Composting Methods

There are three classifications of sludge composting:

• Windrow process

• Aerated static pile process

• In-vessel processes

In the windrow process, the dewatered sludge is agitated periodically for aeration. Agitation also controls the temperature and produces an end product that is uniform. In the aerated static pile process, air is blown through the materials using a blower. In-vessel composting systems are proprietary systems in which composting is performed in a reactor with air forced through the reactor, but with or without periodic agitation. In-vessel systems are enclosed processes. Windrow and aerated static pile processes are not enclosed, but a roof may be provided to protect the compost from precipitation.

The fundamental process of each composting method is similar (see Figure 7.2). The basic steps are as follows:

• A bulking agent is added to the dewatered sludge for porosity and moisture control. Amendments, if needed, are also added in this step to supplement limiting nutrients such as carbon. The bulking agents and amendments can be a wide variety of materials, such as wood chips, ground bark, yard waste, sawdust, wood ash, peat, agricultural residues such as rice hulls, and composted sludge. The mixture must be porous, structurally stable, and capable of self-sustaining the biothermal decomposition reaction.

• Next, a high-rate decomposition step must take place in which the mixture is aerated by mechanical turning, by providing air with a blower, or by both. A temperature in the thermophilic range 40 to 70°C should be attained. This also ensures destruction of pathogenic organisms and reduces the moisture content by evaporation.

• A storage and curing step that allows the compost to further stabilize and cool to lower temperatures.

• If a bulking agent is used, it can be recovered at the end of the high-rate decompositions step or the curing phase. Some compost may be recycled as a bulking agent. Additional air drying may be required between the high rate decomposition and curing step if the compost is too wet for screening.

Generalised Schematic Composting
Figure 7.2 Generalized schematic of composting.

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