FILTER PRESSES

Description

Filter presses are used to remove water from sludge to reduce its volume, and therefore reduce transport and disposal costs. A filter press consists of a series of vertically oriented filter plates, of up to 2 m2 in area, which can be compressed manually, pneumatically, or electrohydraulically. Sludge is fed into the press through a central bore to fill each of the plate cavities and is then squeezed to reduce its liquid content. The pressure is then released and as the filter plates move apart the dry sludge cake drops from the press.

Filter press plates can be constructed from cast iron, polypropylene or fibreglass-reinforced polyester. In a chamber filter press, each plate has a chamber lined with a filter bag made from woven cloth or needle felt. The first solids fed into the cloth bag become trapped in the material to form a barrier to other solids. In a membrane filter press, flexible polypropylene membranes are pressurised with compressed air to squeeze the sludge.

The cake can be discharged with or without its filter cloth. Where the cloth remains in place, it is washed automatically. Filter presses can range from skid-mounted units to large 180 tonne installations.

Application

Filter presses have applications in dewatering sewage sludge, water treatment sludge and industrial sludges. Industrial sludges typically arise from biological treatment plants, mines, chemical plants, power stations, tanneries and the food, ceramic and metal industries.

Filter presses are generally the most cost effective method of producing a relatively dry filter cake with a high solids throughput. Different sludges can be dewatered with different efficiencies, but filter presses generally produce sludge cakes in the range of 30-50 % dry solids. They are able to capture up to 99 % of solids in a sludge and are candidates for sludges that are difficult to dewater by other methods. Membrane filter presses are able to produce a sludge cake of consistent dryness from a sludge feed of variable composition. Filter presses can be used for the dewatering of sludge for incineration.

Sizing

A typical electrical consumption for a filter press is about 1 kWh per cubic metre of sludge to be dewatered. Filter presses are rated between 1 and 15 kW. The feed pump accounts for about 80 % of the electrical consumption; the remainder is due to the hydraulics for opening and closing the filter, a motor for separating the plates and an air compressor.

Filter presses generally comprise 50-180 plates of 009-4 m2 capable of exerting pressures of 700-2500kPa; dewatered sludge cakes are usually 30-50 mm thick.