Disposal Site
Dirty Filter
Backwash
Storage
Equalization
1. Stream A is from wells OW-1, OW-2. OW-4. IW-1 and the French drain.
Average flow is expected to be 20 gpm. Design flow is 50 gpm.
2. Stream B is from mid-canyon wells IW-2 and IW-3. Average flow is expected
to be 40 gpm. Design flow is 80 gpm.
3. Currently (February 1986), influent is trucked from wells and French drain. In
the near future influent will be pumped directly into the plant.
Figure 2. Flow diagram of the Stringfellow leachate pretreatment plant.
Carbon cleTn
Adsorption Backwash
Contact and effluent
Storage
Trucked to
POTW
Carbon
Transfer
Vessel
The applicability of the profiled tech-
nologies to the treatment of hazardous
waste leachate is based on a review of
the 14 case-study sites presented in
Table 1 or, where no experience exists,
on the use of best engineering judg-
ment. As the EPA and its contractors
gain experience in this field, many of
the existing information gaps will be
filled (particularly those in the area of
performance efficiency).
Pretreatment Operations
Equalization entails mixing the in-
coming leachate, which is subject to
large fluctuations in volume and
strength, in a large tank or basin and
discharging it to the treatment plant at a
constant rate. When placed ahead of
chemical operations in the treatment
process train, equalization improves
chemical feed control and process reli-
ability. When placed ahead of biological
operations, equalization minimizes
shock loadings, dilutes inhibitory sub-
stances, stabilizes pH, and improves
secondary settling. In plants that oper-
ate on an intermittent schedule, equal-
ization tanks/basins double as influent
storage tanks. Equalization is generally
reliable and can improve the perform-
ance of sensitive operations such as car-
bon adsorption, biological treatment,
chemical precipitation, and ion ex-
change.
Sedimentation is the gravitational
settling of suspended particles that are
heavier than water in a large tank or
basin under quiescent conditions. Sedi-
mentation is widely used for the re-
moval of settleable solids and immis-
cible liquids, including oil and grease
and some organics. Although haz-
ardous waste leachate typically con-
tains only small loadings of suspended
solids, sedimentation may be included
as a pretreatment step because of the
sensitivity of many downstream proc-
esses to fouling and interference from
suspended solids. Frequently, sedimen-
tation is included in leachate treatment
process trains for separation of solids
generated by chemical and biological
processes. Both circular and rectangu-
lar sedimentation basins (clarifiers) are
used widely and are considered highly
reliable if properly operated and main
tained.
Granular-media filtration is a physica
process whereby suspended solids an
removed from leachate by forcing thi
fluid through a porous medium
Granular-media filtration is useful as
pretreatment step for adsorption proc
esses (activated carbon), membran
separation processes (reverse osmosi:
ultrafiltration), and ion exchange proc
esses, which are rapidly plugged c
fouled by high loadings of suspende
solids. The most common application (
granular-media filtration to hazardoi
waste leachate involves pretreatmei
prior to carbon adsorption. Filtratio
may also be used as a polishing ste
after precipitation/flocculation or bii
logical processes for removal of resi
ual suspended solids in the clarifier c
fluent. Granular-media filters ce
produce an effluent with a suspend!
solids concentration as low as 1 to
mg/liter.
Oil/water separation technology c<
be used to separate immiscible orga
-------
Table 1. Leachate Treatment Case Study Sites
Site/location
Contaminants
Unit treatment processes
Discharge
point
Source
Bofors-Nobel, Inc.
Muskegon, Michigan
CECOS International, Inc.
Niagara Falls, New York
'Gloucester Environmental
Management Services
(GEMS) Landfill
Gloucester Township, New
Jersey
*Helen Kramer Landfill
Mantua Township, New
Jersey
•Heleva Landfill
North Whitehall Township,
Pennsylvania
Hyde Park Landfill
Niagara Falls, New York
"Lipari Landfill
Mantua Township, New
Jersey
Dichloroethylene
Orthochloroaniline
Dichlorobenzidine
Volatile organics
Phenol
Volatile organics
Heavy metals
Volatile organics
Phenols
Heavy metals
Volatile organics
Dissolved organics
Phenol
HET acid
Benzole acid
o-, m-, p-Chlorobenzoic
acid
Heavy metals
Volatile organics
Phenols
Neutralization POTW
Powdered activated carbon treatment/
wet-air oxidation
Equalization POTW
Neutralization
Sequencing batch reactor
Granular-media filtration
Carbon adsorption
Air stripping/vapor-phase carbon ad- POTW
sorption
Meidl and Wilhelmi
1986
Staszak et al. un-
dated
EPA 1985b
Equalization
Precipitation/flocculation/sedimentation
Air stripping/vapor-phase carbon ad-
sorption
Activated sludge
Granular-media filtration
Carbon adsorption
Chlorination
Precipitation/flocculation/sedimentation
Neutralization
Activated sludge
Air stripping
Carbon adsorption
Equalization POTW
Neutralization/sedimentation
Sequencing batch reactor
Carbon adsorption
Equalization POTW
Precipitation/flocculation/sedimentation
Air stripping/vapor-phase carbon ad-
sorption
Granular-media filtration
Carbon adsorption
POTW or sur- EPA 1985c
face water
Surface water EPA 1985d
Ying et al. 1986
EPA 1985e
"Love Canal
Niagara Falls, New York
"New Lyme Landfill
Ashtabula County, Ohio
'Pollution Abatement Serv-
ices (PAS) Site
Oswego, New York
Volatile organics
Semivolatile organics
(acid extractables,
base/neutral ex-
tractables)
Dioxin
Heavy metals
Volatile organics
Refractory organics
Heavy metals
Volatile organics
Semivolatile organics
(acid extractables,
base/neutral ex-
tractables)
Equalization
Sedimentation
Bag filtration
Carbon adsorption
Neutralization/sedimentation
Rotating biological contactor
Precipitation/flocculation/sedimentation
Carbon adsorption
Equalization
Precipitation/flocculation/sedimentation
Carbon adsorption
Neutralization
Granular-media filtration
POTW
Shuckrow, Pajak,
and Touhill 1982
Surface water EPA 1985f
Not specified EPA 1984a
Rothman, Gorton,
and Sanford
1984
-------
Table 1. Continued
Site/location
Contaminants
Unit treatment processes
Discharge
point Source
*Sand, Gravel, and Stone
Site
Elkton, Maryland
'Stringfellow Acid Pits
Glen Avon, California
'Sylvester Site (Gilson Road
Site)
Nashua, New Hampshire
*Tyson's Dump
Upper Merion Township,
Pennsylvania
Heavy metals
Volatile organics
Semivolatile organics
(acid extractables,
base/neutral ex-
tractables)
Heavy metals
Organics
Heavy metals
Volatile organics
Alcohols, ketones
Volatile organics
Equalization
Reduction
Precipitation/flocculation/sedimenta-
tion/sludge dewatering
Neutralization
Filtration
Carbon adsorption
Equalization
Precipitation/flocculation/sedimenta-
tion/sludge dewatering
Granular-media filtration
Carbon adsorption
Precipitation
Neutralization
Filtration
High-temperature air stripping/fume in-
cineration
Activated sludge (extended aeration)
Air stripping/vapor-phase carbon ad-
sorption
Ground EPA 1985g
water/sur-
face water
POTW
EPA 1984b
Ground water EPA 1983
Surface water EPA 1984c
*NPL Superfund site.
ics such as chlorinated solvents and
PCB oils from leachate. Gravity separa-
tors offer the most straightforward, ef-
fective means for phase separation. Co-
alescing separators, which use baffles
in the tank to promote oil droplet ag-
glomeration, provide more effective
separation and can be used in situations
where subsequent treatment processes
cannot tolerate significant concentra-
tions of immiscible organics. The use of
oil/water separation technology is lim-
ited to waste streams that are com-
posed of two immiscible phases having
significantly different specific gravities.
Leachate containing oil that is present
as an emulsion will require the addition
of an emulsion-breaking chemical for
efficient treatment. The efficiency of
gravimetric oil/water separators is a
function of oil concentration and
droplet size, retention time, density dif-
ference between the two phases, and
temperature. The surface area of the
baffles also affects the efficiency of coa-
lescing separators.
Physical/Chemical Treatment
Operations
Neutralization of leachate exhibiting
an extreme pH involves the addition of
a base or an acid to the leachate to ad-
just its pH upward or downward, as re-
quired, to a final acceptable level (usu-
ally between 6.0 and 9.0). In most
hazardous waste leachate treatment ap-
plications, neutralization serves as a
form of pretreatment for optimization of
the performance of pH-sensitive proc-
esses (particularly biological treatment
processes) or for minimization of corro-
sion in more sophisticated physical/
chemical treatments (especially mem-
brane and stripping processes).
Neutralization may also be applied as a
post-treatment operation downstream
of certain chemical processes that yield
acidic or caustic effluents (e.g., oxida-
tion/reduction). The use of post-
treatment neutralization to meet final
discharge criteria is particularly applica-
ble where treated effluent is discharged
to surface or ground water. Perform-
ance of neutralization systems is highly
dependent on the reliability of auto-
mated control systems.
Combined precipitation/flocculation/
sedimentation is the most common
method of removing soluble metals
from leachate. Precipitation involves
the addition of chemicals to the leachate
to transform dissolved contaminants
into insoluble precipitates. Flocculation
promotes agglomeration of the precipi-
tated particles, which facilitates their
subsequent removal from the liquid
phase by sedimentation (gravity set-
tling) and/or filtration. Precipitation/
flocculation/sedimentation is applicable
to the removal of most metals [arsenic,
cadmium, chromium (III), copper, iron
lead, mercury, nickel, and zinc] as wel
as suspended solids and some anionic
species (phosphates, sulfates, and fluo
rides) from the aqueous phase o
leachate. Effluent metal concentration;
of less than 1 mg/liter are theoretically
achievable with precipitation/floccula
tion/sedimentation. In practice, how
ever, theoretical values are seldom at
tained because of the influence o
complexing agents, fluctuations in pH
slow reaction rates, and poor separa
tion of colloidal precipitates.
Oxidation/reduction involves the ad
dition of a chemical oxidizing or reduc
ing agent to leachate under a controlle
pH. Oxidation/reduction of certai
leachate constituents may render ther
nonhazardous or more amenable to re
moval by subsequent processes (e.g
precipitation, ion exchange, or biolog
cal treatment). The most common appl
cations of oxidization/reduction to ha
ardous waste leachate include cyanic
destruction and the reduction of he:
avalent chromium to the less hazardoi
trivalent form. The effectiveness of o>
dation/reduction for a given constitue
-------
is directly related to the time of reaction
and the degree to which interfering or
competing constituents are present.
Carbon adsorption is a separation
technique for removing dissolved con-
taminants from leachate by adsorption
onto granular activated carbon. Carbon
adsorption is a well-developed process
recognized as standard technology for
the treatment of most hazardous waste
leachates. It is especially well suited for
the removal of mixed organic contami-
nants, including volatile organics, phe-
nols, pesticides, PCB's, and foaming
agents. Carbon adsorption is economi-
cally competitive with air stripping for
the removal of relatively low concentra-
tions of volatile organics when VOC air
emissions must be controlled. For
higher contaminant loadings, carbon
adsorption typically is used for effluent
polishing of nonvolatile organics fol-
lowing air stripping. Carbon adsorption
systems usually can be designed to ef-
fect greater than 99 percent removal of
most organic contaminants. Because of
the complex nature of hazardous waste
leachate and the nonselectivity of car-
bon for specific hazardous constituents,
however, effluent concentrations of
target contaminants in the parts-per-
billion range are difficult to achieve.
Air stripping is a mass-transfer proc-
ess that uses air to remove organics that
are volatile and only slightly water-
soluble from leachate. As in the case of
carbon adsorption, this technology has
been widely demonstrated at hazardous
waste sites. The applicability of air strip-
ping for removal of a particular contam-
inant can be predicted by the use of
vapor/liquid equilibria data, which vary
with temperature and the presence of
other constituents. The performance of
air strippers depends on the vapor/
liquid equilibrium behavior of the con-
taminant(s), the dimensions (height, di-
ameter) of the tower, the efficiency of
air-water contact, and the liquid temper-
ature. In hazardous waste leachate ap-
plications, a minimum acceptable re-
moval efficiency is usually defined, and
a system is then designed to meet that
level. Although generally more eco-
nomical than adsorption processes, the
cost advantage of air stripping may be
offset by the need for air pollution con-
trol equipment to remove stripped
VOC's.
Steam stripping, or steam distillation,
is a separation technique in which
steam is used to remove volatile organ-
ics from leachate. Although steam dis-
tillation is commonly used by industry
to recover chemicals from aqueous
streams or to remove contaminants
from manufactured products, it is prob-
ably not practical for direct application
to hazardous waste leachate treatment
(except under unusual circumstances)
because of its high operating costs.
Reverse osmosis is a separation tech-
nique that can be used to concentrate
dissolved contaminants [inorganics and
relatively high-molecular-weight
(greater than 120) organics] in an
aqueous waste stream. To date, reverse
osmosis has not been applied to full-
scale treatment of hazardous waste
leachate, primarily because of the deli-
cate nature of reverse-osmosis mem-
branes and the strength and complexity
of leachate. Steady progress is being
made, however, in the development of
durable membranes and self-cleaning
reverse-osmosis units, and the potential
exists for application of this technology
to future hazardous waste leachate
treatment systems. Reverse osmosis
will probably be limited to use as a pol-
ishing step subsequent to other more
conventional processes.
Ultrafiltration is a membrane process
capable of separating solution compo-
nents on the basis of molecular size,
shape, and flexibility. Ultrafiltration
generally removes high-molecular-
weight (greater than 500) species from
solution, including macromolecules
(proteins, polymers), complexed
metals, oil emulsions, colloidal disper-
sions (clay, microorganisms), and sus-
pended solids. Ultrafiltration (like re-
verse osmosis) has not yet been applied
to the full-scale treatment of hazardous
waste leachate. As membranes exhibit-
ing greater productivity and chemical
resistance are developed, Ultrafiltration
will likely become a more viable treat-
ment alternative.
Ion exchange is a process that re-
versibly exchanges ions in solution with
ions of like charge retained on an insol-
uble resinous solid called an ion-
exchange resin. The ion-exchange resin
has the ability to exchange either posi-
tively charged ions (cation exchange) or
negatively charged ions (anion ex-
change). Ion exchange is used primarily
for the removal of dissolved ionic spe-
cies when a high-quality effluent is re-
quired. The applicability of this process
to the treatment of leachate is probably
limited to use as a final polishing stage
where effluent is discharged to sensi-
tive surface waters. No evidence has
been found that ion exchange has been
applied to the full-scale treatment of
hazardous waste leachate.
Wet-air oxidation is the aqueous-
phase oxidation of concentrated or-
ganic and inorganic wastes in the pres-
ence of oxygen at elevated temperature
and pressure. The wet-air oxidation
process may be applied to any concen-
trated organic or inorganic waste
stream with a COD between 10,000 and
100,000 mg/liter. It is particularly suit-
able for waste streams that are too di-
lute for incineration but too refractory
for chemical or biological oxidation.
The areas of greatest potential appli-
cability for hazardous waste leachate
appear to be treatment of concentrated
liquid waste streams generated by
steam stripping, Ultrafiltration, or re-
verse osmosis; treatment of biological
waste sludges; and regeneration of
powdered activated carbon. No per-
formance data are available on the wet-
air oxidation of hazardous waste
leachate.
Biological Treatment
Operations
The activated-sludge process is a
suspended-growth, biological treat-
ment process that uses aerobic mi-
croorganisms to biodegrade organic
contaminants in leachate. Variations in
the conventional activated-sludge proc-
ess have been developed to provide
greater tolerance for shock loadings, to
improve sludge settling characteristics,
and to achieve higher BOD5 removals.
Process modifications include complete
mixing, step aeration, modified aera-
tion, extended aeration, contact stabi-
lization, and the use of pure oxygen. A
practical upper limit for influent BODs to
an activated-sludge system is 10,000
mg/liter. In general, the activated-
sludge process can readily degrade
simple organic species such as alkanes,
alkenes, and aromatics. Halogenated
hydrocarbons are degraded more
slowly. The performance of an
activated-sludge system is related to
the degree of acclimation of the
biomass. The use of indigenous bacte-
ria from the waste-disposal site can
speed reaction rates and improve total
system performance.
The sequencing batch reactor (SBR) is
a fill-and-draw activated-sludge system.
Unlike conventional, continuous-flow,
activated-sludge systems, the SBR per-
forms all operations in a single tank.
Each cycle of the batch operation in-
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volves five phases of treatment in timed
sequence: fill, react, settle, draw, and
idle. The sequencing batch reactor, like
the conventional activated-sludge proc-
ess, can be used to biodegrade organic
contaminants (e.g., phenol) in leachate.
The SBR is particularly applicable to the
treatment of leachate that is not gener-
ated in sufficient volume to justify a
continuous-flow process. With an SBR,
the leachate can be accumulated in a
holding tank for intermittent treatment.
The SBR also has greater operational
flexiblity to accommodate changing
feed characteristics (flow and/or organic
loading) and can achieve more com-
plete treatment through adjustment of
reaction parameters than the conven-
tional activated-sludge system. Good
treatment performance with leachate
has been demonstrated at the labora-
tory scale under varying conditions of
influent TOC, feed rate, aeration/mixing,
HRT, MLSS concentration, organic load-
ing, temperature, and cycle time. Satis-
factory treatment performance has also
been demonstrated at full scale.
The patented powdered activated car-
bon treatment (PACTR) process (Zim-
pro, Inc.) involves the controlled addi-
tion of powdered activated carbon to
the aeration tank of a conventional
activated-sludge system. Removal of
organics is achieved through a combi-
nation of biological oxidation/assimila-
tion and physical adsorption. The PACT
process is applicable to nearly all
wastewaters with a COD between 50
and 50,000 mg/liter. It is particularly ef-
fective for treatment of wastes such as
leachate that are variable in composi-
tion and concentration, that are highly
colored, and that contain refractive ma-
terials. A number of volatile organic,
acid-extractable organic, and base/
neutral-extractable organic priority pol-
lutants are amenable to treatment by
the PACT process. Laboratory studies
have shown that the PACT process is
capable of better organic removal effi-
ciencies than either activated sludge or
carbon adsorption alone.
The rotating biological contactor
(RBC) is an attached-growth, aerobic bi-
ological treatment process. Rotating bi-
ological contactors can be used for
treatment of leachate containing readily
biodegradable organics. Although not
as efficient as conventional activated-
sludge systems, RBC's are better able to
withstand fluctuating organic loadings
because of large amount of biomass
they support. Rotating biological con-
tactors provide a greater degree of flex-
ibility for meeting the changing needs
of a leachate treatment plant than do
other attached-growth biological proc-
esses. The characteristic modular con-
struction of RBC's permits their multiple
staging to meet increases or decreases
in treatment demands. The hydraulic re-
tention time of the waste and the rota-
tional speed of the disks can be con-
trolled to effect the desired degree of
system performance.
The trickling filter is an attached-
growth, aerobic biological treatment
process in which leachate is continu-
ously distributed over a bed of rocks or
a plastic medium that supports the
growth of microorganisms. Trickling fil-
ters may be used to biodegrade non-
halogenated and certain halogenated
organics in leachate. Although not as
efficient as suspended-growth biologi-
cal treatment processes, trickling filters
are more resilient to variations in hy-
draulic and organic loadings. For this
reason, trickling filters are best suited to
use as "roughing" or pretreatment
units that precede more sensitive proc-
esses such as activated sludge. The ap-
plicability of trickling filters to the full-
scale treatment of hazardous waste
leachate has not yet been demon-
strated.
Post-Treatment Operations
Post-treatment processes are those
operations that occur downstream of
the primary waste treatment stages to
"polish" the system's effluent or pre-
pare it for discharge. Such processes in-
clude filtration to remove residual sus-
pended solids, pH adjustment to return
the effluent to a neutral condition, and
chlorination to disinfect the effluent
prior to its discharge to surface water.
Chlorination is a post-treatment proc-
ess used primarily for disinfection to de-
stroy microorganisms in treated
leachate prior to its discharge to ground
or surface waters. The effectiveness of
chlorination for disinfection depends on
pH, temperature, contact time, mixing,
and the presence of interfering com-
pounds. Performance of chlorination
systems is tied to the reliability of auto-
mated control systems.
Residuals Management
Important considerations in the selec-
tion of a leachate treatment process are
the type and volume of residuals gener-
ated by the process, as these factors af-
fect operating and maintenance costs.
Residuals generated by the technolo-
gies profiled in this document include
sludge, air emissions, concentrated liq-
uid waste streams, and spent carbon.
Table 2 presents a listing of the residu-
als generated by each of these proc-
esses. Current residuals management
practices are discussed under the ap-
propriate headings in the remainder of
this section.
Sludge
Physical/chemical treatment sludges
are generated by the sedimentation of
suspended solids and/or insoluble reac-
tion byproducts. Biological treatment
sludges are generated by the microbial
conversion of soluble organics to cellu-
lar biomass. Because contaminants are
often concentrated in these sludges,
they will require further treatment and
disposal in an environmentally sound
manner.
Sludge dewatering is a physical (me-
chanical) operation used to reduce the
moisture content and volume of
sludges. Moisture reduction, which is
normally required prior to the landfill-
ing or incineration of sludges, facilitates
handling and reduces transportation
and ultimate disposal costs. Chemical
stabilization/solidification involves the
addition of absorbents and solidifying
agents to the sludge. This process is de-
signed to improve the handling anc
physical characteristics of the sludge, tc
decrease the surface area for transpor
of hazardous constituents, to limit th<
solubility of pollutants in the sludge
and/or to detoxify the containec
pollutants. Available methods for thi
stabilization/solidification of sludges in
elude sorption, lime/fly-ash/pozzolai
processes, pozzolan/portland cemen
processes, thermoplastic microencap
sulation, and macroencapsulation (jacl
eting). Biological stabilization c
sludges can be achieved by aerobic c
anaerobic sludge digestion. In aerobi
digestion, microorganisms in the pres
ence of oxygen consume to depletio
the available food in the sludge an
then continue to feed on their own pr<
toplasm to continue living. In anaerob
sludge digestion, organic material is t
ologically converted under anaerob
conditions to methane and carbon dio
ide. Incineration of sludges destro>
some or all of the hazardous co
stituents or characteristics of tl
sludge. High operating and maint
nance costs are associated with incim
ation. Land disposal of sludge requir
that the sludge meet or exceed Su
standards for solids content and thai
8
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Table 2. Residuals Generated by the Various Leachate Treatment Processes
Residuals
Treatment process
Sludge
Air
emissions
Concentrated
liquid waste
stream
Spent
carbon
Pretreatment operations
Sedimentation X
Granular-media filtration
Oil/water separation
Physical/chemical treatment operations
Neutralization X
Precipitation/flocculation/ X
sedimentation
Oxidation/reduction X
Carbon adsorption
Air stripping
Steam stripping
Reverse osmosis
Ultrafiltration
Ion exchange
Wet-air oxidation
Biological treatment operations
Activated sludge X
Sequencing batch reactor X
Powdered activated carbon X
treatment (PACT)
Rotating biological contactor X
Trickling filter X
X
X
X
X
X
X
X
X
contains no free liquids. If the sludge
demonstrates hazardous characteristics
or is a hazardous waste by definition, it
must be disposed of at an EPA-
approved hazardous waste landfill.
Air Emissions
By design, certain leachate treatment
technologies (e.g., air stripping) transfer
VOC's from the liquid phase to the
vapor phase. Other treatment processes
(e.g., activated sludge, rotating biologi-
cal contactor, and sequencing batch re-
actor) strip some VOC's by nature of the
aeration process. Unless provisions are
made for treatment of air emissions,
VOC's will be discharged to the atmos-
phere.
Vapor-phase carbon adsorption is an
effective method for removing VOC's
from the vapor phase. Contaminant-
laden air is passed through a column of
activated carbon. Organics are ad-
sorbed from the air stream, and clean
air is discharged to the atmosphere.
Fume incineration may be useful for the
control of combustible atmospheric
emissions that are generated by air
stripping of organic compounds from
leachate.
Concentrated Liquid Waste
Streams
Liquid waste streams (backwash
water, concentrate, and condensate)
generated by many physical/chemical
treatment operations contain high con-
centrations of suspended solids or pol-
lutants that the particular treatment
process was designed to remove. Back-
wash water is usually returned to the
head works of the treatment plant; how-
ever, if recycling is not practiced, the
backwash water must be treated or dis-
posed of. Options available for the treat-
ment or disposal of concentrates and
condensate include incineration or sta-
bilization/solidification in preparation
for land disposal.
Spent Carbon
Granular and powdered activated car-
bon are used extensively for leachate
treatment and for the control of air pol-
lutants such as VOC's. When the carbon
becomes exhausted, it can either be re-
generated and reused or disposed of by
incineration or land disposal. In most
cases, however, spent carbon is regen-
erated by the supplier and reused.
Carbon regeneration techniques can
be categorized as either thermal regen-
eration or nondestructive regeneration
processes. Thermal oxidation involving
the use of a multiple-hearth, fluidized-
bed, or rotary kiln furnace is the most
prevalent means of regenerating granu-
lar activated carbon. Wet-air oxidation
can be used for thermal regeneration of
powdered activated carbon. Nonde-
structive regeneration of activated car-
bon is accomplished by the use of
steam to remove VOC's, solvents to re-
move a variety of organics, and a pH
shift for weak acids and bases. Other
options for management of spent car-
bon include incineration and land dis-
posal.
Judy L McArdle, Michael M. Arozarena, and William E. Gallagher are with
PEI Associates, Inc., Cincinnati, OH 45246.
Edward J. Opatken is the EPA Project Officer (see below).
The complete report, entitled "A Handbook on Treatment of Hazardous Waste
Leachate," (Order No. PB 87-152 323/AS; Cost: $18.95, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati. OH 45268
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