Composting Process

Composting of wastewater sludge is an aerobic biothermal process that decomposes the organic constituents. It can be described by the formula

It can also be performed as an anaerobic process described by the formula

C6H12O6 = 2C2H5OH + 2CO2 + 27 kcal

The aerobic process provides a higher caloric content. It goes much faster than the anaerobic process, and decomposition of the organic constituents produces a stable humuslike material.

Microbiology Composting represents the combined activity of a succession of mixed populations of bacteria, actinomycetes, and fungi. Although the interrelationship of these microbial populations is not fully understood, it is known that bacteria are responsible for the decomposition of a major portion of the organic matter.

Aerobic Composting Process

Figure 7.1 Phases during composting.

Figure 7.1 Phases during composting.

Composting occurs in three successive phases: the mesophilic, thermophilic, and curing phases. In the initial mesophilic phase, the temperature of the composting pile increases from ambient to 40°C (104°F). In the thermophilic phase, the temperature increases from 40°C to 70°C (104°F to 160°F). In the final curing phase (also known as the cooling phase), the microbial activity is reduced, and the composting process is completed. Figure 7.1 shows the three phases as related to carbon dioxide respiration and temperature.

All three categories of microorganisms are active during all three phases. In the mesophilic phase, acid-producing bacteria metabolize carbohydrates, sugars, and proteins. In the thermophilic phase, thermophilic bacteria become active and metabolize proteins, lipids, and fats. They are also responsible for much of the heat energy produced. Actinomycetes and fungi are present at both the mesophilic and thermophilic stages and are responsible for the destruction of a wide variety of complex organic compounds and cellulose. In the final curing phase, the compost matures through further microbial activity into a stable product. Compost that is insufficiently mature will reheat in storage and upon rewetting.

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