Sludge separation − filtration
Filtration theory defines the way in which water flows through the bed, which is formed of the sludge solids (cake)
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Gravity belt thickening (GBT) thickens sludge by allowing the water (or filtrate) to drain from the sludge under gravity through a permeable medium (a recirculating conveyor belt) on which the sludge sits. The sludge solids concentration is increased along the length of a porous belt as water drains through it, the sludge solids forming a layer on the belt surface.
Filtration theory defines the way in which water flows through the bed, which is formed of the sludge solids (cake)
An overview of sludge thickening − an introduction to theory and related technologies
Processes which act to remove water from the sludge to reduce its volume are called thickening and dewatering
GBT thickens the sludge by allowing the water to drain from the sludge under gravity through a permeable moving belt
A video to demonstrate the workings of the Alfa Laval AS-H gravity belt thickener which can be used for municipal sludge, and industrial sludge that flocculates.
Centrifugal thickening increases sludge concentration by encouraging particles to migrate to the walls of a rapidly rotating
Dissolved air flotation thickens sludges by encouraging the solids to float to the surface where they can be skimmed off
Gravity thickening increases the solids concentration by allowing the particles to settle to the base of a vessel
Membrane thickeners separate suspended solids from liquid by extracting the water through a selective permeable material
Rotary drum thickening increases sludge solids concentration by agitating the solids in a cylindrical drum with porous walls
Conditioning is the pretreatment of sludge upstream of thickening and dewatering to promote solid-liquid separation
Thickening processes concentrate the sludge by removing part of the free water, such that the product can still be pumped
Dewatering processes mechanically remove a significant proportion of the sludge water to produce a 15-45% cake product
In sludge drying the sludge moisture is removed as water vapour by heating, generating a pelleted or powdered product
Sludge stabilisation − alkaline stabilisation, lime and solids dosing, plus anaerobic and aerobic digestion
Thermochemical methods are used to either significantly reduce the sludge solids content or pre-treat sludge upstream of AD
Sludge is the main waste stream from the treatment of wastewater