DoTT: Reduction of sludge volume – Sludge dewatering

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In sludge dewatering mainly mechanical forces such as centrifugal force, external pressure, vacuum, etc., are involved. Such forces are generated in dewatering devices such as:

Decanter centrifuges

operating with the use of centrifugal force are increasingly popular. The concentrated sludge is fed through a pipe into a conical, horizontal centrifuge drum. Inside the drum a sludge collecting screw is installed, rotating in the same direction as the drum, but with lower rotation speed. Sludge in the form of a filter cake deposits on the walls of the drum due to centrifugal force and is collected and moved by a screw towards the conical part, where is being removed through a special hole. The liquid (filtrate) is filtered through drum perforation and removed outside through pipe system. These types of centrifuges work automatically, continuously and the feed of concentrated sludge to the centrifuge automatically regulated. Before being fed to the centrifuge, the concentrated sludge is usually treated with a flocculant in a special tank to achieve a better dewatering effect.

Filtration chamber presses

use the external pressure of the pump feeding the thickened sludge to the press chambers. The maximum pressure that still provides filtration is usually 6 bars. Above, this value the process is stopped, filtration is not carried out. The chamber filter press consists of filter plates covered with a suitable filter cloth. Between the plates empty spaces are created – so called chambers – that have a specific so-called filtering surface. Depending on the number of plates, the filtration surface of a given press is larger or smaller. The plates are usually made of PP, and the compression elements are two steel end plates, which are driven by a hydraulic system of the press. The filtration process involves feeding concentrated sludge into chambers between the plates. The clean filtrate is discharged through the canvases through a system of channels in the plates and discharged outside. The sludge increasing the quality and reuse of sewage sludge remains in the chambers and after filtration, the plates are moved manually or mechanically (depending on the press) and the sludge falls off the plates and is collected under the press through a hopper directly to the transporting system, e.g. a screw conveyor on to a heap, to a truck or big-bag. The proper selection of canvas (structure) is very important in chamber presses. It affects the filtration rate and dryness of the sludge. Standard plate dimensions are 800 x 800 mm and 1200 x 1200 mm. Filtration goes better when adding concentrated flocculant to the sludge.

Belt presses

the principle of operation is based on mixing the sludge with polyelectrolyte and spreading it on a horizontal, porous filter belt. The pre-concentrated sludge is then compressed through a second filter belt and subject to a further dewatering process through a series of rollers with a decreasing diameter. The adjustable wedge drainage zone initiates the gradual introduction of sludge into the high pressure zone. The final dewatering process is achieved through a roller system, working with a minimum of 180° wrapping of the belt. These types of presses guarantee high dewatering rate and consume a relatively low amount of flocculant.

Main parameter: dry matter content level

The dewatering process is the next step of sludge treatment. The sludge after dewatering contains about 13-40% of dry matter. Such sludge may be directed to further thermal treatment in various temperature conditions to get rid of toxic compounds, both organic and inorganic, or transferred directly to the incineration plant, for combustion at high temperatures resulting in heat and / or electricity recovery.

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