Sludge Conditioning

Two sludge conditioning techniques are commonly used in the industry.

• 'Chemical conditioning', which agglomerates tiny particles to form larger ones by coagulation to improve liquid-solid separation. The commonly used coagulants include Al2(SO4)3, FeCl3, lime, and organic polymers. These are also sometimes used in the precipitation process for the primary wastewater treatment.

'Heat treatment', in which the sludge is raised to high temperature (175-230 °C) under pressure (1000- 2000 kPa). This serves to improve the dewater-ing characteristics of sludge. Heat treatment is generally considered superior to chemical conditioning for dewa-tering. However, compared with chemical conditioning, the process has two notable disadvantages: (1) it is more complex in operation and maintenance and (2) it generates highly polluted cooking liquors.

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 etal. (1997)

From 1-3 to 10

Davis and Cornwell (1991)

Secondary treatment


Perry etal. (1997)

Inorganic sludges


Perry etal. (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)




Open aeration tank, settler,


Aerobic reactions by

Convert organic materials


effluent recycle system

heterotrophic biomass

to CO2, H2O, and inert materials


Covered tank with gas or

30(35-37 °C)

Hydrolysis by facultative and

Convert larger organic


mechanical mixer, settler (holding tank), effluent recycle system

Acid fermentation by acid-forming bacteria

Methane fermentation by methane bacteria

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

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