Sludge stabilization, sludge digestion

Sludge stabilization aims the reduction of energy-rich, unstable and easily degrading sludge compounds. A further aim of the sludge stabilization is the decrease of high-water content. Additionally, the minimisation in the number of pathogens should be mentioned as a secondary goal of stabilization.

There are three main methods of sludge stabilizing:
- chemical
- thermal
- biological: aerobic and anaerobic

In the chemical sludge stabilization, the pH of the sludge is usually increased to around 12 by addition lime. This inhibits the biodegradation.

As a thermal method is being used e.g. combustion, which mostly involves, that the sludge has been already dewatered or dried.

The aerobic sludge stabilization takes place either simultaneously or separately. In the simultaneous process, the wastewater treatment takes place together with the stabilization of the raw sludge, under the condition of solids retention time of at least 25 d. In the separate form, the primary and excess sludge is thickened separately from the wastewater treatment and stabilized for approx. 10-15 days.

The anaerobic method means sludge digestion by stabilizing of the sludge under mesophilic conditions.


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