Sludge Stabilization

Sludge stabilization processes serve to break down the organic solids biochemically with the following two prime objectives:

• to make the sludge solids more stable (less odorous and less putrescible) and easier for dewatering and

There are two basic stabilization processes in current practice, namely aerobic digestion and anaerobic digestion. Characteristics of these two processes are listed in Table 14.

From Table 14, it can be seen that the aerobic digestion process is a continuation of the process carried out in the secondary wastewater treatment unit. The major difference between them is that the effluent (supernatant) from the settler must be recycled back to the head-end of the plant due to high concentrations of suspended solids and organic compounds.

In contrast to aerobic digestion, the mechanisms of anaerobic digestion in sludge treatment processes are very different from that in the secondary wastewater treatment, which is aimed at organic phosphorus removal. The differences can easily be identified through a comparison between Table 14 and Figure 6. As in the aerobic digestion process, the effluent (supernatant) must also be recycled back to the plant in order to meet water-quality requirements.

In recent years, egg-shaped anaerobic sludge digesters have been constructed and installed in a number of sludge treatment plants, which are superior to the conventional

Wastewater processing ■<-

Primary settler

Flotable solid removal

Grit chamber

Wastewater feed

Settled grit

Secondary settler

(Optional)

Tertiary settler

(Optional)

Tertiary settler

35 - 60% Solid

Ocean dumping

Land spreading

(Controversial)

Figure 9 A basic sludge-processing scheme.

Sludge blanket

Skimming device

Sludge hopper (hydrophobic solids)

Sludge

Air-charge stream

Figure 10 Air flotation thickener.

Air/sludge distributor

Skimming device

Sludge hopper (hydrophobic solids)

Sludge

Air-charge stream

Thickening & flotation zone

Air/sludge distributor

Thickening & flotation zone cylindrical tanks shown in Figure 3. Two egg-shaped anaerobic digesters are shown in Figure 11.

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