Organic Content

In general, the higher the sludge's organic content, the greater the quantity of heat released during composting. This greater quantity of heat results in the thermophilic phase (55 to 65°C) being reached earlier in the composting process. Raw sludge typically contains 60 to 80% organic material while

digested sludge contains only 30 to 50% organic material. Since raw sludge (from primary clarifiers and secondary clarifiers) contains more organic material than digested sludge, it is reasonable to prepare compost from dewa-tered raw sludge. By utilizing raw sludge, the digesters, pipe, pumps, electrical power, personnel, and so on, normally utilized in the digestion process can be reduced or eliminated.

The composting process not only reduces the organic content in sludge by approximately 25% but also reduces the moisture content of the sludge. The reduction in moisture content is the result of the heat generated during the organic decomposition. The heat generated by composting 1.0 kg of organic material averages 21 MJ. Approximately 4.0 MJ of heat will evaporate 1.0 kg of moisture (taking into account heat losses and heating of the compost material). Thus, the composting of 1.0 kg of organic material facilitates the removal of approximately 5.0 kg of moisture from the residual (21 MJ/4 MJ/kg of water). Figure 7.12 illustrates the change of indexes during composting of

C/N Carbon-to-nitrogen ratio

NO3 Nitrate

NH4 Ammonia

K2O Potassium

N2 Nitrogen P2O5 Phosphate

W Moisture

VOL Volatile solids

C/N Carbon-to-nitrogen ratio

NO3 Nitrate

NH4 Ammonia

K2O Potassium

N2 Nitrogen P2O5 Phosphate

W Moisture

VOL Volatile solids

Figure 7.12 Changes of indexes in raw sludge composting.

dewatered raw sludge with the bulking agent sawdust at the ratio of 1 : 2 by volume (Chertes et al., 1988).

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