Sludge dewatering in a Wastewater Treatment Plant

Sludge dewatering involves separating the solid phase (consisting of organic matter such as microorganisms and waste resulting from their metabolism) from the liquid phase of the sludge.

To achieve optimal dehydration, it is necessary to first thicken the sludge. This thickening can be done naturally by settling at the bottom of the clarifier or by mechanical methods. 

After thickening, the sludge remains in a liquid state with a dryness level of about 10%, meaning a water content of 90%. 

The second phase, dewatering, is carried out mechanically, where water is "forced" to separate from the solid part. This mechanical dehydration results in a pasty sludge with a dryness level that can reach 40%, and up to 80% with solar drying.

Sludge extraction is necessary for the proper operation of wastewater treatment plants and for cost optimization. Indeed, if the sludge volume increases in the aeration basin, energy consumption for oxygenation will need to increase proportionally, thus raising operational costs.

For optimal purification efficiency, it's important to have bacteria of all ages, and if the sludge rate increases, the sludge age will also increase, leading to a decrease in purification efficiency.

Finally, if the sludge quantity increases too much in the basin, there is a risk of sludge discharge into the natural environment.

The goal here is to facilitate the separation of the liquid and solid phases through a preliminary conditioning of the sludge.

To achieve this, the sludge is mixed with polymers that ensure better separation between the solid and liquid phases. 

Flocculants in aqueous solution mixed with the sludge will, thanks to their positive or negative charges, form bonds with the sludge particles.

These bonds, along with a very high molecular weight, help accelerate the liquid/solid separation and form flocs that can settle more quickly.

Depending on the type of sludge, a full range of polymers can be used, which will be charged positively or negatively depending on whether the sludge is more organic or mineral. 

After this treatment step, the polymer-mixed sludge will undergo various treatments to extract as much water as possible. 

The first step is sludge thickening, which can be done naturally at the bottom of the clarifier by settling (without flocculant) or mechanically on a drainage table, for example (with flocculant). 

After this treatment, the sludge remains relatively liquid or pasty.

Belt Filters press

After thickening, the sludge is then poured between two rotating filter cloths that are progressively brought together to increase the contact pressure, allowing water to be removed through the cloths. The dehydrated sludge is recovered at the end of the belt filter for valorization.

Screw Presses

The screw press also requires thickened sludge. The screw slowly turns within a perforated cylinder, gradually compressing the sludge, a compression made possible by the formation of a dehydrated sludge plug at the screw's outlet. The water extracted from the sludge percolates through the perforated cylinder to return to the head of the plant.

Decanter centrifuges

Centrifugation involves applying a centrifugal force to the sludge to accelerate its settling, resulting in two phases: the sediment (dehydrated sludge) and the centrate (supernatant liquid). The material deposited on the inner wall is continuously scraped off by a rotating screw.

Filter press

The plate filter press is the most efficient mechanical filtration and dehydration process that allows dryness levels of 30 to 80% to be achieved. It is used for the dehydration of sludge from industrial and urban effluents.

Geotube

Geotubes are made from a specially designed textile that confines fine solids inside the "sack" while allowing water to percolate through the textile. As the water flows out, the solids continue to densify and consolidate over time.