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                              TECHNICAL PAPER #9
                          UNDERSTANDING AGRICULTURAL
                                WASTE RECYCLING
                                Walter Eshenaur
                             Technical Reviewers
                              Dr. Eldridge Collins
                              Philip R. Goodrich
                                 Martin Wulfe
                       1600 Wilson Boulevard, Suite 500
                        Arlington, Virginia 22209 USA
                    Tel: 703/276-1800 . Fax: 703/243-1865
                  Understanding Agricultural Waste Recycling
                             ISBN: 0-86619-209-3
               [sup.c]1984, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introduction to specific state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their situations.
They are not intended to provide construction or implementation
details.  People are urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis.  Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.  VITA staff included Leslie Gottschalk
and Maria Giannuzzi as editors, Julie Berman handling typesetting
and layout, and Margaret Crouch as project manager.
VITA Volunteer Walter Eshenaur, the author of this paper, is a
research assistant in the Department of Agricultural Engineering
at the University of Minnesota, where he specializes in energy
technologies.  Dr. Eldridge Collins, one of the reviewers of this
paper, is with the department of Agricultural Engineering,
College of Agriculture and Life Sciences, Virginia Polytechnic
Institute and State University.   VITA Volunteer reviewer Philip R.
Goodrich is an Associate Professor with the Department of Agricultural
Engineering, University of Minnesota.   VITA Volunteer
reviewer Martin Wulfe is a consultant in the field of energy development.
He has performed several consultancies in renewable
energy assessment in Africa, Indonesia, and West Sumatra.  Wulfe
has also published several articles and a section in a book on
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.   VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations.  VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
                       By VITA Volunteer Walter Eshenaur
Agriculture is the most important direct source of livelihood and
the largest source of employment in the world's less developed
countries.  The agriculture sector produces food crops, meat and
other animal products, energy crops, and industrial crops.  It
also produces billions of tons of other materials long regarded
as "waste." The main types of agricultural waste are crop residues,
the parts of crop plants that are not eaten, and farm
animal waste products.  In the past, the vast majority of these
materials was indeed wasted.
Agricultural experts are coming to recognize that agricultural
residues can be thought of as a "resource out of place" instead
of simply waste or by-products.   This is a very important change
of perspective which permits the evaluation of waste from a
positive standpoint.  Once an evaluation of waste is embarked
upon, it becomes obvious that this resource represents a partial
solution to the energy dilemma facing agriculture today.  Once
the agricultural waste resource is understood as a tremendous
source of energy, then steps to utilize this energy may be taken.
With appropriate techniques, agricultural wastes can be recycled
to produce an important source of energy and natural fertilizer
for crops.  Recycling agricultural wastes can help a developing
country reduce its dependence on foreign energy supplies and
raise the standard of living in its rural areas.
This paper discusses the general theory involved in recycling
agricultural wastes and several popular methods.   It does not
present detailed practical examples.   It is important to stress
that the choice of recycling method will depend on the type of
waste available and on the end use the farmer has in mind for the
recycled waste.  It is hoped that readers will adapt the general
methods discussed in this paper to their own local conditions.
The main kinds of agricultural waste are crop residues and farm
animal waste.  Most of the energy contained in crop residues is in
the form of carbohydrates and cellulose.   Table 1 shows the chemical
composition of some residues.
                Table 1. Composition of Some Residues
                  Grain    Leaf        Citrus      Manure
                  straw   (grass)       pulp     (poultry)
Dry Matter:
   Organic matter   95        91         93          77
   Ash                5         9          7          23
   Crude protein      3       17          7          32
   Crude fiber       48       27         14          --
    extract          43        44         69          27
Source:  P. van der Wal, "Perspectives on Bioconversion of Organic
Residues for Rural Communities, "Proceedings of Bioconversion of
Organic Residues for Rural Commuinities (Tokyo, Japan:   United
Nations University), 1979, p. 5.
All the residues in Table 1 contain mostly organic matter.  In
developing countries, poultry generally are allowed to forage and
digest much of the organic matter ingested.   Thus, poultry do not
produce as much organic matter as crop residues.   Ash is the waste
that remains unused even after the most rigorous of recycling
processes.  The main content of ash is inorganic substances such
as potassium and phosphorus.   Energy extraction from crude protein
is difficult but is very useful for animal or human consumption
since in this way the protein may be utilized.   Crude fiber may
remain somewhat unused if fed to some animals.   However, if
digested aerobically (in the presence of oxygen), anaerobically
(in the absence of oxygen), chemically (using alkali or ammonia)
or through composting, crude fiber will break down to simpler
carbohydrates that are easily digested either by animals or in
the soil.
The amount of nitrogen-free extract indicates how much nitrogen
is available.  Comparing amounts of organic matter and nitrogen-free
extract indicates approximately how much nitrogen is made
available through digestion or chemical treatment.   A higher percentage
of nitrogen-free extract indicates a lower percentage of
available nitrogen and vice versa.   Nitrogen plays an important
role in soil conditioning and refeeding to animals since it is a
necessary nutrient for both.   Nitrogen also plays an important
role in aerobic and anaerobic processes;however, these processes
change the form of nitrogen, which may influence its availability
to plants, volatility, or leachability.
Grain straw represents the largest component of crop residues.  As
indicated by Table 1, large part of grain straw is crude fiber.
Thus the method for recycling grain straw should include some
type of decomposition process to extract maximum energy.  Grass,
although easier to digest, should receive somewhat the same
treatment as grain straw.
Citrus and vegetable residues are relatively easy to digest and
direct methods of extracting energy such as refeeding or soil
incorporation work well.  However, due to the ease of digestion,
other forms of energy such as methane or alcohol may utilize
these residues more fully since the decomposition process is more
complete.  In conclusion, when recycling crop residues, some type
of decomposition treatment is desirable.
Animal waste includes manure (feces and urine) along with added
bedding, other liquids, and soil.   Other wastes such as milkhouse
and washing wastes, hair, and feathers may also be included
within this category.
The composition of animal manure depends upon "animal specie;
digestibility, protein, and fiber contents of rations; and animal
age, environment, and productivity" (Midwest Plan Service, 1975).
Due to varying diets and wastes, only estimates can be given for
properties and nutrient content of manure.   Table 3 shows manure

uawx4.gif (600x600)

production and characteristics of some popular animals.
Explanations of Table 3 are as follows.   Raw manure includes feces
and urine without bedding.  Feces refers to the solids component
of manure.  Percent raw manure (percent RM) is the percentage of
the raw manure that is made up of feces.   Total solids is the sum
of dissolved and undissolved components of the manure.   Volatile
solids refers to the amount of material that will burn or become
volatile under a temperature of 550 degrees Centigrade.   The
oxygen used for the biochemical oxidation of organic matter is
referred to as the Biological Oxygen Demand (BOD).   The five (5)
refers to the BOD after five days in a 20 degrees Centigrade
environment.  The Chemical Oxygen Demand (COD) is not used in
engineering design but represents the total oxygen demand if all
inorganic and organic material is oxidized.   The COD will always
be a higher value than the BOD.
The main emphasis of Table 3 is to show the various properties of
different animal manures.  It is clear that varying manures differ
in all categories and recommendations may be made as to what
recycling process could be used with each manure.
Table 2 shows daily manure production and is somewhat more detailed

uawx5.gif (600x600)

than Table 3.  Explanations for total solids, volatile
solids and BOD are the same as for Table 3.
Dairy cattle produce more manure than any other individual animal;
poultry produce the least.  However, if production per unit
weight is calculated, poultry produce almost as much as any other
animal.  Poultry manure also contains less water than others.
Total solids and BOD are quite high for poultry, but so are
volatile solids.  Thus, although poultry manure production is
slightly lower than that for cattle (per unit weight), its total
solids or decomposable material is higher.   This is a positive
characteristic for aerobic and anaerobic digestion even though
the BOD is rather high.  Dairy manure is also high in total solids
and therefore provides good digester input (influent).   The ratio
of BOD to total solids is high for swine (0.32) and gradually
decreases from poultry (0.26) to beef (0.23) to dairy cattle
(0.16).  This ratio indicates the relative amount of oxygen necessary
to decompose the solids.  A high number suggests high oxygen
use and a low number suggests low oxygen use.   Thus, for bacterial
processes that require oxygen (most common ones do), dairy manure
will decompose with less oxygen input than will swine manure.  For
decomposition such as aerobic digestion, direct land application,
or composting, dairy manure will provide more decomposed matter
and thus more nutrients per unit time than will swine manure.  In
the controlled decomposition of anaerobic digestion, oxygen deand
is not important because it is not used to a large extent.
However, oxygen demand does reflect indirectly the amount of
organic matter present.  A higher oxygen demand suggests a higher
organic matter content and vice versa.
Data in Tables 2 and 3 were developed by the American Society of
Agricultural Engineers based on work reported in the literature
and represents American or European feeding methods.   These data
may vary by up to 20 percent for animals in confined or semi-confined
quarters.  For animals that are on a total forage diet
(i.e., pasture grazing), the data for a beef cow will be most
accurate.  They will probably give values that are too low, except
for nitrogen, which may be high.
Drying of manure is also used for some recycling processes.  Table
3 shows that swine manure contains the highest water content and
poultry manure the lowest.  Thus for drying it is better to use
poultry than swine manure.
Nutrient content is an important value in determining which
manure will provide the best refeeding or land application
capabilities.  Table 4 shows relative nutrient qualities.
            Table 4. Nutrients Per Manure Quantity
               Element                       Element
        pound/1000 gal. manure        pound/ton raw manure
          N           P         K          N          P         K
         ---         ---       ---        ---        ---        ---
Dairy     41          7.4       27         9.9       1.8        6.6
Beef      45         15         32        11.4       3.7        8.4
Swine     55         18         32        13.8       4.6       9.0
Sheep     97         14         69        22.5       3.3      16.0
 Layer    109        42        47         27.2      10.6      11.6
 Broiler 131         29         41        34.3       7.6      10.6
Horse     48          8         30        11.8       2.0       7.4
Source:  Midwest Plan Service, 1975, p. 5.
Nutrient levels are given only for nitrogen (N), potassium (K)
and phosphorus (P).  Other nutrients are minor and are either
almost totally lost during decomposition or are comparatively
Land application requires that as many nutrients as possible
remain in the soil after decomposition.   In fact, the highest
nutrient content will be without decomposition.   This is somewhat
misleading, however, since nitrogen occurs in several forms, not
all of which are available to be used by plants.   The best form of
nitrogen is ammonia which is easily used by plants.   The most
efficient method of obtaining ammonia is anaerobic digestion, but
the liquid effluent must be used immediately or the nitrogen is
lost.  Composting also produces ammonia but since the composting
materials must be aerated, most of the ammonia is lost.   Table 4
shows that only poultry manure is high in phosphorus.   Phosphorus
and potassium are stable inorganic compounds and are not used in
most decomposition processes.   Thus, both phosphorus and potassium
will remain to be used in the soil after decomposition.   Phosphorus
is a necessary soil nutrient and usually more is needed
than can be provided by animal manure.   Thus, although nutrients
necessary for recycling are present in animal manure, they are
not sufficient to supply the total needs of the recycling process
of most post recycling applications.
This section discusses five popular recycling methods:   anaerobic
digestion, refeeding, land application, composting, and incineration.
The choice of the best method depends on the type of waste to be
recycled and the end use intended for the recycled waste.  General
methods discussed here must be adapted to specific local conditions.
Table 5 gives some potential end uses of organic residues.
Anaerobic Digestion
Anaerobic digestion is used to break down the starch and cellulotic
components of crop residue to produce biogas for lighting or
cooking.  The decomposition of organic matter under anaerobic
conditions produces amino acids, carbon dioxide, hydrogen sulfide,
and methane.  All these gases are either very toxic (hydrogen
sulfide) or contribute to lack of sufficient oxygen (carbon
and methane).  Biogas under most circumstances will burn directly
from the digester.  For applications in internal combustion engines,
the carbon dioxide and hydrogen sulfide must be removed.
But removing these gases requires more complex technology usually
not available in developing countries.   Biogas will provide heat.
The aim of anaerobic digestion is to decompose as much organic
matter as possible and produce as much biogas as possible.  This
requires a high quantity of degradable starch, and a little
cellulose.  Table 1 shows that grain straw, grass, and citrus
residues are not the best organic materials.   Animal manure, on
the other hand, contains much degradable carbohydrates, has little
cellulose, and has a relatively high nutrient level.   More
carbohydrates may be desired depending upon the type of animal
manure being used.  Table 1 shows that poultry manure is lower in
organic material than crop residues and is higher in organic
material than manure from swine or ruminants (cattle, sheep, and
goats).  Thus, crop residue alone is not desirable for the production
of biogas; a mixture of animal manure and crop residue is
most desirable.
               Table 5. Potential End Uses of Organic Residues
Food              microbial biomass
                  fermented foods
                  mushroom production
Feeds             direct use
                  upgrading (physical, chemical, microbial)
                  microbial biomass
Fertilizer        direct use
                  residue of biogas production
Energy            biogas
                  producer gas
                  direct use (combustion)
Construction      boards
 materials         panels
Paper pulp        paper
                  packaging materials
Chemicals         furfural
                  organic acids
Pharmaceuticals   hycogenin
Source:  W. Barreveld, "Availability of Organic Residues as a
Rural Resource," Proceedings of Bioconversion of Organic Residues
for Rural Communities (Tokyo Japan:   United Nations
University), 1079, p. 10.
Nitrogen is an important nutrient in anaerobic digestion and
usually some will remain after digestion is complete.   Other by-products
of anaerobic digestion include phosphorus, potassium,
biogas, organic acids, alcohols, and cellulotic organic matter.
Advantages of anaerobic digestion include:
     *   low initial cost
     *   low operating cost
     *   simple operation (once process has started)
     *   wide variation of loading rates
     *   low nutrient requirement
     *   useful end product: methane
     *   effluent usable as soil conditioner
Disadvantages include:
     *   difficult starting procedure
     *   foul odors
     *  slow rate of microbial growth
     *   best production at elevated temperatures
Anaerobic digestion is becoming more popular because of its
increasing economic viability and improvements in technology.
However, before any attempt to introduce anaerobic digestion into
a particular culture, expert advice should be sought.   Some cultures
do not allow the handling of human wastes and may consider
digestion as imposing on an already viable use.   Great care must
be taken in implementing this technology.
Refeeding of crop and animal wastes works well with ruminants
because this family of animals can utilize the nutrients in their
available form.  The bacteria within a ruminant's stomach system
break down non-protein nitrogen and utilize it as energy.   Monogastric
animals such as horses and swine cannot utilize this form
of nitrogen and do not benefit from direct refeeding without
prior treatment, except for protein utilization.
Some crop residues should be treated before recycling.   Rice straw
or bran will provide the necessary nutrients for livestock without
processing.  However, if soaked in an alkali bath, the digestibility
of these crop residues increases almost twofold.   This
provides, for the same amount of roughage, a great increase in
nutrient availability.  It also allows animals to produce more
milk or realize a greater increase in weight.   Refeeding of leafy
wastes works well and digestibility is good.   However, as with
grain residues, pretreatment is recommended, but not with alkali.
Any animal manure may be refed but poultry seems to be the most
economical since it contains the highest nutrient concentration
per unit weight.  Crop residues are also good for ruminant refeeding
but, as mentioned earlier, processing with alkali or
ammonia will increase the digestibility greatly.
It is very important to process animal manure by drying or ensiling
before refeeding to prevent pathogen transfer.   Drying at
elevated temperatures helps limit pathogen transfer and reduces
the time from excretion to refeeding.   Economically, refeeding of
other than poultry manure is questionable and must be analyzed
for each situation.  Cultural taboos on feeding livestock other
than pasture forage may be strong.   Gaining acceptance may require
a positive demonstration that supplementing pasture forage with
dried or ensiled manure will indeed bring added nutrients and
most likely healthier and strong animals.
Advantages of refeeding include:
     *   up to 75 percent of diet
     *   no change in the taste of milk, meat, or eggs
     *   weight gain remains the same or increases with 75 percent
        of diet
     *   good use of previously unused wastes
Disadvantages include:
     *   feed conversion (conversion from roughage to nutrients)
        less than grams
     *   pretreatment of crop residues necessary
     *   drying of manure necessary
     *   possible negative cultural or economic impacts
For situations where refeeding is culturally and economically
acceptable, refeeding will increase nutrient levels and decrease
dependence on imported feed.
Land Application
One of the most useful methods of recycling is reapplication of
crop residues to the soil.  Several methods are popular.  The
simplest method is reincorporation of residues into the soil
following harvest.  This eliminates the need for postharvest processing.
Much nitrogen is lost, however, through volatilization
of ammonia which is a product of decomposition.   Also, if nitrogen-producing
crops (i.e., legumes) are not grown, the soil will
slowly lose all nitrogen since residues do not return enough to
overcome the loss of nitrogen during the growing season.
A second method uses anaerobic digestion to reduce the crude
fiber content yet retain nutrients necessary for soil conditioning.
Once the digestion process is complete, the effluent is
spread on the soil.
Several important practices must be adhered to in order to maximize
nutrient retention in the soil.   First, most nitrogen contained
within the effluent is in the form of ammonia.   Ammonia has
a low vapor pressure and thus will evaporate quickly.   Also,
ammonia breaks down quickly in the presence of oxygen.   To minimize
the volatilization of the ammonia, immediate incorporation
of the effluent into the soil is necessary.   This practice of
incorporation requires a labor- or energy-intensive system.  In
some situations this may not be possible.
Second, nightsoil and manure constitute better inputs for anaerobic
digestion when combined with crop residues.   Even with the
rigorous decomposition that occurs in the digestion process, some
pathogens and parasites can survive and enter the soil.   This is
very dangerous as these pathogens and parasites, such as hookworm,
can eventually reinvade the human body.   Care must be taken
to ensure that as few pathogens as possible are transferred.  The
most effective method of preventing pathogen transfer is not to
use nightsoil.  Human pathogens are the most harmful and resistant
to treatment.  Another method is to operate the anaerobic digester
at high temperatures.  This will vastly reduce pathogen count.  A
third method would be to dry the effluent for an extended period
of time.  However, since ammonia is quite volatile, nitrogen loss
would be substantial.
Crop residues and animal manure not only fertilize the soil, but
also provide other benefits that are not immediately evident.
Most tropical soils and intensely cultivated soils are poorly
structured, so that the soil is hard and compacted.   This in turn
restricts water movement, plant root penetration, and nutrient
transport, and increases surface erosion and tillage requirements.
Adding crop residues along with animal manure increases
soil aggregation dramatically.   By increasing aggregation, the
soil may be tilled more easily (or not at all in some circumstances),
nutrient and water movement increases, and roots may penetrate
deeper.  Soil productivity is increased substantially while
decreasing tillage needs.
A note of caution must be mentioned here.   To raise the aggregation
of the soil by a substantial amount, large amounts of residue
must be used.  Bulk density relates to the aggregation of the
soil.  Typical bulk densities range from about 1.00 (gram/cubic
centimeter) for highly aggregated soils to 2.00 for very compact
soils.  To decrease the bulk density, the mass of solids must be
decreased.  This is accomplished by adding highly porous residues,
thereby increasing the volume per unit mass.   If the bulk density
of one hectare of land 10 centimeters deep is to be reduced from
1.5 to 1.2, calculations show that 1,500 tons (metric) of residue
must be added.  This is a large amount and may take several years
to accomplish.  This simple example shows that soil conditioning
through residue incorporation can improve soil properties but may
take time to do so.
Advantages of soil conditioning include:
     *   increased soil nutrients
     *   higher soil aggregation
     *   less dependence on imported fertilizer
     *   less tillage required
     *   reduction of surface soil erosion
     *   low capital investment
     *   more soil moisture
Disadvantages include:
     *   large quantities required
     *   residue pretreatment recommended
     *   weed seed concentration
     *   pathogen transport
     *   labor intensive
Economic considerations would include handling and application
procedures.  This is a labor intensive method of recycling and may
be marginally economical in most situations.   Cultural acceptance
is usually directly related to successful demonstration; mores
are usually not an obstacle.   For this recycling method to improve
a situation, full cooperation of producers is important since
large quantities of residue are required.
Composting is the practice of metabolizing residue by using
aerobic microorganisms to break down organic matter into usable
nutrients for application to the soil.   Composting also decreases
the bulk volume of residue, enabling easier transportation and
Composting is accomplished by mounding residue and allowing natural
heat buildup to start chemical metabolization of organic
matter.  This heat also eliminates pathogens and weed seeds and
provides a stable, dry end-product.
For composting to be successful, certain methods must be used.
Moisture content should be maintained at 50 percent by weight and
a temperature of 60 degrees Centigrade maximizes decomposition.
Mixing is important since composting is an aerobic process.  If
mixing is not possible, the composting process may take twice as
long.  Maintaining a pH between 7 and 9 will ensure proper and
rapid metabolization.  Carbon, nitrogen, and phosphorus proportions
are important.  A ratio of 25:1:2 respectively ensures rapid
decomposition and stabilization.   If manure is used, the carbon-nitrogen-phosphorus
ratio will change, making addition of crop
residues necessary.  A ratio of 20:1 of crop residues to manure
gives best results.
Advantages of composting include:
     *   organic matter metabolization
     *   elimination of pathogens and weed seeds
     *   uniform, dry end-product
     *  no insect or rodent problem
     *   no odor
     *   excellent fertilizer and soil conditioner
     *   low capital costs
Disadvantages include:
     *   loss of 50 percent nitrogen
     *   labor intensive
     *   high operating costs
Composting is used in many cultures around the world.   Most cultures
will accept composting as a viable method of obtaining
nutrients.  Composting is labor intensive and could be uneconomical
from that standpoint.  If labor is available, composting and
then soil application is an excellent method of providing nutrients.
Some crop wastes are best used for burning.   Paddy husks and straw
provide a substantial amount of energy when burned.   Other crop
residues may be best used in composting or refeeding, but grain
hulls and straw provide large amounts of energy when simply
In the paddy husk furnace, paddy husks and straw are burned with
good aeration.  The exhaust may be routed through a heat exchanger
with the heated air used for drying grain, etc.   Incineration of
materials other than grain chaff and straw may not use the energy
available in the most appropriate manner and all nitrogen is
Advantages of incineration include:
     *   efficient energy extraction
     *   up to 80 percent volume decrease
     *   heat produced is easily utilized
     *   low capital investment
Disadvantages include:
     *   good aeration is necessary
     *   burned residue is of little value
     *   labor intensive
     *   nitrogen is destroyed.
Culturally, incineration is usually easily incorporated.  Positive
demonstration of viability is important as with most new technologies.
Incineration is one recycling method that has proved to be
economical in most situations.   From simple, open fire burning to
furnace incineration, this technology is efficient and easily
Agricultural waste recycling is of great importance in the world
today.  Recycled agricultural waste represents a valuable
resource.  Agricultural waste may be utilized through many methods
of recycling.
A general overview of theory has been presented in this paper.
Several methods of recycling have been discussed and general
guidelines set forth.  It is hoped that this information will
provide the basis for particular waste recycling projects.
The possibilities of waste recycling are endless.   It is left to
the individuals responsible in particular situations to be
innovative and apply the knowledge presented herein to the
continuing and difficult task of returning a great resource to
its place.
Barreveld, W. "Availability of Organic Residues as a Rural
     Resource."  Proceedings of Bioconversion of Organic Residues
     for Rural Communities.  Tokyo, Japan: United Nations University,
Bewick, M.W.M. Handbook of Organic Waste Conversion. New York,
     N.Y.: Nostrand Reinhold Co., 1980. 418 pp.
Bruttini, A. Use of Waste Materials. Westminster, England: P.S.
     King and Son, Ltd., 1923. 367 pp.
Cox, G.W. and Atkins, M.D. Agricultural Ecology. San Francisco,
     CA: W.H. Freeman and Co., 1979. 721 pp.
Midwest Plan Service.  Livestock Waste Facilities Handbook.
     [City?], Iowa: Midwest Plan Service, 1975. 94 pp.
Palz, W., and Chartier, P. Energy From Biomass in Europe. Essex,
     England: Applied Science Publishers Ltd., 1980. 234 pp.
Pequegnat, C., ed. "Waste Recycling and Canadian Agriculture."
     Proceedings on Commercial Uses of Wastes and Animal Feed.
     Ottawa, Canada: Agricultural Economics Research Council of
     Canada, 1975. 253 pp.
van der Wal, P. "Perspectives on Bioconversion of Organic
     Residues for Rural Communities." Proceedings of Bioconversion
     of Organic Residues for Rural Communities. Tokyo,
     Japan: United Nations University, 1979.