A problem with composting raw sludge as compared to digested sludge is the higher-intensity odor that can be released due to the higher percentage of organic material in raw sludge. Various methods can be used to control the odor, but the method favored is the addition of quicklime (CaO) to change the pH of the sludge. Experiments show that organic material loses its odor when the pH is raised from the typical 5.5 to 6.5 to a pH of 10.0 to 10.5.

The hydration of the quicklime (absorbing moisture from the sludge) causes the quicklime to release heat to the composting mixture. During the process of hydrating 1 kg of chemically pure quicklime (100% CaO), 320 g of moisture is absorbed from the sludge and 1152 kJ of heat is produced. This release of heat shortens the time span of the mesophilic phase (25 to 40°C) and drives the process to the thermophilic phase (55 to 65°C) quicker, resulting in an overall reduction in the composting time.

In addition to raising the pH, absorbing moisture, and releasing heat, the quicklime also binds with heavy metals that might be present in the residual. This binding of the heavy metals provides a better environment for the growth of the microorganisms needed for the composting process.

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