EPA Report Number
January 1980
ENGINEERING ASSESSMENT
OF VERMICOMPOSTING
MUNICIPAL WASTEWATER SLUDGES
by
Camp Dresser & McKee Inc.
Boston, Massachusetts 02108
Contract No. 68-03-2803
Project Officer
Roland V. Villiers
Municipal"Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research Labora-
tory, U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the view and
policies of the U.S. Environmental Protection Agency, nor does mention of
trade names or commercial products constitute endorsement or recommendations
for use.
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..... ABSTRACT
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 practitioners, and visits
to sites where vermicomposting is being attempted on an experimental scale,
the process has been found to be feasible and potentially competitive eco-
nomically with conventional sludge-stabilization techniques such as land-
spreading of liquid sludge and static-pile composting. The question of
whether yermicomposting is the equivalent of conventional processes in
stabilizing sludge and reducing pathogens in it remains to be answered at
demonstration scale.
•s
In feeding on sludge and defecating undigested remains as "castings", worms
alter the physical structure of the sludge, changing it in composition from
an amorphous mass to small discrete particles. The resultant enormous in-
crease in surface area accelerates drying, aeration, and microbial activity.
The castings have a faint odor of fresh earth or compost and are quite dry
and pebbly in consistency. Because their nutrient value is roughly the
equivalent of the sludge from which they were derived, they might be of
agricultural benefit if used as a "top dressing" or soils amendment, provided
that concentrations of toxic heavy metals in the original sludge were at
levels considered acceptable for agricultural use.
Vermicomposting proceeds most rapidly in moist, aerobic conditions, with
temperatures maintained within a moderate range of 13°C to 22°C. Anaerobi-
cally-digested sludges cannot support a thriving worm population. If liquid
sludge is used as a feed, it is necessary to apply it to a bulking agent
(such as sawdust) to reduce the substrate's moisture content and assure
aerobic conditions. A wide variety of sludge feeds have been used in vermi-
composting experiments, but analysis of actual dry solids loading rates at
six vermicomposting operations show sludge:worm (S:W) ratios to fall within
the relatively narrow range of 0.12 to 0.27 Ib dry solids to Ib worms per
day.
Of the different techniques of vermicomposting currently being used, only
vermicomposting of liquid primary and waste activated sludges appears com-
petitive at present stages of development. Vermicomposting of dewatered
sludge in raised indoor beds is, at present, too labor-intensive for eco-
nomical scale-up. Costs for labor alone would be about $300 per ton. Out-
door windrowing techniques are not suitable for year-round use in most areas
of the United States. Costs developed for the vermicomposting of liquid
sludges in protected facilities are estimated at $105 to $235 per ton,
iv
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depending on the type of structures used.
An S:W ratio of 0.20 was assumed for determination of vermicomposting costs.
Area! requirements were assumed to follow those generally used in vermi-
culture, or about 0.4 Ib of worms per square foot of bedding. The product
of the S:W ratio and areal requirement was found to yield a loading density
of 0.08 Ibs of sludge per day per square foot of area; this density was used
as the basis for developing cost estimates. Loading densities of up to four
times this figure have been reported to be successfully used, but feasibility
of the higher loadings remains to be demonstrated at full scale.
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-TABLE.OF CONTENTS
Foreword
Abstract
Acknowledgements
Section 1. INTRODUCTION
Purpose of Study
Format of Assessment
Section 2. PROCESS OPERATION
Definition of Vermicomposting
History of Vermicomposting
Current Status
Description of the Process
Sludge Feed
Requirements for Supplemental Substrate
Problem Constituents
Bed Depth and Configuration
Biological Parameters
Earthworm Species Used in Vermicomposting
Life Cycle in Earthworm Species Used in
• Vermicomposting
Conditions of Culture
Performance in Vermicomposting
Rates of Vermicomposting
Physical Effects
Chemical Changes
Section 3. PHYSICAL FACILITIES REQUIRED FOR VERMICOMPOSTING
Basis of Design
Liquid Sludge
Process Operation
Equipment Requirements
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4
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5
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VI
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TABLE OF CONTENTS (Cont.)
Building Requirements
Dewatered Sludge
Process Operation
Equipment Requirements
Building Requirements
Section 4. VERMICOMPOSTING PRODUCTS AND PRODUCT MARKETING
Worm Castings as a Product
Agricultural Value
Anticipated Market Development
Earthworms as a Product -
Recreational Market
Inoculation of Soils
Fertilizer or Soil Supplement
Worm Stock for Vermiculturists
Animal Feed
Human Nutrition
Market Prospects
Section 5. ENVIRONMENTAL AND PUBLIC-HEALTH ASPECTS
OF VERMICOMPOSTING
Potential On-Site Problems
Odors
Vermi n
Site Runoff and Leachate
Workers' Safety
Potential Risks in Dispersal of Products
Toxic Substances
Pathogens
Section 6. ECONOMICS OF VERMICOMPOSTING
Basis of Cost Estimates
Costs of Vermicomposting Liquid Sludge
Page
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28
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32
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34
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36
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vii
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TABLE OF CONTENTS (Cont.)
Capital Costs
Annual Costs
Comparison of Unit Costs
Costs of Vermicomposting Dewatered Sludge
Section 7. FINDINGS AND RECOMMENDATIONS
Major Findings
Research and Development Needs
Basic Research
Demonstration-Scale Applied Research
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51
52
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54
56
56
58
References
60
Appendix. SITE VISITS
64
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..- LIST OF ILLUSTRATIONS
Figure Ti tle Page
Figure 2-1 Vermicomposting windrows being formed at 6
Ridgefield, Washington
Figure 2-2 Applying aerobiGaily-digested municipal waste- 7
water sludge to vermicomposting windrows at
Ridgefield, Washington
Figure 2-3 Structures, enclosing vermi compos ting . -•_„:>• 7
facilities adjacent to municipal wastewater-
treatment plant, Lufkin, Texas
Figure 3-1 Sludge force main and laterals for distributing 26
raw primary and waste activated sludges to worm
beds, Lufkin, Texas
Figure 3-2 End-on view of simple vermicomposting structures 27
suitable for use in warmer climates, Lufkin,
Texas
Figure 3-3 Typical mechanical rotating harvesting device 29
for separating earthworms and castings
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..LIST OF TABLES
Table Title Page
2-1 Biological Data on Four Species of Lumbricid 12
Worms
2-2 Feeding on Eisenia foetida at Different 14
Temperatures
2-3 S:W Rates at Six Vermicomposting Operations 19
2-4 Mineralization, Reduction of Nitrogen, and 22
Effects on C:N Ratio in Sludge with and
without Worms
4-1 N-P-K Ratio in Sludge and Castings 32
4-2 Composition of Eisenia foetida ' 39
4-3 Amino Acid Analyses (%) of High-Protein 39
6-1 Costs of Vermicomposting One Dry Ton per Day 53
of Liquid Sludge
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ACKNOWLEDGMENTS
The "Engineering Assessment of Vermicomposting Municipal Wastewater Sludges"
was prepared under the direction of Mr. Paul W. Prendiville, officer-in-
charge, and Dr. Albert B. Pincince, project manager. Principal investigators
were Mr. John F. Donovan (project engineer), Mr. John E. Bates, and Mr.
David F. Young. The assistance of Ms. Suzanne M. Belleville, Ms. Ellen M.
Connors, Ms. Suzanne G. White, and Ms. Beverley J. Williams is gratefully
acknowledged.
EPA project officer was Mr. Roland V. Villiers, who provided valuable direc-
tion and guidance.- Dr. Joseph B. Parrel! Reviewed an earlier draft of this
document and offered several useful suggestions that have been incorporated
into this final report.
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Section 1. INTRODUCTION
Purpose of Study
The "Engineering-Assessment of Vermicomposting Municipal Wastewater
Sludges" is an engineering and scientific feasibility study of vermicom-
posting as a means of stabilizing, disinfecting and converting municipal
sludges to a usable soils amendment. The report contains:
• .Discussion of the state-of-the-art of vermicomposting of munici-
pal sludges
• Engineering analysis of the technical and economic aspects of
vermicomposting municipal sludges
• Recommendations as to the applicability of vermicomposting to
present and future sludge-management needs
• Recommendations for further study
Work was carried out as a work effort (WE-2) under Contract No. 68-03-
2803, U.S. Environmental Protection Agency (EPA), Office of Research and
Development, Municipal Environmental Research Laboratory. The investiga-
tion combined review of pertinent technical and nontechnical literature,
extensive written and telephone contact with numerous representatives of
the vermiculture industry and researchers in the field, and visits to
seven sites where vermicomposting of sludge is being practised and/or
research is being conducted. Literature references were obtained through
contacts with individuals working in the field, as well as through a
computer-based literature search of related entries in the following data
bases: A6RICOLA, BIOSIS PREVIEWS, COMPENDEX, EPS, NTIS, POLLUTION
ABSTRACTS, and SSIE CURRENT RESEARCH.
Format of Assessment
In successive chapters, this report covers aspects of the vermicomposting
process (Section 2) and its facilities (Section 3), vermicomposting
products and product marketing (Section 4), environmental and public-
health issues (Section 5), economic considerations (Section 6), and
findings and recommendations (Section 7). The material presented .is
based on- critical review of the available literature, site inspections,
and discussions with those working in the field. As a result of the
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technical evaluations described in Sections 2 and 3, we have identified
two methods of vermicompost ing that are currently being practised with a
degree of success and have developed in Section 6 a range of unit cost
estimates for vermicomposting. The implications of these costs for ver-
micomposting are among the major findings discussed in Section 7.
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Section 2. PROCESS OPERATION
Definition of Vermicomposting
Vermicomposting involves the degradation 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 will feed on
the sludge, digest a portion of the organic matter and expel the undi-
gested 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 defe-
cated. The rate of sludge decomposition is accelerated over what would
occur without worm activity, due primarily to the small size of the
expelled castings and, therefore, to the qreatly increased surface area
they 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 low
order of technology. Beds can be raised or on the ground. Some worm
beds might 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
products should be arranged (automatic rotary harvesting screens are
available, but other methods might be feasible).
Vermicomposting has been compared to static-pile composting of sludge,
as developed by U.S. Department of Agriculture (USOA) at Beltsville,
Maryland. Although both processes require moist, aerobic conditions,
however, they are neither identical nor compatible. Static-pile com-
posting involves degradation of sludge by a wide variety of heat-tolerant
bacteria, whose metabolic activity generates appreciable increases in
temperature (up to 70°C, at the most active stage). Vermicomposting, on
the other hand, depends on the maintenance of moderate temperatures in
the range of 13°C to 22°C.
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History of Vermicomposting
The role of earthworms in nature has been recognized since ancient times
and was studied extensively by the biologist Charles Darwin in the late
19th century. Despite this awareness, and despite the fact that success-
ful culture of earthworms need involve no new technology, the practices
of raising worms (vermiculture) and using them for waste management
(vermicomposting) have not been advanced until recently.
There are now numerous individuals, gardeners and entrepreneurs raising
worms in a soilI/peat/manure bedding in indoor bins or outdoor plots; most
of those practising vermiculture depend heavily on the baitworm market to
realize some income from the business.
To our knowledge, only since 1970 has the vermicomposting of wastes been
attempted at any more than barnyard scale. Pioneering efforts commonly
mentioned in the literature (1, 2) include a demonstration project at
Hollands Landing, Ontario (Canada), which was begun in 1970 and has since
been operated under private ownership, andv a pilot-scale, one-time
demonstration of vermicomposting municipal solid waste (MSW), which was
conducted in 1975 at Ontario, California. The Hollands Landing facility
has vermicomposted small amounts of manure, food-processing wastes, and
sludge (Carl Klauck, owner, personal communication); the Ontario,
California project involved composting and vermicomposting of nine tons
of hand-sorted municipal refuse over a period of 120 days (1).
Other work has been carried out in Japan, where some pulp and food-
processing industrialists have turned to vermicomposting techniques for
management of sludges and waste byproducts (3). Information obtained
through sources in the vermiculture industry indicates, however, that
only a handful of privately-owned installations are operating in Japan
(as in the United States), the largest of which can process about 30 TPD
of pulp and food wastes. Operation appears to be rather labor-intensive,
and the economics appear to depend heavily on disposal fees and on sale
of freeze-dried worms (as fish feed) and castings (Shizuro Aobuchi, per-
sonal communication).
Current Status
At present, vermicomposting is being practised at several installations
across the United States. Some of these projects represent funded
demonstrations, such as ongoing work being carried out under National
Science Foundation grants in San Jose, California and Syracuse, New York.
Others — in Maryland, Washington and Florida, for example -- are being
conducted by vermiculture entrepreneurs who are eager to demonstrate the
potential of vermicomposting in waste management. And in Texas, the
state and the municipality itself are supporting demonstration of ver-
micomposting sludge at the Lufkin wastewater-treatment plant.
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In the course of this investigation, seven Vermicomposting operations
were visited in order to confirm details of process operation and to view
at first hand the techniques and facilities used. The following visits
were made between July and October 1979:
Keysville, Maryland — Vermicomposting in indoor beds of approx-
imately 50 pounds per day (dry solids) of aerobically-digested,
concentrated and air-dried sludge. -
Louisville, Colorado -- Pilot-scale Vermicomposting of composted
yard wastes
Lufkin, Texas -- Vermicomposting of 900 pounds per day (dry solids)
of thickened primary and waste activated sludge sprayed over
sawdust beds
Ridgefield, Washington -- Experimental Vermicomposting of
aerobically-digested liquid sludge applied to windrows
San Jose, California -- Windrow vermjcomposting of sludge that has
been dried in a lagoon for several years
Syracuse, New York — Laboratory-based research into Vermicom-
posting of digested sludges
Titusville, Florida -- Experimental Vermicomposting of approxi-
mately 140 pounds per day (dry solids) of sludge
Accounts of these site visits are included as the Appendix to this report.
Description of the Process
Sludge Feed
The sludge feed to Vermicomposting operations can take a variety of
forms, ranging from liquid sludge to sludge cake to dried sludge. To our
knowledge, 10 wastewater-treatment plants in the United States are
currently contributing a port-ion of their sludge for use in Vermicom-
posting. Septage (pumpings from septic tanks, in this case dried to 16
percent solids) has reportedly been used for Vermicomposting in the
Eugene, Oregon area (4).
Researchers at the State University of New York at Syracuse have found
that anaerobically digested sludges are unsuitable for Vermicomposting
(5). If freshly obtained from the digester, the sludge is toxic to
worms; if aged for several weeks after digestion, the sludge still fails
to support a thriving earthworm population. It is not known what
constituent(s) in the anaerobically digested sludge is toxic to worms.
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Liquid Sludge. Vermicomposting operations in Ridgefield, Washington and
Lufkin, Texas are based on use of liquid sludge. At Ridgefield,
aerobically-digested sludge from three wastewater-treatment plants
(WWTP) is applied to unprotected windrows in a field (Figures 2-1 and
2-2), while at the Lufkin WWTP, combined primary and waste activated
sludges are gravity-thickened to about four-percent solids and pumped to
enclosed vermicomposting facilities located onsite (Figure 2-3).
Sludge Cake. At Keysville, Maryland, a pilot-scale vermicomposting
operation uses a sludge cake obtained from the New Oxford, Pennsylvania
WWTP, where the sludge has been aerobically digested, conditioned with
polymers, and dewatered to about 12-percent solids on a belt filter
press. The operator then air-dries the sludge to about 18-percent solids
before applying it to the vermicomposting beds.
A vacuum-filter cake from another WWTP was formerly used at Keysville.
When the method of conditioning sludge at that WWTP was changed from
polymers to lime and ferric chloride, however, the sludge was found to be
unsuitable for vermicomposting.
In an Akron, Ohio study, raw waste activated sludge was dewatered by
centrifuge to approximately 10 percent solids before vermicomposting (6).
Dried Sludge. In San Jose, California, sludge dried in large lagoons to
more than 80-percent solids has been shaped into windrows for a vermicom-
posting demonstration project. The sludge, generated at the San Jose
WWTP, was anaerobically digested and dried in the beds for two years or
more prior to its use in vermicomposting. The vermicomposting operator
found it necessary to rewet the material for a 14-day period before
introducing earthworms to the windrows.
Figure 2-1 Vermicomposting windrows being formed at
Ridgefield, Washington
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2-2.
& x
Figure 2-2 Applying aerobically-digested municipal
wastewater sludge to vermicomposting
windrows at Ridgefield, Washington
Figure 2-3 Structures enclosing vermicomposting facilities
adjacent to municipal wastewater-treatment plant,
Lufkin, Texas
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Requirements for Supplemental Substrate
From the preceding discussion, it is evident that a wide range of sludge
processes can be used prior to vermicomposting. Some operators have
found sludge alone to provide a suitable substrate for the worms, pro-
vided that it can be kept in an aerobic condition and that its moisture
content is within the range considered ideal for the worms' feeding (see
"Conditions of Culture"). Those installations handling liquid sludge,
however, require supplemental substrate in the wormbeds in order to main-
tain aerobic conditions and ideal moisture content in the vermicomposting
mass.
The supplemental substrate can be materials such as sawdust, woodchips,
cardboard, other waste products including municipal solid waste, or
mineral soil. At Lufkin, Texas, an eight-inch layer of sawdust is used
as a bedding material to absorb liquid sludge that is sprayed over it.
The Lufkin operators originally started with only 2 to 3 inches of
bedding, but increased the depth in order to maintain aerobic conditions
and offset consumption of sawdust by earthworms. At some other installa-
tions, sludge is mixed with cardboard or other wood byproducts.
A mineral-soils: substrate might support the fastest rates of sludge con-
version and worm growth (7). Possible reasons given for this include
changes in cation exchange capacity, the assistance of the soils in
grinding food in the worm's gizzard, and microbial effects — but these
are all somewhat speculative. More research is needed to determine
whether such a substrate is needed or whether proper processing of sludge
prior to vermicomposting eliminates the need.
Problem Constituents
Some materials in sludge or substrate materials can cause reduced feeding
activity among the worms or can have toxic effects on them. Unfortun-
ately, only very limited research has been conducted in this area. Some
guidelines for avoiding such problems have been developed in manuals of
vermiculture, but the best prevention is said to be 12-hour jar-testing
of the material using a small sample of live worms (8). In some cases,
leaching of marginal materials — animal manures, some peat-moss pro-
ducts, and pulp products containing aromatic oils — will make them
suitable for use.
As discussed previously, anaerobic sludges are toxic or harmful to worms.
Use of ferric chloride, lime and alum for phosphorus removal and chemical
conditioning of sludge can reduce worm activity (Paul France, personal
communication). Ammonium salt added as acetate at 1,000 ppm was lethal
to worms after two weeks, and copper added as copper sulfate at 2,500
mg/1 was lethal to worms within one week (9).
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Extremes of pH In either sludge or bedding will be harmful to worms.
Pulp obtained from processing coniferous trees can contain resins that
will cause worms to migrate out of the beds (8).
A number of synthetic organic chemicals used as agricultural agents can
be harmful or toxic at high dosages, according to researchers working on
field tests. Fumigants, carbamate fungicides and carbamate insecticides
were found to be generally toxic; organophosphate and organochlorine
pesticides generally had harmful effects. Most herbicides were not found
to have direct harmful effects (10).
For the purposes of this investigation, it can be assumed that aerobic
sludges obtained from most facilities treating predominantly domestic
wastewater flows will be safe for use in vermicomposting.
Bed Depth and Configuration
Outside of the need to shape wormbeds so that they are fully accessible
to operators for maintenance, the principal criterion for bed con-
figuration is based on the absolute requirements for moist, aerobic,
moderate-temperature conditions within the beds. These requirements can
be met by maximizing the surface-area-to-volume ratio. Shallow beds or
windrows provide the greatest exposure of sludge to air, helping to main-
tain aerobic conditions in the sludge mass, preventing accumulation of
excess moisture at depth, and dissipating excess heat generated by micro-
bial decomposition of the sludge. In addition, since the worms used in
vermicomposting are found in nature in the uppermost layers of soil, the
use of shallow beds conforms to the worms' instincts for vertical
distribution.
The best way to maintain optimum vermicomposting conditions year-round is
to cover or enclose the wormbeds. At Ridgefield, Washington, vermicom-
posting is conducted using windrows located out-of-doors. During the
winter, however, the earthworms reportedly migrate to lower depths in the
ground to protect themselves against freezing; feeding activity is sig-
nificantly reduced. Other outdoor vermicomposting operations have
sustained worm losses from heavy rains (San Jose, California and Akron,
Ohio) and hurricane-force winds (Titusville, Florida).
Biological Parameters
Earthworm Species Used in Vermicomposting
Only two species of earthworms — Eisenia foetida and Lumbricus rubellus
are commonly mentioned in the literature as suitable for use in waste-
vermicomposting operations. Researchers have focused almost exclusively
on the vermicomposting performance of these two species, which often
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share the common name of red worm. Other species have reportedly been
utilized in pilot-scale studies, including Lumbricus terrestris (5) —
the nightcrawler — and Allolobophora caliginpsa (1) — the field worm,
and these same species often will invade the lower reaches of composting
windrows (11), but comparative discussions of their survival and perfor-
mance in vermicomposting are not available. Recently, some interest has
been shown in the so-called "African nightcrawler", Eudrilus eugeniae,
but this worm requires near-tropical conditions of culture, and its per-
formance in vermicomposting has been explored only in laboratory-scale
studies (12, 13).
Not surprisingly, the two worms that appear best suited to the conditions
of culture in vermicomposting occur in nature in enriched organic
substrates. Both are small- to mid-sized earthworms classified by biolo-
gists among the lumbricids (Family Lumbricidae). They are pigmented
surface-dwellers, found in nature in the upper 8 cm of the soil surface.
Their surface-dwelling habit is not a coincidence: in nature, fresh
organic matter is concentrated in the upper soil layers.
The distribution of E. foetida and L. rubellus in nature tends to be
highly localized, as opposed to the widespread distribution of larger
earthworm species that inhabit stable agricultural lands, fields, and
woods. The "vermicomposting species" cannot thrive in unenriched
environments; they require concentrations of organic matter. Conversely,
the larger "agricultural species" -- such as the nightcrawler and field
worm — breed'more slowly, adjust poorly to the managed conditions of
vermicomposting, and cannot tolerate the temperature increases that can
accompany bacterial decomposition of organic matter (11).
»
E. foetida is commonly known as the brandling worm (also red worm, red
wiggler, manure worm, red-gold hybrid (11)). A relatively small worm of
4 to 8 mm diameter and 100 mm in length (10, 11), it is found in nature
only in areas of high organic concentration — in decaying logs, compost
heaps, dung heaps. The brandling worm cannot, in fact, survive for long
in average soils containing a greater proportion of mineral matter. Two
other characteristics distinguish it from most other earthworm species.
E. foetida can tolerate somewhat higher temperatures than can most of the
subsurface, burrowing species (14); this enables E. foetida to survive
and feed in a compost heap sooner after the completion of its active,
heating phase than can the other species (except L. rubellus). E.
foetida is also more prolific than most other worms; it is the only
cultivated species known to produce, on the average, more than one viable
offspring per egg capsule (or cocoon).
L. rubellus (red worm, red wiggler, hybrid red worm, English red, Georgia
red, California red) is similar in habit to E. foetida; it is found
naturally in stream banks, organic leaf litter, under dung pats in agri-
cultural fields. Like E. foetida, this red worm breeds rapidly and has a
relatively short development time to sexual maturity. Both L. rubellus
and E. foetida can be found through all levels of a stabilized compost
heap, whereas larger agricultural species are generally found only in the
lower parts of the heap.
10
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Table 2-1 presents vital biological•data on E. foetida and L. rubellus,
along with data on two non-vermicomposting species, L. terrestris and A_._
caliginosa, for comparative purposes. (From this point on in the report,
references to "earthworms" should be taken to mean E. foetida and L_._
rubellus, unless otherwise specified.)
It is evident, from the table, that data gaps exist in some areas and
that conflicts in data obscure true values in others. Some of the data
conflicts can undoubtedly be attributed to differences in experimental
techniques and to the fact that some observations were obtained in the
field and others in controlled conditions favorable to growth. In the
field, the period of growth is markedly affected by conditions when
growth began.
Sound experimental techniques will one day resolve these discrepancies and
fill in other gaps in our knowledge of earthworm biology as it applies to
vermicomposting. In the meantime, it is advisable to use the conser-
vative values, although earthworms entrepreneurs have made a practice of
using the outer-limit values to support their claims of high production
and growth in their earthworm populations.,
Life Cycle in Earthworm Species Used in Vermicomposting
Like all other earthworms, E. foetida and L. rubellus are hermaphroditic,
each individual worm possessing both male and female sex organs.
Reproduction normally occurs through copulation and cross-fertilization,
following which each of the mated pair can produce cocoons (oothecae)
containing one to 20 fertilized ova. Production of cocoons and emergence
of offspring in these species are summarized in Table 2-1.
The resistant cocoons, tiny and roughly lemon-shaped, are usually depo-
sited near the surface of the ground, except in dry weather when they are
left at deeper levels. After an incubation period that varies with cli-
matic conditions, the cocoons hatch. Young earthworms, white and only a
few millimeters in length on emerging from a cocoon, gain their adult
pigmentation within a day. Assuming favorable conditions, they will grow
to sexual maturity within several weeks of emergence, although much
depends on temperature, season, and conditions of culture.
Mature individuals are easily distinguished by the presence of a cli-
tellum, the pale- or dark-colored swollen band located at the "front"
end of these worms, somewhat behind the genital pores. (The clitellum
secretes the fibrous cocoon, and clitellar gland cells produce a nutri-
tive albuminous fluid contained in the cocoon.) They can continue to
grow in size for several months after completing their sexual develop-
ment.
Cocoon production, development and growth are much affected by seasonal
climatic conditions. Cocoon production for L. rubellus, for example, is
greatest in the months from June through September (11), and falls off
11
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Table 2-1
BIOLOGICAL DATA ON FOUR SPECIES OF LUMBRICID UO{tMS*
Vermi compos ting Species
Characteristic
Common name
Color
Elsenia foetlda
. Brandling worm (red worm,
manure worm)
. Brown and buff bands
Lumbricus rubellus
. Red worm
. Reddish brown
Non-Verm1compost1ng Species
Lumbricus terrestrls
. Night crawler
. Brown violet
Allolobophora callglnosa
. Field worm
. Rose or brown red
Size of adult worms
Weight
No. of cocoons/year
Size of cocoons
Incubation period
No. of worms hatched/
cocoon
4-8 mm diameter,
50-100 mm length
2-3 mg at hatching
0.4 g average adult
Up to 2.4 g 1n controlled
conditions
11 (field conditions)
Up to 100 in controlled
conditions
3.87 mm x 3.17 mm
3 weeks (25°C)
11 weeks (field)
1.6 - 3.6 mean, varied
seasonally
Average of 2.6 (and up to
6) among those that hatched—
21.5% did not hatch
4 mm diameter,
70-150 mm length
79-106
3.18 mm x 2.76 mm
16 weeks (field)
Usually 1-2
8 mm diameter,
80-300 mm length
Average 5.0 g
Low
5.97 mm x 4.69 mm
Usually 1-2
4 mm diameter,
40-200 mm length
. 27
**
. 19 weeks
. Usually 1-2
Development to
maturity
Lifespan
. 5-9 weeks (under controlled
conditions, 18-28°C)
.. 47-74 weeks (field)
. 4-1/2 years (protected)
. 37 weeks (field)
**
. 52 weeks (field)
. 6 years (protected)
. 55 weeks (field)
. Long
*From (7, 9, 10, 11, 15) and (Neuhauser, personal communication)
**Information not found
-------
directly with decreasing soil temperatures (we have not seen any
discussion of whether it may also reflect an endogenous rhythm).
Although, theoretically, earthworms living in controlled conditions might
be capable of extremely high rates of population growth -- with popula-
tion doubling times measured in only a few days (8) — most ver-
miculturists assume population doubling times of 60 to 90 days (11, 16).
It is not known what generation times can be expected under the
controlled, but periodically disturbed, conditions of vermicomposting.
In temperate zones, the seasons of greatest developmental activity are
the spring and fall. Earthworms will enter a quiescent state (or
diapause) when conditions are hot and dry, or very cold. Under such con-
ditions, production of cocoons is ceased, and worm growth and development
are slowed. Cold weather also lengthens the incubation period for
cocoons. Cocoons produced during cold-weather months generally will not
hatch until spring. Relatively little growth occurs in summer and
winter, and individuals that emerge from cocoons in the height of summer
can take up to twice as long to reach full development as those that
emerge in autumn or spring.
In general, conditions of heat and drought are more dangerous to earth-
worms than those of wet and cold. (Some worms have been shown to survive
weeks of immersion in water, provided the water is kept aerated.)
Cocoons, however, can survive extremes of hot and cold within the range
of normal climatic conditions.
E. foetida reportedly have lived for more than four years in controlled
conditions. In the field, average lifespans probably range from one to
three years. Among natural hazards, in addition to temperature and
moisture extremes, are internal parasites (some microorganisms, platy-
helminth worms, rotifers, nematodes and fly larvae) and predators (many
birds, badgers, hedgehogs, moles, some snakes, certain beetles and their
larvae, centipedes, and a few species each of carnivorous- slugs, leeches
and flatworms).
Conditions of Culture
While details on the biology of earthworms seem scant or somewhat contra-
dictory, the practical experience of worm breeders, worm farmers, and
researchers has built a relatively consistent body of information on
optimum conditions for supporting a worm population.
Temperatures. Worms exhibit a fairly complex response to changes in tem-
perature. In general, conditions that promote the most rapid feeding and
conversion of waste to castings are found in the temperature range of
13°C to 22°C, averaged from results obtained by most workers. Both E.
foetida and L. rubellus will prefer substrate temperatures within this
13
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moderate range, but the upper limit of temperature preference is somewhat
lower for L. rubellus. at 18°C. Table 2-2 illustrates the effect of tem-
perature on feeding rates.
Table 2-2
FEEDING OF EISENIA FOETIDA
AT DIFFERENT TEMPERATURES*
Temperature
5
10
15
20
25
Feeding rate**
(g dry wt sludge/
g dry wt worm/day)
0.132
0.434
1.699
v 1.526
0.876
*From (17)
**As determined by egestion of castings
These moderate temperatures represent practical criteria for design and
operation of vermicomposting systems.
At soil temperatures below 10°C, worms' feeding activity is described as
greatly-reduced to nonexistent; below 4°C, production of cocoons and
development of young earthworms cease. Worms will tend to hibernate
and move to deeper layers for protection. Worms can become acclimated
during the fall months to the temperatures they will encounter at deeper
substrate levels in the winter, but they cannot survive in freezing
temperatures.
At temperatures above the optimum range, up to 25°C to 27°C, the worms'
performance depends in part on their acclimation to the higher tem-
peratures. Worms raised from their hatching to adulthood under con-
trolled conditions and 25° temperatures have been shown to feed and
grow well, and to develop and reproduce at rates faster than worms raised
at lower temperatures or in the field (7, 11). For worms not acclimated
to higher temperatures, activity is significantly reduced at 25°, and
there may be loss-of-weight and mortality (5, 18).
14
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The unfavorable effect on worms of high (25°C and above) temperatures is
not entirely a direct effect. Warm temperatures also support accelerated
growth and activity of microorganisms in the substrate; the increased
microbial activity tends to use up available oxygen, to the worm's detri-
ment (5).
Although the ultimate goal of sludge processing is the breakdown and
recycling of complex constituents in the sludge, the increased microbial
catabolism of sludge at higher temperatures probably is not desirable in
vermicomposting. Depletion of available oxygen by microorganisms in the
substrate will interfere with the worms' activity; the worms' reduced
feeding will result in significantly lower turnover of sludge into
castings. The signal advantage of vermicomposting is the rapid conver-
sion of sludge to a relatively aerobic, odorless product that can then be
decomposed by microbes at rates three times faster (18) than can sludge
that has not been digested by worms. The vermicomposting operation
should be designed to maintain temperatures within the 13° to 22° range
found to be optimum for feeding.
pH. Earthworms generally prefer neutral s,oils (1), and both E. foetida
and L. rubellus find their optimum at pH 7.0 to 8.0, neutral to mildly
alkaline (11). Worms will avoid acid soils of pH less than 4.5, and pro-
longed exposure to such soils acts as a violent contact poison with
lethal effects (10).
Minor increases in acidity caused by addition of fresh wastes to the ver-
micomposting bed will be neutralized by the worms' intestinal secretions
and excreted ammonia (10). Operators can also control acidity by adding
lime (19) or limestone flour (8), as necessary.
Moisture Content. Under the right conditions, earthworms have extraor-
dinary capabi1 iities to survive either submersion in water or dehydration.
L. rubellus and other worm species have been shown to survive 31 to 50
weeks in total submersion, provided the water is aerated. In fact
cocoons were produced and hatched, and young worms fed and grew under
water (10). In practical experience, however, worms used in composting
have been reported drowned by "exceptionally heavy rains" (6) or to have
migrated out of vermicomposting windrows during periods of rain (20). It
is likely that the adverse effects are due more to depletion of oxygen in
saturated sludge than to the moisture itself.
The threat of drying — desiccation — is more serious. Earthworms are
up to 85 percent water by weight, and this water is easily lost through
the worms' skin and protective outer covering. Worms can lose 75 percent
of their moisture content and still survive, provided they are revived
immediately by immersion in water.
E. foetida will drive down to deeper soil levels in dry conditions (11).
Prolonged drying of composting beds or windrows will result in loss of
feeding activity and death of the worms. Every author on vermiculture
stresses the need to keep bedding materials moist.
15
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In nature, the greatest number of worms will be found in soils of 12 to
30 percent moisture (11). In vermicomposting, the optimum range of
moisture content has been reported at 50 to 90 percent. Above 80 percent
moisture, conditions of poor oxygen transfer might interfere with the
worms' feeding. Sludge that has been dried to less than 50 percent
moisture might be too hard for E. foetida to burrow through (5).
The worms' feeding rate apparently is independent of moisture content as
long as a threshold value of moisture is present (17).
Aeration. Earthworms have no specialized respiratory organs; oxygen dif-
fuses in through skin layers of the body wall, and carbon dioxide dif-
fuses out. Earthworms are sensitive to anaerobic conditions. Their
respiration rates are depressed in the presence of low oxygen con-
centrations -- by 55 to 65 percent, for example, in the presence of oxy-
gen at one-fourth its normal partial pressure (10). Feeding activity
might be reduced (7). E. foetida has been reported to migrate en masse
from a water-saturated substrate in which oxygen is being depleted, or in
which carbon dioxide or hydrogen sulfide is accumulating. The situation
is made more critical by oxygen requirements that may increase by a fac-
tor of 10 as temperatures increase from 9aC to 27°C (10).
In the vermicomposting operation, aeration requirements can be met by (1)
maximizing surface area in the compost beds or piles, (2) protecting
against bed saturation, (3) adding .bulking agents such as wood chips, or
sawdust to the beds, (4) maintaining temperatures within the optimum
range for feeding, (5) mechanically turning or tilling the beds at regu-
lar intervals (say, every two to three weeks.) Some researchers argue
against mechanical turning, both because "it can cause trauma to the worms
and because it can redisperse castings into the substrate, with possibly
toxic effects on the worms (7).
Nitrogen and Other Substrate Minerals. Earthworms reportedly thrive in a
medium of 9 to 15 percent protein (19). Fresh bovine feces contain about
14 to 15 percent protein (19), sludge can vary from 12 to 38 percent pro-
tein, and unsorted mixed municipal refuse may contain only 4.2 percent
protein, depending on sources (21). A low to moderate carbon-to-nitrogen
(C:N) ratio is considered desirable, with worms showing maximal weight
gain in the C:N range of 15:1 to 35:1 (22). This optimal range
corresponds well to that found in most sludges. Only the most biodegra-
dable fractions of municipal solid waste, however — such as yard wastes
and food wastes — fall within this optimum range; unsorted or processed
mixed municipal refuse will have a C:N ratio on the order of 50:1 (21).
Application of high-nitrogen fertilizers is not recommended, as these can
create unfavorable acidic conditions (11).
A lower threshold for calcium of 800 mg/kg has been shown to permit
growth of the field worm, A11 piobophora caliginosa. No upper limit is
known, and concentrations as high as 3,600 mg/kg have supported growth
(11).
16
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Concentrations of potash and phosphorus have not been shown to have a
direct effect on worm growth. Soil-surface concentrations of copper of
260 ppm are toxic to earthworms (11). Many industrial and agricultural
chemicals are toxic to earthworms, as discussed in a previous section.
Generally speaking, mixing of soil into the substrate appears to promote
worm growth and development, perhaps due to inorganic constituents found
in soil (22). - - -.- -
Light. Worms have concentrations of light-sensitive cells in their skin,
and they will migrate toward weak sources of light, away from strong
sources of light (10). Installation of protective shading will not only
prevent worms from burrowing deeper into the compost bed, but also
help to keep summer temperatures at moderate levels favorable to feeding.
(Ideally, the shading can be removed in winter months to take advantage
of solar heat.)
Substrate Compaction and Maintenance. The field worm, A. caliginosa,
has been shown to migrate freely from loose soil into highly compacted
soils (23). While compaction itself might not pose problems to worms,
failure to keep substrate loose and aerated can kill worms or cause
their migration out of the substrate.
Of prime importance, substrate must be replenished before it is
exhausted. Worm castings have toxic effects if eaten by the species of
worm that produced them, as has been observed in laboratory conditions
(7), and worms will migrate from composting beds when adequate food is
not available (Collier, personal communication).
Density. Recent laboratory results obtained by Neuhauser et al (24)
relate worm survival and growth to population density. The findings show
that growth rates during the worm's rapid phase of growth (from age three
weeks to about 9-12 weeks) are considerably depressed by crowding. At 4
worms per 250 g of centrifuged activated sludge (11% solids) on 50 g of
soil, E. foetida grew at an average rate of 250 mg per week; at 16 worms
in the same 78 cm' area, growth fell to 170 mg per week. The effects of
crowding are attributed to competition for food, diversion of energy into
production of cocoons as opposed to production of body tissue, and to the
accumulation of castings, which are toxic to E. foetida. The researchers
found that activated sludge dewatered to 11-percent solids had a high
population carrying capacity; by extrapolation, 2900 g of worms could be
supported in one square meter of space if sufficient sludge were present.
This biomass would be about 10 times that found in naturally occurring
populations (10).
17
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Performance in Vermicomposting
As the worms eat and digest sludge, they expel the digested material as
"castings", which are nothing more than worm feces. Castings, lying next
to uneaten sludge, are immediately distinguishable: the large particles,
irregular shapes or amorphous surfaces of sludge are reduced by the worms
to much smaller particles of relatively uniform shape and size.
In fact, when dry, castings have the appearance of sand pebbles of
roughly oblong shape, with approximate dimensions of 0.5 mm in diameter
and 1.0 mm in length (25). Their odor is that of fresh earth or compost
and is not noticeable unless an effort is made to smell the castings
close up. Because of their relatively uniform shape and size, castings
can be considered aesthetically superior to many other sludge products.
Rates of Vermicomposting
The rate of biological decomposition is controlled 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.
In vermicomposting, a logical expression for feeding rate would be dry
weight of sludge consumed per day per unit weight of worms. This feeding
ratio can be expressed as follows:
(Dry weight of sludge) (1-)
(Wet weight of worm) (Days to total conversion)
We have termed this ratio the sludge-worm ratio, or S:W ratio. The S:W
ratio can be applied to any vermicomposting operation, regardless of the
moisture content of the sludge received or of the depth to which sludge
is applied.
In some biological systems, such as in the activated-sludge process, the
density of organisms is measured per unit volume. In surface-limited
systems, such as in trickling filters, the density of organisms is
measured per unit area. Given an adequate depth of substrate, worms can
be considered to be surface limited. So it is reasonable to express the
density of worms in terms of their weight per unit area.
Loading rates at six different vermicomposting operations were evaluated
to determine what S:W ratios have been used: consistently, daily feeding
ratios in the range of 0.12 to 0.27 were obtained (Table 2-3). The rela-
tively narrow range of values obtained is remarkable, because the criteria
used in deciding on loading rates at most of the facilities cited here
were based more on observation and previous experience than on rigorous
computation of optimum loadings. More study is required to determine
18
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whether this range defines "optimum" worm feeding rates under constant
conditions. If confirmed or modified experimentally, the S:W ratio could
serve as a principal criterion in design of future siudge-vermicomposting
facilities, as it is tied directly to process requirements. !
— Table 2-3
S:W RATES AT SIX VERMICOMPOSTING OPERATIONS
Location
and Loading Percent Worms Days to S:W
Reference (wet weight) Solids (wet weight) Conversion Ratio
Akron, OH
(6)
800 Ib
Keysville, MD 300 Ib
(site visit)
Lufkin, TX 2,500 Ib
(site visit)
Eugene, OR
(4)
2200 Ib
Syracuse, NY 30 parts
(25) by weight
Syracuse, NY ". 4 g
(25)
15-20
18
4
16
11
20
400 Ib
N
100 Ib
800 Ib
300 Ib
1 part
by weight
1 9
2
1
10
14 +
4
0.15-0.20
0.27
0.125
0.12
£0.24
0.2
Note that the S:W ratio depends heavily on definition of "total
conversion", the point at which conversion of sludge to castings is
complete. There exists, at present, no standard definition of when the
vermicomposting process is complete; operators often make judgments based
on visual inspection of the wormbeds. (Note, too, that there is no need
to consider worm weights on a dry basis: worms retain a relatively
constant proportion of moisture — approximately 80 to 85 percent of body
weight — under controlled conditions of vermiculture or vermicomposting.)
A more theoretical estimate would take into account only the volatile
fraction of the sludge, but data are not yet available to extend the
analysis.
19
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Data on areal densities of worms are presented by Minnich (11). Worm
densities reported for several small-scale vermicultural operations range
from about 0.17 to 0.50 Ib/sq ft, with an average of about 0.35 Ibs/sq
ft. At only two of the sludge-vermicomposting operations investigated in
this study were areal densities reported. The estimated worm density at
Lufkin's vermicomposting operation -- 0.42 Ib/sq ft -- is close to the
average density reported for vermiculture.
The density of worms used in vermicomposting dewatered sludge at
Keysville, Maryland is much greater, at 1.33 Ibs/sq ft. (Even higher
densities have reportedly been attempted in this operation, but slime
exuded by the worms was found to make the sludge bedding and castings
difficult to handle and process.) If the areal density used at Keysville
can be confirmed at other operations, it would suggest that densities of
about three times those conventionally used in vermiculture might be
appropriate for vermicomposting of sludge.
As noted above, the product of the siudge-to-worm ratio and the areal
density provides a loading rate in terms of weight of sludge per unit
time per unit area. At Lufkin, this produqt is approximately 0.05 Ibs of
dry solids per day per square foot. At Keysville, the ratio is about
0.36 Ibs/d/sq ft. Because of the wide ranges found in practice in both
the S:W ratios and the areal densities of worms, we suggest that research
be conducted to better define the S:W ratio and the optimum worm density
in order to obtain suitable design criteria defining the rate of sludge
application per unit area.
Physical Effects
The most comprehensive analyses run on sludge and castings to date have
been performed by Hartenstein jit jal_, and most of the discussion that
follows is derived from a paper completed by these researchers in 1979
(25).
The eating and defecating of sludge by worms significantly changes the
physical and chemical properties of the sludge. The single most impor-
tant change is the physical reduction of the sludge from an amorphous
mass to the small cloddy particles called castings. Carmody has esti-
mated that vermicomposting of a processed (shredded) solid waste will re-
duce particle size by 40 to 60 percent (1), which would signify a
two-fold increase in surface area. For sludge, the reduction in size and
concomitant increase in surface area are many times higher.
Three important effects of the increase in surface area might reasonably
be expected: (1) the castings should dry more rapidly than sludge that
has not been exposed to worm action, (2) the great increase in surface
area should accelerate microbial activity over that in the sludge, and
(3) the material should remain aerobic and relatively odor-free. Results
of experiments have confirmed that all three expected effects do occur.
20
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Drying. Side-by-side comparison of castings and the sludge from which
the castings were derived shows that castings dry 1.5 to 2.1 times faster
than the sludge (25). Spread in a layer 2.5 cm thick and kept at 105°C,
castings were essentially dry 12 days after they were produced; sludge
kept under the same conditions took 21 days to reach the same state.
Because castings dry more rapidly than sludge, sludge mass is reduced
much more quickly when sludge is exposed to earthworm action under proper
conditions. ' "" *
It has been observed that worms secrete around the castings a membrane
that might serve to protect the worms from their feces (castings are
generally toxic to the species of worm that produces them). A side
effect of this process, however, might be that castings are kept sepa-
rated from each other, thereby increasing the exposure of surface to air.
Microbial Activity. One measure of microbial activity in sludge is the
rate at which oxygen is consumed by the sludge. Aerobically-digested
sludge, dried .to 20 percent solids and then maintained at 25°C, will
reach a low steady state of oxygen consumption after about eight days.
The same sludge, when exposed to earthworm activity at 25°C, does not
reach the steady-state level any more quickly, but it does show markedly
higher oxygen-consumption rates during days 2 to 8. Hartenstein et al
have calculated that the increased microbial activity supported on
castings, as indicated by the elevated oxygen-consumption rates, means
that an additional 19 percent of the sludge is oxidized as a result of
the worms' eating (25). Actually, this bacterial increase could reason-
ably be attributed either to improved substrate conditions (increased
surface area) or to a possible "bloom" of decomposer bacteria prompted by
favorable growth conditions in the earthworm's gut. Whatever the cause,
the effect — significantly improved growth of microorganisms on earth-
worm castings — has been observed in nature as well as in vermicom-
posting (10).
Aerobic Conditions. Because the castings remain aerobic, the odor-
producing compounds — such as amines, sulfides and phenolic compounds
normally associated with anaerobi-c sludges — are either oxidized or con-
densed (25). The castings have no offensive odor.
Chemical Changes
The earthworms' feeding on sludge changes its chemical composition in a
number of different ways. Some of the changes are directly attributable
to the worms' metabolism of the sludge: for example, some 0.5 percent of
the nitrogen in sludge is assimilated by the worms for growth and cocoon
production (25). Other chemical changes -- including a much more sub-
stantial reduction in nitrogen — are due to microbial activity after the
castings leave the worm; the activity is accelerated over what might
occur in the absence of worms.
21
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Hartenstein et ^1_ have conducted a series of experiments yielding the
following summary of chemical changes in sludge subjected to worm activ-
ity (25). In their experiments, they used aerobically digested sludge
centrifuged to 11 percent solids and maintained the sludge samples, with
and without worms, at 25°C.
pH. The pH of sludge was depressed by E. foetida from levels of 7.0 to
7.1 to levels in the castings of 6.4 to 6.5.[Researchers working in
field tests have recorded the opposite effect, such that addition of
worms to mildly acid soils will gradually cause an elevation in pH (10, 11).)
Ash Content, Nitrogen. C:N Ratio. Side-by-side comparisons of sludge and
castings over a four-week period yielded the following additional infor-
mation, as summarized in Table 2-4.
Table 2-4
MINERALIZATION, REDUCTION Of NITROGEN, AND
EFFECTS ON C:N RATIO IN SLUDGE WITH AND WITHOUT WORMS*
Ash content
Carbon
Nitrogen
C:N
Original
Sludge
29.3 + 0.06
39
5.78 JK 0.02
6.74
Sludge without Worms
after Four Weeks
36.0% + 0.04
36%
5.08% + 0.22
7.08
Sludge with Worms
after Four Weeks
38.4% +0.10
34%
4.56% + 0.03
7.46
.* From (25)
The feeding activity of E. foetida accelerates the rates of mineraliza-
tion and reduction of nitrogen. The fact that proportionally more nitro-
gen than carbon is lost due to earthworm action results in a higher C:N
ratio in castings: 7.46 with earthworms present after four weeks versus
7.08 without earthworms and 6.74 in the original sludge. The decreases
in nitrogen content are due in some small part to uptake of nitrogen by
the worms, as mentioned earlier in this section, and probably in large
part to activity of denitrifying bacteria and loss of ammonia to the
atmosphere. Discussions of N-P-K concentrations and suitability of
castings for agricultural use are contained in Section 4.
22
-------
Heavy Metals and Other Elements. The aging of sludge causes a propor-
tionate increase of heavy metals and other refractory materials, due to
decreasing sludge mass from mineralization. As noted in Section 5, it is
not clear whether the worms' activity changes the availability of these
materials to the plants.
23
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Section 3. PHYSICAL FACILITIES REQUIRED FOR VERMICOMPOSTING
Basis of Design
Because the approaches to vermicomposting used to date vary so widely
from one installation to another, description of "an optimum process" or
"typical facilities" is not possible. Sludge feed, application rates,
methods of separating worms from castings, and wormbed configuration are
among components of the vermicomposting process that are still in experi-
mental stages. Nonetheless, based on what is known about vermiculture,
sludge management, and the performance of worms in vermicomposting,
facilities must be provided for the following functions:
• Wormbeds — Since these worms are surface dwellers, and since it
is crucial to maintain aerobic conditions and moderate tem-
peratures in the substrate, the vermicomposting beds or windrows
- must.be shallow.
\
• Sludge conveyance — Depending on the nature of the sludge feed,
sludge can be moved and distributed by manual, mechanical or
hydraulic means.
• Shelter — Structural facilities must protect against extremes of
cold, heat, drought and moisture, all of which can cause loss of
activity, migration, or death.
• Bed harvesting — Collection of worms and castings can be
accomplished by manual or mechanical means or by use of mobile
equipment, such as a front-end loader.
• Separation of products -- Cylindrical rotating screens are com-
monly used for mechanical separation of worms and castings. Some
other techniques require no special facilities or equipment.
In order to describe more specifically the types of equipment and facili-
ties required for vermicomposting and to estimate their associated costs,
we have developed as cases for study two installations that are based
directly on facilities operating in Lufkin, Texas and Keysville,
Maryland. The Lufkin facility accepts for vermicomposting a raw liquid
sludge pumped directly from the city's wastewater-treatment plant, while
the Keysville facility uses an aerobically-digested and dewatered sludge.
Reasonable design parameters are the sludge:worm ratio and the areal worm
density (see Chapter 2). As shown in Table 2-3, the weight ratio of
sludge (dry) to earthworms (wet) varies somewhat from site to site, but
it averages about 0.20 Ib/day/lb. And a worm density of about 0.4 Ib/sq
ft is common in vermiculture, although the Keysville operator reportedly
uses more than three times that rate. The S:W of 0.2 Ib/day/lb and the
worm density of 0.4 Ib/sq ft yield a loading rate of 0.08 Ib/day/sq ft.
24
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Liquid Sludge
Process Operation
For a facility processing liquid sludge, the sludge must be applied to an
appropriate substrate material in order to maintain aerobic conditions in
the bed. At-Lufkin, this-is accomplished by placing a 6- to 8-in. layer of
sawdust as a bedding base and spraying sludge onto the sawdust using an
in-place distribution system. To provide 0.08 Ib/day/sq ft, about 0.24
gallons of 4-percent-solids sludge would be dosed per square foot of bed
area. This dosage would be applied on a daily basis. At two-month
intervals, a 1- to 2-in. layer of sawdust would be added to the beds.
The operator at Lufkin believes this is required because there is some
consumption of the sawdust by the earthworms (Ed Green, personal
communication). Eventually, the castings would build up'on the bottom of
the bed, since they are a denser material than the sawdust.
The castings/earthworms mixture would be removed (harvested) from the
beds every 6 to 12 months arid fresh beds constructed. Although material
had not been removed from the Lufkin beds at the time of this writing,
the operator reported that he plans to use a migration technique.
(Mechanical screening technique is discussed below for the dewatered-
sludge case.) The migration technique would be accomplished in two
steps. In the first step, a small front-end loader, tractor or other
vehicle with a blade attached would drive onto the bed and windrow the
vermicomposted material. Next, a food source such as sludge would be
spread adjacent to the windrow(s). After a day or two, nearly all the
earthworms would be expected to have migrated to the new material. The
windrows, which now would consist primarily of castings and substrate,
would then be removed, leaving a high-density pile of earthworms in the
adjacent temporary bed. The earthworm would then be used to stock a new
bed, and the cycle would be started again.
Equipment Requirements
The equipment required for liquid-sludge vermicomposting includes that
needed for sludge distribution and for harvesting. At Lufkin, sludge is
pumped via a force main from the wastewater-treatment facility over a
distance of several hundred feet to the vermicomposting beds. The force
main connects to a lateral along each long side of the 20-ft x 95-ft beds
(see Figure 3-1). Spring-loaded valves placed at intervals of every 25
to 30 feet distribute the sludge over the bed area.
Alternative methods of bed configuration and distribution are possible.
The pumping and distribution system should be designed to minimize the
amount of sludge remaining in the pipe after the daily dosing. This con-
'dition results in septicity and has caused some problems at Lufkin.
Should small-diameter piping be used, it might be necessary to install a
25
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grinder or mazerator ahead of the piping, depending on the nature of the
sludge used. Pumping and'distribution system controls should be located
within view of the beds so that the operator can monitor the dosing
operation.
6 y 3
Figure 3-1 Sludge force main and laterals for distributing
raw primary and waste activated sludges to worm
beds, Lufkin, Texas
Mobile equipment requirements consist of a small front-end loader for
initial bed construction and harvesting. The operator at Lufkin has
suggested that a more specialized machine could be used to remove
material from the bed, such as a "poultry-house cleaner" used commer-
cially in the poultry-raising industry to remove bedding and poultry
manure for later agricultural use.
Building Requirements
Building requirements are based on the rate of sludge conversion per unit
of area. As was discussed above, a loading rate of 0.08 Ibs of dry
sludge per day per square foot is reasonable. For a one-dry-ton-per-day
facility, approximately 25,000 square feet of bed area.would be required.
Based on area requirements for pumping and walkways of an additional 15
percent, a 29,000-sq-ft building would be required. (If worm density
could be tripled, as is done at Keysville, the total area needed would
drop to about 10,000 sq ft).
26
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At 'some vermicomposting facilities, the building requirement is met by
using an unused portion of a covered sludge-drying bed or by leasing an
unused barn or similar structure. For this report, we have considered
two types of structures. The first is a prefabricated "greenhouse-type"
building that might be utilized in northern climates. The building is
capable of withstanding snow loading and could be heated by solar heat or
by conventional means. The second type of structure is like the one used
at Lufkin and is appropriate for warmer climates (Figure 3-2). It con-
sists of a simple arched steel frame with two layers of 6 mm polyethylene
cover material. The outer layer of polyethylene is dark, the inner layer
is transparent. The outer layer can be removed during the winter months
to allow the sun to heat the beds though the inner layer. Alternatively,
with both layers in place and the ends of the structure open, the beds
can be kept at reasonable temperature levels during the summer months.
The life of the polyethylene cover might be only about two years,
depending on method of handling and climatic conditions.
.Figure 3-2 End-on view of simple vermicomposting structures
suitable for use in warmer climates, Lufkin, Texas
27
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Dewatered Sludge
Process Operation
For a facility using dewatered sludge, the operation might be similar to
the existing pilot-scale operation in Keysville, Maryland. This facility
is processing only about 54 Ib of dry solids per day. ;
At Keysville, vermicomposting takes place on two 75-sq-ft beds.
Dewatered sludge, concentrated and air-dried to about 18-percent solids,
is shoveled onto the beds from a wheelbarrow. The sludge is leveled to a
uniform depth of about two inches, and the earthworms are then weighed
and distributed evenly atop the beds. Over a two-day period, the earth-
worms convert the sludge into castings. At the end of this period, the
castings-earthworm mixture is removed from the beds. The earthworm-
castings mixture is fed the slightly elevated "feed" end of a cylindrical
rotating screen. As the material is tumbled by gravity down the length
of the screen, the castings fall through the screen and the earthworms
pass through to the discharge end where they are collected. The earth-
worms are then available for reuse. Labor requirements total approxi-
mately two hours per day to load, unload, and screen materials.
The wheelbarrow-shovel operation used at Keysvilie would be inappropriate
for a larger facility (a 1-ton-per-day facility would, after all, process
•about 40 times more sludge than is currently being processed at
Keysville). A large operation would require mechanization. Dewatered
sludge cake would be transported to the bed by conveyor and leveled using
an automatic device. After two days, the bed would be unloaded automati-
cally. This might be accomplished by designing each bed as a movable
belt conveyor. In this way, the material could be discharged directly to
another conveyor which would transport material to the screening device.
The operator at Keysville reports that about one percent of the earth-
worms are lost in the harvesting process per week. Due to the low resi-
dence time in the beds and the intermittent disturbance caused by
harvesting, earthworms in the vermicomposting beds might have little or
no opportunity to breed and maintain the population level. In addition,
many of the juvenile worms that do hatch in the beds fall through the
harvesting screen with castings. Therefore, in order to make up for worm
loss, new stock must be purchased regularly or several undisturbed beds
must be set aside strictly for breeding activity.
Equipment Requirements
The major items required for vermicomposting of dewatered sludge are the
beds and material-transfer and harvesting equipment. The beds used at
Keysville are constructed of wood and wire screens. The sides of the bed
are approximately 4 inches high, which provides for a 2-inch freeboard
above the surface of the mixture. The freeboard is required to keep
28
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material from dropping to the floor during bed loading and unloading.
The bottom of the bed consists of a fine wire-mesh screen (the operator
recommends an eighth-inch screen, but is currently using common window
screening). Screening, rather than wood, is used for the bed bottom, in
order to promote aerobic conditions throughout the 2-inch layer of
material.
The operator at Keysville has suggested that the beds could be stacked to
save space. Such a stacking arrangement is very common in the earthworm-
breeding industry.
A harvesting device used at several installations consists of a
cyclindrical screen from 8 to 12 feet long and of about 2 feet in
diameter (see Figure 3-3). The screen is rotated by a small-horsepower
motor via a belt or by direct drive along a central shaft. The screen is
usually divided into two or three sections of different mesh sizes in
order to screen products of varying sizes.
3-3
Figure 3-3 Typical rotating harvesting device for
separating earthworms and castings
29
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Building Requirements
A building for handling sludge cake would be more compact than one for
liquid sludge, because the worm beds would likely be stacked. Three-
tiered operation would cut area to about one-half the requirement for
liquid sludge - about 15,000 sq ft per ton/day of sludge for a worm
density of 0.4 Ib/sq ft and about 5,000 sq ft per ton/day of sludge
for a worm density of 1.2 Ib/sq ft.
As was suggested earlier, vermicomposting beds can easily be constructed
within an existing building. The two types of structures described
earlier for different climatic conditions would probably be generally
acceptable for vermicomposting of dewatered sludge.
30
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Section 4. VERMICOMPOSTING PRODUCTS AND PRODUCT MARKETING
Successful techniques of vermiculture, including vermicomposting, produce
castings and earthworms. Potential markets exist for both products; it
has been suggested that income derived from the sale of vermicomposting
byproducts can help to offset process costs. - -
Markets for earthworms can be divided between markets for live earthworms
(bait for sports fishing, agricultural uses) and dried or processed
earthworms (soil additive, animal feed, high-protein diet supplement for
humans). Castings have been marketed successfully as a component in
potting mixes for horticulture and have been distributed in bulk for land
application.
Recognition of the growth of vermiculture among private entrepreneurs
prompted the California State Legislature in 1978 to define vermiculture
byproducts as an agricultural commodity entitled to the market promotion
and safeguards and product-research provisions contained in the
California Agricultural Code under the Marketing Act of 1937. The
California Farm Bureau has taken an active role in development and sup-
port of vermiculture enterprises, through conduct of discussion on
industry research and regulations (J.W. Field, State of California
Department of Food and Agriculture, personal communication).
Similar legislation was enacted in 1979 in the State of Washington. It is
not known what other states, if any, have promulgated regulation of ver-
miculture or have authorized its support.
Worm Castings as a Product
The worms' castings must be periodically removed from the vermicomposting
beds, as the castings apparently contain substances that are toxic to the
earthworms (12). The methods used to separate earthworms from castings are
discussed in Chapter 2.
The castings have obvious agricultural and horticultural appeal, repre-
senting, as they do, a natural, "organic" soil amendment with attractive
structural properties and low-order plant nutrient values. The castings
have a favorable appearance: they lack the offensive odor of sludge
(although they might not be'entirely odor-free), and, when sifted and
dry, they are granular, 0.5 to 2.5 mm in diameter, and of a brownish-gray
color.
Passage of sludge through the earthworm's gut significantly alters'the
physical structure of the sludge. Large sludge particles are broken down
into numerous smaller particles, with a resultant enormous increase in
31
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surface area (17). As a result of the increase in surface area (as dis-
cussed in Section 2), the castings dry about twice as fast as the original
sludge, remaining odor-producing sulfides are completely oxidized, micro-
bial respiration is accelerated by a factor of 3, and Salmonella bacteria
are destroyed about twice as rapidly as in the original sludge (7, 25).
The chemical composition of castings derived from sludge reflects that of
the original sludge. Increases will be observed in the concentrations of
some constituents (such as sodium, calcium and heavy metals), and
decreases in others (such as carbon and nitrogen) as described in Section
2, Performance in Vermicomposting.
Remarkable claims have been made for other characteristics of worm
castings, including greatly accelerated humification and formation of
water-stable aggregates. Results of more rigorous research show,
however, that these characteristics pertain more to castings produced by
surface-casting species such as Lumbricus terrestris and Allolobophora
Tonga than to those produced by vermicomposting worms. In fact, the per-
centage of humic acid in castings produced by E. foetida has been shown
to decrease to below the percentage for control sludge.(sludge without
worms), over a 28-day period, despite an initial increase in the first
five days (17).
Agricultural Value
Table 4-1 shows nitrogen-phosphorus-potassium (N-P-K) concentrations in
aged, aerobically-digested sludge from a wastewater-treatment plant in
New York State. These values would give an N-P-K ratio for «the sludge of
5-3.5-0.5. E. foetida fed on this sludge produce castings with a vir-
tually identical N-P-K ratio of 4.5-4-0.5. For purposes of comparison,
heat-dried sludges produced as soil supplements in Houston, Chicago and
Milwaukee have N-P-K ratios of 5-4-0, 5-4-0.5, and 6-2-0, respectively.
Raw sludges subjected to thermophilic composting have N-P-K ratios on the
order of 1-1-0.2 (28).
Table 4-1
N-P-K RATIO IN SLUDGE AND CASTINGS*
Sludge Sludge
(original values) (after four weeks) Castings
N
P as ?2®5
K as K?0
5.78
3.30
0.48
5.08
3.78
0.51
4.56
4.05
0.57
—• •
*From (25)
32
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The nitrogen in castings is not all available immediately for plant
uptake (29); this means that castings could act beneficially as a slow-
release, low-order nitrogen fertilizer. The actual suitability of
castings for agricultural use, however, depends on the composition of the
original sludge feed. Castings derived from an aged anaerobic sludge in
San Jose, California were found by an independent laboratory to be accep-
table for use as a soil amendment in terms of nutrients and salinity,
sodium and pH values, but excesses of boron and, possibly, of phosphorus
rendered the material unsuitable for direct use as a planting soil (30).
Analysis of castings derived from a wide variety of feeds (not
necessarily sludge) showed most contained amounts of sodium or other
salts that would be detrimental to plant growth (29).
Although castings are not generally suitable for use as a sole-source
planting medium, mixture of castings with other materials can yield an
acceptable potting soil. The laboratory analyzing the San Jose product
recommended that castings be incorporated into top layers of agricultural
soils, in bulk, or be mixed in specified proportions with sphagnum peat
moss, Perlite, potassium nitrate, calcium carbonate lime, and iron
sulfate for use as a potting soil, provided that concentrations of heavy
metals were found to lie within acceptable ranges. Other worm growers
have used different mixes; for example, a 1:1:1 mix of castings, peat
moss and Perlite (31).
Anticipated Market Development
The existing or potential markets for castings include:
•
• Use as an ingredient in potting mixtures
• Sale or distribution as an organic soil amendment
t Sale as an organic fertilizer
The first of these markets has some potential, at least on a local or
regional level. Castings-based potting soils have been successfully
marketed in some areas of the United States, including California,
Washington, Colorado and Ohio. Consumers have, in fact, shown themselves
to be willing to pay a small premium for potting soils that contain
earthworm castings, apparently because of good growing performance
obtained through use of the product. One vermicomposting operation is
currently producing and marketing a castings potting mix that is sludge
derived, but regulations in other states might preclude sale of such a
product.
Competition in this market can be expected from conventional potting
mixes, which are often less expensive, and from vermiculturists selling
high-grade mixtures derived from worms feeding on a variety of non-sludge
substrates.
33
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The second market -- bulk sale or distribution of casting as an agri-
cultural soils' amendment — is more compatible with the scope and purpose
of sludge-processing operations. Competition here would be represented
by other sludge "products" offered for land application. The advantages
of castings over these other products lie in their benign odor and
appearance and uniform quality, all of which are consistent with charac-
teristics of other non-sludge fertilizers and amendments currently used
in-agriculture. — •- - -
The third market, sale as an organic fertilizer, is not a particularly
viable one. In this case, the costs of production are relatively high as
compared to those for other fertilizers containing the same or a greater
amount of nutrients. In addition, the sale of castings as fertilizer
might be constrained by state regulations that define fertilizers in
terms of nutrient value. (Arizona, for example, requires that soil addi-
tives sold as fertilizer have a nitrogen content of at least 4 percent.
The material is also subject to a tax of $0.20 per dry ton.)
Whether sold as a fertilizer or soil amendment or distributed in bulk for
application to public lands or farmlands,xcastings must satisfy the same
criteria as sludges that are proposed for land application. Passage of
sludge through the earthworm's gut and its subsequent mineralization
increase the concentrations of heavy metals that are present in the
sludge. Some of these metals can be injurious to plant growth, and some
can be accumulated by organisms in the food chain to levels that might be
harmful to humans. This constraint is discussed further in Chapter 5.
In summary, some nutrients of agricultural benefit are present in
castings produced by worms feeding on sludge. Castings could, therefore,
be a valuable soil amendment, provided that the original sludge contained
normal proportions of plant nutrients and was not heavily contaminated by
potentially toxic substances. In rural and suburban areas, there might
be sufficient demand for this product to realize some income from sale of
the castings to farmers and property owners. In other areas, the product
might be used in bulk by municipal parks and grounds departments or
public works departments. It could also serve as a nutritive substrate
mixed with soils for reclamation of disturbed lands.
Earthworms as a Product
According to a report prepared by the University of California
Cooperative Extension,-"the major use of earthworms today is as bait for
freshwater sport fishing...Some worms are also sold to home and organic
gardening enthusiasts for soil improvement and composting of organic
refuse" (32). Entrepreneurs in the vermiculture industry have made claims
for a virtually unlimited market serving the following sectors:
34
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• Sport fishing (32)
t Inoculation of horticultural (16) or agricultural soils or
reclaimed lands (8, 11)
• Fertilizer or soil supplement (33)
• Animal feed (34) - - - -- -•• -
• Worm stock for vermiculturists (8, 11)
• Human nutrition (16, 35)
Of these market sectors, the only "large stable market" for vermicul-
turists, at present, involves sale of baitworms (36). The remaining sec-
tors must be described as speculative, at best. For vermicomposting
operations, additional constraints operate, as discussed below.
Recreational Market >
L. rubellus and E. foetida are reported to be satisfactory baitworms, but
both are rather small -- particularly when raised under high-density con-
ditions of composting—and so are rather difficult to handle. Some
anglers consider nightcrawlers — not a "domesticated" species — to be
superior baitworms (11, 37), due primarily to their larger s'ize.
Nationally, the market for worms used as bait by sport fishermen has been
estimated variously at $5 million (McNelly, personal communication), $26
million (11), $50 million (37, 38), and $80 million (8). The U.S.
Department of Agriculture (USDA) has made no objective analysis or pro-
jection of the baitworm market, nor has it played any role in regulating
or promoting the market (Mr. J. Schwartz, USDA, Beltsville, Maryland,
personal communication).
It is difficult to make a valid analysis of the market, because much of
the local demand is filled by youthful entrepreneurs selling collected
nightcrawlers at roadside stands and by small-scale growers. Other
segments of this market are handled through mail order by large-scale
growers and wholesalers selling in bulk to retail outlets such as bait
shops, tackle shops and fishing resorts.
Retail prices for baitworms range from $1.25 per hundred to $0.50 per
dozen, depending on area and the local supply of baitworms. Some retail
enterprises reportedly rely on vending machines placed in strategic loca-
tions near fishing centers (8).
The University of California Cooperative Extension notes that on occasion
the established "local markets have become saturated" by the entry of new
worm growers into the business (36).
35
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Beyond the constraint posed by lack of market capacity, distribution of
live earthworms that have been raised in sludge could pose public health
hazards through exposure of buyers to pathogens contained in or on the
worms' bodies or in the substrate. The difficulty of separating live
earthworms from disease-producing microorganisms would appear to elimi-
nate the bait market from serious consideration for a sludge-vermicompost-
ing operation, despite the central importance of this market to the ver-
miculture industry. Although this problem might be solved by keeping
market-ready worms in a clean substrate for a period of time prior to
sale (as is done in shellfish depuration), the added cost of this step,
even if feasible, would affect the competitiveness of the sludge-raised
worms in the baitworm market.
Inoculation of Soils
The USDA is on record with the view that earthworms are indicators -- and
not agents — of good agricultural soil. The University of California
does not recommend use of earthworms for soil improvement "because we do
not know of research data which substantiate claims sometimes made for
this" (36). And, although industry sources claim this market "is so large
and varied that it is almost impossible to list and discuss", these sources
are unable to give even an approximation of its value (8).
Even if a market were available, the demand could not be addressed by a
vermicomposting operation. The vermicomposting species, E. foetida and
L. rubellus, are suited only to life in soils containing a very high per-
centage of organic material. Placed in agricultural soils, these worms
are unlikely to survive more than one season (11); their activity during
this season is unlikely to improve soil humification or water-holding
characteristics. Incorporation of both substrate and worms into farm
soils might provide a sufficient organic base to extend the worms' useful
lifespan somewhat, but this practice would be subject to the same regula-
tory constraints as apply to land application of sludge. The worms could
not be supported in the soil without additional applications of sludge at
periodic intervals (11). Since the worms would provide no benefit that
could not be provided by indigenous populations of the same or different
species, the higher cost and management requirements for implanting worms
would appear to be unjustified.
Fertilizer or Soil Supplement
It has been projected that conversion by worms of the nation's 16 million
tons of wastewater sludge and 120 million tons of city refuse annually
would yield a byproduct of dehydrated earthworms totalling 150,000 tons
of 10-percent nitrogen material (33). The economics of the worm market,
however, argue against the use of dried worms as a high-nitrogen fer-
tilizer or soil additive. Worms are currently selling at $2.50 per Ib,
wholesale. As worms are more than 80 percent water by weight, more than
36
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5 Ib of live worms would be required to produce 1 Ib of dehydrated
worms. A 5-lb bag of 1.0-1-1 fertilizer from dried earthworms would cost
the wholesaler $62.50. Even in poor market conditions, vermiculturists
can find more realistic — if limited — markets in sale of live worms to
other breeders, organic gardeners, and sport fishermen.
As for the vermicomposting operation, it seems unlikely that worms would
be available in such-excess as to justify-wasting of worm stock for use
as a nitrogen additive. Loss of one percent of the worm population per
week — as is estimated at the intensively cultured indoor vermicomposting
operation at Keysville, Maryland -- requires substantial makeup. Excess
worm stock at such a facility must be maintained in breeder beds both for
routine make-up and as backup in case of system failure.
Even if use of worms as fertilizer could be justified on economic
grounds, environmental constraints would operate on this market.
Unpublished results obtained by Neuhauser show that concentrations of
nickel, copper, zinc, lead, chromium and cadmium all increase in worm
bodies within four weeks of first exposure to sludge. A side-by-side
analysis-performed on castings and worms.from a vermicomposting operation
in San Jose shows cadmium at 32 mg/kg in castings and at 43 mg/kg in the
worms. Elevation of cadmium or other heavy-metals levels in any soils-
amendment product could jeopardize its acceptability.
Worm Stock for Vermiculturists
Vermiculturists consider this market to be second only to the fishbait
market in its potential (8, 11). Assuredly, it is a market that has
served some entrepreneurs well indeed. Some practices in this market,
however, have been legally questionable and have cast a shadow on the
larger industry. "Binning" and "buy-back" arrangements, in which large
distributors sell starter packages to home growers with agreement to
purchase all worms produced, have often been violated, resulting in the
shutting down of worm-distribution operations in several states
(including Florida, Oregon, Wisconsin, Colorado, and California). Buy-
back agreements with fixed price guarantees are subject to regulation by
the U.S. Securities and Exchange Commission (8).
This is not a viable market for the composting operation, due in part to
the potential transmission of disease by pathogenic bacteria and viruses
attached to worms that are raised in sludge. Some have speculated that
sale of cocoons, rather than live worms, might be feasible (39, 40).
Potential advantages of this approach include the light weight, longevity
under proper storage, and relative hardiness of cocoons (as compared to
earthworms) and the possibility that external surfaces of cocoons could
be disinfected by low heat or air dryi'ng without harming the developing
worms inside.
37
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No reliable estimate has been made of market potential for the sale of
either worms or cocoons to other growers, although those in the industry
claim that demand exceeds supply.
Typical 1974 prices (8) are as follows:
• Breeder stock — $6.50 to $18 per 1,000 with discounts up to 80
percent in quantities of 50,000 or over
• Bed-run (mixed sizes) stock — $5.50 to $12 per 1,000 with
discounts up to 80 percent in quantities of 25,000 and over
• Established beds — $100 to $300 each with no contractual (buy-
back) agreement, $350 to $600 each with a contractual agreement.
These prices appear to be in line with current prices. A typical adver-
tisement in an industry journal (The Vermiculture Journal, Vol. 2, No. 1,
January 1979) reads:
- "Lively fat redworms. Satisfaction
guaranteed. Dealers are welcome.
1,000 - $5.50; 5,000 - $25.00.
Free information..."
No reliable analysis has Been made of market potential; there is every
likelihood that entry of a few major new cocoon or worm suppliers would
swamp the market and drastically reduce prices.
Animal Feed*
The composition of E. foetida is high in protein, as shown in Table 4-2.
A comparative analysis of amino-acid composition in worm meal and
commercial-grade meat meal and fish meal, as shown in Table 4-3, indi-
cates that "the earthworm product has a relatively high level of the essen-
tial amino acids, particularly the important sulphur-containing ones
(cysteine and methionine)" (35).
Worm meal can be prepared by washing worms clean and then freeze-drying
or low-temperature hot-air drying the worms (35). Two University of
Georgia researchers have found that dried earthworm meal was palatable to
domestic cats (19). Earthworm meal has been compared to meat meal in
small-scale feeding experiments on broiler chickens, and to meat meal and
a commercial preparation in an experiment with weanling pigs. In both
cases, animals raised on worm meal showed no ill effects and, in fact,
grew as rapidly as those raised on the more conventional diets. In addi-
tion, the broilers gained weight as rapidly on the worm-meal diet while
consuming 13 percent less food (35).
38
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Table 4-2
COMPOSITION OF EISENIA FOETIDA*
Dry matter (%) 20-25
Composition of-dry matter -
(worm meal)
Crude protein (% Kjeldahl
Nx6.25) 62-64
True protein (%) 60-61
Fat (%) - 7-10
Ash (%) 8-10
Calcium (%) 0.55
Phosphorus (%) 1.00
Gross Energy (Kcal/Kg) 3900-4100
*From (35)
Table 4-3
AMINO ACID ANALYSES (%) OF HIGH-PROTEIN MEALS*
Arginine**
Cysteine
Glycine
Histidine**
Isoleucine**
Leucine**
Lysine**
Methionine**
Phenylalanine**
Serine
Threonine**
Tyrosine
Valine**
Crude protein
Worm
Meal
4.1
2.3
2.9
1.6
2.6
4.8
4.3
2.2
2.3
2.9
3.0
1.4
3.0
61.0
Meat
Meal
3.5
1.1
7.1
1.0
1.3
3.5
3.1
1.5
2.2
2.2
1.8
1.3
2.2
51.0
Fish
Meal
3.9
0.8
4.4
1.5
3.6
5.1
6.4 .
1.8
2.6
-
2.8
1.8
3.5
60.9
*From (35)
**Essential amino acids in nutrition, as higher vertebrates
cannot synthesize these from nitrogen sources.
39
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Those in the vermiculture industry who have investigated this market (4)
report that, in order to be competitive, worm meal must be priced in or
near the range of $0.10 per pound (1979 prices for meat and bone meal) to
$0.17 per pound (for fish meal). Current wholesale worm prices are two
orders of magnitude higher.
The USDA reports no current use of worm meal as a feed product, either
for livestock or for pet food. The University of California Cooperative
Extension, in its 1978 investigation of worm markets (36), could identify
no markets for use as animal feed. The California Department of Fish and
Game reported to the Extension that fish farmers use pelleted food — and
not earthworms -- to feed fish (although at least one waste-vermicomposting
operation in Japan is providing worm meal to fish farmers). Other sources
of protein meal, including soybeans, are known to be less expensive than
worm meal (8). •
The same market constraints as are reported for other uses of live worms
apply to the feed market. The presence of pathogens in the sludge
substrate and attached to the worms, the accumulation or concentration of
certain heavy metals in worm body tissueN and the fact that earthworms
are known intermediate hosts and passive agents in transmission of para-
sites to poultry, swine and small mammals -- all make it essential that
distribution, sale and use of sludge-raised earthworms be regulated and
monitored. Steps used in processing of worm products could control
transmission of biological agents, but only source-control measures and
limits on worm residence time in sludge could prevent excessive accumula-
tion of heavy metals in worm tissues.
Human Nutrition
The public-health constraints that apply to any of the market sectors for
earthworms apply the more forcefully to use of sludge-raised worms for
human nutrition. This is not a viable market for the composting opera-
tion.
Market Prospects
Most markets that have been proposed for castings and worms appear to be
severely limited by lack of market demand and by potential public-health
considerations that must be addressed in distributing any sludge-derived
product. The nutrient value of castings, however, and their aesthetic
and handling characteristics make them a desirable agricultural soils
amendment. Provided that the sludge from which the castings are derived
is relatively "clean", in terms of heavy metals and toxic synthetic
organics, and provided that pathogen removal can be demonstrated (or
developed) as part of the vermicomposting process, castings have the
40
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potential of filling local or regional demand for a low-order fertilizer
and soils amendment. The material could be mixed with other potting
materials for sale as a planting medium, bagged for wholesale distribu-
tion, or stockpiled for pickup by local gardeners and farmers.
While prospects are dim for product income offsetting production costs,
this market would serve, at the very least, to reduce or eliminate a
municipality's current costs for final disposal of sludge.
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Section 5. ENVIRONMENTAL AND PUBLIC HEALTH
ASPECTS OF VERMICOMPOSTING
The potential benefits and risks associated with use of a sludge-vermi-
composting product include the following:
• Beneficial addition of nutrients and organic material to soil
• Risk of heavy-metalsV buildup in soil and subsequent uptake by
plants
t Potential for pollution of ground and surface water by nitrites
in sludge
• Restricted land use at application site
t Potential dispersal of pathogens
Vermicomposting must address all of these areas of concern, as well as
potential risks at the vermicomposting site.
Potential On-Site Problems
Odors
Most problems of sludge odor are aggravated or caused by anaerobic
conditions in the sludge. As earthworms cannot long survive anaerobic
conditions nor thrive in anaerobically digested sludge, successful vermi-
composting will rely on maintenance of aerobic -- and, therefore, rela-
tively odor-free -- conditions.
The use of vermicomposting techniques that hasten conversion of sludge
into castings will virtually eliminate the odor nuisance; castings have
no objectionable odor and apparently will not develop odors even when
stored for a period of time under adverse conditions. In a test directed
by the Texas Department of Water Resources and conducted by the Angelina
& Neches River Authority of Texas, worm castings were sealed in an air-
tight jar and maintained at 70°F. At the end of seven days, the seal
was broken, and tests were made for the presence of odors, hydrogen
sulfide, and other indications of anaerobic conditions. The test report
states that the only odor detected was a moist earthy smell and that
there were no indications of hydrogen sulfide or anaerobic conditions
(41).
Odors were not found to be a problem at any of the pilot- or full-scale
vermicomposting operations visited for this study.
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Vermin
If not controlled, flies and other vermin can cause severe nuisance prob-
lems at any sludge-handling facility where sludge is left exposed. Con-
ceivably, these vermin could act as vectors in transmitting pathogens.
Several aspects of the vermicomposting process might alleviate the prob-
lem, however. First, it appears that sludge can be vermicomposted quite
rapidly, under certain conditions, thereby reducing the amount of time
that raw or freshly digested sludge is exposed at the site. Second, the
fact that turnover is so high — and that worms are actively burrowing in
the. sludge throughout this period — means there should be little or no
opportunity for fly larvae to hatch and thrive in the sludge (1). And
third, as the conditions of vermicomposting require that structural faci-
lities be erected to protect the operation from climatic extremes in most
areas of the United States, it should be relatively simple to screen the
facilities to reduce numbers of flies and other animals at the vermicom-
posting facility.
Some flies were observed at the Lufkin, Texas vermicomposting facility.
According to he operator, the flies at the vermicomposting site were no
more populous than he had observed at the"nearby water-pollution control
plant. Flies posed no nuisance problems at any other vermicomposting
facility visited.
Enclosure of the facility or installation of protective screens would
also prevent predation by ground squirrels, moles, armadillos and birds,
all of which have been known to eat worms. Collier notes that small ani-
mals_have caused no severe problems at his windrow operation, despite the
fact that it is located near a sanctuary for birds and other animals (42).
Site Runoff and Leachate
Where facilities are enclosed, runoff and leachate will not be problems.
In fact, the optimum moisture content for sludge that is to be vermicom-
posted is low enough that even its placement in screen-bottomed beds (as
is done at Keysville, Maryland) results in no dripping or pooling of
moisture that would have to be controlled. The use of unprotected beds
or windrows for vermicomposting, however, would necessitate site-design
and process-operation features to control potential pollution of ground-
water and surface water by high biochemical and chemical oxygen demands,
suspended and dissolved solids (including chlorides and nitrates), patho-
genic bacteria, color, iron and other metallic ions, and low pH.
If control -of surface runoff is necessary, it should be collected for
discharge to sewers, or be stored in holding tanks for later disposal to
sewers or for separate treatment by irrigation or other means.
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Leachate at an open or semi-enclosed vermicomposting facility can be
controlled either by selecting a facility site that is underlain by an
impervious soil layer or by installing an underlying impervious layer of
soil or synthetic material. This will prevent the flow of polluting
nutrients or toxic materials into groundwater supplies.
Workers' Safety ......
Potential risks to workers' safety and health appear to be fewer at a
vermicomposting facility than at a conventional wastewater-treatment
plant. Some operational activities, however, could pose problems.
Spraying liquid sludge onto the wormbeds might generate aerosols carrying
fungal spores, pathogenic bacteria, or viruses. The hazard would be more
severe is spraying were done within the confines of an unventilated
structure. Another possible health problem could be caused by the dusty
conditions sometimes created when mechanical screening devices are used
to separate earthworms from castings. Routine safety and hygienic prac-
tices, including masking of operators during critical operations, will
help to protect workers' health.
V
Access to a vermicomposting facility should be controlled in order to
avoid unnecessary exposure of the general public to the pathogens that
can be present in sludge and castings. Some existing pilot-scale facili-
ties are basically backyard or barnyard operations that are easily
accessible to children. The fact that vermicomposting might be an
"appropriate technology" in some rural and suburban areas should not be .
permitted to encourage a casual approach to the handling and management
of wastewater-derived sludges.
Potential Risks in Dispersal of Products
As discussed in Section IV, the vermicomposting operation yields two pro-
ducts — castings and worms — for which there are potential markets.
Some of these potential markets are constrained by real limits on demand,
and others, by environmental problems that might result from increased
dispersal and application of sludge-derived products. Of primary concern
among these potential environmental problems are the toxic substances and
pathogenic microorganisms that are present in raw sludge.
Toxic Substances
Use of worm castings as an agricultural soils amendment, landscape "top
dressing", or potting-soil ingredient represents the most viable poten-
tial market for a vermicomposting product. Because the castings are de-
rived from sludge, however, there exists a potential for contamination of
the product by heavy metals, chlorinated hydrocarbons and other toxic
44
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substances. Considerable research has .been done in this area, and the
best information to date indicates that vermicompost ing does nothing to
solve the problem of toxic constituents in sludge. Castings will contain
concentrations of heavy metals as high as or higher than the sludge from
which the castings were derived.
Sludges that are relatively free of heavy metals and toxic organics will
be suitable for vermicomposting and subsequent use in agricultural appli-
cations; "borderline" sludges and those that are high in concentrations
of toxic substances will not be acceptable. '
Heavy Metals. Constituents of concern include the heavy metals
cadmium, copper, nickel, zinc, lead, and chromium, all of which can have
toxic effects on plants (25). High levels of copper will kill earth-
worms. Cadmium in soils can accumulate in plants to levels believed to
be harmful to humans.
Hartenstein jt _al_ have shown that aging of sludge over four weeks' time
will significantly increase the concentrations of all of these heavy
metals, due primarily to the mineralization and loss of organic mass that
occur as sludge ages. Because vermicomposting promotes these aging
effects, concentrations of heavy metals increase even more. Unfortun-
ately, mass balances have not been done yet, so the exact fate of the
metals is not known.
It is not clear whether worm activity changes the availability of metals
to plants. According to research reported in Edwards and Lofty (10), the
availability of lead and zinc (and calcium) is increased by worm acti-
vity, but Neuhauser has stated that conversion of aerobic sludge to
castings neither increased nor decreased plant-available cadmium, copper,
nickel, lead and zinc (9).
Some of these metals are also accumulated in worm tissue, which could be
of concern if the worms were to be disposed of haphazardly or otherwise
dispersed into the environment. Worms apparently- are quite capable of
concentrating some heavy metals -- such as cadmium -- to levels high
enough to be toxic to birds or small mammals preying on them (43, 44).
Most researchers have found that worms accumulate the following heavy
metals: cadmium (9, 43, 44, 45, 46), copper (43, 37), nickel (44), mer-
cury (48), zinc (43, 44, 45, 49, 50) and lead (44, 45, 49, 50). Whether
this concentration actually occurs in worms feeding on sludge under con-
ditions of vermicomposting is another question. Apparently, much depends
on the worms' residence time in the substrate, on the water solubility of
the metal in the substrate, and on the level of the metal in the
substrate.
Over short residence times of a month to a month-and-a-half, worms
(Aporrectodea tuberculata) have been shown to accumulate readily cadmium,
copper, mercury, silver and zinc, if those metals are supplied to the
worms in soluble form (43). These metals might not all be soluble or
45
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biologically available to earthworms, however, in sludges obtained
directly from a wastewater-treatment plant. A four-week laboratory study
of E. foetida feeding on dewatered waste activated sludge showed no
significant accumulation of cadmium, chromium or nickel, but did show
accumulation of copper, zinc and lead (47). Mercury as present in sludge
was not taken up by worms, and chromium was not taken up in any form
(43).
Effects of aging and microbial action in sludge might gradually release
the heavy metals in sludge in forms available for uptake by worms.
Helmke recorded cadmium levels of up to 118 ppm in worms collected from a
former 60-ton-per-hectare sludge application plot (43); the sludge itself
contained 102 ppm of cadmium. Ireland found that worms grown in heavily
contaminated soils contained proportions of soluble zinc and lead (and
calcium) that were significantly higher than those in the worms' castings
or in the soil itself (50).
Other Toxic Substances. In addition to heavy metals, a number of other
toxic substances can be accumulated or concentrated in worm tissues.
Among them are the organochlorine insecticides such as DDT. It is not
known to what degree, if at all, the process of vermicomposting hastens
degradation of these persistent substances.
Uptake of pesticides by earthworms has been reported to the following
levels:
DDT and residues 9.Ox to 10.6x soil levels
Aldrin 3.3x soil levels
Endrin 3.6x soil levels
Heptachlor 3.Ox soil levels
Chlordane 4.Ox soil levels
The worms' uptake of DDT is relatively rapid; at a concentration of 1 ppm
in the substrate, worms will reach background levels within nine weeks
(10).
Organophosphorus insecticides such as parathion do not appear to be con-
centrated by worms (10).
When toxic substances in sludge limit its suitability for use as a soils'
amendment, it will usually be the heavy-metals' component that is in
violation of EPA guidelines. Nevertheless, as discussed above, worms
used in vermicomposting are known to concentrate other toxic, persistent
substances in their tissues. More research is needed in this area of the
environmental aspects of vermicomposting.
Pathogens
Because pathogenic microorganisms in sludge are known to live up to six
months after sludge is applied to land (51), the presence of these
pathogens could limit the market acceptability of sludge-derived castings
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proposed for use as a soils amendment. Certain features of the vermi-
composting operation might lessen this constraint, however. For example:
• Salmonella bacteria present in sludge are destroyed about twice
as fast in the presence of E. foetida as in the worms' absence (7)
t Evaporation of moisture in sludge dramatically increases the
inactivation of viruses present in the sludge (52); since
castings dry about twice as fast than the sludge from which they
are produced, vermicomposting could accelerate inactivation of
viruses.
The Texas Department of Health found no Salmonella in sludge-derived
castings or in live earthworms used in a Shelbyville vermicomposting
operation. At the time, the Shelbyville facility was vermicomposting raw
sludge obtained from the Center, Texas wastewater-treatment facility (41).
There are various methods to sterilize or reduce the number of pathogens
in sludge. Research conducted at Sandia Laboratories in Albuquerque, New
Mexico has shown that survival- of viruses is reduced by four orders of
magnitude as sludge is air-dried at 21°C from b-percent solids to 83-
percent solids (52). Castings spread in a 2.5-cm layer at 25°C will be
dried to 83-percent solids within 10 days of egestion (25), and this
drying process should kill or inactivate most pathogens present in the
sludge (unless some unknown characteristic of the castings "harbors"
pathogens and protects them from inactivation).
Castings might, be sterilized by steam treatment, open-flame heating, or
exposure to 100-percent methyl bromide gas for a 24-hour period
(J. McClarran, personal communication), but the efficacy of any of these
methods is not known.
It is not clear how the current EPA criteria for landspreading of sludge
would be applied to a vermicomposting product. Criteria published on
13 September 1979 link degree of sludge treatment to proposed agricul-
tural use.
Under some circumstances, according to the EPA criteria, sludge need only
be treated by one of the specified "Processes to Significantly Reduce
Pathogens". These processes -- aerobic and anaerobic digestion, air
drying, low-temperature composting, lime stabilization, and "equivalent
methods" — are considered adequate for treatment of sludges that are to
be applied to non-agricultural land or agricultural land where the edible
portion of the crop will not come into contact with the sludge. "Pro-
cesses to Further Reduce Pathogens" — high-temperature composting, heat
drying, heat treatment, thermophilic aerobic digestion, and equivalent
processes -- are necessary for treatment of sludge applied to croplands
for human food (although any of the "significant reduction" processes, if
followed by pasteurization or irradiation, also suffices for this pur-
pose). Application of any treated sludge to land must be accompanied by
control of public access for at least 12 months and prevention of grazing
by meat and dairy animals for at least one month after application.
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Vermicomposting is a low-temperature process that does not itself reduce
pathogen levels; apparently, however, it creates conditions that sub-
sequently accelerate the reduction of pathogens to a significant degree.
It might be necessary to establish specific conditions of vermicom-
posting, such as maintenance of a defined "curing" period for castings,
before vermicomposting of raw sludge can be considered a "significant
reduction" process.
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Section 6." ECONOMICS OF VERMICOMPOSTING
Basis of Cost Estimates
We have used as a basis for developing cost estimates the structural and
equipment requirements identified in Section 3. All costs are based on
present-day prices. They might be optimistic, because vermicomposting is
now in a developmental stage. The technique is being tested and
demonstrated in order to establish technical feasibility and not to
define associated costs. Costs have not been closely documented by
operators at any of the facilities researched or visited in the course of
this investigation.
In this section, costs are estimated for vermicomposting one dry ton
per day of sludge — an amount of sludge that could be expected from a
one-mill ion-gallon-per-day (1-mgd) facility treating domestic waste-
waters. A treatment plant of this size normally would serve a municipality
of 10,000 to 15,000 persons. The costs developed are based on a sludge
loading rate of 0.08 Ib/day/sq ft (sludgerworm ratio of 0.20 Ib/d/lb and
a worm density of 0.4 Ib/sq ft).
Costs of Vermicomposting Liquid Sludge
Initial capital costs and total annual costs for vermicomposting a liquid
sludge are presented in Table 6-1. Included among the capital costs are
land and site development costs, a building, and initial earthworm stock
and equipment.
Capital Costs
Land and Site Development. As was shown in Section 3, the total building
area required for this vermicomposting facility would be about 29,000
square feet. The actual area of land to be purchased would depend on its
proximity to the wastewater-treatment plant, site topography, and other
factors. A vermicomposting facility located at the treatment plant and,
therefore, having only minimal buffer requirements would require about 1
acre of land in order to allow for access roads and other ancillary
facilities.
In this estimate, costs are based on $5,000 per acre for land and an
additional $15,000 per acre for site development. These costs will vary
considerably from site to site. Costs will be greater in urban areas or
where extensive grading is required.
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Building. Total costs might be reduced by leasing space in underutilized
buildings. The experience at all vermicomposting operations except
Lufkin has involved purchase or lease of abandoned sludge drying beds,
barns and other structures. The principal consideration in converting
and utilizing existing structures is what additional transportation costs
might be incurred.
Costs for a building-are based on the types of structures described in
Section 3. The low end of the estimated range of costs in this category
is based on the "Lufkin-type structure", which was constructed at an
actual cost of approximately $2 per square foot. The second type of
building described in Section 3, a prefabricated metal structure or
greenhouse, was used as the basis for the upper end of the estimated
range of costs. Such a structure might cost approximately $20 per square
foot including equipment for heating and ventilation. Based on the esti-
mate of 29,000 square feet, the building will cost from $58,000 to
$580,000, depending on the type of structure used.
The cost of constructing a building represents the single largest cost in
vermicomposting liquid sludge. Depending, on the geographical location of
the facility, the building represents 20 to 60 percent of the estimated
annual cost of this process. Although it might be possible to operate a
vermicomposting facility in windrows and beds constructed out-of-doors
(as has been done on a small scale at Titusville, Florida and Ridgefield,
Washington), the facility would probably be limited to seasonal opera-
tion, and conversion rates presented in this report would probably not be
consistently maintained. The worms' performance is best if temperatures
are kept to between 13°and 22°C, as discussed in Section 2.
Earthworm Stock. Earthworms (Eisem'a foetida and/or Lumbricus rubellus)
would be purchased in order to stock the vermicomposting beds. At a S:W
ratio of 0.20 Ib of sludge per Ib of earthworms, approximately 10,000 Ib
of earthworms are required for a 1-TPD facility. The cost of earthworms
at Lufkin, Texas has been reported at $1.20 to $3.75 per pound; for this
report, we have used an average cost of $2.50 per pound. (A pound of
earthworms represents a population of approximately 1,000 worms of mixed
sizes.)
The cost of the initial stock of earthworms is $25,000. We have assumed
that this stock would be purchased only once and that the worms' breeding
in the vermicomposting beds would maintain or increase the population
throughout the life of the facility. Earthworms might also be sold at
the end of the project life at a cost equal to or exceeding the initial
cost. For planning purposes, however, this income cannot be counted on.
Some vermicomposting operators have suggested that only one third to one-
half of the total stock required should be purchased initially, with the
process to be phased into full operation over a period of several months
as excess earthworms are produced. Research is needed to determine the
rates at which earthworms will breed under conditions of vermicomposting.
50 .
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Equipment. Equipment required for vermicomposting liquid sludge includes
a sludge-pumping and -distribution system and a front-end loader or other
device for transfer of materials. As the harvesting of castings (or
castings and earthworms) occurs only periodically, existing municipal
equipment, if available, might fill this requirement. For this report,
we have used a cost of $40,000.
Total Capital Costs. Total capital costs are $143,000 to $615,000,
depending on the type of structure used. To express the costs in terms
of equivalent annual costs, at a 7-percent interest rate, varying service
lives were used. The Lufkin-type building and all equipment were amor-
tized over 10 years. All other costs, including land and site develop-
ment, the prefabricated building, earthworms, and equipment, were
amortized over 20 years. The total equivalent annual costs ranged from
$18,000 to $66,000.
Annual Costs
Annual.costs include labor-for operation, utilities, and. maintenance, in
addition to the amortized capital costs developed above.
Operation. Labor costs are low for this type of facility, because daily
tasks consist of no more than applying sludge to the beds, routine tests
and maintenance which should take no more than 2 hours per day. Every 6
to 12 months, the harvesting operation will require approximately 8
workhours per 2,000 square feet of bed area. For a 25,000-sq-ft bed
handling 1 dry ton per day, a total of about 2-1/2 workweeks would be
required annually for harvesting. Up to an equivalent amount of time
might be required to construct-new beds. Total labor would amount to
one-third to one-half of an operator's time. Costs are based on an
annual salary of $15,000, including fringe benefits. This cost will vary
somewhat, depending on the region.
Utilities. Utility costs include power for pumping and lighting (if
required) and fuel for heating and machine operation. Based on the esti-
mates of the Lufkin facility, these costs should be about $2,500 to
$3,000 per year. An annual cost of $5,000 was used in this report to
account for heating costs that would be incurred in northern climates.
Maintenance. Costs are included for maintenance and materials, at
approximately 5 percent of equipment costs.
Miscellaneous. Miscellaneous costs include allowance for sawdust and
replacement parts. Based on yearly harvesting, about 8 inches of sawdust
are required per year, or about 600 cubic yds per year for a 1-TPO facil-
ity. Approximately 400 additional cu yds per year might be required
periodically to replenish the beds. Sawdust costs vary widely, depending
on the local supply. At Lufkin, sawdust is obtained free of charge from
a local mill, and the transportation cost to the municipality is low.
Costs for sawdust in this report are based on $2.00/cu. yd.
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The polyethylene covers used at Lufkin have a life of only about 2 years.
The reported installed cost at Lufkin was approximately $0.25 per sq ft
of bed area. Based on 29,000 sq ft of area and replacement every other
year, annual costs for this type of facility would increase by about
$3,500.
Total Annual Costs. Total annual costs for vermicomposting of liquid
sludge are $38,000 to $86,000 per year including amortized capital costs.
Actual costs would depend on the location of the facility and the type of
structure built.
Comparison of Unit Costs
A unit cost for vermicomposting liquid sludge can be computed by
dividing annual costs by the number of dry tons processed yearly (365
tons). Unit costs of about $105 to $235 per dry ton are computed for a
loading of 0.08 Ib/day/sq ft.
Two variables relating to worm density couldv affect the unit costs devel-
oped for vermicomposting. As was presented "in Section 3, the S:W ratios
encountered in this investigation ranged from approximately 0.12 to 0.27;
0.2 was used as a basis for estimates in this section. Worm densities
per unit area of up to three times those used as a basis for cost esti-
mates in this section have reportedly been successfully used in some ver-
micomposting operations. If feasible for full-scale practice, the use of
higher sludge loading rates might halve the unit costs of vermicom-
posting.
These unit costs are quite reasonable when examined against those for
other, more conventional processes for facilities of similar size. For
example, trucking and land-spreading one dry ton per day of liquid sludge
might be expected to cost about $70,000 per year or about $190 per dry
ton. Other options might include dewatering and static-pile composting
or dewatering and landfill. Costs for these systems might be expected to
range between $175 and $250 per dry ton, depending on transportation
costs.
These costs place vermicomposting of liquid sludge in a very competitive
posture with other contemporary sludge-management practices.
Costs of Vermicomposting Dewatered Sludge
The process as presently practised at Keysville is too labor-intensive
for any facility much larger than the present operation. At Keysville,
about 300 Ib of 18-percent-solids sludge (54 Ib) are processed per day.
Labor requirements total approximately two hours per day to load, unload
and screen material from a 75-square-foot bed. At this rate, seven
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operators — shoveling cake and castings full time —would be required
for a facility processing one.dry ton per day. Depending on the loading
rates, total annual costs would amount to about $360 per dry ton, with
labor alone accounting for about $300 per ton. Not only is this cost
high, but also the problem of hiring and motivating staff for so menial a
task appears insurmountable. This method of operation clearly is limited
to small facilities that would be required to use their labor force only
part time for vermicomposting. - - - — -
A municipality planning for vermicomposting of dewatered sludge would
undoubtedly construct a highly mechanized system that would reduce the
.high.labor requirements. At this point, however, no such system has been
designed nor are there reliable estimates of its cost. If mechanization
of this vermicomposting process could be developed at reasonable cost
(thereby reducing labor requirements), the process might be competitive
with conventional practices. .
- • Table 6-1
>
COSTS OF VERMICOMPOSTING ONE DRY TON PER DAY OF LIQUID SLUDGE
Capital Costs
Land and Site Development $20,000
Building 58 - 580,000
Earthworms 25,000
Equipment 40,000
Subtotal $143 - 665,000
Annual Costs •
Amortized Capital Costs $18 - 66,000
Operation 7,500
Utilities 5,000
Maintenance 2,900
Miscellaneous 4,700
TOTAL $38 - 86,100
UNIT COST
($ per dry ton processed) $105 - 235
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Section 7. FINDINGS AND RECOMMENDATIONS
Major Findings
1. Vermicomposting is-in its infancy. Vermicomposting of municipal
wastewater sludges has been considered seriously only within the last
10 years. Research and demonstration have been conducted only at
small scale. No one technique of vermicomposting has been accepted
as optimum; no full-scale operating experience of any duration has
been obtained. Two facilities highlighted in this report (Lufkin,
Texas and Keysville, Maryland) have each been in operation for about
one-half a year. The Lufkin facility, processing 3 to 4 dry tons of
sludge per week, is the largest to date in this country.
2. The technology is being developed largely by private entrepreneurs.
Nearly all vermicomposting operations are directed by individuals
associated with the worm-growing industry. Often,, they can devote
only part-time effort to the operation; as a result, record-keepng is
sometimes poor, and results are poorly documented.
3. The S:W (sludge:worm) ratio appears to offer a reasonable engineer-
ing design parameter.Valid comparisons can be drawn among dif-
ferent vermicomposting operations only if some consistent indication
of performance is used. This can be found in the ratio of sludge
weight (dry) consumed per day to earthworm weight (wet), which we
have termed the S:W ratio. Examination of loading rates used in six
widely-varying vermicomposting operations r*evealed a reasonably
narrow range of performance, as measured by S:W — 0.12 to 0.27 Ib
dry solids to Ib weight of earthworms per day, with an average of
0.2. This parameter might be an important tool in the study and
design of vermicomposting facilities.
4. The worm density to area ratio (W:A) is also an important engineering
design parameter.For this report, we used a ratio of 0.4 which
corresponds with experience in the worm breeding industry and with
vermicomposting at Lufkin, Texas. Higher W:A ratios were also
observed, however, and, if proven feasible, use of the higher density
ratios would result in reduced area requirements and lower unit costs
for the technique. This parameter should be evaluated more fully.
5. Earthworm "castings" might be suitable for sal-e or distribution as an
agricultural soils amendment.Castings derived from sludge, other
waste products, and high-quality feeds have been marketed success-
fully in horticultural potting mixes. The nutrient properties of
castings, and their physical structure, appear to make them suitable
for agricultural use, provided concentrations of heavy metals in the
original sludge are within established limits. Potential product
income will probably not play a significant role in cost-
effectiveness analysis of future operations.
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Other markets that have been proposed for the castings and worms
generated in vermicomposting are severely constrained by environ-
mental and public-health considerations and by lack of market capa-
city.
6. Environmental effects of vermicomposting are similar to those of most
other processing methods involving eventual land application^
Although the nutrient value of castings lies within a range that
could make them of value in agricultural use, the process of ver-
micomposting does nothing to reduce levels of heavy metals present in
the sludge. In fact, the concentrations of these metals -- some of
which are toxic at low concentrations to plants or animals — increase
significantly in castings, due to accelerated drying and mineraliza-
tion effects. Castings will be acceptable for agricultural use when
heavy-metals' values in the original sludge are low, as is the case
for most of the smaller municipal facilities where vermicomposting
might be used.
There is evidence that the accelerated drying of castings hastens
reduction of Salmonella levels>. It is not known whether other patho-
genic bacteria or viruses in sludge die off more quickly as a result
of vermicomposting.
7. Costs of vermicomposting a liquid sludge appear to be competitive for
small installations^Operating costs projected from experience at
existing vermicomposting facilities are high. Unit costs for ver-
micomposting of liquid sludge at a daily loading ratio of 0.8
Ib/day/sq ft are reasonable — $105 to $235 per dry ton for a 1-ton-
per-day facility. When compared to other options for facilities of
the same size, vermicomposting of liquid sludge might prove to be
cost-effective.
The economics of composting dewatered sludge are not as attractive
($360/ton). As currently practised, labor requirements for this pro-
cess are very high. Mechanization of bed loading and harvesting must
be developed and demonstrated. If this is done, vermicomposting of
dewatered sludge might prove to be competitive with other process
options.
8. Vermicomposting has not yet been demonstrated to be the equivalent of
conventional sludge-stabilization techniques.Comparison should be
made at demonstration scale between vermicomposting and conventional
processes such as digestion and thermophilic composting, in order to
scale the performance of vermicomposting in stabilizing sludge and
removing pathogens in it. Specific research and development needs
are discussed in the next section.
55
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Vermicomposting is a feasible process for municipal wastewater
sludges.It is currently being used, somewhat successfully, to treat
a portion of the sludge generated at about 10 wastewater-treatment
plants across the U.S. Although it is in its infancy as a tech-
nology, it does show potentially favorable economics as compared to
other available processes. Two most-likely applications of the ver-
micomposting process are:
• Vermicomposting of raw or aerobically-digested liquid sludge
pumped directly to wormbeds and applied to an appropriate
bedding substrate, such as sawdust, as is practiced at Lufkin,
Texas.
t Vermicomposting of a centrifuged or other polymer-conditioned
sludge with a solids content of 18 to 20 percent. Dewatered
cake would be conveyed directly to the wormbeds, but, since
mechanization of this method has not been demonstrated, capacity
would be kept to a level requiring only part-time operation.
In either case, the sludge would havex to be derived from pre-
dominantly domestic wastewater flows. Some provision might have to
be made for product sterilization or stabilization, depending on the
intended end use of the castings.
Research and Development Needs
Recommendations for additional research in vermicomposting fall under two
major headings: basic research and demonstration-scale applied research.
Basic research, conducted in laboratories or in conjunction with opera-
tion of a demonstration facility, could yield answers to outstanding
questions about the optimum conditions of vermicomposting and about the
performance of this process as compared to accepted techniques of sludge
stabilization. The demonstration-scale research would be applied to
optimizing techniques of vermicomposting, determining requirements of
scale-up, and developing reliable cost information. Both types of
research must be based on design of experiments from which valid
inference can be drawn and on rigorous documentation of test conditions
and results.
Basic Research
1. The daily loading ratio of sludge-sol ids:worms (S:W) must be con-
.firmed experimentally as a meaningful indication of performance under
constant conditions of vermicomposting. As the ratio depends in part
on determination of when conversion of sludge to castings is
complete, some consistent, measurable determinant of conversion must
be established (e.g., average particle size, percent loss in mass,
56
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original sludge, sludge pH,and effects over time. The availability
of metals in castings to uptake by plants and animals should also be
documented.
7. Effort should be devoted to identifying the sludge-processing tech-
niques that are and are not compatible- with vermi compost ing. It is
presently believed that anaerobically-digested sludge is toxic to
earthworms and that some sludge-conditioning agents (lime, ferric
chloride) create unfavorable conditions for vermtcomposting.
8. Reliable estimates should be developed for worms' generation time
under conditions of vermicomposting. Generation times (rates of
population doubling) could vary according to worm density, food (as
dry solids), moisture levels, temperature. This research might help
to determine optimum worm-residence times in vermicomposting beds and
establish whether there is a need to maintain, as an adjunct to the
vermicomposting operation, managed beds for breeding and growing new
worm stock.
9.~ Effects of moisture content should be tested in order to establish
the limits within which the S:W ratio remains relatively constant.
Decline in S:W at low or high moisture levels will indicate practical
limits for the vermicomposting operation.
Demonstration-Scale Applied Research
1. Sound documentation of the capital and operating costs of vermicom-
posting is required in order to help engineers make valid economic
comparisons between vermicomposting and conventional sludge-handling
techniques. Among the most sensitive cost areas are those associated
with structural requirements, labor, and purchase of new worm stock.
Requirements in these areas should be documented at a demonstration-
scale facility.
2. Mechanization for loading wormbeds with sludge cake might make ver-
micomposting of dewatered sludge competitive with other stabilization
techniques, as present vermicomposting systems of this type are
labor-intensive. Additional facilities should be developed for
unloading and transporting castings away from the worm beds.
3. Present techniques for harvesting -- separating worms from castings
-- include mechanical rotary harvesting and "migration" or "baiting"
techniques. The relative efficacies of these methods require eval-
uation, as do their effects on the economics of vermicomposting.
New methods should be examined, including mechanical techniques —
such as slow-moving conveyor systems -- that take advantage of the
worms' tendency to migrate to fresh sources of food.
58
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Worm growth, development, and reproduction under conditions of full-
scale vermicomposting all need documentation. There is not, at
present, good evidence that initial worm stocks will be maintained or
expanded in vermicomposting beds, given the worms' natural survival
times, the periodic disturbance of beds, high food loadings, constant
conditions of moisture and temperature, trauma incurred in har-
vesting, and so forth. Optimum conditions for vermicomposting might
not be optimum for worm breeding; in this case, it might be necessary
to culture fresh worm stock in undisturbed beds both to make up for
normal worm loss and to act as a buffer against system "crashes".
Or, depending on the loss rates (if any) projected in demonstration-
scale operation, costs of periodic purchase of new worm stock must be
considered in the economic evaluation of vermicomposting.
From all of these demonstration-scale investigations, the problems
and economies of system scale-up should be anticipated. Comparison
should be made between laboratory and "real-life" conditions of ver-
micomposting, in order to put laboratory-based research results in
proper context. Results obtained at the demonstration facility
should be fully documented to assist in future consideration of ver-
micomposting as a feasible sludge-management alternative.
59
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percent change in moisture). Test plots can then be "established with
varying the unit mass of dry sludge per given worm density in order
to find out what ratio provides optimum (fastest) conversion, as
defined, under constant ambient conditions. This loading ratio will
be a key to future design of physical facilities for vermicomposting;
from it can be determined the facility's process and area require-
ments.
2. The S:W ratio will vary according to the conditions of vermicom-
posting. Although the ratio appears to be independent of sludge
moisture content (within a fairly broad range), it will be affected
by variables such as temperature, pH, supplemental substrate used,
and species of worms used. Optimum conditions of vermicomposting can
be determined by testing the S:W ratio obtained under different sets
of conditions. Interactions among the variables (including worm
density) might help develop specific sets of operating conditions
that are feasible for different geographical areas of the United
States.
3. Worm density ratios (the W:A ratio expressed in terms of Ib worms to
unit area of bedding) vary quite widely from one operation to
another. Provided that higher worm densities do not interfere with
the worms' feeding behavior or degrade product quality, it may be
possible to reduce space requirements and unit costs of vermicom-
posting by increasing the W:A ratio. Research should be directe to
optimizing areal densities and establishing clearly their rela-
tionship to and effects on the S:W ratio.
4. By far the greatest amount of research work in vermicomposting has
been dedicated to the performance of Eisenia foetida and, to a lesser
extent, Lumbricus rubellus. Comparisons should be developed on
L. rubellus and other species of worms, including Allolobophora
chlorotica and Dendrobaena subrubicunda— both of which occur in
nature in areas of high concentrations of organic matter.
5. Although conversion of sludge to castings might hasten the die-off of
Salmonella, specific documentation of this effect should be devel-
oped. Effects of vermicomposting on other pathogenic bacteria and
on viruses should be examined, with relationships drawn between pres-
ence of pathogens in the sludge feed and, subsequently, presence in
castings at the time of defecation and at intervals over a subsequent
"curing" period. If curing, is necessary to make vermicomposting the
equivalent of other recognized sludge-stabilization techniques that
significantly reduce pathogens, a protocol of curing should be estab-
lished.
6. The exact fate of heavy metals present in sludge fed to worms should
be determined. Materials' balances should be performed to determine
how much of each metal is taken up (accumulated) by worms and how
much is expelled in castings. These rates should be related to other
factors, such as concentration and solubility of the metals in the
57
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REFERENCES
1. Carmody, F.X. Vermicomposting: An assessment of the state of the art
as of May, 1978.
2. Gaddie, R.E., Sr. and D.E. Douglas 1977. Earthworms for ecology and
profit, Vol II. Bookworm Publishing Company, Ontario, CA.
3. New York Times. Sept. 1977. Japanese industrialists and farmers look
to earthworms for help.
4. Solid Waste Resources, Ltd. Brochure on an earthworm recycling system.
Eugene, OH.
5. Hartenstein, R., 1978. The most important problem in sludge management
as seen by a biologist. Conf. Proc. on utilization of soil organisms
in sludge management. State University of New York (SUNY) Coll. of
Environm. Sci. Forestry, Syracuse, NY.
6. Arete Vermicomp, Inc. 1979. Final report of the vermicomposting
project at the Botzum sewage treatment plant in Akron, OH.
7. Hartenstein, R., E.F. Neuhauser and D.L. Kaplan. A progress report on
the potential use of earthworms in sludge management. SUNY Coll. of
Environm. Sci. Forestry, Syracuse, NY. In Press.
8. Gaddie, R.E., and D.E. Douglas 1977. Earthworms for ecology and profit,
Vol I. Bookworm Publishing Company, Ontario, CA.
9. Neuhauser, E.F. 1978. The utilization of earthworms in solid waste
management. Conf. Proc. on utilization of soil organisms in sludge
management. SUNY Coll. of Environm. Sci. Forestry, Syracuse, NY.
10. Edwards, C.A. and J.R. Lofty, 1977. Biology of earthworms. Chapman
and Hall Ltd., London. 333 pp.
V
11. Minnich, J. 1977. The earthworm book. Rodale Press, Emmaus, PA.
372 pp.
12. Kaplan, D.L., R. Hartenstein, and E.F. Neuhauser. Coprophagic relations
among the earthworms Eisenia foetida. Eudrilus eugeniae and Amynthas
spp. SUNY Coll. of Enviornm. Sci. Forestry. Syracuse, NY. In Press.
60
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13. Neuhauser, E.F., D.L. Kaplan and R. Hartenstein. Life history of the
earthworm Eudrilus eugeniae. SUNY Coll. of Environm. Sci. Forestry.
Syracuse, NY. In Review.
14. Minnich, J., M. Hunt and the editors of "Organic Gardening" magazine,
1979. The Rodale guide to composting. Rodale Press, Inc., Emmaus, PA.
405 pp.
15. Watanabe, H. and J. Tsukamato, 1976. Seasonal change in size class and
stage structure of lumbricid Eisenia foetida population in a field
compost and its practical application as the decomposer of organic waste
matter. Rev. Ecol. Biol. Sol. Vol. 13, No. 1.
16. McNelly, J. 1979. Planet Earthworms, Inc. Automated vermicomposting
system for municipal - solid waste disposal. Application to U.S.
Department of Energy Region VIII Appropriate Technology Small Grants
Program.
17. Mitchell, M.J., R.M. Mulligan, R. Hartenstein, E.F. Neuhauser. 1977.
Conversion of sludges into "topsoils"^by earthworms. Compost Sci.,
Vol. 18, No. 4.
18. Mitchell, M.J., 1978. Role of invertebrates and microorganisms in
sludge decomposition. Conf. Proc. on utilization of soil organisms in
sludge management. SUNY Coll. of Environm. Sci. Forestry, Syracuse, NY.
19. Fosgate, O.T. and M.R. Babb. 1972. Biodegradation of animal waste by
Lumbricus terrestris. Journal of Dairy.Science, Vol. 5, No. 5.
20. Collier, J., 1978. Use of earthworms in sludge lagoons. Conf. Proc.
on utilization of soil organisms in sludge management. SUNY Coll. of
Environm. Sci. Forestry, Syracuse, NY.
21. Niessen, W.R. (Principal Investigator). 1970. Systems study of air
pollution from municipal incineration. The division of process control
engineering, National Air Pollution Control Administration, U.S. Depart-
ment of Health, Education and Welfare. Arthur D. Little, Inc.,
Cambridge, Massachusetts, March 1970.
22. Neuhauser, E.F., D.L. Kaplan,, M.R. Malecki and R. Hartenstein.
Materials supportive of weight gain by the earthworm Eisenia foetida
in waste conversion systems. SUNY Coll. of Environm. Sci. Forestry,
Syracuse, NY. In Press.
23. Dexter, A.R. 1978. Tunnelling in soil by earthworms. Soil Biology
and Biochemistry, Vol. 10, No. 5.
24. Neuhauser, E.F., R. Hartenstein and D.L. Kaplan. Growth of the earth-
worm Eisenia foetida in relation to population density and food ration-
ing. SUNY Coll. of Environm. Sci. Forestry, Syracuse, NY. In Press.
61
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25. Hartenstein, R., A.L. Leaf, E.F. Neuhauser, D.H. Bickelhaupt and F.
Hartenstein, 1979. Physiochemical changes accompanying the conversion
of activated sludge into castings by the earthworm. SUNY Coll. of
Environm. Sci. Forestry, Syracuse, NY.
26. Collier, J. 1978. Vermicomposting: A practical approach to managing
municipal wastes. Part I - Sewage Sludge. The Vermiculture Journal,
Vol. 1, No. 1. '
27. Langer, B.W., 1979. The earthworm and resource recovery -- the recy-
cling of biodegradable wastes — theoretical aspects. The Vermiculture
Journal. Vol. 2, No. 1.
28. Parr, J.F., G.B. Willson, and E. Epstein. 1978. Current research on
composting of sewage sludge. Conf. Proc. on utilization of soil organ-
isms in sludge management. SUNY Coll. of Environm. Sci. Forestry.
Syracuse, NY.
29. Stark, N., P. Pawlowski, and S. Bodmer, 1978. Quality of earthworm
castings and the use of compost on ar-id soils. Conf. Proc. on utiliza-
tion of soil organisms in sludge management. SUNY Coll. of Environm.
Sci. Forestry, Syracuse, NY.
30. Collier Worm Ranch. 1978. Agricultural suitability analysis. Soil
and Plant Laboratory, Inc.
31. Kluin, G. 1978. Castings: A horticulture blue-ribbon winner. The
Vermiculture Journal. Vol 1., No. 1.
32. Black, J.H., R.M. Hawthorne and J.P. Martin. 1977. Earthworm: biology
and production. Division of Agricultural Sciences University of
California.
\
33. Hall, S.I. 1978. Vermology: A viable alternative for better world
management. The Vermiculture Journal, Vol. 1, No. 1.
34. Sabine, J.R. 1978. The nutritive Value of earthworm meal. Conf. Proc.
on utilization of soil organisms in sludge management. SUNY Coll. of
Environm. Sci. Forestry, Syracuse, NY.
35. Sabine, J.R. 1978. Earthworms: A new form of proteins. The Vermi-
culture Journal. Vol. 1, No. 1.
36. Nelson, L. Jr. 1978. Earthworm Enterprises in California. University
of California Cooperative Extension.
37. Gilbert, B. August 1979. They crawl by night. Sports Illustrated.
38. Scaglione, C. 18 February 1979. Breeder builds army of pollution
fighters. San Diego Union, San Diego, CA.
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39. McNelly, J. 1977. Wormglenn: An organic waste, recycling center.
Planet Earth Worms. Louisville, CO.
40. American Organic, 1979. Birth of a new agricultural industry: the
worm rancher.
41. Texas Department of Health, letter to City of Lufkin City Manager,
7 October 1977; Angelina & Neches River Authority, letter to City of
Lufkin City Manager, 18 December 1978.
42. Collier, J. 1978. Conversion of municipal wastewater treatment plant
residual sludges into earthworm castings for use as topsoil, First
Annual Report to NSF. NSF Grant ENV77-16832.
43. Helmke, P.A., W.P. Robarge, R.L. Korotev, and P.O. Schomberg. 1979.
Effects of soil-applied sewage sludge on concentrations of elements in
earthworms. J. Environ. Qual., Vol. 8, No. 3.
44. Moreau, J.P. 1977. Effects of municipal sewage effluent irrigation
on the trace metal content of earthworms. A thesis, SUNY Coll. of
Environm. Sci. Forestry, Syracuse, NY\
45. Van Hook, R.I. 1974. Cadmium, lead and zinc distribution between
earthworms and soils: Potentials for biological accumulation. Bull.
of Environm. Contam. and Toxicology. Vol. 12, No. 4.
46. France, V.P. 1979. The vermiculture industry today. Arete Vermicomp,
Inc., Canton, OH.
47. Hartenstein, R., A.L. Leaf, E.F. Neuhauser and D.H. Bickelhaupt.
Composition of the earthworm Eisenia foetida and assimilation of fifteen
elements from sludge during growth. SUNY Coll. of Environm. Sci.
Forestry, Syracuse, NY. In Review.
48. Bull, K.R., R.D. Roberts, M.O. Inskip and 6.T. Goodman. 1977. Mercury
concentrations in soil, grass, earthworms and small mammals near an
industrial emission source. Environm. Pollut. Vol. 12.
49. Ireland, M.P. 1976. Excretion of lead, zinc and calcium by the earth-
worm Dendrobaena rubida living in soil contaminated with zinc and lead.
Soil Biology and Biochemistry Vol. 8, No. 5.
50. Ireland, M.P. 1975. The effect of the earthworm Dendrobaena rubida
on the solubility of lead, zinc and calcium in heavy metal contaminated
soil in Wales. Journal of Soil Science, Vol. 26, No. 3.
51. France, V.P. 1979. The effect of earthworms on human pathogens in
sewage sludge. Arete Vermicomp, Inc. Canton, OH.
52. Ward, R.L. 1978. Pathological aspects of inactivation of enteric
viruses in dewatered wastewater sludge compost facilities, Chicago, IL.
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Appendix. SITE VISITS
In the course of this investigation, seven vermicomposting operations
were visited in order to confirm details of process operation and to view
at first hand the techniques and facilities used. Projects currently
underway range from the laboratory bench-scale studies conducted at State
University of New York (SUNY) in Syracuse to the nearly full-scale ex-
perimental vermicomposting techniques being practised at Lufkin, Texas.
The following descriptions, which are based on on-site observations and
discussions with facility operators, provide perhaps the best overview of
the present state-or-the-art of vermicomposting in the United States.
Keysville, Maryland
Owner/Operator: Mr. V. Paul France, Arete Vermicomp, Inc.,
. . . .-. Canton,. Ohio/Mr.v Robert Bowers, Keysville,
Maryland
Date of Visit: 18 October 1979 (Facility not in operation due
- to flooding at WWTP during previous week)
Sludge Feed: Aerobically digested sludge, concentrated to
12 - 14 percent solids and air-dried to 17 - 18
percent solids.
•
Capacity: Approximately one (1) wet ton per week.
Background. Since June of 1979, sludge trucked from the New Oxford,
Pennsylvania wastewater-treatment plant (WWTP) has been processed by ver-
micomposting at the Keysville facility. Prior to June, Mr. France had
for several months obtained and vermicomposted sludge from a WWTP in
Laurel, Maryland.
Sludge Processing. Sludge produced at the New Oxford, WWTP is aerobi-
cally digested and then concentrated to 12 to 14 percent solids on a
Carter belt filter press. A portion of the WWTP's sludge is hauled two
or three times per week by open trailer or dump truck to the Keysville
site, where is is spread in a 2-inch layer to dry in a semi-enclosed
facility. Drying to an estimated 17 to 18 percent solids takes place
over two or three days; the sludge is raked periodically to improve
drying.
The Laurel WWTP sludge used prior to June, 1979 was an aerobically di-
gested sludge conditioned with polymers and dewatered on vacuum filters.
The plant operators' subsequent switch to "hot" (dehydrated) lime and
ferric chloride for conditioning proved, however, to make the sludge
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unsuitable for vermicomposting. Mr. France says the plant's sludge pro-
cessing will be converted to belt-press operation in December, 1979, at
which point vermicomposting of the Laurel sludge will be resumed.
Operation. Once dried, the sludge is hauled in to an enclosed barn and
shoveled onto two tables, each of approximate dimensions 30 ft. long and
2l/£ ft. wide. Aprroximately 300 Ib. of sludge is placed on each table and
is spread to-a uniform depth of life to 2 inches.
The bottom of each table consists of common windowscreen supported by
chicken wire and wooden cross-braces. The open bottom provides increased
surface area, helping to dry the sludge further and to keep it in an
aerobic condition.
Approximately 100 Ib. of earthworms, Eisenia fpetida, are weighed out and
distributed evenly over the sludge in each table.The worms quickly
migrate into the sludge and reportedly convert all of the sludge to
castings within a 48-hour period. A black plastic cover is placed over
the sludge bed for approximately 6 to 8 hours during the 48-hour ver-
micomposting process, in order to ensure that worms feed at the surface
of the sludge mass.
Conditions of Culture, Temperatures in the vermicomposting room are re-
portedly maintained between 18°C and 22°C, although the temperature was
14°C at the time of the site.visit. Temperatures below 16°C or above
26°C reportedly slow the vermicomposting process by 10 percent or more.
Higher worm:sludge ratios have been attempted as a means of reducing ver-
micomposting time, but the result has been a wetter and less manageable
castings product. Lower ratios increase vermicomposting time.
Fresh worm stock is maintained in semi-enclosed and enclosed structures
using standard techniques of vermiculture.
Processing of Products. After 48 hours (or when conversion is determined
by inspection to be complete), earthworms and castings are removed manu-
ally from the beds and placed in a cylindrical rotating screen. Castings
fall through the screen into tubs, while earthworms travel through the
length of the drum for collection and recycling into another sludge bed.
Some worms are lost in the harvesting process, due to their small size or
to trauma, but these losses are estimated at only one percent per week.
At present, castings are being stockpiled for future use.
Marketing of Products. No products of the Keysville operation are cur-
rently being marketed. Mr. France" reports, however, that he is marketing
in Ohio a "Nutri-Loam" potting soil, that contains castings produced at
his similar vermicomposting operations at Gal lion and Canton, Ohio.
Castings make up about 15 percent of the Nutri-Loam mixture, by volume;
other ingredients include Vermiculite, peat moss, Perlite, sand, kelp and
other ingredients. The product, selling for $1.75 per four-quart bag,
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has been available in selected supermarkets and garden shops since
August. Mr. France is currently setting up distributorships in Ohio and
several middle-Atlantic seaboard states. He also plans to start pro-
ducing Nutri-Loam at the Keysville facility. Mr. France states that his
long-term interest is in development of an agricultural product suitable
for use in bulk.
Research and Analysis. Most of the process development underway at Keys-
ville is focused on optimizing the conditions of sludge vermicomposting.
Analyses of sludge and castings constituents have reportedly been carried
out by laboratories at Pennsylvania State University and the University
of Montana (one analysis of sludge constituents is included in Section 2
of this report, in Description of Sludge Feed). Dr. Rufus Chaney of the
U.S. Department of Agriculture has also obtained a castings' sample for
analysis.
Louisville. Colorado
. Owner/Operator:. Mr.. Jim McNelly,. Planet Earth Worms
Date of Visit: 14 September 1979
Feed: Composted yard wastes (grass clippings)
Background. Mr. McNelly has been doing research in vermicomposting for
the last five years; he produces and markets several organic materials,
one of which includes castings.
Material Used and Operation. Composted grass-clippings and other yard
wastes are obtained from the City of Boulder, which operates a windrow-
type process. The compost is fed to earthworms maintained in an indoor
windrow-type facility in order to produce castings. Neither sludge nor
municipal solid waste have been utilized at Louisville.
«
Marketing of Products. A "NaturSoil" potting soil containing earthworm
castings (1.5 percent by weight) is packaged and marketed. The castings
include those produced at Louisville as well as others purchased from
worm growers who have gone out of business. Other ingredients include:
domestic and Canadian peat moss, limestone, bark mulch, topsoil, sand,
vermiculite and diatomaceous earth.
The suggested retail price is $1.75 per 8-1b package, although prices
throughout Colorado range from $1.40 to $2.50. The mixture is also sold
in bulk to greenhouses for $280 per truckload (approximately 8 cu yds).
McNelly sells to brokers, who distribute the product to supermarkets,
health food stores, retail plant stores and hardware and garden shops.
Research. Most of the experience at Louisville is with marketing. Re-
search has been conducted on capsule production using a slurrying/screen-
ing technique. Research is geared toward developing a market for earth-
worms as a future protein source.
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Lufkin, Texas "
Owner/Operator: City of Lufkin, Texas/Mr. Edward Green,
Early Bird Farms (project consultant)
Date of Visit: 7 September 1979
Sludge Feed:- Primary and waste activated sludge at
3.5 to 4.0 percent solids
Capacity: 1,800 gallons per day
Background. This demonstration project, still in the start-up phase at
the time visited, involves vermicomposting of a portion of combined raw
sludges from the 4-mgd City of Lufkin WWTP. The City's primary objective
is to reduce total sludge-management costs and not to market a castings
or earthworms product.
Sludge Processing. The City of Lufkin operates a secondary wastewater-
treatment facility that processes about 4 mgd of domestic and .industrial
wastewater. The wastes are of high strength, due to flows contributed
from two poultry-processing facilities. Primary and waste activated
sludges are combined prior to thickening in a gravity thickener; a small
portion of the thickened sludge is then pumped to the vermicomposting
operation at 3.5 to 4.0 percent solids. Most of the sludge from the
facility 'is heat treated and vacuum filtered prior to landfill ing.
Operation. The City has constructed 12 experimental vermicomposting
beds. Each bed is of dimensions 20 ft. by 95 ft., or 1900 sq. ft. in
area, and each is covered with a plastic roof over a steel frame. The
roof consists of two layers of plastic, an inner layer of clear plastic
and an outer black layer that can be removed during the winter to create
a greenhouse effect. . .
At the time of the site visit, six of the 12 beds were in operation.
Within the beds are a 2-in. deep layer of sawdust and the earthworms.
Since, the..site visit, the bed depth has been increased to about 8 inches.
The City has purchased 4800 Ibs of worms at an average cost of $1.50/lb
and has advertised for another 5200 Ib. The earthworm density is
approximately 800 Ib per 1900 sq ft (0.42 Ib/sq ft).
The earthworms feed near the top few inches of the beds, and castings
eventually become compacted near the bottom of the beds.
Approximately 300 gallons of 3.5 to 4.0 percent sludge is sprayed onto
each bed each day; the solids loading rate is approximately 100 lb/1900
sq ft. When all 12 beds are in operation, about 10 percent of the plant
sludge will be processed by vermicomposting. The sludge spraying system
is automatic and consists of a central header with laterals along each
side of the 12 beds. A spring-loaded valve is used to distribute the
sludge along the beds.
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Processing of Products. Earthworms and castings have not yet been separ-
ated but have simply been left in the six active beds. Although mechani-
cal rotary screen is available for separating worms from castings, the
project consultant has recommended that the City use instead a baiting
technique for moving earthworms. This technique would consist of wind-
rowing a finished bed and placing fresh sludge and sawdust adjacent to
the windrow. The earthworms would migrate to the better food source,
leaving a windrow of finished castings. The technique should be less
costly than mechanical-screen harvesting and might also present less risk
of trauma to the worms.
Marketing of Products. No attempt has been made to market castings or
worms.
Research and Analysis. Although only in the start-up phase, the Lufkin
facility offers a potentially valuable source of information on vermicom-
posting, provided that the research is carrried out in a manner that will
produce good documentation of the results.
Ridgefield. Washington
Owner/Operator: Mr. J. "Red" McClarran, American Organics, Ltd.
Date of Visit: 13 September 1979
Sludge Feed: Raw sludge at 2 percent solids
Background. American Organics is a firm that markets several organic
agricultural/horticultural materials, some of which are made from earth-
worms castings. Reportedly, over the last several years, some 400 tons
of horse manure and other manures, paper-mill sludge and cedar-toe
sawdust (from the production of cedar shingles) have been vermicomposted
at the Ridgefield facility. Mr. McClarran has also conducted pilot studies
on the vermi compost ing of municipal sl,udge.
Most of the experience at Ridgefield is related to product packaging and
marketing. In fact, a large portion of the castings used in the firm's
potting mix is bought from wormgrowers who have gone out of business.
There is not a great deal of documentation of results obtained using
various types of organic materials.
Operation. Mr. McClarran has constructed several test plots using vari-
ous bedding and feed materials. In one field plot, organic materials
were placed in windrows approximately 10 feet wide and 1 foot deep (dur-
ing the cold winter months, the depth is increased to 3 or 4 feet)'. To
date, approximately 750 linear feet of windrows have been constructed.
Since July 1979, approximately 80,000 gallons of raw sludge at 2% solids
(6.5 dry tons) has been applied to the windrows. Sludge is trucked in
two or three times per week from the Ridgefield, La Center and Woodland
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WWTPs. The liquid sludge is either sprayed on the windrow of the materials
described above or discharged at the higher end of a 150-ft. -long windrow.
The liquid sludge flows by gravity along the length of the windrow.
The loading rate is approximately 20 gallons/ft, during favorable weather
and 3 gallons/ft, during winter conditions.
The present operation is very Tabor-intensive, but AO has begun aerating
the windrows mechanically, using a tractor that pulls a rotating comb. The
comb breaks up the top 4 to 6 inches of material.
Three types of earthworms are present; night-crawlers, red worms, and
common garden worms. Predators are reportedly not a problem. Some flies
are present, and slight odors are noted after windrow aeration. Leachate
is not a problem, due to clay soils at the site.
Processing of Products. AO has not separated castings from the windrows
to which sludge has been applied. The Southwest Washington Health District
does not presently allow marketing of this sludge-derived material.
' ' * \
Castings from non-sludge windrows are sterilized to control nematodes and
other pests, prior to use in a potting mix. According to Mr. McClarran,
three sterilization methods are available: steam, open flame, and methyl
bromide.
The material sells for $3 to $4 per eight-quart bag in grocery and depart-
ment stores and lumber suppliers and nurseries. A 12-quart bag is also sold.
Marketing of Products. Approximately 100,000 IDS. (100,000 quarts) of
"Black Gold" potting mix have been marketed. "Black Gold" consists of
earthworm castings, perlite and Canadian peat moss.
San Jose, California
Owner/Operator: City of San Jose/Mr. Jack Collier, Collier's
Earthworm Compost Systems, Inc. (CECOMS)
Date of Visit: 11 July 1979
Sludge Feed: Anaerobically-digested sludge, dried in a
lagoon for several years to about 80 percent
solids
Capacity: 20-25 cubic feet per week
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Background. Several years ago, Mr. Collier approached the City of San
Jose with a proposal to compost sludge using earthworms. He was granted
access to two 3^-acre sludge lagoons at the 160-mgd San Jose-Santa Clara
water pollution control plant. He also applied for and received a grant
from the National Science Foundation to pilot a vermicomposting process.
Sludge Processing. The sludge is an anaerobically digested sludge that
has been dried two or more years in the sludge lagoons to an 80-percent
solids content.
Operation. One of the lagoons was first stripped of vegetation
(tumbleweeds had grown to a five-foot height). About five inches of
dried sludge was then scraped from the lagoon and formed into windrows,
each of dimensions 5 feet wide, 50 feet long and about 15 inches deep.
The sludge was wetted with potable water to an unknown moisture content
and covered with a canvas to retain moisture. A V-shaped trench was cut
along the top of the windrow and approximately 125 pounds of earthworms
(E. foetida) were distributed over the 50-foot length, or somewhat less
than 0.5. Ib of worms per square foot of surface area. After a few days,
the worms migrated out into the sludge calce. After three or four months,
the process was considered accomplished.
Most windrows that have been constructed have never been harvested. In
fact, some do not contain adult worms, which reportedly have abandoned
the windrows in search of a new food source. During the last several
months, Mr. Collier has been attempting to persuade the City to authorize
the use of 50 acres of lagoons for a full-scale production facility.
Vermicomposting has not been studied as an alternative by the City's con-s
sultant in its current 201 facilities plan for sludge management.
The vermicomposting process developed by CECOMS is based entirely on the
San Jose experience. There is no control parameter to ascertain when the
material is considered composted. Mr. Collier stated that the density of
earthworms seeded in thh windrow might be doubled from 0.75 Ib/cu. ft. to
1.5 Ib/cu. ft., but what corresponding decrease in retention time might
be achieved is not known.
Processing of Products. Some materials have been removed from windrows
to a rotating-screen harvester, where castings and worms were separated.
Marketing of Products. Mr. Collier plans to market the castings as a
potting soil.
Research^and Analysis. Castings produced at the San Jose facility have
been analyzed by an independent laboratory; some of the results are
reported in Section 4 of this report.
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Syracuse, New York
Institution/Principal State University of New York, College
Investigators: of Environmental Science and Forestry/
Dr. Roy Hartenstein, Dr. Edward F. Neuhauser
- Date of Visit: 4 October 1979
Sludge Feed: Primarily aerobically digested, air-dried
to 11 percent solids; also aerobically
digested sludge centrifuged to 11 - 20
percent solids
Capacity: Laboratory and pilot-scale research
Background. Since July 1976, work by Dr. Hartenstein and his associates
has been funded by successive grants from the National Science Founda-
tion. The work also received in 1979 joint funding by the State of New
York. The research has concentrated on laboratory experiments using E.
foetida feeding on sludges obtained from local wastewater-treatment ficfil-
ities. Some recent research has covered the biology and ecology of
Eudrilus eugeniae (African nightcrawlers) and pheretemoid worms.
Research and Analyses. Two compendia have been published under National
Science Foundation grants: "Utilization of Soil Invertebrates in Stabi-
lization, Decontamination and Detoxification of Residual Sludges from
Treatment of Wastewater", June 1977, and "Utilization of Soil Organisms
in Sludge Management", June 1978. The latter contains papers presented at
a SUNY-hosted conference. Numerous additional papers have been publi-shed
or submitted for publication in technical journals; the following repre-
sentative titles indicate the scope of research completed or underway at
SUNY:
"Effects of Different Sewage Sludges on Some Chemical and Biological
Characteristics of Soil", M.J. Mitchell et a! (14)
"A Study on the Interactions of Enzymes with Manures and Sludges", L.
Theoret et al (17)
"A Progress Report on the Potential Use of Earthworms in Sludge Man-
agement", R. Hartenstein et a! (107)
"Growth of the Earthworm Eisenia foetida in Relation to Population
Density and Food Rationing", E.F. Neuhauser et al (108)
Reproductive Potential of the Earthworm Eisenia foetida", R. Harten-
stein et al (116)
"Physicochemical Changes Accompanying the Conversion of Activated
Sludge into Castings by the Earthworm Eisenia foetida", R. Harten-
stein et al (120)
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"Materials Supportive of Weight Gain by the Earthworm Eisenia foetida
in Waste Conversion Systems", E. F. Neuhauser et al (1W]
"Composition of the Earthworm Eisenia foetida and Assimilation of
Fifteen Elements from Sludge during Growth", R. Hartenstein et al (124)
"Soil Organisms and Stabilizing Wastes", D. L. Dindal (16)
"Conversion of Sludges into Topsoils by Earthworms", M. J. Mitchell
et al (15)
Because the SUNY research is carried out under carefully controlled con-
ditions in small-scale laboratory tests, it is particularly valuable in
helping to establish a reliable base of information on chemical, biologi-
cal and physiological parameters of vermicomposting. Two qualifications
must be attached, however. First, the SUNY results might not be directly
applicable to full-scale vermicomposting operations in the field. The.
E. foetida used in SUNY experiments are acclimated to the constant 25°C
temperatures at which the experiments are run. Performance of individual
worms in glass dishes might be either improved or depressed in larger
beds with high densities of worms. Second, we are aware o? no comparable
research being conducted in the United States; therefore, there is lack-
ing a measure of competition and criticism that might improve the
accuracy and applicability of the SUNY findings."
Among the particularly pertinent results obtained at the SUNY laboratory,
Dr. Hartenstein and his associates determined in 1977-78 that anaerobic
and anaerobically digested sludges are unsuitable for vermicomposting.
Freshly obtained, anaerobically digested sludge is toxic to E. foetida;
even if aged prior to vermicomposting, the sludge still fails to support
a thriving worm population.
In laboratory experiments at 25°C, E. foetida consume about 0.5 g of
aerobically digested of raw sludge per day per gram of worm weight (wet).
A typical experiment shows that 25 g of earthworms require four weeks to
consume a 350 g sludge sample.
Titusville, Florida
Owner/Operator: Ms. C. Carlson
Date of Visit 6 September 1979
Sludge Feed: Fresh sludge, thickened to 10 percent solids
Capacity: 4,000 to 6,000 gallons per week
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Background. This facility has been operated on an experimental basis for
the past two years. The operator is not interested in marketing of pro-
ducts, but rather in demonstrating the feasibility of vermicomposting for
sludge management. For the most part, experiments are conducted without
data collection or documentation of findings.
Feed Materials. A variety of organic feed materials has been tried, in-
cluding mixtures of cow manure and sawdusty siudge and sawdust, and
sludge and cardboard. Sludge is obtained from the City of Titusville
WWTP at 3 percent solids; it is delivered to the site two or three times
per week in a 2000-gallon tank truck. Sludge is discharged to a ditch
adjacent to the facility, where it is allowed to thicken to about 10-per-
cent solids.
i
Operation. Three types of earthworm beds have been used: two 10-ft by
60-ft inground beds, four 3-ft by 8-ft concrete beds and three 4-ft by 9-
ft wooden beds. The larger, in-ground beds reportedly are the most suc-
cessful. Bed depths range up to 1 foot.
Sludge is manually removed from the storage ditch and transported by
wheelbarrow approximately 50 feet to the worm beds. The worm beds have
an initial layer of cardboard placed in them. A 1-inch layer of sludge
cake is shoveled onto the cardboard in the beds. This is repeated sever-
al times to create a 3- to 4-inch layer of fresh material.
The worms in the bed reportedly migrate up from lower, older layers to
feed on the new cardboard-sludge mixture. Consumption of this mixture
reportedly takes place in six to eight weeks, depending on weather con-
ditions and season, and as judged by observation. In warmer weather, up
to twice as long may be required for conversion; in cooler weather, as
little as half as much time is required. Cool-weather temperatures in
Florida result in bed temperatures in the 15°C range, which are preferred
by earthworms to hot weather conditions (27°C and higher).
Processing and Marketing of Products. The operator is not separating
earthworms from castings; wormbeds are simply used to store the finished
product.
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