Filter presses for sludge treatment

Filtration is the most widely used method in the treatment of sludge produced by wastewater treatment. It can consist just in dreinage though sand beds or it can be mechanical under vacuum middle or high pressure conditions which require more sophisticated equipment.
Filter presses operate applying very high pressures to the cake (from 5 to 15 bars and sometimes even more).

Plate filter presses

This pressing technique is the most widespread despite its intermittent operation and its high investiment cost.

Description

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.
This vertical plate layout forms watertight filtration chambers allowing easy mechanisation for the discharge of cakes. Filter chlotes finely or tightly meshed are applied to the two groowed surfaces in these plates.
Orifices feed the sludge to be filtered under pressure in the filtration chamber. They are usually placed in the center of the plates allowing a proper distribution of flow, right pressure and better drainage of sludge within the chamber. Solids sludge gradually accumulates in the filtration chamber until the final compacted cake is formed. The filtrate is collected at the back of the filtration support and carried away by internal ducts.

Filtration cycle

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. Closing pressure is self regulated through filtration.

2- Filling: During this short phase chamber are filled with sludge for filtration. Filling time depends on the flow of the feed pump. For sludge having good filterability 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: Once the chamber have been filled continuous arrival of sludge to be dewatered provokes a rise in pressure due to the formation of an increasingly thick layer of filter sludge on the cloths. This filtration phase can be stopped manually, by a timer or more convenientely by a filtrate flow indicator which issues a stop alarm when the end of filtration rate has been reached. When the filtration pump has stopped, the filtrate circuits and central duct, which is still filled with liquid sludge, are purged by compressed air.

3- 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 pulles out the plates one by one. The speed of plate separation can be adjusted to account to the cake texture.

4- Washing: Whasing 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). Whasing is sincronized with separation of plates.

Filtration capacity

The production capacity of a filter press is somewhere between 1.5 and 10 kg of solid per m² of filtering surface. For every the filter press model the chamber volume and the filtering surface depend on the number of plates in the filer.

In practical terms pressing times are less then four hours.
Filtration time depends on:

- cake thikness
- sludge concentration
- specific resistance
- compressibility coefficient.

One of the advantages of the filer press is that it can accept sludge with average filterability. It is always advantageous to optimally thicken sludge before filter press operations. Although sludge presenting a high filterability enables better production capacities, a filter press still accepts sludge with low conditioning precision. This tolerance means that the device offers greater owerall operational safety.

Performance

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.

More information about sludge treatment and reduction are on the following pages:

Stabilisation
Thickening
Dewaterig
Sludge drying
Copious growth of filamentous organisms - problems and solutions
Centrifugation and centrifuges

Source: Water treatment handbook, Degremont 1991