Compost Process Design and Operational Considerations

The principal design considerations associated with the aerobic biological decomposition of prepared solid wastes are presented in Table 3.18. It can be concluded from this table that the preparation of a composting process is not a simple task, especially if optimum results are to be achieved. For this reason, most of the commercial composting operations that have been developed are highly mechanized and are carried out in specially designed facilities. Because of their importance, pathogen and odor control are considered further below. Additional details on the design and operation of compost processes may be found in Refs. 13 and 16.

Pathogen Control Pathogenic organisms and weed seeds exposed to the higher temperatures for the times indicated in Table 3.19 will be destroyed. However, because of the nature of solid waste, the processes used, and the

TABLE 3.18 Important Design Considerations for Aerobic Composting Process

Item

Comment

Particle size

Carbon to nitrogen (C/N) ratio

Blending and seeding

Moisture content

Mixing/turning

Temperature

Control of pathogen

Air requirements pH control

Degree of decomposition

Land requirement

For optimum results, the size of solid wastes should be between 25 and 75 mm (1 and 3 in).

Initial C/N ratios (by mass) between 25 and 50 are optimum for aerobic composting. At lower ratios ammonia is given off. Biological activity is also impeded at lower ratios. At higher ratios, nitrogen may be a limiting nutrient.

Composting time can be reduced by seeding with partially decomposed solid wastes to the extent of about 1-5% by weight. Wastewater sludge can also be added to prepared solid wastes. Where sludge is added, the final moisture content is the controlling variable.

Moisture content should be in the range between 50 and 60% during the composting process. The optimum value appears to be about 55%.

To prevent drying, caking, and air channeling, material in the process of being composted should be mixed or turned on a regular schedule or as required. Frequency of mixing or turning will depend on the type of composting operation.

For best results, temperature should be maintained between 122 ° and 131 °F (50 ° and 55 °C) for the first few days and between 131 ° and 140 °F (55 ° and 60 °C) for the remainder of the active composting period. If temperature goes beyond 151 °F (66 °C), biological activity is reduced significantly.

If properly conducted, it is possible to kill all the pathogens, weeds, and seeds during the composting process. To do this, the temperature must be maintained between 140 ° and 158 °F (60 ° and 70 °C) for 24 hours.

The theoretical quantity of oxygen required can be estimated using the stiochiometrc equation for the conversion of organic matter. Air with at least 50% of the initial oxygen concentration remaining should reach all parts of the composting material for optimum results, especially in mechanical systems.

To achieve an optimum aerobic decomposition, pH should remain at 7-7.5 range. To minimize the loss of nitrogen in the form of ammonia gas, pH should not rise above about 8.5.

The degree of decomposition can be estimated by measuring the reduction in the organic matter present using the chemical oxygen demand (COD) test. Another measurement that has been used to determine the degree of decomposition is the use of respiratory quotient (RQ)

The land requirements for a plant with a capacity of 50 tons/day will be 1.5-2.0 acres. The land area required for larger plant will be less.

Source: G. Tchobanoglous, and F. Kreith, Solid Waste Handbook, 2nd ed., McGraw-Hill, New York, 2002; and U.S. EPA, Composting of Municipal Solid Wastes in the United States, EPA, Washington, DC, 1971.

TABLE 3.19 Temperature and Time of Exposure Required for Destruction of Some Common Pathogens and Parasites

Organism

Observations

Salmonella typhosa

No growth beyond 46 °C; death within 30min. at

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