This stage consists in mechanically dehydrating the sludge until it is reduced to 15-30% dry matter.
The sludge extracted from the latest treatment facilities has a dry content of 20-50g per liter and,
aside from subsequent treatment through wet oxidation, all other types of treatment require a higher dry content. This concentration takes place either on a fixed unit in facilities serving over 3,000 population equivalents or on a mobile unit, which travels around between several rural plants. The investment burden, which would be prohibitively expensive for any one of these authorities to bear on its own, can thus be shared.