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United States
Environmental Protection
Agency
Municipal Environmental Research ~
Laboratory
Cincinnati OH 45268 /"
Research and Development
EPA-600/S2-82-014 May 1982
Project Summary
Engineering Assessment of
Hot-Acid Treatment of
Municipal Sludge for Heavy
Metals Removal
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A hot-acid method for treating
sludge was developed by the Walden
Division of Abcor, Inc., to remove
heavy metals from municipal waste-
water sludge. Sulf uric acid is added to
the sludge at a rate of about 20 to 30
percent of the sludge dry solids. The
mixture is then heated to 95° C for a
30-min reaction time.
The hot-acid process effectively
solubilizes 50 to 90 percent of the
selected heavy metals, and it produces
an essentially pathogen-free product.
Based on cadmium application rates
stipulated by the U.S. Environmental
Protection Agency {EPA), land appli-
cation of the treated sludge requires
only a fourth of the land area required
by untreated sludge. Primarily because
solubilized metals are retained in the
aqueous phase entrapped in wet cake,
a highly contaminated sludge will still
retain substantial concentrations of
heavy metals even after treatment.
Such sludges might not have concen-
trations for each metal reduced below
the concentrations suggested by the
U.S. Department of Agriculture;
nevertheless, even for such sludges,
potential for utilization on land is
increased.
A disadvantage of the hot-acid treat-
ment is that in addition to metals, it
also solubilizes the nitrogen, phospho-
rus, and organic contents of the
sludge to varying degrees, thus de-
creasing the fertilizer value of the pro-
duct. Although improved dewaterabil-
ity was one of the objectives of the
experiment and was borne out in
bench-scale tests, a test on a full-scale
centrifuge indicated poor dewater-
ability.
The hot-acid treatment of sludge
would cost about $440/ton for a 2-
mgd wastewater treatment plant and
about $140/ton for a 20-mgd plant.
Though hot-acid treatment is costly
when compared to conventional sludge
treatment practices, its cost effective-
ness may be quite good when com-
pared with other systems aimed
specifically at controlling the heavy
metal contents of sludge.
This Project Summary was devel-
oped by EPA's Municipal Environ-
mental 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
information at back).
Introduction
The sludge generated as a byproduct
of municipal wastewater treatment
contains nutrients of value to some agri-
cultural applications. At the same time,
however, the high concentrations of
heavy metals present in the sludges of
some municipalities render them unsuit
able for application toagricultural lands.
The reason is that the uptake of heavy
metals by plants grown on sludge-
amended soils can have toxic effects on
the plants and on animals that feed on
the plants. Concern over the potential
public health effects posed by the
uncontrolled use of wastewater sludge
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has led the EPA to develop mandatory
guidelines for land disposal of sludges.
(40 CFR Part 257. Criteria for Classifica-
tion of Solid Waste Disposal Facilities
and Practices. Federal Register, Vol 44,
No. 179, Thursday, September 13,
1979.)
Though control of land application
practices can reduce the potential
hazards posed by spreading municipal
sludges on agricultural lands, it also
tends to increase the constraints that
are imposed on this solution to the
sludge disposal problem. Removal of
heavy metals from wastewater sludge
would ease these constraints. To this
end, the Walden Division of Abcor,
Incorporated (Wilmington, Massachu-
setts) has developed the hot-acid pro-
cess, which subjects sludge to acid
treatment that renders heavy metals
soluble and therefore removable from
the sludge during the dewatering step
that follows.
The process is fully described in
Walden's 1979 report, "Evaluation of
Hot-Acid Treatment for Municipal
Sludge Conditioning."This report docu-
ments results obtained in bench-scale
tests to establish the technical and eco-
nomic feasibility of the process. EPA
engaged Camp, Dresser and McKee,
Inc. (COM) in 1980 to review the report
and perform an economic and engineer-
ing assessment of hot-acid treatment.
Process Description
In the hot-acid treatment of sludge,
acid and heat are applied to municipal
wastewater sludge; sulfuric acid is
added to decrease its pH between 1.5 to
2.5, and the mixture is heated to just
below the boiling point (95°C). The acid
is added at a dosage of 20 to 30 percent
of the sludge dry solids; reaction time is
30 minutes.
The aim of the process is threefold:
• To stabilize the sludge (that is, to
make it less putrescible)
• To condition the sludge for de-
watering
• To make heavy metals soluble
The treated sludge goes directly to
dewatering. Since a significant concen-
tration of heavy metals has gone into
solution at the low pH level, these dis-
solved heavy metals are separated from
the sludge during dewatering and
removed with the process sidestream.
The metals in the sidestream can then
be precipitated by neutralization and
subsequently dewatered and disposed
of in secure landfills. The sludge cake,
which contains lower levels of heavy
metals than the raw sludge, may be
more suitable for soil conditioning pur-
poses.
Assessment of the Process
The hot-acid process removed about
50 to 90 percent of selected heavy
metals under optimum conditions of
solubilization. Cadmium, zinc, and
nickel are removed by 80 to 85 percent
at acid doses of 20 percent. Significant
removals of chromium and copper can
be obtained only by higher acid doses
(up to 30 percent). Lead is not easily
removed by this process, according to
data from other sources. Acid consump-
tion at optimum metals solubilization
conditions are presented in Table 1.
The process also makes soluble (and
therefore removable) some other con-
stituents that are desirable in sludge
destined for application to agricultural
lands. Nitrogen, organics, and solids
were reduced by 15 to 40 percent; other
sources have reported even greater
phosphorus removals. Thus a 30-min
detention time is recommended for the
sludge/acid mixture instead of the 60
min that was found to be optimum for
solubilization of heavy metals.
Though the hot-acid process yields
good reductions of heavy metals in
sludge, it may not produce the desirable
effects on dewatering that were antici-
pated. Bench-scale tests indicated good
dewatering; however, in full-scale cen-
trifuge dewatering tests, only 25 per-
cent of the sludge solids could be
separated from the treated sludge. Nor-
mally, an opera torcould expect recovery
of more than 75 percent of sludge sol-
ids. The poor dewaterability of hot-acid-
treated sludge appears to be a major
problem in thesystem'sfeasibility,even
though it may be improved by the use of
non-ionic polymers as flocculants.
The cases of two cities (Milwaukee
and New York) were evaluated to deter-
mine how the hot-acid process might
affect the disposal alternatives available
to major municipalities whose sludges
contain significant concentrations of
heavy metals. The assumption was that
successful dewatering of hot-acid-
treated sludge could be achieved, yield-
ing 85 percent recovery of solids in a
20-percent solids cake.
Milwaukee
Cadmium levels in Milwaukee sludges
limit the amount of sludge that can be
applied annually to a given plot of land
(according to 40 CFR 257, only 0.5 kg
Cd/ha of land canbeappliedannuallyto
certain croplands through the land
application of sludge). If it is assumed
that hot-acid-treated sludge can be suc-
cessfully dewatered, reduction of cad-
mium in Milwaukee's sludge could
reduce land requirements for a 20-ton-
per-day (tpd) sludge operation (dry sol-
ids basis) from 1640 ha to 450 ha.
Though land costs rarely limit accept-
ability of the land application alternative,
this savings in area could be valuable if
only a limited area of land were avail-
able.
New York City
Concentrations of several heavy
metals in sludges generated at 11 New
York City treatment plants exceed the
upper limits recommended by the U.S.
Department of Agriculture (USDA) for
sludges intended to be applied to agri-
cultural lands. Hot-acid treatment of
these sludges (again, assuming suc-
cessful dewatering) could make one of
Table 1. Average Solubilizations Obtained at Optimum Metal Solubilization
Conditions1
Sludge
Constituent
Cadmium
Zinc
Nickel
Chromium
Copper
Solids
Organics (COD)
Nitrogen (TKN)
Solubilization
r/o>
92
92
84
53
53
24
16
28
Acid
Usage (%f
28
28.5
28.5
28.5
28.5
29
29
29
PH
1.87
1.85
1.85
1.85
1.85
1.75
1.75
1.75
1 Data were obtained in three series of bench-scale tests. Constituents were not all
measured in the same series.
2 Acid usage is expressed as a percentage of dry sludge solids.
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the 11 sludges suitable for land applica-
tion and six additional sludges suitable
except for their copper concentrations.
Hot-acid treatment could clearly de-
crease metals levels to within accept-
able ranges in borderline cases.
Treatment Facilities and Costs
The actual facilities required for hot-
acid treatment of sludge include compo-
nents for the hot-acid process, sludge
storage and pumping facilities, de-
watering, sidestream treatment and de-
watering, and neutralization of de-
watered sludge solids (see Figures 1
and 2). Ancillary facilities would include
components for treating corrosive acid
vapors emanating from hot-acid diges-
tion, and components for providing
additional aeration capacity in the treat-
ment plant to handle recycled side-
stream BOD of 1,500 to 5,000 mg/L.
Sizes and costs can be developed for
all process units except the dewatering
equipment. (As mentioned before, full-
scale test performance data indicated
that hot-acid treatment alone does not
effectively condition waste-activated
sludge for dewatering by continuous
solid-bowl centrifuge; it is not known
whether the use of flocculating aids
such as non-ionic polymer would result
in effective performance.)The corrosive
Thickened primary
and waste
Activated sludge
Mixed sludge
holding
tank
Acid-sludge
mixing tank
Grinder
Acid-Sludge
Holding
Tan
7 ""> Liquid
Figure 1. Facilities for hot-acid treatment of sludge.
Cake\
f
. j Sidestream
To secondary treatment system
Acid sludge
\ Coagulant
', system
Sidestream
Ume
system
ob ob
Cake Lime Floccut
mixing tan
tank
i -jaiiiiiiy i r i
<^tank/
Cation Tf
k
1 ^ pewatering\ Q""^
1 Metal -hydrc
txide sludge cake __
Low metal sludge cake
Figure 2. Facilities for handling the dewatering sidestream.
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nature of heated, acidified sludge will
affect the costs of all equipment to be
used in full-scale hot-acid treatment
facilities. Fiberglass-reinforced polyes-
ter (FRP) would be suitable for holding
tanks and mixing tanks, and Carpenter
20 stainless steel would be best for
moving parts, piping, and heat-ex-
changer/digester components ex-
posed to the highest operating tempera-
tures.
Costs were developed for installation
and operation of hot-acid process equip-
ment at hypothetical wastewater treat-
ment plants of 2- and 20-mgd capacity.
Compared with another environmen-
tally acceptable sludge-stabilization
alternative (anaerobic digestion), capital
costs and net unit costs of hot-acid
treatment are high (see Table 2).
These costs do not include those for
dewatering hot-acid sludge or anaerobi-
cally digested sludge; actual costs
would thus be significantly higher. Even
when the benefits of metal removal by
hot-acid treatment are considered, the
cost difference is affected only mini-
mally. Land is not normally purchased
for land-application disposal, so the on ly
cost savings obtained would be those
that resulted from shorter haul distan-
ces to smaller sites. The latter could be
used because of the higher application
rates possible with sludge bearing
lower levels of heavy metals.
Though hot-acid treatment is costly
when compared with conventional
sludge stabilization practices such as
anaerobic digestion, it maybe more cost
effective than other systems that are
specifically aimed at controlling con-
centrations of heavy metals in sludge.
One such system is source control, or
the removal of heavy metals by pretreat-
ing industrial wastewater flows tribu-
tary to a municipal wastewater treat-
ment plant. Source control can be
expensive. In New York City, for exam-
Table 2. Net Unit Costs of Hot-Acid Treatment and Anaerobic Digestion
Cost/Dry Ton
Process
2 mgd
20 mgd
Hot-acid treatment
Anaerobic digestion
$440
114
$140
42
pie, estimates show that an 80-percent
reduction in heavy metals contributed
by industries only (a 38-percent reduc-
tion in city's total load of heavy metals)
would cost $480/dry ton of sludge. This
i s more tha n th ree ti mes the cost of hot-
acid treatment. Furthermore, hot acid
can remove a greater percentage of
metals (assuming successful dewater-
ing of hot-acid sludge). On this basis,
hot-acid treatment of sludge appears to
be economically promising.
Conclusions
The major advantages of the hot-acid
treatment are the decreased land area
requirements for land application and
the increased marketability of sludge
with low metal levels. Another distinct
advantage is that the process reduces
pathogen density beyond levels nor-
mally expected in processes such as
anaerobic digestion.
The major drawbacks of the hot-acid
process are the apparently poor de-
waterability of the sludge following tht
treatment and its loss of nutrients sucf
as nutrition and phosphorus, which are
solubilized along with the heavy metals
The loss of nutrients detracts from the
fertilizer value of the final sludge pro
duct.
Capital costs and net unit costs of the
hot-acid process are high when com
pared with conventional sludge treat
ment systems. Overall costs are ever
higher if the cost of dewatering the acid
treated sludge is included. But overal
costs of the process shou Id be compa rec
with those of other methods that coulc
be more expensive. For example, hot-
acid treatment appears to be economi-
cally promising when compared with
source control of heavy metals.
The full report was submitted in fulfill
ment of Contract No. 68-03-2803 by
Camp, Dresser & McKee, Inc., under the
sponsorship of the U.S. Environmental
Protection Agency.
The authors are with Camp Dresser & McKee, Inc., Boston, MA 02108.
B. V. Salotto is the EPA Project Officer (see below).
The complete report, entitled "Engineering Assessment of Hot-Acid Treatment
of Municipal Sludge for Heavy Metals Removal," (Order No. PB 82-189 655;
Cost: $9.00, subject to change} will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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