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Polychlorinated biphenyls (PCBs) total

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aMust not be used on crops grown for direct human consumption (i.e., crops consumed without processing to minimize pathogens). Can be used on food chain crops and other agricultural and horticultural uses. Must not exceed 10 mm (0.39 in.) particle size.

6Must be restricted to use on nonfood chain crops. Must not exceed 25 mm (0.98 in.) particle size. Source: New York State, Municipal Energy Recovery Facilities Handbook, New York State Environmental Facilities Corporation, Albany, 1988.

aMust not be used on crops grown for direct human consumption (i.e., crops consumed without processing to minimize pathogens). Can be used on food chain crops and other agricultural and horticultural uses. Must not exceed 10 mm (0.39 in.) particle size.

6Must be restricted to use on nonfood chain crops. Must not exceed 25 mm (0.98 in.) particle size. Source: New York State, Municipal Energy Recovery Facilities Handbook, New York State Environmental Facilities Corporation, Albany, 1988.

use of the compost. A typical listing of permissible metal concentrations in compost is presented in Table 3.16. For pathogen reduction purposes, the temperature of the mixture must be maintained at or above 131 °F (55 °C) for at least three consecutive days.

The total composting time, including curing, is determined by the material, process used, and exposure to the elements. Two weeks to as much as 18 months may be required for complete stabilization and curing of the compost. Thus, a plant location distant from habitation is recommended, as odors may become a problem. Also, because the demand for compost may be seasonal, provision must be made for compost storage.

Composting Process

Composting involves the biological decomposition of organic materials (substrates) under controlled conditions that allow for the development of an end product that is biologically stable and free of viable pathogens and plant seeds and can be applied to land beneficially. The key concepts and objectives contained in the definition of compost are as follows:

• Composting is a biological process (e.g., aerobic anaerobic).

• Composting results in the production of a biologically stable end product.

• The end products free of viable pathogens.

• The end product is free of viable plant seeds.

• The end product can be applied to land beneficially.

FIGURE 3.20 Overview of windrow composting operation: (a) preparation (size reduction) of yard waste for composting and (b) processed waste placed in windrows to undergo the composting process (see also Figure 3.21).

To meet the above objectives, the composting process, as illustrated in Figure 3.20, usually involves the following three basic steps:

1. Preprocessing (e.g., size reduction, seeding, nutrient addition, and addition of bulking agent)

2. Decomposition and stabilization of organic material (two-stage process comprised of a first-stage high-rate phase followed by second-stage curing phase)

3. Postprocessing (e.g., grinding, screening, bagging, and marketing of compost product)

Composting of mixed solid waste should be preceded by a separation and recycling program, including glass, plastic, and metal separation; then usually shredding or grinding; and a program for the periodic collection of household hazardous waste. Industrial and other hazardous waste must be excluded.

The two-stage decomposition and stabilization of organic solid waste to a compost process can be described by the following reaction:

Proteins Amino acids

As shown by this reaction, essentially all of the organic matter with the exception of cellulose and lignin are converted during the compost process. It should be noted that, in time, both the cellulose and lignin will undergo further biological decomposition, primarily through the action of fungi and actinomycetes.

Postprocessing will typically include screening and nutrient and other amendment additions, depending on the application. Many municipalities make the compost available to the residents for a nominal price.

Composting Technologies

The three composting methods used most commonly in the United States are (1) windrow, (2) aerated static pile, and (3) in-vessel methods. It should be noted that over the past 100 years more than 50 individual compost processes have been developed. The more important of these processes based on function and/or the type of reactor used for the process are summarized in Table 3.17. Some of the processes are described next.

Although many process variations are in use, odor control is a major concern in all processes. Aeration and controlled enclosed processing facilities can be used to minimize the problem. Provision must also be made for vector control, leachate collection, and the prevention of groundwater and surfacewater pollution. The stabilized and cured compost may be ground but is usually screened before sale. Storage space is required.

Lipids Carbohydrates Cellulose Lignin Ash

+ O2 + Nutrients + Bacteria ^ Compost + Newcells + CO2 + H2O + NO- + SO-2 + Heat

(Principal components comprising the organic fraction of MSW)

(Principally cellulose, lignin, and ash)

TABLE 3.17 Municipal Composting Systems Grouped by Function or Reactor Configuration

Function or Configuration

Commercial Process

Heaps and windrows, natural aeration, batch operation

Indore/Bangalore

Organic Gardeners Composting

Organic Gardeners Composting

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