Environmental Microbiology of Aquatic and Waste Systems

292 11 The Disposal of Municipal Solid Wastes

When the curing stage is complete, the compost is
considered “stabilized” or “mature”. Any further
microbial decomposition will occur very slowly.

Microbiology of Composting
Composting proceeds through a succession of non-
actinomycete mesophilic bacteria, followed by actino-
mycetes, then fungi; the simplest substrates are
metabolized quickly, while substrates which are more
complex or difficult to metabolize remain. In the com-
posting of grass straw, the sugars from cell wall poly-
saccharides are degraded initially, while the less
available cellulose is decomposed in the latter stages
of composting.
In the first stage of composting (see “A” in Fig. 11.6),
the temperature rises from ambient into the thermo-
philic range. Mesophilic bacterial populations, (mainly
non-actinomycete) multiply very rapidly utilizing
simple and readily available substrates.
The activity of the mesophilic organisms (non-
actinomycete bacteria followed by mesophilic fungi)
generates heat which is retained within the compost
pile. As the temperature rises to about 45°C, meso-
philic populations die off and thermophilic bacterial
populations begin to flourish (B in Fig. 11.6).
Thermophilic non-actinomycete bacteria, thermophilic
fungi, and thermophilic actinomycete in that order
become dominant.
In the third stage (see “C” in Fig. 11.6), the tem-
perature drops as the activity by thermophilic declines
because they have used up available food in the
compost.
In the fourth and final stage (see “D” in Fig. 11.6),
the curing stage, the temperature once again returns to
the ambient and mesophilic bacteria increase, some-

times even beyond their initial original numbers. The

mesophilic fungal and actinomycete populations grad-

ually attack the more complex materials such as cel-

lulose and lignin. Overall, microbial activity drops

progressively to very low rates,

Some of the mesophilic bacteria encountered in

compost are Alcaligenes faecalis, Bacillus brevis, B.

circulans complex. Among the thermophilic bacteria

are: Bacillus subtilis, B. polymyxa, B. pumilus, B.

sphaericus, and B. licheniformis, B. stearothermophi­

lus, B. acidocaldarius, and B. schleglii, B. circulans

complex types i and ii, and B. subtilis.

Mesophilic actinomycetes include Actinobifida

chromogena, Microbispora bispora, Micropolyspora

faeni, Nocardia sp., and Streptomyces rectus, while

some thermophilic actinomycetes are Thermomonospora

sp., and Thermoactinomyces vulgaris

Mesophilic fungi include: Absidia corymbifer, A.

ramose, Mortierella turficola Mucor miehe, M. pusil­

lus, and Rhizomucor sp., while thermophilic fungi are

Aspergillus fumigatus, Chaetomium thermophile,

Humicola lanuginosa, Mucor pusillus, Thermoascus

aurantiacus, and Torula thermophila. Of these, only

Chaetomium thermophile has been found to be

cellulolytic.

11.3.1.5 Landfills Used in the Disposal of MSW

An MSW landfill is not simply a hole in the ground

where refuse is dumped. Rather it is a carefully engi-

neered structure built into the ground (where a natural

valley exists) or on top of the ground (if the terrain is

flat) in which trash is isolated from the surrounding

environment, especially groundwater, but also from rain

and the surrounding air. This isolation is accomplished

10 A

B C

TIME

pH

Temperature

Thermophilic

bacteria

bacteria

Mesophilic

Mesophilic fungi

Thermophilic fungi

Thermophilic actinomycetes

D

5

6

7

8

pH

Log

10

No of Organisms

9

10

25

40

TEMPERATURE,

C

55

Fig. 11.6 Microbial 70
groups dominant at each
temperature regime during
composting (Modified
from Palmisano and Barlaz
1998 )
Stages in the production
of compost: A, mesophilic
stage; B, thermophilic
stage; C, cooling stage;
D, maturation stage