Page 252 - Materials Chemistry, Second Edition

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Composting MSW 223
8.5.8 PH
The optimum pH range in composting is so broad that difficulties due to an excessively high or low
pH level are rarely encountered. Organic materials of a pH range from 3 to 11 can be composted.
Optimum values, however, are between 5.5 and 8. Recall from above that bacteria are the key cat-
alysts in organic matter transformations, and typically prefer a near-neutral pH. In contrast, fungi
develop better in an acid environment.
During the early stages of composting the pH level normally decreases (perhaps to as low as
5.0) because of the production of organic acids (e.g., formic, acetic, and pyruvic). These acids serve
as substrates for succeeding microbial populations. As the acids are decomposed, pH rises and often
stabilizes at approximately neutral. In some cases, compost pH may reach as high as 8.5.
Because it is unlikely that the pH will drop to inhibitory levels, there is no need to buffer the
feedstock by adding liming materials (e.g., limestone, calcium hydroxide). The addition of lime
should be avoided because it can lead to excessive losses of ammonium nitrogen. The lime does,
however, promote the formation of aggregates which in turn improves physical properties such as
air and water movement.
The pH of the compost pile is commonly measured using a standard glass electrode pH meter.
A known mixture, for example, a 2:1 ratio of water/solids is mixed and the resultant pH is read on
the meter.

8.6 THE COMPOSTING STAGE

After MSW feedstock is preprocessed it is introduced into the composting operation. At this time,
indigenous microorganisms actively decompose the feedstock; most of the physical and chemical
changes to the feedstock occur during this stage.
The actual compost process can be established in a number of environments, from simple out-
door piles to sophisticated reaction vessels with controlled temperature, airflow, and humidity.
Some popular composting methods are:

Open systems
● Turned piles
● Turned windrows
● Static piles using air blowing or suction
Closed systems
● Rotating drums
● Tanks

All systems are designed and operated to establish optimum conditions for composting. These
conditions directly influence the growth and metabolism of the microorganisms that are responsi-
ble for the process. The factor that can be most influenced by technology, around which compost-
ing designs are developed, is the availability of oxygen.

8.6.1 TURNED PILES
Turned piles are a widely used method for composting MSW due to their simplicity of operation.
As the name implies, the feedstock is mixed periodically using a front-end loader or similar equip-
ment. Turning of the feedstock maintains oxygen, moisture, and temperature at adequate levels for
microbes. The outer layers are incorporated within, where they are exposed to higher temperatures
and more intensive microbial activity. The frequent turning allows for the introduction of oxygen
and also releases excess heat from the center of the pile. Turning therefore promotes uniform