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Sludge dewatering employs a variety of different processes, such as centrifugation or pressing, to reduce the sludge moisture content by mechanical means. The energy consumption is higher than that associated with thickening processes and the moisture content of the processed product correspondingly lower.
Dewatering processes provide a concentrated, consolidated product − retaining most of the solids from the original sludge – along with a diluted stream which is predominantly water.
Dewatering processes apply a significant mechanical force to achieve increased water removal over that possible from thickening.
A belt filter press provides sludge dewatering by pressing the sludge to force the water through a permeable medium.
The process produces a cake (the dewatered product) having a dry solids (DS) content of 30% or more in the case of primary sludge. This is to be distinguished from the lower-energy gravity belt thickener process which achieves a maximum DS content of ~10%.
A screw press provides sludge dewatering by conveying the sludge along the inside of a permeable cylinder. It is based on a slowly-rotating (~5 RPM) Archimedean screw within a cylindrical screen.
It is normally inclined by ~20° to the horizontal to assist with the draining of water into the sump. The screen generally has an aperture rating below 0.5 mm and is based on wedge-wire or occasionally perforated metal.
The filter press is the only mechanical thickening or dewatering technology that is limited to batchwise (rather than continuous) operation.
It is also the only dewatering technology capable of routinely achieving high concentrations of the dewatered solids cake product – between 35 and 45% dry solids depending on the feed sludge origin and the chemical conditioning applied.
A rotary press is a dewatering technology that removes water by passing the sludge through a relatively narrow, rotating parallel-flow channel with porous walls.
It is relatively low in energy demand and is a completely enclosed system, providing very good odour control, as well as low noise levels. The technology is well established, having first been implemented for sewage sludge dewatering in the mid-1990s.
Using centrifugation for dewatering demands more energy than for thickening. This is because more water must be removed to achieve the higher solids concentrations.
The dewatered product, whose dry solids content may be as high as 50%, takes the form of a cake: a deformable semi-solid which forms lumps rather than being a free-flowing fluid − this means it can only be transported using a conveyor belt.
Sludge drying beds provide sludge dewatering by allowing the liquid to both drain under gravity through a permeable medium on which the sludge sits, and to evaporate under ambient conditions.
Lagoons provide dewatering through evaporation alone.
Electro-dewatering (EDW) technology provides enhanced removal by non-mechanical means through the application of an electric field.
EDW of sludge removes water from the sludge through the principle of electro-osmosis, where movement of a solid relative to the surrounding liquid can take place on the basis of the charge at the solid−liquid interface when a direct electric field is applied.
