Page 250 - Materials Chemistry, Second Edition

CAT3525_C08.qxd 1/29/2005 10:03 AM Page 221
Composting MSW 221
2. Molecular weight of the compost product is

Carbon 14(12) 168
Hydrogen 20(1) 20
Oxygen 9(16) 144
Nitrogen 1(14) 14
Total 346
The kg-mol of finished compost per kg-mol at the start of the process is n 180 / (1.00)(346) 0.52
The following values are to be used in the calculations:
a 35 w 14
b 67 x 20
c 31 y 9
d 1 z 1
The r and s values are determined as

r 0.5{67 0.52 (20) 3[1 0.52(1)]} 27.58
s 35 0.52 (14) 27.7
4. From Equation 8.3, the quantity of oxygen required to complete the composting process is

W 0.5 [0.52(9) 2(27.7) 27.58 31] (1.00)(32) 906.6 kg
5. The above data can be checked with a materials balance.

Inputs
Substrate 1000
Oxygen 907
Total in 1907 kg

Outputs
Compost 180
CO 2 1.00(27.7)(44) 1218.8
H O 1.00(27.58)(18) 496.44
2
NH 3 [1 0.52(1)](1.00)(17) 8.16
Total out 1903 kg

Note: Since air (not O ) is being applied to the pile, its requirement is 907/0.232 3909 kg. This
2
value is equivalent to 3909/1000 3.909 kg air/kg substrate.
8.5.6 MOISTURE CONTENT

The preferred moisture content and oxygen availability for composting are closely interrelated. The
interstices in the waste feedstock will contain either water or air, so the presence of one will directly
affect the concentration of the other. The optimal moisture content for successful composting varies
depending on the physical state and size of the particles and on the composting system used.
Regular monitoring of the chemical and physical properties of the pile, previous experience with
MSW composting, along with a review of the literature should serve as a practical guide to opti-
mum moisture content.
Less moisture in the pile will result in dehydration, which slows biological processes. Water is
required for numerous cellular processes, and properties including nutrient transport, waste
removal, turgidity, and as a component in innumerable biochemical reactions. Excess water inter-
feres with aeration by clogging pores. If the moisture content of the mass is so high as to displace
most of the air from the interstices, anaerobic conditions develop within the mass. Therefore, the