Page 268 - Materials Chemistry, Second Edition

CAT3525_C08.qxd 1/29/2005 10:03 AM Page 239
Composting MSW 239
2. Acid generation. The low-molecular-weight amino acids, fatty acids, and monosaccha-
rides are converted into lower molecular weight intermediate compounds by the action
of nonmethanogenic, acetogenic bacteria, also known as acid formers. Acidogens are fac-
ultative and obligate anaerobes. Products include carbon dioxide and hydrogen and a
number of organic acids and alcohols including acetic acid (CH COOH), propionic acid
3
(CH CH COOH), butyric acid (CH CH CH COOH), and ethanol (C H OH).
3
2
5
3
2
2
2
An example of an acid-forming reaction is shown below:
C H O → 2C H OH 2CO 2 (8.9)
12
6
6
2
5
3. Methanogenesis. The acids and alcohols produced in phase 2 are converted into methane
and carbon dioxide by the action of methanogenic bacteria, which are strict anaerobes.
Methanogenesis reactions include the following:
CH COOH → CH CO 2 (8.10)
4
3
2C H OH CO → CH 2CH COOH (8.11)
5
2
4
3
2
CO 4H → CH 2H O (8.12)
2
4
2
2
As we shall see in Chapter 10, these identical reactions occur in a cell of a sanitary landfill, with
the potential for production of enormous volumes of methane gas.
8.12.1 PROCESS DESCRIPTION
There are four basic steps involved in high-solids anaerobic digestion of MSW to produce methane:
1. Initial processing. Commingled MSW is received at the facility, sorted to obtain the
organic fraction, then shredded.
2. Preparation and conditioning. In order to prepare a material suitable for biological pro-
cessing, moisture and nutrients are added, the input is blended, pH is adjusted to near-
neutral, and the slurry mix is heated to between 55 and 60°C.
Moisture and nutrients are added to the wastes in the form of wastewater sludge or ani-
mal manure. Additional nutrients may also have to be added, depending on the chemical
composition of the added wastewater or manure.
Anaerobic digestion is typically conducted in a continuous-flow reactor whose contents
are completely mixed. In some operations, a series of batch reactors are used. Foaming
and the formation of surface crusts have caused problems in MSW digestion; therefore,
adequate mixing is extremely important in system operation.
3. Recovery, storage, and separation of the gas components.
4. Dewatering and disposal (or application) of the digested sludge.

8.12.2 SUBSYSTEMS
Several subsystems have been developed to treat MSW anaerobically. The categories include
(Chiang Mai University, 1998):
Batch vs. continuous

● Batch — The reactor vessel is loaded with raw feedstock and inoculated with digestate
from another reactor. It is sealed and allowed to react until degradation is complete. The
digester is emptied and a new input of organic mixture is added.