United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
v/EPA
Research and Development
EPA-600/S2-81-076 June 1981
Project Summary
Lime Stabilization and
Ultimate Disposal of
Municipal Wastewater Sludges
Robert M. Otoski
The report summarized here docu-
ments the successful use of lime to
stabilize sludge at 28 municipal waste-
water treatment plants. Sludge stabi-
lized with lime can be simple and in-
expensive and can be used as a backup
or interim system, or to upgrade a
system, or instituted as a less costly
system. Bacterial analyses demon-
strated that liming a sludge to pH of 12
is an effective means of inactivating
total and fecal coliform, although
organisms can regrow as the pH drops
in stockpiled sludge. The lime-stabi-
lized product can be landfilled, land
applied as a liquid sludge or as a cake.
or stockpiled before landfilling or land
application.
This Project Summary was devel-
oped by EPA's Municipal Environmen-
tal Research Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully documented
in a separate report of the same title
(see Project Report ordering information
at back).
Introduction
The process of lime stabilization in-
volves adding lime to wastewater-
derived sludge in quantities sufficient to
raise the pH of the sludge to 12.0 for a
contact period of at least 2 hours. The
high pH reduces levels of pathogenic
(disease-causing) bacteria and viruses
in the sludge and controls putrefaction
and odors. The process has been ac-
cepted by the U.S. Environmental Pro-
tection Agency (EPA) as a "Process to
Significantly Reduce Pathogens,"
meaning that lime-stabilized sludge
meets minimum requirements for dis-
posal on controlled land-application
sites.*
In some areas of the country, the
technique of lime stabilization and land
disposal has gained in popularity as an
alternative to processing sludge by heat
treatment techniques or disposing of it
by incineration, both of which have
been affected by rising costs and un-
certain availability of fuel. Many waste-
water treatment plants at which lime
stabilization is practiced were not
originally designed for this process. The
technique has been instituted by plant
operators to replace more costly sludge-
handling processes, to meet interim
sludge-handling demands, to back-up
existing techniques of sludge manage-
ment, or to meet seasonal demands.
Although some design guidance is
available (e.g., EPA's Process Design
Manual "Sludge Treatment and Dispos-
al"), the technique has presently been
defined more by practice than by design.
Consequently, the study reported here
was undertaken to sample facilities at
which lime stabilization is practiced and
to produce a g uide to serve as a practica I
source of information for designers,
operators, and others. The Guide also
describes the effectiveness of lime
stabilization for pathogen kill and
investigates the cost of lime stabilization.
*40 CFR Part 257. Criteria for Classification of Solid
Waste Disposal Facilities and Practices Federal
Register, Vol 44, No 179, September 13, 1979
-------
This Project Summary was developed
by EPA's Municipal Environmental Re-
search Laborabory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Survey of Plants
The investigators visited 28 waste-
water treatment facilities in New
England and New York at which lime
stabilization of sludge was reportedly
being practiced on a permanent, interim,
or seasonal basis. (In fact, true stabiliza-
tion (pH > 12) was not achieved at some
of the plants visited; several routinely
limed sludge to pH levels slightly below
12.)
Three general techniques of lime
stabilization were observed:
• Adding lime to a liquid sludge
before land application. Typically,
this is carried out as a batch process.
Lime is added to a tankf ul of sludge,
and the lime-stabilized sludge is
then pumped to a land-application
vehicle for removal from site.
• Adding lime to a liquid sludge
before dewatering. With the use of
conventional equipment originally
installed for conditioning the sludge
before dewatering by mechanical
means, operators simply increase
the lime dose to obtain the pH
levels required for stabilization.
• Adding lime to a dewatered sludge
cake. Hydrated lime is added directly
to a sludge-cake screw conveyor,
and rotation of the conveyor serves
to mix the lime and sludge.
On the average, operators applied to
the sludge a dose of 20 to 25 percent
lime (as CaO) based on dry sludge solids
content; data from other sources indicate
that this dose should be sufficient for
sludges thicker than 2 percent solids.
For thinner sludges, lime doses of 30
percent or more would be required.
Operators' tests for total coliform and
fecal coliform bacteria indicate com-
plete inactivation of these indicators in
sludge limed to pH 12.0 and higher.
Below pH 12.0, numbers of these bacteria
are greatly reduced from numbers in the
raw sludge, but they are detectable.
Regrowth of bacteria in stockpiles of
dewatered, lime-stabilized sludge can
be correlated with the gradual decay in
pH that occurs after several weeks.
The lime-stabilized sludge product is
either disposed of in a landfill, applied to
land as a liquid sludge, applied to land as
a cake, or placed in stockpiles for future
landfilling or land application.
Findings
Required Facilities
Lime stabilization can be simple and
inexpensive. Required facilities consist
of a dry storage area for bagged lime or a
storage tank with mixer for slurry, a
steel tank with a mixer and dust collector
for slurry makeup (or without the dust
collector if the tank is being used only
for slurry dilution), a slurry metering
pump, and a mixing tank to provide the
necessary detention period for the
sludge/lime mixture. Stabilized by this
method, the liquid sludge can then be
pumped directly to land-application
trucks or can be dewatered before
disposal.
In the simplest of systems—the batch
process—only a sludge/lime mixing
tank and land application truck are
required. Lime slurry can be purchased
when needed and used immediately.
Running the system requires little train-
ing and little technical knowledge.
Suitable Applications
Lime stabilization is well suited to the
sludge management requirements of
smaller plants. Twenty of the twenty-
eight plants visited in the study have
current average wastewater flows of
0.25 mVs (6 mgd) or less. Of the five
plants treating flows of more than 0.45
mVs (10 mgd), only one is practicing
true lime stabilization on a full-time
basis.
Two factors make the process more
attractive for application in smaller
plants than in larger. First, process costs
are operation-and-maintenance (O&M)
rather than capital intensive. Second,
the costs of chemicals—a major portion
of the process' total costs—show little
economy-of-scale. Typical costs of lime
stabilization of liquid sludge in a batch
operation (the only lime stabilization
method rated acceptable without quali-
fication in this study) are illustrated in
Figure 1; costs for converting and using
existing lime-conditioning equipment
for operation as a lime stabilization
system are presented in Figure 2. These
curves are derived from criteria listed in
Table 1. In both cases, costs of lime are
included. It is assumed that waste
carbide lime is used for the batch
operation and that the most economical
500
Q
Q
°200
c:
-------
IUU
2
£™
Thousands of Do
-* N)
a o
Cost of
Stabilizat
on Tank
_i— -H
X"
7a6/e 7. Criteria Used for Cost Study
2 5 10
Plant Flow (mgd)
a Construction Cost
20
50
o
Q
n
-»
O
to
3
Q
•s
to
Q
O
"to
t*
^C
Stabilization Tank
Increase Dose
\
Increase Dose Only
12 5 JO 20
Plant Flow (mgd)
b. Operation and Maintenance Cost
Figure 2. Cost of conversion to lime
stabilization at a facility
with vacuum filtration.
(mgd X 0.0438 = m3/s)
• as a technique to upgrade or im-
prove the degree of stabilization
presently being obtained at a facil-
ity, or
• as a less costly replacement for
incineration or heat treatment
processes.
Back-up Systems
Because lime is widely available,
suitable for storage, and often already in
use at a facility in another capacity, the
lime-stabilization system can be brought
on-line on short notice, following rela-
tively simple start-up procedures. When
designed for use as a back-up process,
the temporary operation of which can be
Sludge Production
Lime Dose for Stabilization
Capital Cost Base
Capital Cost Amortization
Salary
Electrical Cost
Maintenance
Cost of Lime (delivered as CaO)
Bagged Hydra ted Lime
Carbide Lime
Quicklime
0.24 kg/m3 (2000 Ib/mil gal)
wastewater treated
20 percent (as CaO)
3135(ENR, April 1980)
20 years; 7 percent
$13.50 per hour (including benefits)
$ 0.06 per kwh
3 percent of equipment capital cost
(annually)
$135/metric ton ($125/ton)
$94/metric ton ($85/ton)
$72/metric ton ($65/ton)
labor-intensive without disrupting plant
activity, the capital cost of the lime
stabilization system is very low.
Interim Systems
If an interim sludge handling system
is required, lime stabilization can be
done inexpensively with the use of equip-
ment already available at the plant site.
At a number of plants visited in the
course of this study, lime stabilization
took place in available sludge conditioning
tanks, existing aerobic digesters, or
unused sludge tanks, or by directly
applying lime to sludge in existing
sludge-conveyance systems.
Upgrading Systems
At the study sites, the operators' use
of lime stabilization upgraded their
existing sludge processing systems in
three ways:
• The process was used to upgrade
the degree of stabilization achieved
by the aerobic and anaerobic
digesters.
• It was applied to systems that had
been disposing of conditioned, but
unstabilized, sludge.
• It was instituted as a means of
controlling odors.
Typically, the equipment required was
either available on-site or acquired at
little expense.
Replacement Systems
Rising fuel costs have greatly increased
the costs of sludge incineration and
heat treatment. The problem can be
especially severe at smaller treatment
plants, where fuel must be wasted
during low-load periods to keep inciner-
ator temperatures up. At 1 2 of the
plants studied, with capacities up to 16
mgd, lime stabilization has replaced
heat treatment processing or incinera-
tion, which had been found to be too
costly.
Operational Considerations
Type of Lime
The economical form of lime depends
on plant size and location. Bagged
hydrated lime and lime slurry are the
least expensive forms for smaller plants
(up to about 0.2 mVs(5 mgd); quicklime
is appropriate for larger plants. If carbide
lime (which is a waste product) can be
economically delivered, its use is appro-
priate for a wide range of plant sizes.
Effects of Stockpiling
Stockpiling may adversely affect the
bacteriological quality of lime-stabilized
sludge. Although lime stabilization
results in a high cohform kill, the
process does not destroy volatile sludge
organics, which represent a source of
food for the microorganisms surviving
in the sludge. These surviving bacteria
can "regrow" or can recover from
inactivation if sludge pH drops and
causes the sludge to become "unstabi-
•lized." Evidence of this was noted at
various sites. Stockpiles of a few months
to a year old, when broken down, emitted
an odor signifying bacteria regrowth.
Stockpiling sludge that has been
lime-stabilized, dewatered, and allowed
to dry on sandbeds is a significant
improvement over the stockpiling of wet
cake.
* US GOVERNMENT PRINTING OFFICE 1981-757-012/7154
-------
Land Application
Developing a successful program for
applying lime-stabilized sludge to land,
subject to provisions of 40 CFR, Part
257, is clearly possible. Some treatment
plant operators report that farmers who
had been skeptical about using the
product on their lands have obtained
improved crop growth and have come to
welcome the availability of the sludge.
In New England, particularly, where
farmers routinely add lime to agricultural
soils to maintain pH, use of lime-
stabilized sludge reduces or eliminates
the farmer's need to purchase lime.
Lime-stabilized sludge has been suc-
cessfully used as a soil builder. Soils
that have lost top-soil by poor manage-
ment or mining practices are improved
by the organic content and moisture-
holding capacity of the sludge.
Operations involving land application
of lime-stabilized sludge have had odor
problems when stockpiled sludge was
applied on a warm day. The odors,
however, have been described as being
less offensive than those from spread
manure.
Effects of Poor Process Control
Lime stabilization systems may be
subject to poor operational procedures.
Because the effects of incomplete
stabilization are not immediately appar-
ent and do not occur onsite, an improper
lime dose may not be recognized. In
addition, where lime is added before
dewatering, the lime dose required for
stabilization may be confused with the
dose required for sludge conditioning.
This may result in a lower sludge pH
than is required for stabilization. Cutting
lime dose is an easy, but improper,
means of cutting chemical costs.
Treatment plant operators should be
educated on the goals of lime stabiliza-
tion—be more aware of the importance
of adding an appropriate quantity of lime
to the sludge and supplying adequate
mixing and detention time—to reduce
problems caused by poor operation and
cases of incomplete stabilization. Addi-
tionally, operators must understand
that the primary goal of stabilization is
not to control odors but to protect public
health.
Conclusions and
Recommendations
The only lime stabilization method
rated fully acceptable is the addition of
lime to mixed batch tanks. Adding lime
before dewatering with the use of
equipment intended for sludge condi-
tioning requires close operator attention
to ensure successful stabilization. Add-
ing lime directly to dewatered sludge
cake has not yet been demonstrated to
be consistently effective in bacterial
kills. Although both of these latter
techniques offer the advantge of low
capital cost, their effectiveness and
reliability in killing pathogens should be
demonstrated.
Stockpiling provides flexibility in land-
application schedules—sludge can be
applied when climatic and agricultural
conditions are most favorable. If, how-
ever, pH drops, bacteria can regrow in
the stockpiles. When such piles are
broken down, the sludge gives off
offensive odors. The regrowth of indi-
cator and pathogenic bacteria overtime
in stockpiles should be studied. Sand-
bed drying of stabilized sludge before
stockpiling appears to reduce odors
occurring when piles are broken down,
but this effect, too, should be quantified.
Bacteria analyses for total coliform or
fecal coliform bacteria were performed
at the study sites. Fecal streptococcus
bacteria are indicators that have proven
resistant to inactivation by lime. In
future studies, researchers should
consider studying fecal streptococcus
bacteria and, possibly, actual patho-
genic bacteria as well.
The full report was submitted in
fulfillment of Contract No. 68-03-2803
by Camp Dresser & McKee, Inc., Boston,
MA, under the sponsorship of the U.S.
Environmental Protection Agency.
Robert M. Otoski is with Camp, Dresser & McKee, Inc., Boston, MA 02108.
Roland V. Villiers is the EPA Project Officer (see below).
The complete report, entitled "Lime Stabilization and Ultimate Disposal of
Municipal Wastewater Sludges," (Order No. PB 81-198 160; Cost: $15.50,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati. OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use S300
RETURN POSTAGE GUARANTEED
Third-Class
Bulk Rate
V £r'A
S uEARrtOWN SI
CHICAGO 1L 60604
------- |