(best examples and good practices) on household organic waste ...

71
The basic system is composed of a top roller that presses down on to two
bottom rollers <strong>and</strong> the drums rotate to facilitate the passage of the material on
a conveyor belt (Orsat et al., 1999). This system is well adapted to combining
with electroosmotic dewatering. larger groove angles can help to further reduce
the moisture content during roller pressing of sugar cane bagasse.
Screw press dewatering
In a screw press, the material is introduced in a perforated chamber where an
endless screw forces the material along the length of the chamber towards the
discharge. The pressure force of the screw drives the water out through the
perforations of the holding chamber. For this type of dewatering process, the
waste feed must have a certain particle size large enough not to clog the
perforations of the holding system <strong>and</strong> to flow through without excessive
resistance.
Rotary <strong>and</strong> centrifugal presses
A centrifugal dewatering system consists of a basket or a solid bowl <strong>and</strong> a
conveyor, both of which can rotate at high speed. As the bowl rotates, the
heavier solids gravitate to the bowl wall where they accumulate. The separation
of solids from the liquid depends on the G-force, time <strong>and</strong> permeability of the
waste mass (Leung., 1998).
Membrane filter press
A membrane filter press comprises a stack of filter plates held tightly closed by
pressure. The filter plates have a filtration drainage surface that supports a filter
media, in most cases a polypropylene filter cloth held in place by a more rigid
polypropylene structure. The mixed solid–liquid waste is pumped into the
chambers under pressure. The filtered liquid passes through the filter cloth,
against the drainage surface of the plates, <strong>and</strong> is directed towards discharge
collectors. The pressure gradient between the cake <strong>and</strong> the filter material
provides the driving force for the flow. Solids are retained on the filter cloth
forming a filter cake. The filter plates are separated <strong>and</strong> the filter cake is
discharged. At this stage a vacuum step may be introduced to further reduce
the moisture content. In a study by (El-Shafey et al. 2004), brewer’s spent
grain was dewatered to a low moisture level of 20–30% when combining
membrane filter pressing (500 kPa) with vacuum drying.
Electroosmotic dewatering
Electroosmosis is caused by the electrical double layer that exists at the
interface of suspended particles subjected to an applied voltage across a solid–
liquid mixture. In waste slurries, the solid particles possess a slight electric
charge known as the zeta potential. Hence, when exposed to an electric field,
the charged particles <strong>and</strong> the liquid fraction are entrained to move in opposite
directions: one towards the anode, the other towards the cathode (Orsat et al.,
1996). On the one h<strong>and</strong>, electrophoresis is the movement of charged particles