A filter comprises a set of vertical, juxtaposed recessed plates, presses against each other by hydraulic jacks at one end of the set. The pressure applied to the joint face of each filtering plate must withstand the chamber internal pressure developed by the sludge pumping system.
The filter press is an intermittent dewatering process. Each press operation includes the following steps:
1- Closing of the press: as the filter is completely empty, the moving head activated by the jacks calmps the plates.
2- Filling: For sludge having good filter ability it is best to fill the filter very quickly so as to avoid the formation of a cake in the first chamber before the last ones have been filled.
3- Filtration: When the filtration pump has stopped, the filtrate circuits and central duct, which is still filled with liquid sludge, are purged by compressed air.
4- Filter opening: The moving head is drawn back to disengage the first filtration chamber. The cake falls has a result of his own weight. A mechanised system pulls out the plates one by one.
5- Washing: Washing of clothes should be carried out every 15-30 processing oprations. For mid- or large units this take place on press using water sprayers at very high pressure (80-100 bar).
The production capacity of a filter press is between 1.5 and 10 kg of solid per m2 of filtering surface. For every filter press model, the chamber volume and the filtering surface depend on the number of plates.
In practical terms pressing times are less then four hours.
Filtration time depends on:
- cake thickness
- sludge concentration
- specific resistance
- compressibility coefficient
One of the advantages of the filter press is that it can accept sludge with average filterability. It is always advantageous to optimally thicken sludge before filter press operations.
The filter press is suitable for almost all types of sludge:
Hydrophilic organic sludge: inorganic conditioning is often recommended to enable satisfactory cake release due to minimal aderence to filter cloth.
Hydrophilic inorganic sludge: the filer press generally requires the addition of lime only.
Hydrophobic inorganic sludge: it is very dense and ideal for the filter press. It is dewatered without any preliminary conditioning.
Oily sludge: the filter press can be used to treat sludge containing light oils, the presence of grease can sometimes impair the smooth running of the filter; clothes have to be degreased at frequent intervals.