United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600S2-81 -075 June 1981
Project Summary
Engineering Assessment of
Vermicomposting Municipal
Wastewater Sludges
John Donovan
Vermicomposting, the biological
degradation of organic matter that
occurs as earthworms feed on waste
materials, has been advocated by
some as a means of stabilizing and
disposing of municipal wastewater
sludges. Based on review of available
literature, discussions with practi-
tioners, and visits to sites where
Vermicomposting is being attempted
on an experimental scale, the process
has been found to be feasible and
potentially competitive economically
with conventional sludge stabilization
techniques such as land spreading of
liquid sludge and static pile compost-
ing. The question of whether vermi-
composting is the equivalent of con-
ventional processes in stabilizing
sludge and reducing the pathogens in
it remains to be answered at demon-
stration scale.
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
Vermicomposting involves the degra-
dation of organic wastes by earthworm
activity Some species of earthworms
(although not the common nightcrawler
and garden worm) thrive in managed
conditions on a diet and substrate
composed almost entirely of organic
matter. When these worms are added to
shallow beds or windrows of sewage
sludge, they feed on the sludge, digest a
portion of the organic matter and expel
the undigested remains as feces, or
castings.
Breakdown of organic constituents of
the sludge inside the worm's gut is
followed by continued decomposition of
the material after it is defecated. The
rate of sludge decomposition is acceler-
ated over what would occur without
worm activity primarily because the
small size of the expelled castings
greatly increases the surface area the
castings offer for exposure to air and
attachment by microorganisms.
After the worms have fed on the
sludge and converted it into castings,
more sludge can be added. Eventually,
however, the worms must be separated
from the castings and provided with
new sources of food. Worms can be
recycled into new beds of sludge or,
possibly, marketed in some form. The
castings, once dried, have properties
that might make them a desirable soils
amendment. The end products of sludge
Vermicomposting, therefore, are worms
and castings.
Typically, the facilities associated
with Vermicomposting are of a loworder
of technology. Beds can be raised or on
the ground. Some worm beds must be
set aside for propagation of new stock.
Protection from weather extremes must
be provided. Some means of delivering
and spreading sludge should be included,
and a technique for separating the
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products.should be arranged. Automatic
rotary harvesting screens are available,
but other methods may be feasible.
In 1979, Camp, Dresser and McKee,
Inc., undertook for the U.S. Environ-
mental Protection Agency (EPA) a study
of the feasibility of vermicompostmg
municipal wastewater sludge (Contract
No. 68-03-2803, Office of Research and
Development, Municipal Environmental
Research Laboratory). The investigation
combined review of pertinent literature,
discussions with representatives of the
vermicomposting industry and researchers
in the field, and visits to eight sites
where vermicomposting is being prac-
ticed or research is being conducted. At
two of the eight sites visited, facilities
and practices were developed fully
enough to warrant consideration of
applying the technique to full-scale
operation:
• Keysvitle, Maryland—Vermicom-
posting in indoor beds of aerobically
digested, concentrated and air-
dried sludge
• Lufkin, Texas—Vermicomposting
of thickened primary and waste
activated sludge sprayed over saw-
dust beds
Process Considerations
Only two species of earthworms,
Eisenis foetida and Lumbricus rubellus,
are commonly mentioned in the litera-
ture as suitable for use in waste-vermi-
composting operations. These worms
can thrive and reproduce in managed
conditions and readily feed on high
concentrations of organic matter. The
requirements of culture for these species
can be summarized as follows:
• Temperature—The most rapid
feeding and conversion of waste to
castings occur in the range of 13°C
to 22°C.
• pH—Both species prefer neutral
soils, pH of 7.0 to 8.0.
• Moisture — Optimum range of
moisture for worms in vermicom-
posting beds is reported as 50 to 90
percent. (Processed sludge is usu-
ally 80 to 98 percent moisture.)
• Aeration—Earthworms are sensi-
tive to anaerobic conditions, par-
ticularly at higher temperatures.
• Nitrogen—Earthworms reportedly
trive in a medium of 9 to 15 percent
protein (Sludge can vary from 1 2
to 38 percent protein.)
The characteristics of some municipal
wastewater sludge are compatible with
these requirements. Some researchers
have reported, however, that anaero-
bically digested sludges are toxic to
earthworms. Liquid sludges must be
applied to a supplemental substrate m
the beds—sawdust, cardboard, or min-
eral soils—to maintain aerobic condi-
tions and ideal moisture content. The
beds themselves should be laid out so as
to provide maximum possible surface
area. In all but the most benign climates,
it is necessary to protect beds against
extremes of rain, drought, heat, and
cold.
Performance in
Vermicomposting
The rate of biological decomposition
achieved in vermicomposting is con-
trolled by two variables: (1) the feeding
rate of the individual organisms and (2)
their density. The product of these two
values provides sizing criteria for rate of
substrate decomposition per unit volume
or unit area. A logical expression for
feeding rate in vermicomposting would
be dry weight of sludge consumed per
day per unit weight of worms. This
feeding ratio can be expressed as:
(Dry weight of sludge)
(Wet weight of worm)
(Days to total conversion)
This can betermedthesludge—worm
ratio, or S:W ratio. Evaluation of feeding
rates at six different vermicomposting
operations revealed a fairly consistent
daily feeding ratio of 1 20 g to 275 g
sludge (dry weight)/kg worms (0.12 to
0.27 Ib sludge).
The second variable of concern is
worm density (worm-density-to-area
ratio, or W:A). At the Lufkin, Texas,
operation, the estimated worm density—
2000 g worms/m2 (0.42 Ib worms/ft2)
of area—falls near the average density
reported among some vermiculturists
(hobbyists or worm growers who practice
worm culture).
The product of the S:W ratio and areal
density provides a loading rate in terms
of weight of sludge converted to castings
per unit time per unit area, under
constant conditions. At Lufkin, this
loading rate is approximately 245 g dry
solids/mVday (0.05 Ib dry solids/day/
ft2). Higher feeding rates and wo'rm
densities reported at the Keysville,
Maryland, facility yield a loading rate
there of 1750 g/mVday (0.36 Ib/day/
ft2), which suggests that much higher
loading rates may be possible under
proper conditions. The Keysville facility.
however, is operated at much smallei
scale than is Lufkin's, and the techniques
used are not demonstrated to be feasible
or economical at full scale.
Case Study
In 1979, the City of Lufkin, Texas,
constructed twelve 175 m2 (1900 ft2;
enclosed vermicomposting beds, each
capable of receiving 1200 L/day (30C
gallons/day) of liquid sludge at 3.5 tc
4.0 percent solids. The sludge is sprayed
over beds of sawdust and earthworms
The system has worked quite success-
fully, although its full cycle of operation
has not yet been demonstrated. A 15-
cm to 20-cm (6-in. to 8-m.) layer of
sawdust was used as a bedding base. At
2-month intervals, a 2.5 cm to 5.0 cm (1
in. to 2 m.) layer of sawdust is added to
the beds.
It is planned that castings/earthworm
mixture will be removed from the beds
every 6 to 1 2 months and fresh beds
constructed. The castings and earth-
worms will be separated by means of a
two-step migration technique. In the
first step, a small front-end loader will
drive on the bed and windrow the
mixture. Next, a food source will be
spread adjacent to the windrow(s). After
2 days, nearly all the earthworms would
be expected to migrate to the new
material. The windrows, which now
consist of castings and substrate, will be
removed, leaving a high-density pile of
earthworms. The earthworms will then
be used to stock a new bed, and the
cycle will be started again.
Analysis of Full-Scale Facility
Requirements and Costs
Using the experiment at Lufkin as £
model, we considered vermicompostinc
of a quantity of sludge generated a1
municipal wastewater treatment facilitv
serving a population of 10.OOOto 15.00C
persons. About 1 metric dry ton ol
sludge per day (or close to 1 dry ton ol
sludge per day) would be expected
Based on an average conversion rate ol
about 400 g/m2/day (0.08 Ib/day/ft2),
approximately 2400 m2 (25,000 ft2) ol
bed area would be required. Considering
areas for sludge pumping equipment
and access, a total of about 2700 m:
(29,000 ft2) of building would be required.
Capital costs (Table 1) include land
and site development, a structure,
earthworm stock, and equipment. The
cost of a building would be the large^l
single variable in the costs of compost"
ing If a new structure were erected, the
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Table 1. Costs for Vermicomposting J Dry Ton 1907 kg) Per Day of Liquid Sludge
Capital Costs Cosf Annual Cost.
Land and Site Development $ 20,000
Building
Earthworms
Equipment
Subtotal (rounded)
Operating Costs
Labor
Utilities
Maintenance
Miscellaneous
Subtotal (rounded)
TOTAL (rounded)
Unit Cost ($ per dry ton
60,000 to 600,000
25,000
40,000
$140,000 - $680,000
$140,000 - $680,000
processed)
$18,000
7.500
5,000
2,900
4,700
- $66,000
$20,000
$38,000 - $86,000
$105-3235
costs would depend on the geographical
location of the facility and the expected
service life. An inexpensive structure,
such as the one constructed at Lufkin,
Texas, would cost only about $21.50m2
($2/ft2). A more conventional, prefabri-
cated metal structure or a greenhouse,
which would be more suitable for
northern climates, might cost about
$215/m2 ($20/ft2). Based on these unit
costs, the building could cost between
$60,000 to $600,000.
About 4540 kg (10,000 Ib) of earth-
worms would be required to initially
stock the 2400 m2 (25,000ft2) area beds
2000 g/m2 (0.4 Ib/ft2). At an average
cost of about $5.80/kg ($2.50/lb), the
initial earthworm cost would be $25,000.
The cost analysis assumes that earth-
worms would be purchased only once
and that the worms' breeding in the
beds would maintain or increase the
population throughout the life of the
project. This assumption however,
needs to be proven in demonstration
projects.
Total capital costs would be approxi-
mately $140,000 to $680,000, depend-
ing on the type of structure used. The
total equivalent annual costs at a 7
percent interest rate and varying service
lives would be $18,000 to $66,000.
Annual operating costs including
labor, utilities, maintenance, and saw-
dust would be about $20,000 per year. It
is estimated that, on a yearly basis at a
municipal wastewater treatment facility,
a vermicomposting operator would be
required only 30 to 50 percent of the
time.
Total annual costs of vermicomposting
would be $38,000 to $86,000 or about
$115 to $260/metnc ton ($105 to
$235/dry ton) processed. These unit
costs are quite reasonable compared
with other conventional disposal
methods at facilities of similar size. For
example, options involving (1) land
spreading of liquid sludge, or (2) de-
watering and composting or landfilling
might cost between $165 and $2757
metric ton ($150 and $250/ton) de-
pending on transportation.
Findings
Vermicomposting of municipal waste-
water sludges has been considered
seriously only within the last 10 years.
The technology has been developed
largely by private entrepreneurs asso-
ciated with the worm-growing industry;
by analyzing their data, useful engineer-
ing design parameters such as the S:W
and W:A ratios can be developed.
Vermicomposting of sludge is a feasi-
ble process for use at small wastewater
treatment plants producing a raw,
aerobically digested or mechanically
dewatered sludge. Costs of composting
a liquid sludge that is pumped and
sprayed onto wormbeds filled with an
appropriate bulking substrate are rea-
sonable compared with other available
processing and disposal options
The castings produced by worms that
have been fed on sludge are dry, virtually
odorless, and suitable for use as a soil
amendment or low-order fertilizer. Con-
centrations of heavy metals and syn-
thetic organic chemicals must, however,
fall within acceptable ranges, this
would probably be the case if the waste-
water is originally collected from pre-
dominantly domestic sources. Reduc-
tions in levels of pathogenic bacteria
and viruses by this process have not
been demonstrated. Government regu-
lations on sludge-derived products
constrain the sale of castings, and no
municipality pursuing vermicomposting
as an option should assume that reve-
nues from sale of castings can offset
process costs
Research and
Development Needs
Research and development needs in
vermicomposting may be divided into
basic research and demonstration-
scale applied research. Basic research
should continue on the parameters of
feeding rate and density as they affect
loading rates (and, therefore, costs) of
this process; the earthworms' ability to
breed and maintain viable populations
under vermicomposting conditions; and
the fate of heavy metals and pathogens
during vermicomposting.
Demonstration-scale applied research
should seek to document the capital and
operating costs of vermicomposting,
including structural requirements, labor,
and purchase of earthworms. Develop-
ment of mechanical methods of loading
and unloading beds with dewatered
sludge is needed, as is an analysis of
various methods of separating earth-
worms and castings. To put laboratory-
based research results in proper context,
laboratory and "real-life" conditions of
vermicomposting should be compared.
Results obtained at the demonstration
facility should be fully documented to
assist in future consideration of vermi-
composting as a feasible sludge-man-
agement alternative.
The full report was submitted in
fulfillment of Contract No. 68-03-2803
by Camp, Dresser and McKee, Inc.,
under the sponsorship of the U.S.
Environmental Protection Agency.
> US GOVERNMENT PRINTING OFFICE 1981-757-012/7151
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John Donovan is with Camp, Dresser & McKee, Inc, Boston, MA 02108.
Roland V. Villiers is the EPA Project Officer (see below).
The complete report, entitled "Engineering Assessment of Verm/composting
Municipal Wastewater Sludges," (Order No. PB 81-196 933; Cost: $9.50,
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 Protect/on Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research '
Information
Cincinnati OH 45268
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