Thermophilic composting is becoming increasingly used for processing of a wide range of organic wastes. However, the process does not always produce high quality products that have potential for soil and land improvement. Over the last 20 years interest has increased progressively into the potential of a related process which involves the use of earthworms to break down organic wastes. In 1881, Charles Darwin first drew attention to the great importance of earthworms in the breakdown of dead plant organic material and the release of essential nutrients from it in his book “The Formation of Vegetable Mould Through the action of Worms” and his conclusions have been fully confirmed and utilized during the last century. However, only in recent years has the potential of earthworms for systems of breaking down organic wastes been explored in more depth. The basic research, which began at the State University of New York, Syracuse in the 1970s, under the leadership of Dr. Roy Hartenstein, mainly at the use of earthworms for processing sewage solids. This was expanded in the early 1980s to field-scale practical methods for disposing of poultry, pig and cattle wastes in an interdisciplinary research program under the leadership of the author, at the Rothamsted Experimental Station, U.K., involved nearly 50 scientists, including biologists, agricultural engineers, economists, and representatives of a range of commercial enterprises. These studies, which have since been complemented by research by other workers in France, Germany, Italy, Spain, and Australia, have demonstrated the very considerable economic potential of using earthworms to convert a wide range of organic wastes into valuable and efficient plant growth media.
Earthworms fragment organic wastes extremely rapidly and increase microbial activity in them dramatically. The main difference between composting and vermicomposting is that whereas composting is a thermophilic process reaching temperatures of 60-70ºC, vermicomposting systems must be maintained at temperatures below 35ºC. Exposure of the earthworms to temperatures above this, even for short periods, will kill them, and to avoid such overheating careful management of the wastes is required. Earthworms are active and will consume organic wastes in a relatively narrow aerobic layer of 6-9 inches, that is close to the surface of a bed or container. The key to successful vermicomposting lies in adding organic wastes to the surface in successive thin layers at frequent intervals, so that any thermophilic heating that occurs does not become excessive, although if well managed, some heating will maintain the activity of the earthworms at a high level of efficiency, since vermicomposting works best at temperatures between 20ºC and 25ºC.
Almost any agricultural, urban or industrial organic wastes can be used for vermicomposting, but some may need some form of preprocessing to make them acceptable to earthworms. Such preliminary treatments can involve washing, precomposting, macerating or mixing. Wastes from the brewing, soft drink, processed potato and paper industries, sewage solids and yard, garden and food wastes as well as sewage biosolids are particularly suitable for vermicomposting. Often, mixtures of several different wastes can be processed more readily than individual wastes, are easier to maintain aerobically, and result in a better product.
There is an extensive but small-scale cottage industry that grows earthworms for fish bait in a variety of organic wastes. These use, almost exclusively, outdoor ground beds or windrows. Such systems require large areas of land for large-scale production and are relatively labor-intensive, even when machinery is used for adding wastes to the beds. More importantly, windrow systems process wastes relatively slowly, taking anywhere from 6 to 18 months for processing of a layer 18” deep to be complete. Since this is usually an outdoor process, there is good evidence that a large proportion of the essential plant nutrients, that are in a relatively soluble form, are either washed out or can volatilize during this long processing period. Such nutrient losses are undesirable, since they can contribute to groundwater pollution, and result in a poor, low nutrient product with relatively low potential as a plant growth medium.
The Scientific Basis for Vermicomposting
A few species of earthworms can consume organic wastes very rapidly and fragment them into much finer particles, by passing them through a grinding gizzard, an organ that all earthworms possess. The earthworms obtain their nourishment from microorganisms that grow upon the organic wastes; at the same time they promote further microbial activity in the wastes, so that the casts, or vermicompost, that they produce, is much more fragmented and microbially active than the organic wastes the earthworms consume. During this process, the important plant nutrients that the wastes contain, particularly nitrogen, phosphorus, potassium and calcium, are released and converted into forms that are much more soluble and readily available to plants than those in the original waste. The retention time of the waste in the earthworm is short and very large quantities are passed through an average population of earthworms. In the traditional aerobic composting process, organic wastes have to be turned regularly, or aerated in some way, in order to maintain aerobic conditions in the waste. This may often involve extensive engineering and machinery to process the wastes as rapidly as possible on a large scale. In vermicomposting, the earthworms, which survive only under aerobic conditions, take over both the roles of turning over the waste and maintaining it in an aerobic condition, thereby lessening the need for expensive engineering.
The major constraint to vermicomposting is that, in contrast to traditional composting, which is a thermophilic process that can raise temperatures in the waste to more than 70ºC, vermicomposting systems must be maintained at temperatures below 35ºC. The processing of organic wastes by earthworms occurs most rapidly at temperatures between 15ºC and 25ºC (60º to 79ºF) and at moisture contents of 70% to 90%. Outside these limits, earthworm activity and productivity and the rate of waste processing can fall off, and for maximum efficiency, the wastes should be maintained as close to these environmental limits as possible. The earthworms are also sensitive to certain conditions in the wastes. In particular, earthworms are very sensitive to ammonia and salts and certain other chemicals. For instance, they will die quite quickly if exposed to wastes containing more than 0.5 mg of ammonia per gram of waste and more than 0.5% salts. However, salts and ammonia can be washed out of organic wastes readily or dispersed by precomposting. Contrary to common belief, earthworms do not have many serious natural enemies, diseases or predators and can survive exposure to many adverse conditions.
Types of Vermicomposting Technology Available
A number of species or earthworms that are specific to organic wastes have been used in vermicomposting. The temperate species most commonly used world-wide is Eisenia fetida (the tiger or brandling worm). Another suitable temperate species is Lumbricus rubellus (the red worm), and two tropical species, Eudrilus eugeniae (the African night-crawler), and Perionyx excavatus, an Asian species; the latter two species are very productive, but cannot withstand temperatures below 5ºC. Each species has its particular favorable environmental requirements and it is important to choose the best species for any climate and waste.
The traditional methods of vermiculture have been based on beds or windrows on the ground containing waste up to 18″ deep, but these have numerous drawbacks, particularly in terms of land and labor requirements. They also have the major drawback that when the vermicompost is collected it is necessary to use rotating mesh trommels or other mechanical means to remove the earthworms from the processed materials.
It is possible to use batch vermicomposting systems involving bins or larger containers, often stacked one above the other in racks. Such container systems, particularly if large, have the drawback of needing considerable handling and lifting machinery, and there are also problems in adding water to maintain the moisture contents and in adding additional layers of waste at frequent intervals. Hence, large-scale batch systems have not been used much on a commercial basis.
However, small-scale container systems have been used extensively to process domestic and institutional food wastes. They range from simple raised containers, described by Mary Appelhof in her book “Worms Eat My Garbage” (1997), to more sophisticated stacking systems marketed under names such as “Can O’Worms”, the “Worm Factory,” “Wriggly Ranch” and the “Worm Gin.” Such systems have been very popular in schools, which involve the children in their operation and with certain municipal waste authorities, such as in Vancouver, Canada, and various cities in Australia which subsidize their purchase and use.
In recent years, much more efficient systems of vermicomposting have been developed. These are based on large containers raised on legs above the ground, that allow wastes to be added at the top from mobile gantries and collected mechanically through mesh floors at the bottom using breaker bars. Such methods were developed and tested extensively by engineers at the National Institute for Agricultural Engineering, Silsoe, in England. The methods they designed ranged from relatively low technology systems using manual loading and waste collection systems, to large (128 ft long x 8 ft wide x 3 ft deep), completely automated and hydraulically-driven continuous flow reactors, that have operated successfully for several years, using the original earthworm population, which reaches an equilibrium population of about 2 lbs or 500-1000 earthworms per square foot, and involving short retention times within the reactor. For instance, such reactors can fully process three-feet deep layers of suitable organic wastes in 30-45 days. Although these systems require more capital outlay, the cost of the reactor can usually be recouped in 1-3 years, and they can be operated on a large scale with minimal labor requirements. An automated reactor that will process 1000 tons of waste per year can be built for $15,000-30,000 and the manually-operated, lower technology systems that work on a similar principle cost much less. Economic studies have shown such reactors to have much greater economic potential to produce high grade plant growth media very quickly and efficiently than windrows or ground beds. A large-scale system in southern France, developed by Dr. Marcel Bouché and based on similar principles, is part of a total recycling waste system for a small town, involving separation and sorting of the wastes followed by composting, vermicomposting, and sieving. This system converts 27% of the total waste stream for the town into a valuable vermicompost.
Marketing of Products
There are many commercial attractions to finding methods of converting urban and industrial organic wastes into materials that do not have to go into landfills. Such economics become even more attractive if the process produces a value-added horticultural plant growth medium with considerable commercial value.
Extensive plant growth trials at the Ohio State University have shown that substitution of 10% to 20% of the best horticultural plant growth media by vermicomposts, increased the rates of germination, growth, flowering, and fruiting of a wide range of ornamental and vegetable crops very considerably. This places considerable commercial value on vermicomposts, although the market is only recently being fully exploited. Even without a proper marketing structure, vermicomposts can be sold readily for $30-40 per cubic yard but they also have been readily marketed for high-grade horticultural use for as much as $150 per cubic yard, after appropriate standardization, formulation, and packaging. Such standardization often involves some acidification (or mixing with peat). As much as $300 per ton has been offered for good quality export vermicomposts, and with good marketing, even higher returns have been realized. The more standard and well-formulated the product, the greater is its market potential.
Additional Information Sources
Appelhof, M. 1997. Worms Eat My Garbage. Flower Press, Michigan, 162 pp.
Edwards, C.A. and Neuhauser, E.F. (Eds.) 1988. Earthworms in Waste and Environmental Management. SPB Academic Publ. The Hague, Netherlands, 392 pp.
Edwards, C.A. and Bohlen, P.J. 1996. The Biology and Ecology of Earthworms. 3rd Ed. Chapman and Hall, London, 426 pp.
Edwards, C.A. (Ed.) 1998. Earthworm Ecology. CRC Press/St. Lucie Press, Boca Raton, 389 pp.
Edwards, C.A. 1995. Historical Overview of Vermicomposting. BioCycle, June 1995, 56-58.
Subler, S., Edwards, C.A. and Metzger, J. 1998. Comparing Vermicomposts and Composts. BioCycle, July 1998, 63-65.