A volume reduction of the sludges with relatively low solid matter contents encountered during water treatment is a requirement for the economical treatment and disposal of sludges. Therefore, during expansion of anaerobic sludge stabilisation plants, new methods are increasingly being applied that provide for mechanical pre-thickening of the excess sludge. At present, there is no safe database describing the influence of the higher solid matter concentration and the resulting increased raw sludge viscosity on the sludge treatment. This paper introduces various results of a targeted survey regarding this subject characterising differences in the methodology and operation during sewage sludge treatment between plants with and without separate mechanical excess sludge thickening. It turns out that plants that have a mechanical excess sludge pre-thickening unit are more frequently subjected to mechanical and apparatus-related operation problems, although in practice the increase of the solid matter concentration in the raw sludge is only 20–25% compared to conventionally operated plants.

The maximum achievable solid matter content in the thickened excess sludge - and therefore that in the raw sludge - is especially dependent on the choice of the mechanical pre-thickening unit and the application of a flocculant. At the same time, plants with mechanical pre-thickening of excess sludges achieve a higher gas yield and degree of stabilisation. On the whole the results of the survey lead to the conclusion that mechanical pre-thickening of the excess sludges is better applied for the training of sludge treatment plants, which show sludge solid matter contents of less than 4% DSM (dried solid matter) under normal operating conditions.

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