Process Description 721 Factors Influencing Composting

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Composting represents the combined activity of a succession of mixed populations of bacteria, actinomycetes, and fungi at different stages of the process. The principal factors that affect the biology of composting are moisture, temperature, pH, nutrient concentration, and oxygen supply.

Moisture Decomposition of organic matter depends on moisture. Less than 40% moisture may limit the rate of decomposition. The optimum moisture content is 50 to 60%. Moisture content is also important for the structural integrity and sufficient porosity of the composting pile. If the initial compost mixture has more than 60% moisture, proper structural integrity will not be achieved and the mixture will not decompose well.

Dewatered municipal sludges are usually 18 to 35% solids (65 to 82% moisture), depending on the type of dewatering equipment used. Such sludge cakes are too wet for composting. Mixing the cake with a dry bulking material can reduce the moisture content of the sludge cake. Table 7.1 lists some of the typically used bulking agents and their characteristics. Figure 7.3 shows the effect the solids content of dewatered sludge cake has on the required mixing ratio of wood chips to sludge by volume. This illustration is site specific and the curve will shift depending on the relative volatility and solids contents of the sludge and bulking material. In this illustration, the amount of wood chips needed for sludge with 18% solids is about three times the amount required for a 35% sludge cake. High-solids sludge cake will reduce the amount of bulking materials required and the material management costs for mixing and screening the bulking materials. However, sludge cake of 30 to 35% solids may not break into small clumps uniformly when mixed with bulking agents. This necessitates more sophisticated mixing equipment, which can add cost and maintenance requirements. If the mixture is not homogeneous, airflow through the composting materials will not be uniform, allowing some zones to become anaerobic and cause odor. A site-specific economic analysis or pilot testing on a given feed stock with various bulking agents and blending strategies should be made to determine the best approach for a particular composting facility.

Temperature For the most efficient operation, composting process depends on temperatures of 50 to 65°C, but not above 70°C. For best results, temperatures should be maintained between 50 and 55°C for the first few days and between 55 and 60°C for the remainder of the composting process. Temperatures above 65 to 70°C for a significant period of time are detrimental to microbial activity. However, thermophlic microflora consists of bacteria whose temperature activity can oscillate between 60 and 75°C. Moisture content, aeration rates, size and shape of pile, atmospheric conditions, and nutrients

TABLE 7.1 Characteristics of Bulking Agents

Bulking Agent


Wood chips

Typically, will be purchased.

High recovery rate in screening (60 to 80%).

Good source of supplemental carbon.

Shredded bark

Typically, will be purchased.

Medium recovery rate in screening (50 to 70%).

Good source of supplemental carbon.

Chipped brush

Possibly available as a waste material.

Low recovery rate in screening (40 to 60%)

because of high percentage of fines.

Good source of supplemental carbon.

Long curing time for compost because of continued

decomposition of unrecovered fines.

Sawdust and ground

Possibly available as a waste material.

waste lumber

Good source of supplemental carbon if relatively


Poor source of supplemental carbon if old and

extremely dry because more volatile forms of

carbon are missing.

Very low recovery rate, resulting in a large volume of


Leaves and yard waste

Available as waste materials.

Must be shredded.

Wide range of moisture content requires close

attention to material balance.

Readily available source of supplemental carbon.

Relatively low porosity.

Not recoverable, resulting in a large volume of


Shredded tires

Often mixed with other bulking agents.

Provide no supplemental carbon.

Nearly 100% recoverable.

May contain metals.

Agricultural residues

Availability is regional.

such as rice hulls

Possibly available as a waste material.

Good source of supplemental carbon.

Very low recovery rate.

Source: Adapted in part from WEF, 1998.

Source: Adapted in part from WEF, 1998.

influence the distribution of temperature in a composting pile. For example, if excessive moisture is present, heat will be carried off by evaporation, and thus the temperature rise will be less. On the other hand, low moisture content will decrease the rate of microbial activity, thereby reducing the rate of heat evolution and retention.

Bio Solids Affect The Local Populations

10 20 30 40 50

Figure 7.3 Effects of solids content on the ratio of wood chips to sludge by volume.

10 20 30 40 50

Percent Solids in Sludge

Figure 7.3 Effects of solids content on the ratio of wood chips to sludge by volume.

pH The pH of composting mixture should generally be in the range 6 to 9. Optimum pH range for the growth of most bacteria is between 6 and 7.5 and between 5.5 and 8 for fungi. Although the pH in the pile varies throughout the composting period, it is essentially self-regulating. It is difficult to alter the pH for optimum biological growth. Therefore, it has not been found to be an effective operational control. The effect of pH is described further later in the chapter.

Nutrient Concentration Both carbon and nitrogen are required as energy sources for the growth of microorganisms. Approximately 30 parts by weight of biodegradable carbon are used by microorganisms for each part of nitrogen. Therefore, the most desirable carbon-to-nitrogen ratio in the composting mix is in the range 25 : 1 to 35 : 1 by weight. Lower ratios increase the loss of nitrogen by volatilization as ammonia, resulting in the loss of nutrient value of the compost and the emission of ammonia odor. Higher ratios lead to progressively longer composting time, and the organic material remains active well into the curing stage (Poincelot, 1977).

The carbon-to-nitrogen ratio of wastewater sludge is generally in the range 20 : 1 to 40 : 1. Therefore, sludges with lower than a 25 : 1 ratio require additional biodegradable carbon for active microbial growth. Bulking agents and amendments provide the supplemental carbon and improve both the energy balance and the carbon-to-nitrogen ratio. In wood chips, only a thin layer of wood is available as carbon. Carbon in amendments such as sawdust is more readily available.

Oxygen Supply Oxygen concentration in the composting mass should be maintained between 5 and 15% by volume of gas mass. Although in the windrow process oxygen concentrations as low as 0.5% have been observed without anaerobic symptoms, a minimum of 5% is generally required for aerobic conditions. Oxygen concentrations higher than 15% will result in a temperature decrease because of the higher airflow. Oxygen should reach all parts of the composting materials for optimum results, especially in in-vessel systems.

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