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 diges tion. 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 com parison 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 diges ters have been constructed and installed in a number of sludge treatment plants, which are superior to the conventional cylindrical tanks shown in Figure 3. Two egg shaped anaerobic digesters are shown in Figure 11 .

Sludge blanket

Skimming device


Air-charge stream

Figure 10 Air flotation thickener.

Sludge hopper (hydrophobic solids)

Air flow

Air flow

Thickening & flotation zone


Hydrophilic solids

Air/sludge distributor

Table 13 Solid concentration achieved in thickening processes


Sludge source and classification

Solid concentration (%)

Data source (in further reading)

Gravity settler

Primary treatment (organic)


Perry et al. (1997)

From 1-3 to 10

Davis and Cornwell (1991)

Secondary treatment


Perry et al. (1997)

Inorganic sludges


Perry et al. (1997)


Not specified

From 0.5-1 to 3-6

Davis and Cornwell (1991)

General thickening

Not specified


Werther and Ogada (1999)

Table 14 Stabilization processes



Retention time (days) Mechanisms


Aerobic Open aeration tank, settler, 15-20

digestion effluent recycle system

Anaerobic Covered tank with gas or 30 (35-37 °C) digestion mechanical mixer, settler

(holding tank), effluent recycle system

Aerobic reactions by heterotrophic biomass

Hydrolysis by facultative and anaerobic bacteria

Acid fermentation by acid-forming bacteria

Methane fermentation by methane bacteria

Convert organic materials to CO2, H2O, and inert materials Convert larger organic compounds to soluble smaller ones

Convert small soluble compounds to volatile organic acid (acetic acid) Split acetic acid to CO2 and CH4, and produce CH4 from CO2+H2

Figure 11

Egg-shaped anaerobic digesters. Photo courtesy

Figure 11

Egg-shaped anaerobic digesters. Photo courtesy

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