Organic material is broken down into plant nutrients by two primary processes, aerobic and anaerobic. Aerobic processes require oxygen and anaerobic must occur in the absence of oxygen. Each process has some advantages, when properly instituted, and conversely, each has some disadvantages. Aerobic digestion is best implemented on a smaller scale, while anaerobic digestion, especially if natural gas production is your goal, requires a rather large physical plant in order to be essentially trouble- free.
Our purpose here is to discuss aerobic digestion, since this process is the most practical for individuals to use. The basic requirements for aerobic digestion are: a source of organic material, aerobic organisms, ( bacteria, fungus, and in some operations, earthworms ) water, and oxygen.
The compost pile is not a putrid heap of worthless material, but a home for millions of organisms which provide an absolutely essential function for Earth's life cycle. These life organisms find the compost a delicious smorgasbord, especially when properly prepared.
The formula for the compost mix is not extremely critical, since different organisms will predominate according to the make–up of the pile. However, a few simple considerations will keep the pile healthy and thereby keep the odors to a minimum. A blend of roots, shoots and fruits will help keep nature's order in balance. Roots can come from pulling up weeds when the soil is moist. Don't make too great an effort to remove soil from the roots. The soil contains additional strains of bacteria and other organisms. These organisms most likely will improve the organic family in your digester. Shoots can include sticks, wood chips, stems, bark and the like. Fruits include kitchen garbage, grass clippings and leaves. Together, these items can be blended to make a healthy compost pile.
All life organisms are Ph sensitive and will only thrive within a very narrow Ph range. This, plus the fact that the growth cycle requires rather large quantities of calcium, an element that has a rather low solubility factor. It is recommended that approximately ½ cup of "garden lime" ( CaSO4 ) be added for each bushel of compost material. This, too, will help keep odors under control.
In general, there are enough organisms present in the environment to start your compost culture. However, there are many commercially available products (compost starters) that will help speed things along in the beginning and perhaps provide additional "hardy" strains of aerobic organisms. Once your culture is started, additional bacteria is not required, especially if you leave a small quantity of material in the composter when adding new material.
One of the advantages of aerobic digestion is that rather large quantities of heat are released. This is useful for destroying weed seeds and unwanted organisms. If the compost is kept properly moist, in a few days the weed seeds will germinate. Then, if the temperature of the compost material is allowed to rise sufficiently, the weed sprouts will be killed. This will give you a peat–like material that is free of weed seeds. The high temperatures will also kill insects and other parasites in the material. If the composter maker is sufficiently insulated, as some commercially available units are, it is possible to kill almost all of the culture bacteria resulting in a rather sterile medium. Mushroom growers use this technique to prepare their medium for mushroom spores. A quantity of "culture material" should be removed before temperatures are allowed to reach this level or else you will need to restart your culture from scratch.
Control of the moisture content of the medium is extremely important. Too much water will force oxygen out of the medium and "drown" the organisms. Too little allows the medium to dry out and your culture organisms will "die of thirst". The medium needs to be thoroughly moist with no "standing water".
Insuring sufficient Oxygen in the medium is very critical. The medium organisms will quickly suffocate if the oxygen level becomes too low or the carbon dioxide level becomes too high. The main disadvantage of the "compost pile" method is the difficulties encountered in "turning" the pile on a regular timely basis. Inadequate mixing results in areas where there is too much water, too little oxygen, insufficient organisms and hot spots where many organisms are killed off. Thus, if rapid odor free digestion is your goal you need to adopt a composting method that maintains uniform conditions throughout the compost mixture.
The horizontal rotating drum system embodies the basic concepts required for a simple home composting system. The drum consists of narrow slats spaced about ¼ inch apart to allow air in and excess water to drain out. The slats need to be cedar or redwood in order to be durable since they are wet constantly and other woods would rot. Pressure-treated "green Lumber" should be avoided since this material contains toxic substances. Plastic materials could be used but these would need to be custom manufactured and would only be suitable for large scale manufacturing operations. The units have a crank that is used to rotate the drum every couple of days to mix the compost and maintain uniformity in the medium. A well–designed unit would use a perforated pipe as a spindle with a swivel hose connection so that water could be introduced during each mixing session. Care should be taken not to add too much water, since this will flush out needed organisms and nutrients.
This type of system can also be used to make "compost tea" after the composting process is complete. The tea is suitable for feeding potted plants and areas of your garden where it would be difficult to add the composted material. Although the remaining material in the composter will have reduced fertilizing value, it still is high in fiber and will serve to improve your soil permeability.
As mentioned before, some composting units have insulating jackets to allow for pasteurization of the medium and to improve performance during colder times of the year when the medium has a tendency to freeze. The drum system can be equipped with a plastic "space blanket" to accomplish this purpose. It could be easily removed for filling or emptying, or for the early stages of digestion, when slightly lower temperatures are desirable.
In general, small systems require more attention than larger ones, but with careful attention to design, they can be highly effective without becoming burdensome. The optimized environment of a properly designed digester can quickly return nutrients to the soil and improve the over all performance of your garden.
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