Page 248 - Materials Chemistry, Second Edition
P. 248

CAT3525_C08.qxd  1/29/2005  10:03 AM  Page 219
                       Composting MSW                                                              219

                                     TABLE 8.2
                                     Carbon/Nitrogen Ratios of Various Wastes and Materials

                                     Material                      C:N
                                     Sawdust                       200-500:1
                                     Wheat straw                   125-150:1
                                     Grass clippings               12-20:1
                                     Corn stalks                   60:1
                                     Humus                         10:1
                                     Activated sludge              6:1
                                     Cow manure                    18:1
                                     Horse manure                  25:1
                                     Poultry manure                15:1
                                     Food scraps                   15:1
                                     Mixed MSW                     50-60:1
                                     Adapted from Diaz, L. F. et al., Composting and Recycling Municipal Solid
                                     Waste, 1994. Reproduced with kind permission of Lewis Publishers, an imprint
                                     of CRC Press. Boca Raton, FL.



                       8.5.5 AERATION
                       The atmosphere within the interstices of the composting mass will vary significantly during decom-
                       position. When the organic feedstock is delivered to the compost site, the oxygen supply available
                       to microbes occurs from the diffusion of ambient air, and in air originally trapped within the voids.
                       However, the rate of diffusion of ambient air into the mass is very limited; hence, interstitial air is
                       the major source of oxygen.
                          At the outset, the composition of air in voids is similar to that of ambient air (i.e., approximately
                       20.9% O and 0.03% CO , v/v). Within a short time (hours to days), however, the heterotrophic pio-
                                           2
                              2
                       neer communities become activated and begin decomposition of the raw organic substrates, with the
                       concurrent increase in CO content and decrease in O level. With a closely monitored composting
                                                                  2
                                            2
                       system, the oxygen content can be varied from 15 to 20% and CO from 0.5 to 5% for the process
                                                                            2
                       to be successful.
                          During aerobic respiration, organic chemicals are oxidized to carbon dioxide and water or other
                       end products using molecular oxygen as the terminal electron acceptor. Aerobic respiration occurs
                       under highly oxygenated conditions. The reaction for the aerobic oxidation of a glucose molecule is
                                              C H O   6O → 6CO   6H O   energy                    (8.1)
                                                 12
                                               6
                                                                  2
                                                          2
                                                                       2
                                                    6
                       If the O concentration falls below approximately 15%, facultative anaerobic microorganisms are
                             2
                       activated and rapidly become dominant. Fermentation and anaerobic respiration reactions take over.
                       Undesirable products such as acetic acid, ethanol, methane, and ethane will form. These are odor-
                       iferous and may inactivate beneficial compost microorganisms:
                                              C H O → 2C H OH   6CO   6H O                        (8.2)
                                                          2
                                                                            2
                                                            5
                                                                      2
                                               6
                                                 12
                                                    6
                                              glucose   ethanol
                       Additional reactions under anaerobic conditions are described later.
                          The decomposition of organic materials is significantly faster and more complete in the pres-
                       ence of oxygen. The energy available in Equation 8.1 is approximately 14 times greater than that
                       for anaerobic decomposition of glucose (Equation 8.2) (Zubay, 1983).
   243   244   245   246   247   248   249   250   251   252   253