DoTT: Reduction of sludge volume – Sludge thickening

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Sludge thickening at WWT plants aims at sludge volume reduction and free water removal. This process can be chemically aided e.g. with polyelectrolytes acting as flocculants resulting in generation of bigger and heavier agglomerates, that facilitate decantation thru e.g. gravity forces.

Both gravity and mechanical forces (e.g. centrifugal force or external pressure) are engaged in the thickening process. Devices that use gravity forces include:

Settling tanks

 (e.g. Dorr type) with a conical bottom, in which the decanted sludge is periodically scraped off with a low-speed “rake” scraper, towards the hole located in the centre of the conical bottom and pumped out for further processing. This type of settling tanks usually has a diameter of a few to several meters, depending on the size of the treatment plant. Due to their size, this type of settling tanks is made of reinforced concrete, protected with a special anti-corrosive coating.

Tanks for consolidated sludge

in which sludge decantation also occurs, and from which sludge – like in case of decanters – is periodically pumped out for further treatment. These types of tanks have a diameter of few meters and are made either of special stainless steel, e.g. 316 L or carbon steel, protected with an appropriate anti-corrosion coating. Tanks made of plastic, e.g. PP or polyester resins, e.g. derakane type with adequate mechanical strength are becoming increasingly popular.

Sedimentary plots

the hydrated sediment is poured into pools properly protected against leachate penetration into the soil, in which the decantation takes place, the sediment settles and the water is pumped from the surface using e.g. leverage.

Belt thickener

built of a filter tape slowly rotating between the rollers, where water is drained and the sediment is pushed out of the tape. The thickener is an enclosed device allowing the odours to be sucked out. Special movable baffles are built on the belt. Their task is to evenly feed the sludge to the belt. In order to obtain a better thickening effect, flocculant (polyelectrolyte) is added to the hydrated sludge in a special mixing tank. The construction of such a thickener is made of 304 or 316 stainless steel due to corrosive environment.

Drum thickener

uses mechanical forces, its operating principle is that hydrated sludge is introduced into the interior of the perforated drum, covered with filter cloth. The drum rotates very slowly for not to disintegrate / destroy the sediment structure. Water from the sludge is drained through filter cloth. Processing or recycled water can be used for the thickening process. As a rule, before the sludge enters the thickener drum, a flocculant in the form of a polyelectrolyte is first introduced into the hydrated sludge in a special small reactor-mixer. The drum thickener is usually made of 304 or 316 stainless steel due to corrosive environment. Works in a closed housing, often with odours suction available.

Main parameter: dry matter content level

Sludge thickening causes slight dehydration. The dry matter content level of 2-7% can be achieved depending technology used. 5-7% dry matter content can be obtained on drum or belt thickeners only with addition of flocculant in polyelectrolyte form. Such sludge is still a slightly concentrated solution and is mainly directed to further treatment, i.e. dewatering process.

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