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SLUDGE THICKENING / DEWATERING TECHNOLOGY
- Picket Fence Thickener
Sewage sludge suspended solids carry electrostatic charges that attract water causing the solids and water to aggregate into large particles that are referred to as floccs. The water incorporated into the floccs is referred to as 'bound' water and the water in the spaces between the floccs is referred to as 'free' water. Floccs containing sewage solids and water are denser than water and therefore settle under gravitational force, displacing free water as gravitational settlement reduces the spaces between the floccs. A picket fence thickener comprises a series of vertical rods supported on a structure that spans the consolidation tank. As the picket fence slowly rotates about the centre of the tank a series of vertical voids are produced which permit the passage of separated water and micro-bubbles released from the floccs to rise to the surface.
- Gravity Belt Thickener
The gravity belt thickener consists of an 'endless' horizontal filter belt on which thickening takes place in two phases; gravity drainage and compression. The sludge must be conditioned with polyelectrolyte prior to introduction to the belt to form large stable floccs and a clear separation between the solid and liquid phases (flocculation). The flocculated sludge is then fed onto a continuously moving horizontal belt that is supported by a polythene grid. As the belt progresses free water drains from the sludge and through the weave of the belt. A series of ploughs fitted on the upper side of belt the turn the sludge to enhance gravity compression and also create a series of clear areas to assist with drainage. At the discharge end of the belt a small inclined ramp turns the sludge back on itself causing a further compression effect, which expels additional water from the sludge, which again drains through the weave of the belt.
- Filter Belt Press
The filter belt press is a further development of the gravity belt thickener and utilises two endless belts between which a sludge layer is compressed and subjected to increasing pressure as the belts pass through the machine. The initial stage of the machine is as the gravity belt thickener to allow for gravity drainage of the free water content of the sludge. At the end of the gravity drainage section an upper belt is introduced in a compression zone in which the two belts gradually come together to form a belt/sludge/belt sandwich and applying an increasing pressure/shear force. The belts then progress through the machine around a series of progressively decreasing diameter rollers that gradually increase the pressure and shear force applied to the sludge. At the discharge of the machine the two belts part allowing the dewatered sludge cake to be discharged. As with the gravity belt thickener, the two belts are continuously washed during operation
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