Page 255 - Materials Chemistry, Second Edition
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                       226                       Waste Management Practices: Municipal, Hazardous, and Industrial
                       the blower is controlled. The blower can be run continuously or intermittently. Continuous opera-
                       tion permits lower airflow rates because oxygen and cooling are supplied constantly. Intermittent
                       operation of the blower is achieved with a timer or temperature feedback system.

                       8.6.4 IN-VESSEL SYSTEMS

                       In-vessel systems are relatively sophisticated units in which composting is conducted within a fully
                       enclosed chamber. Environmental conditions are mechanically controlled and usually automated.
                       An in-vessel system may be suitable for MSW composting if:

                           ● The process must be finished rapidly
                           ● Odor and leachate control are a significant concern
                           ● Space is limited

                          In-vessel technologies range from simple to complex. Two categories of in-vessel technologies
                       include rotating drums and tank systems. Rotating drums rely on a tumbling action to mix contin-
                       uously feedstock materials. Figure 8.8 illustrates a rotating drum composter. The drums typically
                       are long cylinders, approximately 3 m (10 ft) in diameter which are rotated slowly, usually at less
                       than 10 r/min (CRS, 1989). Oxygen is forced into the drums through nozzles from air pumps. The
                       tumbling action allows oxygen to be maintained at high and uniform levels throughout the drum.
                          Tank systems are available as horizontal or vertical types. These tanks are long vessels in which
                       aeration is accomplished through the use of external pumps that force air through the perforated bot-
                       tom of the tanks. Mixing is accomplished by mechanically passing a moving belt, paddle wheel, or flail-
                       covered drum through the feedstock. The agitation breaks up clumps and maintains porosity. Solids are
                       retained in this system for 6 to 28 days and then cured in windrows for 1 to 2 months (U.S. EPA, 1994).


                       8.7 THE CURING STAGE
                       Once the materials are adequately stable, they must be cured. Oxygen uptake and CO evolution
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                       measurements will indicate the degree of maturity of compost. One method to measure pile matu-
                       rity is to monitor the internal temperature of the compost pile after it is turned. If reheating of the
                       pile occurs, then the material is not yet ready for curing.
                          During the curing stage, compost is stabilized as the remaining microorganisms metabolize the
                       remaining available nutrients. For the duration of the curing stage, microbial activity diminishes as
                       available nutrients are depleted. Curing is a relatively passive process compared with the primary
                       composting operation, so less intensive methods and operations are used. In general, materials are
                       formed into piles or windrows and left until the specified curing period has ended (Figure 8.9).



















                       FIGURE 8.8 A drum composter (Bedminster Bioconversion Corporation).
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