General Background Considerations

The following parameters affect the aerobic digestion process:

1. Nature and characteristics of the sludge

2. Rate of sludge oxidation

3. Sludge age

4. Sludge loading rate

5. Temperature

6. Oxygen requirements

Generally, the majority of VSS digestion in the aerobic process occurs during the first 10 to 15 days of aeration. Figure 7.44.1 shows both the VSS reductions for a variety of sludges and the major portion of the reduction that occurs during the initial 10 days. The mixed pulp and paper waste sludge is most resistant to digestion (typical of paper mill sludge due to their high content of lignins and cellulose material).

Figures 7.44.3 and 7.44.4 show that maximum stabilization occurs within the 15 days of digestion. Figure 7.44.4 also reflects the increased reduction of volatile matter with increasing temperature.

The sludge age is defined as the ratio of the weight of VSS in the digestor to the weight of VSS added daily. The maximum sludge age is when no significant reduction occurs in the concentration of VSS. Laboratory studies indicate that VSS removal efficiency correlates with sludge age. One such study, summarized in Figure 7.44.5, was conducted at organic loadings between 42 and 112lb per 1000 ft3 and detention times of 15 to 30 days. The waste was a mixture of activated sludge and primary sludge. The following equation describes the relationship between the sludge age and VSS removal efficiency:

FIG. 7.44.3 Effect of detention time on aerobic digestion of activated sludge.
FIG. 7.44.4 Temperature effects on aerobic digestion.

%VSS Reduction = 2.84 + 35.07 log10(sludge age) 7.44(3)

Specific oxygen uptake rates (gm O2 used per gram VSS per day) vary with detention time. Figure 7.44.6 shows the data reported for excess activated sludge.

FIG. 7.44.5 VSS reductions as a function of sludge age.
FIG. 7.44.6 Typical oxygen requirements for the aerobic digestion of sludge.

4. Efficiency of VSS reduction required

5. Solids loading rate

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