Your web browser is out of date.
Update your browser for better security, speed and to get the best experience on this website.
Thickening processes remove water from the sludge to reduce its volume by mechanical means at relatively low energy consumption rates. Processes used include sedimentation, dissolved air flotation, centrifugation and draining through a belt or a rotating drum.
Thickening processes concentrate the sludge by removing part of the free water so that the finished product retains the liquid, free-flowing characteristics of the feed sludge and so can still be conveyed by pumping.
Thickening processes are those which retain most of the solids from the original sludge – along with a diluted stream which is predominantly water.
The finished product retains the liquid, free-flowing characteristics of the feed sludge, so that it can still be conveyed by pumping. Thickening normally increases the dry solids (DS) content to 4−6%.
Gravity thickening increases the solids concentration by allowing the particles to settle to the base of a vessel, producing a thickened solids stream at the vessel base and a supernatant stream at the surface.
Sedimentation is the simplest of the thickening processes. A gravity sludge thickener has the same design features as a primary sedimentation tank used for wastewater treatment, and most often has cylindrical geometry.
Dissolved air flotation (DAF) thickens the sludge by encouraging solids to float to the surface.
DAF is applied to increase the sludge solids concentration when the solids are neutrally buoyant (i.e. neither sink nor float) and so cannot be readily removed by settling. This is usually the case for waste activated sludge which demands a relatively low solids loading rate of 20−30 kg dry solids per m2 tank area per day (kgDS/(m2d)) if treated by gravity thickening, compared to almost five times this rate for DAF.
Gravity belt thickening increases the sludge solids concentration by allowing the water to drain from the sludge under gravity through a permeable medium (a moving belt) on which the sludge sits.
There are two types of belt-based processes for increasing sludge solids concentration. Gravity belt thickening is a low-pressure process which operates by allowing the water to drain from the sludge under gravity. A belt filter press, on the other hand, operates by squeezing water from the sludge under pressure, and achieves a higher dry solids concentration as a result.
In rotary drum thickening, the sludge solids concentration is increased by agitating the solids in a slowly-rotating vessel with porous walls though which the water drains.
In a rotary drum thickener (RDT), water drains from the sludge through a retaining porous medium. For an RDT, the porous medium is the cylindrical wall of a 0.5−1.5 m diameter drum which rotates at speeds between 5 and 20 RPM while the sludge continually passes through it.
Centrifugal thickening thickens the sludge by encouraging the particles to migrate to the walls of a rapidly rotating cylindrical vessel under the influence of a centrifugal force.
There are a number of configurations of centrifuge used for reducing the water content of sludge. They can be used for both thickening and dewatering. Configurations include solid bowl, basket, and disc-nozzle.
In principle, sludge can be thickened using membrane technology − the membranes provide separation of suspended solids from liquid though a highly-selective permeable polymeric or ceramic material. Water is extracted through the membrane, allowing the sludge solids concentration to increase accordingly.
Membranes provide 100% suspended solids capture since the solids are completely rejected by the sub-micron pores of the membrane. The quality of the filtrate, i.e. the extracted water, produced is therefore extremely high.
