vvEPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-00-052
September 2000
Biosolids
Technology Fact Sheet
Alkaline Stabilization of Biosolids
DESCRIPTION
Biosolids are primarily organic materials produced
during wastewater treatment which may be put to
beneficial use. Biosolids are used in home
gardening, commercial agriculture, silviculture,
greenways, recreational areas and reclamation of
drastically disturbed sites such as those subjected to
surface mining. Biosolids are often rich in nutrients
such as nitrogen and phosphorus, and contain
valuable micro nutrients. The Environmental
Protection Agency's (EPA) 40 CFR Part 503,
Standards for the Use and Disposal of Sewage
Sludge, (the Part 503 Rule) requires that
wastewater solids be processed before they can be
beneficially used. The processing is described in this
fact sheet as stabilization. Stabilization helps to
minimize the potential for odor generation, destroys
pathogens (disease causing organisms), and reduces
the material's vector attraction potential. One
method of stabilization is to add alkaline materials
to raise the pH level to make conditions unfavorable
for the growth of organisms (such as pathogens).
Figure 1 is a picture of alkaline stabilized biosolids
being dropped from an overhead conveyor into
windrow curing piles at Middlesex County Utility
Authority's facility in New Jersey.
The Part 503 Rule defines two types of biosolids
with respect to pathogen reduction: Class A (no
detectable pathogens) and Class B (a reduced level
of pathogens). Both classes are safe, but additional
requirements are necessary with Class B materials.
These requirements are detailed in the Part 503 Rule
and include such things as limiting public access to
the site of application, limiting livestock grazing,
and controlling crop harvesting schedules. Class A
biosolids are not subj ect to these use restrictions and
can generally be used like any commercial fertilizer.
Alkaline stabilization can achieve the minimum
requirements for both Class A and Class B biosolids
with respect to pathogens, depending on the amount
Source: Parsons Engineering Science, Inc., 1999.
FIGURE 1 ALKALINE STABILIZED
BIOSOLIDS
of alkaline material added and other processes
employed. Generally, alkaline stabilization meets
the Class B requirements when the pH of the
mixture of wastewater solids and alkaline material
is at 12 or above after 2 hours of contact.
Class A requirements can be achieved when the pH
of the mixture is maintained at or above 12 for at
least 72 hours, with a temperature of 52°C
maintained for at least 12 hours during this time. In
one process, the mixture is air dried to over 50
percent solids after the 72-hour period of elevated
pH. Alternately, the process may be manipulated to
maintain temperatures at or above 70°C for 30 or
more minutes, while maintaining the pH requirement
of 12. This higher temperature can be achieved by
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overdosing with lime (that is, adding more than is
needed to reach a pH of 12), by using a
supplemental heat source, or by using a combination
of the two. Monitoring for fecal coliforms or
Salmonella sp. is required prior to release by the
generator for use.
The Part 503 Rule also allows for meeting Class A
pathogen reduction requirements through
monitoring for pathogens before and after
processing. For example, Class A Alternative 3
requires that the unprocessed wastewater solids be
monitored for enteric viruses and helminth ova. The
process is monitored for lime dosage and pH and
the final product must have no detectable levels of
enteric viruses or helminth ova.
For more specific details on the requirements for
achieving Class A or B, please refer to the Part 503
Rule.
Materials that may be used for alkaline stabilization
include hydrated lime, quicklime (calcium oxide), fly
ash, lime and cement kiln dust, and carbide lime.
Quicklime is commonly used because it has a high
heat of hydrolysis (491 British thermal units) and
can significantly enhance pathogen destruction. Fly
ash, lime kiln dust, or cement kiln dust are often
used for alkaline stabilization because of their
availability and relatively low cost.
The alkaline stabilized product is suitable for
application in many situations, such as landscaping,
agriculture, and mine reclamation. The product
serves as a lime substitute, source of organic matter,
and a speciality fertilizer. The addition of alkaline
stabilized biosolids results in more favorable
conditions for vegetative growth by improving soil
properties such as pH, texture, and water holding
capacity. Appropriate applications depend on the
needs of the soil and crops that will be grown and
the pathogen classification. For example, a Class B
material would not be suitable for blending in a top
soil mix intended for use in home landscaping but is
suitable for agriculture, mine reclamation, and
landfill cover where the potential for contact with
the public is lower and access can be restricted.
Class A alkaline stabilized biosolids are useful in
agriculture and as a topsoil blend ingredient.
Alkaline stabilized biosolids provide pH adjustment,
nutrients, and organic matter, reducing reliance on
other fertilizers.
Alkaline stabilized biosolids are also useful as daily
landfill cover. They satisfy the federal requirement
that landfills must be covered with soil or soil-like
material at the end of each day (40 CFR 258). In
most cases, lime stabilized biosolids are blended
with other soil to achieve the proper consistency for
daily cover.
As previously mentioned, alkaline stabilized
biosolids are excellent for land reclamation in
degraded areas, including acid mine spoils or mine
tailings. Soil conditions at such sites are very
unfavorable for vegetative growth often due to acid
content, lack of nutrients, elevated levels of heavy
metals, and poor soil texture. Alkaline stabilized
biosolids help to remedy these problems, making
conditions more favorable for plant growth and
reducing erosion potential. In addition, once a
vegetative cover is established, the quality of mine
drainage improves.
APPLICABILITY
Where lime or another alkaline additive (for
example, recycled kiln dust), is relatively
inexpensive, alkaline stabilization is often the most
cost-effective process for wastewater solids
stabilization. This is particularly true where
dependable markets for the alkaline product can be
developed, such as in areas where alkaline materials
are routinely applied to agricultural soils to
maximize crop yields.
Alkaline stabilization is practical at small wastewater
treatment plants that store wastewater solids for
later transportation to larger facilities for further
treatment. It is also applicable as an expansion of
existing facilities or as anew facility to reduce odors
and pathogens. The technology is especially useful
at wastewater treatment facilities with flows that
vary greatly since the process adjusts easily to
changing flows. This adaptability also makes
alkaline stabilization an appropriate choice as a
secondary or backup stabilization method because
these facilities can be started and stopped relatively
quickly and easily. Facilities can also be designed to
handle either liquid or dewatered wastewater solids.
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In general, alkaline stabilization is not a proprietary
process, meaning that no fee is required to be paid
to a patent holder to use the process. However,
several variations on the basic process are
proprietary, such as:
BioFIX Process (marketed by Bio Gro
Division of Wheelabrator Clean Water
Systems, Inc.)
RDP En-Vessel Pasteurization System
(marketed by RDP Company.)
N-Viro Advanced Alkaline Stabilization
with Drying (marketed by N-Viro
International Corporation.)
ADVANTAGES AND DISADVANTAGES
Alkaline stabilization offers several advantages,
including:
Consistency with the EPA's national
beneficial reuse policy. Results in a product
suitable for a variety of uses and is usually
able to be sold.
Simple technology requiring few special
skills for reliable operation.
Easy to construct of readily available parts.
Small land area required.
Flexible operation, easily started and
stopped.
Several possible disadvantages should be considered
in evaluating this technology:
The resulting product is not suitable for use
on all soil. For example, alkaline soils
common in southwestern states will not
benefit from the addition of a high pH
material.
The volume of material to be managed and
moved off-site is increased by approximately
15 to 50 percent in comparison with other
stabilization techniques, such as digestion.
This increased volume results in higher
transportation costs when material is moved
off-site.
There is potential for odor generation both
at the processing and end use site.
There is a potential for dust production.
There is a potential for pathogen regrowth
if the pH drops below 9.5 while the material
is stored prior to use (U.S. EPA, 1992.)
The nitrogen content in the final product is
lower than that in several other biosolids
products. During processing, nitrogen is
converted to ammonia, which is lost to the
atmosphere through volatilization. In
addition, plant available phosphorous can be
reduced through the formation of calcium
phosphate.
There are fees associated with proprietary
processes (Class A stabilization.)
Environmental Impacts
There are several potential environmental impacts
associated with alkaline stabilization of wastewater
solids. Odor problems may occur at the point of
processing or use due to the release of ammonia and
ammonia related compounds and amines. These are
generally considered nuisance issues without long-
term environmental impact. Handling of the
material, such as loading, unloading, or spreading,
all potentially cause release of ammonia and amines.
The amount of ammonia released from the alkaline
stabilized product depends on the nitrogen content
of the wastewater solids and the pH and
temperature achieved through the process. The
extent of amine released will depend in part on the
nature of the dewatering chemicals used.
In addition, small amounts of parti culate matter may
be emitted by the processing facility, but these are
easily mitigated.
Land application of any biosolid product can
increase the concentrations of trace elements in the
soil. Alkaline stabilized biosolids help to create soil
pH conditions in which metals are insoluble,
minimizing plant uptake and movement of metals to
groundwater.
Soils which have a low pH will benefit greatly from
the alkaline material and will be more fertile. Lime
is usually low in metals and, when blended with
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wastewater solids, can improve the quality of the
product with respect to metals.
DESIGN CRITERIA
There are many factors that must be considered in
designing an alkaline stabilization facility for
biosolids. The most critical are:
Percent solids of infeed wastewater solids.
Desired results (Class A versus Class B)
which affect the amount of alkaline material
needed and mixing time, which, in turn,
impacts equipment size.
Source and volume of alkaline material to be
used.
Odor control equipment at the processing
facility.
Storage and curing areas.
The equipment necessary for alkaline stabilization is
relatively easy to install and operate. Typical
equipment includes the following:
Wastewater solids feed/conveyance
mechanism.
Lime storage (silo, 1000 or 50 pound bags,
etc.)
Lime transfer conveyor.
Mixer.
Air emission control equipment to minimize
odors and dust.
Figure 2 presents a typical flow diagram for alkaline
stabilization. A list of general design parameters
and criterion for Class B alkaline stabilization is
found in Table 1.
Designing a facility to meet Class A stabilization
requirements may require additional lime storage to
allow an increased lime dose, additional curing
capacity, and/or the provision of supplemental heat.
Storage Silo
->-Treated Air Vented to Atmosphere
Exhaust Air
Treatment
Process Air
Alkaline
Additive
Biosolids
f Mixing f V
Supplemental Heat
Source or Alkaline
Mixing to Achieve
^^ Class A ^.
i
T
i
[ Product Curing |
!_ _ ai2d/?torage _ ]
Product
Distribution
Source: Parsons Engineering Science, Inc., 1999.
Note: Dashed lines indicate optional equipment.
FIGURE 2 FLOW DIAGRAM OF A TYPICAL ALKALINE STABILIZATION OPERATION
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TABLE 1 TYPICAL DESIGN CRITERIA
FOR CLASS B ALKALINE STABILIZATION
Parameter
Design Criterion
Alkaline dose
Retention time in mixer
Retention time in curing
vessel
0.25 pound per pound of
wastewater solids at 20
percent solids
1 minute
30 minutes
Source: National Lime Association, undated.
The end use of the material is another important
factor when designing a biosolids management
program, including alkaline stabilization. The
resulting material is suitable for many uses,
including agricultural application, mine reclamation
and landfill cover. The amount of land that must be
available differs with the use. Alkaline stabilized
biosolids are generally lower in nitrogen than other
biosolids products because nitrogen is converted to
ammonia during processing. The material contains
approximately one to two percent nitrogen; one
percent phosphorus; and negligible potassium,
nutrients of primary importance for vegetative
growth. In agricultural application, alkaline
stabilized biosolids are often applied more for their
pH adjusting characteristics at a typical application
rate of two to five tons per acre. Nutrients supplied
to the crop are a secondary benefit. In reclamation
applications, the material is often applied only once
rather than annually or periodically as in agricultural
applications. Therefore, the material is usually
applied at a higher rate of between 60 to 100 dry
tons per acre. In landfill cover applications, the
amount of material used is dictated more by
regulatory requirements than nutrient content.
Alkaline stabilized biosolids can be used as daily
cover, or to amend the final cover, in accordance
with federal regulations which require material to
enhance conditions for vegetative growth. For daily
cover, a minimum of six inches of soil is required.
Equivalent thickness of alkaline stabilized material
must be approved on a case by case basis. Typical
application rates for incorporation into final cover
material are similar to those at mine reclamation
sites.
PERFORMANCE
Alkaline stabilization is frequently selected by
wastewater treatment facilities in regions where
soils have a tendency to be acidic and where low
cost alkaline materials are readily available. In areas
where soils are acidic, the end product has greater
value. Table 2 shows location, size, and end use for
representative alkaline stabilization facilities.
Alkaline stabilization systems are generally quite
reliable and flexible. The same equipment can often
be used to produce either Class A or B biosolids
with minor process modifications, such as a larger
dosage of alkaline material or the addition of
supplemental heating (pasteurization.)
With respect to pathogen reduction, Class B alkaline
stabilization has been demonstrated to reduce total
coliform, fecal coliform, and fecal streptococci
bacterial concentrations by more than 99.9 percent.
Concentrations of Salmonella and Pseudomonas
aeruginosa have been shown to be reduced below
the level of detection. Pathogen concentrations in
Class B alkaline stabilized biosolids range from 10
to 1,000 times less than those in anaerobically
digested sludge (U.S. EPA, 1979.) Alkaline
stabilization can result in an exceptional quality
product when it meets Class A pathogen reduction,
vector attraction reduction and the highest standards
for metal concentrations. This higher level of
processing can result in a more valuable product
because there are no restrictions on end use. The
high level of disinfection achieved in Class A
products makes them easier to handle and apply.
For example, if a farmer purchases an exceptional
quality product, he will not have to restrict access or
limit grazing and harvesting times.
The appearance, odor causing potential, and
handling characteristics can vary greatly depending
on the type of alkaline stabilization process used.
Processes that add larger amounts of alkaline
material or include added heat, drying, or curing will
produce a drier product that resembles topsoil.
Other alkaline stabilization processes that simply
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TABLE 2 REPRESENTATIVE LIME STABILIZATION FACILITIES
Solids
Location Production Disposal/End Use
(dry tons/day)
Hampton Road, VA
Reedsville, Wl
Lancaster, OH
Raleigh, NC
Howard Co., MD
Cookeville, TN
Charlotte, NC
Washington, DC
Middlesex, NJ
Toledo, OH
Greenville, SC
Tarpon Springs, FL
Syracuse, NY
Urbana-Champagne, IL
Upper Gwynedd, PA
Bergen County, NJ
8
1-2
3
20
20
<1
20
270
146
63
10
3
30
5
2-3
45
Land Application
Land Application
Land Application
Land Application
Land Application
Land Application
Agricultural Liming Agent
Land Application
Landfill Cover
Agricultural Liming Agent
Agricultural Liming Agent
Landfill Cover
Land Application
Agricultural Liming Agent
Land Application
Land Application
Landfill Cover
Pathogen
Reduction
Class A
Class A
Class A & B
Class A
Class A & B
Class A
Class A
Class B
Class A
Class A
Class A
Class A
Class A
Class B
Class B
Class A
Process Employed
Bio*Fix
EnVessel
Pasteurization
Bio*Fix
N-Viro AASAD
Bio*Fix
EnVessel
Bio*Fix
Post-Lime Stabilization
N-Viro AASAD
N-Viro AASAD
N-Viro AASAD
N-Viro AASAD
N-Viro AASAD
N-Viro
Generic
Bio*Fix
Source: National Lime Association, 1995; RDP Technologies, Inc. web site, 1999;
personal communication with facility operators.
N-Viro International Corp. web site, 1999, and
add enough lime to increase pH to 12 for 2 hours
may still resemble biosolids cake from the
dewatering equipment. The characteristics of the
end product should be considered when evaluating
the methods and feasibility of storage. Drier
alkaline stabilized products tend to have fewer
odors after 30 days or more of storage. On the
other hand, cake-like products should be used as
soon as possible to minimize odor complaints at the
application sites.
The odors associated with alkaline stabilized
products are also dependent on the characteristics of
the wastewater solids. Plant operators who
minimize sludge age in the their wastewater
treatment facility will also minimize odor generation
both at the processing facility and at the end use
site. In addition, 25 percent total solids cake is
easier to process and requires less lime than 20
percent total solids cake. The odors also result
from the dewatering chemicals used.
OPERATION AND MAINTENANCE
Alkaline stabilization systems are relatively
uncomplicated facilities operated with the skills
found in typical wastewater treatment plant
personnel. Labor requirements include heavy
equipment operators, maintenance personnel, and
instrumentation/computer operators.
The caustic nature of the alkaline additive requires
higher maintenance on these systems than on
stabilization systems that do not involve caustic
materials. Proper design and operation of the
mixing equipment is necessary to ensure a
consistent, homogeneous product.
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COSTS
It is difficult to estimate the costs of stabilizing
biosolids with alkaline materials without specific
details, such as wastewater solids characteristics and
quantities. One study estimated costs for Class A
alkaline stabilization ranging from $139 to $312 per
dry ton of wastewater solids processed by facilities
designed to serve wastewater treatment plants
ranging in capacity from 10 to 60 million gallons per
day. This estimated range demonstrates the
economy of scale associated with larger systems.
The capital costs cited in this same study ranged
from $1.5 to $4.0 million and annual costs were
estimated to range from $1 million and $4 million.
This study concluded that alkaline stabilization was
less expensive than composting or thermal drying
(Sullivan, 1996.)
Although exact costs for alkaline stabilization
cannot be provided, the following items must be
considered in estimating costs for any alkaline
stabilization facility:
Processing equipment purchase and
installation.
Product curing and storage facilities.
Loading facilities.
Transport of product to point of use.
Royalty and operating fees for proprietary
processes.
Equipment maintenance and fuel.
Alkaline additive.
Labor.
Odor control equipment and chemicals.
Marketing costs/revenues.
Regulatory compliance, such as permit
applications, site monitoring, biosolids
analyses, and regulatory record keeping and
reporting.
The incremental capital cost for meeting Class A
requirements through alkaline stabilization is the
lowest among stabilization alternatives such as
thermal drying and anaerobic and aerobic digestion.
The incremental unit cost (including capital and
operation and maintenance) associated with creating
a Class A product from a system currently making
a Class B product and serving a 5 million gallon per
day wastewater treatment facility was estimated to
be $39 per dry ton. Again, this is significantly less
than the unit costs to increase pathogen treatment
through aerobic or anaerobic digestion, which were
cited as $88 and $103 per dry ton, respectively
(National Lime Association, 1998.)
Some generators of alkaline stabilized biosolids sell
the product for approximately $3 to $5 per wet ton.
REFERENCES
Other Related Fact Sheets
Odor Management in Biosolids Management
EPA 832-F-00-067
September 2000
Centrifugal Dewatering/Thickening
EPA 832-F-00-053
September 2000
Belt Filter Press
EPA 832-F-00-057
September 2000
Land Application of Biosolids
EPA 832-F-00-064
September 2000
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owmitnet/mtbfact.htm
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Backmann, F., 1942. Emergency Treatment 9.
of Army Camp Sewage. Engineering News-
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Christy, P.O., 1990. Process Equipment 10.
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Christy, R.W. Sr., 1990. Sludge Disposal 12.
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Forste, J., 1996. Agricultural Use of Lime
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J.D. Robinson. Arlington, Virginia.
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N-Viro International Corporation's Web
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Smith, K.A., Goins L.E., and Logan T.J.,
1996. Effect of Lime Dose on Thermal
Reactions and Physical Properties of
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16. U.S. EPA, 1994. A Plain English Guide to
the EPA Part 503 Biosolids Rule.
EPA/832/R-93/003, U.S. EPA,
Washington, D.C.
17. U.S. Environmental Protection Agency,
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Valley Forge Sewage Authority
Rick Taylor
333 Pawling Road
Phoenixville, Pennsylvania 19460
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U.S. Environmental Protection
Agency.
21. Wheelabrator Water Technologies, Inc. Bio
Gro Division, 1996. Bio*Fix Alkaline
Stabilization.
ADDITIONAL INFORMATION
District of Columbia Water and Sewer Authority
Chris Peot
5000 Overlook Avenue, SW
Washington, D.C. 20032
Middlesex County Utility Authority
Richard Fitamont
One Main Street Extension
Sayreville, New Jersey 08872
National Lime Association
200 North Glebe Road, Suite 800
Arlington, Virginia 22203
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
1200 Pennsylvania Ave., N.W.
Washington, D.C. 20460
MTB
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