In order to better understand the description of the operating and control parameters, a brief explanation of the composting process is included here. There are two stages to the biological activity associated with composting. The first stage is the active stage in which there is a high rate of biological activity taking place. The temperature of the compost mass increases to 60 degrees Celsius. During this time the readily biodegradable material is decomposed to less readily degradable components. As this occurs the temperature of the mass drops because of a slowing of microbial activity. This second stage is known as curing or the maturation stage and ends when the material reaches the required degree of stability. Hence, an important consideration in the evaluation of a compost system is the fact that the compost process is finished only upon completion of the curing stage.
4.1 Time Requirements
The compost process is complete when the original organic material cannot be identified and the compost has a fresh earthy smell. At this point, the mass has been stabilized to a point where it can be stored without causing nuisances and can be used without inhibiting plant growth. The curing interval is particularly important because it must be long enough for the composting material to have reached the final level of stability.
The length of time required to complete the composting process will depend upon the nature of the waste and the system employed. For example, yard and park debris, including grass clippings and leaves with the proper balance between the nutrients of carbon and nitrogen (C/N), composting can be accomplished in four to eight weeks. Leaves without the proper C/N ratio can take two to three years to compost. Municipal solid waste, with or without sewage sludge, can take eight to 24 weeks. Sewage sludge alone (with suitable bulking) can take six to eight weeks. Animal manure and assorted food processing wastes (that have been appropriately bulked) can take six to eighteen weeks to produce compost.
4.2 Temperature Requirements
Unless there is an obvious problem no effort is required to ensure the adequate temperature is achieved. The pile reaches the highest temperature, between 55 to 60 degrees Celsius, when the degree of microbial activity is greatest. The temperature of the pile will rise quickly as this is the most active stage of microbial activity, which is referred to as the thermophilic stage.
Eventually the microbial activity decreases and the temperature cools. This return to ambient temperature is referred to as the mesophyllic stage.
Typically the highest temperature is achieved in the first 10 days of the composting process and then gradually returns to the initial temperature over a period of six to eight weeks depending upon the nature of the material.
If the temperature of the pile is too high, microbial activity will be impaired and efforts may have to be taken to lower the temperature depending upon the type of system being used. The usual approach is to increase the rate and extent of aeration.
If the temperature rises too slowly or there is no temperature rise then not enough microbial activity is taking place. The attainment and maintenance of thermophilic temperatures for a time period is required for weed and pathogen control. If the temperature rise is inadequate the reason may be an operational or other problem.
4.3 Moisture Requirements
Ideally the moisture content should be between 45 and 55 percent. If the moisture content of the mass is eight percent or lower, microbial activity will cease.
Too much moisture prevents air from being supplied to the microbes. For this reason it is necessary to add bulking agents such as wood chips, straw, or leaves to wastes that have a high moisture rate such as cannery wastes, sewage sludge, and fresh manure.
4.4 Oxygen Requirements
Although composting can occur without oxygen (anaerobic systems), composting systems which use oxygen (aerobic systems) are considered more efficient, reliable and can tolerate sudden changes in environmental conditions. Aerobic systems are less likely to cause nuisance conditions such as objectionable odours.
If the composting mass is producing foul odours, then this could be an indication that there is not enough oxygen present in the pile. A slow temperature rise during the active stage of the composting process or an unexpected drop in temperature in later stages is another indication that the pile needs better mixing to provide additional oxygen to the pile. Other signs include the slowing in the breakdown of the organic matter and the absence of expected physical changes in the composting mass.
In practice the rate of aeration should be determined by experimenting with the waste to be composted. If aeration is achieved by turning then how often the turning occurs becomes the important consideration in trying to supply enough oxygen. With composting systems which used forced aeration the rate and volume of throughput air is the primary factor to be considered.
While it is possible to accelerate composting by adding pure oxygen to the input air stream it is unlikely that the benefits would out-weigh the cost of such an approach.
4.5 Nutritional Requirements
The organisms which create the compost, like all living things, need moisture, air and a source of nutrients. The most important nutrients to supply the microbes in a compost mass are carbon and nitrogen. Other nutrients including cobalt, manganese, magnesium and copper should also be present but in smaller amounts. Calcium can also be important to ensure that the pile can resist changes in acidity (pH). Nitrogen is probably the only nutrient which needs to be added to ensure that a suitable carbon/nitrogen ratio is achieved.
The nutrients must be present in a form that the microbes can use or digest. Some substances will be very resistant to breakdown by the composting process even under ideal conditions. These can include wood, straw and paper as well as feathers and shellfish.
Experience has shown that, with the exception of carbon and nitrogen, most organic wastes contain nutrients in the amounts and ratios required for composting. The ideal carbon/nitrogen ratio is 25 to 30 parts carbon to one part nitrogen. The composting process becomes increasingly slower as the ratio rises beyond this range. The carbon/nitrogen ratios for a number of organic materials are shown in table 1.