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aMaterials listed as recyclable are those for which large-scale markets exist in areas where the recycling industry is well developed.

bDerived from Table 10.3.1. Currently recycled materials are not included. cA substantial portion of this category is readily recyclable, and a substantial portion is not. Some of the material listed here as nonrecyclable can be recovered in recyclable condition by an efficient ferrous recovery system at a combustion facility.

aMaterials listed as recyclable are those for which large-scale markets exist in areas where the recycling industry is well developed.

bDerived from Table 10.3.1. Currently recycled materials are not included. cA substantial portion of this category is readily recyclable, and a substantial portion is not. Some of the material listed here as nonrecyclable can be recovered in recyclable condition by an efficient ferrous recovery system at a combustion facility.

A portion of finished MSW compost cannot be marketed and must be landfilled.

In MSW discharged from compactor trucks, most glass containers are still in one piece, and most metal cans are uncrushed. Most glass and aluminum beverage containers are in recyclable condition. Many glass food containers and steel cans are heavily contaminated with food waste, however. Some of the recyclable paper in MSW received at disposal facilities is contaminated with other materials, but 50% or more is typically in recyclable condition.

The ratio of carbon to nitrogen (C/N ratio) is an indicator of the compostability of materials. To maximize the composting rate while minimizing odor generation, a C/N ratio of 25/1 to 30/1 is considered optimum. Higher ratios reduce the composting rate, while lower ratios invite odor problems.

Table 10.5.2 shows representative C/N ratios of com-postable components of MSW. Controlled composting of food waste, with a C/N ratio of 14/1, is difficult unless large quantities of another material such as yard waste (other than grass clippings) are mixed in to raise the ratio. The C/N ratio moves above the optimum level as quantities of paper are added to the mixture, however.

Paper, leaves, and woody yard waste serve as effective bulking agents in composting MSW, so the addition of a bulking agent such as wood chips is generally unnecessary.

The metals content of MSW is a major concern in composting because repeated application of compost to land can raise the metals concentrations in the soil to harmful levels. Compost regulations usually set maximum metals concentrations for MSW compost applied to land. Most regulations do not distinguish between different forms of a metal. For example, the lead in printing ink on a plastic bag is treated the same as the lead in glass crystal even though the lead in printing ink is more likely to be released

TABLE 10.5.2 REPRESENTATIVE C/N RATIOS OF COMPOSTABLE COMPONENTS OF MSW

Waste Category

C/N Ratio

Yard waste

29/1

Grass clippings

17/1

Leaves

61/1

Other yard waste

31/1

Food waste

14/1

Paper

119/1

Newspaper

149/1

Corrugated & kraft

165/1

High-grade paper

248/1

Magazines & mail

131/1

Other paper

85/1

Disposable diapers

95/1

Fines

23/1

Note: Derived from Table 10.3.4.

Note: Derived from Table 10.3.4.

into the environment. Similarly, the hexavalent form of chromium found in lead chromate is treated the same as the elemental chromium used to plate steel even though the hexavalent form is more toxic than the elemental form.

Two extensive, recent studies of metals in individual components of MSW yielded contradictory results. A study in Cape May County, New Jersey found toxic metals concentrated in the noncompostable components of MSW (Camp Dresser & McKee Inc. 1991; Rugg and Hanna 1992). A study in Burnaby, British Columbia, however, found higher metals concentrations in the compostable components of MSW than were found in Cape May (see Table 10.3.6) (Rigo, Chandler, and Sawell 1993).

Disposable diapers are listed as compostable in Table 10.5.1 despite their plastic covers. The majority of the weight of disposable diapers is from the urine, feces, and treated cellulose inside the cover, all of which is com-postable. Note, however, that most people wrap used diapers into a ball with the plastic cover on the outside, using the waist tapes to keep the ball from unraveling. Vigorous size reduction is required to prepare these diaper balls for composting.

Wood is biodegradable but does not degrade rapidly enough to be considered compostable. The same is true of cotton and wool fabrics, included in the textiles/ rubber/leather category in Table 10.5.1.

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