Gemwater is able to provide any type of solution relevant to sludge thickening and dewatering. The aims of this treatment are to increase the solids content and to reduce the volume of free water, which gives the following advantages:
• reduced cost for sludge disposal due to reduced volume;
• minimized the unit load on downstream processes such as digestion and dewatering.
Thickening and dewatering are carried out in different stages, depending on the degree of humidity required.
Sludge thickening can be obtained by:
3. Chemical conditioning associated with the above systems.
1. The sludge suspension is fed into a special tank (thickener) with floor slope where it remains for a long time, so that the sludge can become compacted and then extracted from the bottom, while the interstitial liquid is drawn off the top.
Thickeners can be static or mechanized. The static ones are usually constructed as conical or pyramidal tanks, where the thickening is just obtained by water compression and residence time. The mechanical ones are normally round shaped and equipped with sludge thickening arms.
2. Flotation: the purpose of flotation-thickening is to inject small air bubbles to suspended solids and cause the solids to separate from the water in an upward direction (solid particles have a specific gravity lower than water when bubbles are attached). The sludge suspension is fed from the bottom and the thickened sludge is collected to the surface. This system has the following advantages:
• smaller surface and volume
• higher concentration of thickened
3. Even after thickening, the sludge still contains a big proportion of water (90-98%); if a lower humidity is required, other treatment steps need to be provided. This is the purpose of conditioning, which may be either chemical or thermal.
Chemical conditioning is usually performed through mineral or organic agents, such as ferric chlorides, ferrous sulphates (both eventually associated with lime dosages), polyelectrolyte.
Thermal conditioning (heating between 150-200°C for approx. 30-90 min) destroys colloidal gels, disinfects the sludge and the water can be recirculated at plant’s inlet.
Release of the residual water can be obtained by gravity drainage of sludge on drying beds or by mechanical means.
Drying beds are filled with sand-gravel on which the sludge is spread and left to drain/evaporate for a certain time. By simple drainage a max of 20% solid-in water-sludge can be achieved.
Mechanical dewatering can be achieved by:
• Filter press: separation process used to separate liquids and solids that utilizes the principle of pressure drive,
provided by a slurry pump. A filter press is fed into the machine, after which a moving plate clamps a series
of filter plates together.
• Centrifugal decanter: solids particles are accelerated by fast rotation thanks to the centrifugal forces to the
periphery of a cylindrical or conical separator, from where is removed by a screw conveyer or blades.
It is a very compact system.
• Belt press: dewatering technology that applies mechanical pressure to a chemically conditioned slurry, which
is sandwiched between two tensioned belts. Typically, a belt filter press receives a slurry ranging
from 1-4% feed solids and produces a final product of 12-35% cake solids.
• Vacuum filter: works on the principle that inside a drum rotating in the sludge is created a vacuum, and consequently
sludge is deposited on the drum surface and water is sucked inside. The moisture content in the sludge cake varies
with the type of sludge, typically from 80-84% (for raw activated sludge) to 60-68% (for well digested primary sludge).