Filter presses for sludge drying in wastewater treatment plants

April 5, 2024 (Reading 11 mins)
Jordi Fabregas

1. Introduction to sludge drying

A common objective in most wastewater treatment plants is to reduce the volume of sludge generated in both primary and secondary treatment as much as possible.

There are several technologies for dewatering these sludges, the most economical being mechanical systems, among which the following stand out: centrifugal decanters, belt filters, screw presses and filter presses, the latter being the ones that can achieve higher degrees of dewatering, in addition to having lower energy and reagent costs.

Drying process% dryness mudEnergy consumptionReagent consumption
Centrifugal decanter18 - 23Very highVery high
Band filter18 - 22UnderVery high
Screw press22 - 26UnderVery high
Filter press25 - 50Very lowUnder

The filter press system is the one that achieves the highest sludge dryness, but, unlike the other systems, it has discontinuous operation.

Filter presses have various applications in the industrial field, and in many cases, the dehydrated sludge obtained can be marketed, as in the following cases:

  • Ceramic industries: It is used in the ceramics industry to improve the qualities of porcelains, grey, marbles, quartz and pigments.          
  • Mining industries: In mining, the filter press is used to dewater coal slurries and other minerals, so that dry solids of appreciable purity are obtained. It is used as part of the process of metal extraction, such as iron, lead, copper, zinc, gold, etc.
  • Food industries: In this type of industry, it is used to purify and improve the quality of syrups, syrups, beers, wines, yeasts and sugars, among other products.
  • Pharmacy: In the pharmaceutical industry, filter presses are commonly used to separate liquids, blood serum being the most important of all, as it serves as raw material for the synthesis of various drugs.
  • Chemical: In the chemical industry this type of filter is used to concentrate products and separate residues.

In this article we will focus on the use of filter presses for the treatment of sludge produced in wastewater treatment plants.

What is a filter press?

A filter press is a machine mounted on a support structure that houses a set of chambers with filter cloths inside. The sludge is pumped by means of a pump, which is usually pneumatic, to avoid operating problems due to the large amount of solids present in the liquid to be filtered.

A filter press consists of:

Marking or filter press frame

It is the structure that forms and supports the whole filter press.

Filter chamber

The chambers, which are concave in shape and are opposed to each other, are the spaces between the filtering plates. They act as a mold for the sludge that forms inside them. The plate pack is kept closed and sealed at high pressures due to the action of an electro-hydraulic unit that uses a central piston to pressurize the chamber pack.

The plates can be of various dimensions. The most common range from 80 x 80 cm to 250 x 250 cm. The construction materials are usually plastic, such as polypropylene.

Filter presses usually operate at pressures in the order of 6 bar, but there are also high pressure equipment up to 16 bar, and even higher, whose objective is to achieve the highest possible dehydration. The number of chambers depends on the volume of sludge to be obtained in each drying process and is usually between 12 and 50.

To calculate the number of chambers required, simply divide the amount of sludge to be dewatered by the volume of a chamber, taking into account the density of the sludge and the concentration that can be achieved with the filter. This concentration value is obtained in laboratory tests and depends on each type of sludge.

Filter cloths or media

These fabrics are normally made of plastic materials (PP, polyester, polyamide, etc.). These fabrics are permeable to the passage of liquid and act as filters retaining suspended solids.

The sludge concentration increases with increasing pressure, up to the design limit obtained in the laboratory.


A collector is a part of the filtration system that is used to separate solids from liquids in the filtration process. In the context of a filter press, the collector may refer specifically to the system or components that collect the filtrate (the liquid that has passed through the filter media) after the suspension (the mixture of solid and liquid) has been pressed through the filter plates.

3. Sizing of a filter press for sludge drying.

We develop an example with the following data:

  • Sludge inlet flow rate = 1.25 m3/h. - Sludge inlet MS = 3%.
  • Sludge density: 1.1 Kg/l. - Sludge production hours: 8 h/day.

We calculate the quantity of DM and liquid, based on the daily flow of discharges:

  • Q = 1.25 m3/h x 8 h/d = 10 m3/d=> 10 x 1.1 = 11,000 kg/d. of sludge.
  • The solids content of these sludges is 3%.
  • DM = 11000 kg/day x 3% = 330 kg/day of sludge.
  • While the rest is liquid fraction = 11,000 - 330 = 10,670 kg/d of liquid to be drained.
  • If the sludge cakes reach a dryness of 30% => Daily sludge production = 330/0.3 = 1100 kg/day of sludge dewatered by the filter press.
  • Of which SS = 330 kg/day and drained liquid = 10670 kg/day.

In summary, the u of sludge generated from cake is obtained as follows:

  • If the sludge cake has a density d = 1.4 Kg/l, the volume of sludge cake generated from 30% sludge will be = 1100 Kg/day / 1.4 Kg/l = 785.7 liters/day.
  • The filter press must remove a cake volume equal to 785.7 liters in one day.

Let us assume that the filtration system operates 8 h/day and that a filtration process (including filling, filtering, opening and various options) takes 4 h. Under these conditions, 2 filtration operations will be necessary to dewater the sludge produced in one day, and each drying will produce = 785.7/2 = 392.9 liters of sludge at 30 % dryness.

Filter press volume = 785.7 liters/2 = 392.85 liters.

To determine the number of plates needed, we start from:

  • Considered plate size = 1000 x 1000 x 25 mm
  • Volume per plate = 19.7 l. Surface per plate = 153.1 dm2

The number of plates for each drying is = 393 liters / 19.7= 19.9, i.e. 20 chambers. As the number of filter plates is n+1 chambers => 20 + 1 =21 filter plates. And the filtering surface of the filter press will be = 20 x 1,53 m = 30,3m2

4. Operation of a filter press

The liquid sludge stream enters the chambers through a manifold from which a feed to each chamber is derived. This sludge is propelled into the chambers by a pneumatic pump.

The drained water is collected on the opposite side once it has been filtered. In this way, the filter cloths and the accumulating sludge itself facilitate the increase in pressure, the increase in concentration and the formation of sludge cakes. 

The feed sludge must fill the filter chambers quickly in order to form uniform cakes (time approx. 5 - 6 minutes). For this reason, it is common to have low pressure pumps for support.

Depending on the type of solid to be separated, a specific flocculant, or lime slurry, is dosed into a sludge tank beforehand. In some cases, coagulant reagents, such as Fe(OH)3 or PAC, are also added beforehand.

While flocculant consumption does not usually exceed 4 kg/mt DM, lime consumption reaches values of 15% of the sludge DM, which translates into a higher degree of dehydration, at the expense of higher sludge (DM) production.

As the sludge cake forms, the pressure inside the chambers increases until it reaches a point where it becomes asymptotic and the pneumatic pump slows down its own rate until it practically stops. At this point the circulation of drained water also stops, which is detected by a flow sensor. The end of the pressing cycle is detected by a pressure switch signal, which alerts the operator of the situation. It is at this moment that the sludge cakes are ready to be discharged.

The filter press assembly is usually mounted on an elevated structure, so that a container can be placed under the chamber package in which the pressed solids can be collected. These containers usually have an emptying device in case water or malformed cakes get inside. These filters are usually placed outside and have a canopy to prevent rain from reaching the sludge tank.

Unloading the cakes is very simple. You only have to decompress the electro-hydraulic group that keeps them closed, and then open them and separate them from each other so that they fall into the tank under their own weight.

In small filters (with plates up to 1 m), discharge can be done manually, or even with the help of a lever to separate them from the plates to which they may remain attached. For larger sizes, automatic unloading systems are used that move the plates and accompany the openings with vibrations or rattles that cause the cakes to fall. In any case, it is imperative to check that no cakes remain adhered, either totally or partially on the plates, to avoid incorrect sealing and sludge leakage in successive drying processes. A common solution is to pass air currents to further dry the cakes and ensure that all the water has been filtered out.

To optimize the collection of dry sludge, cake breaking systems are installed to reduce the volume occupied inside the tanks.

The maintenance of this type of filter is very simple, since it is limited to keeping the cloths associated with the plates in good cleaning conditions. For this purpose, they must be cleaned with a certain frequency, which implies a closed-circuit washing operation, or they must be disassembled and cleaned separately, and then reassembled once they have been cleaned.

5. Types of filter presses

There are several types of filter presses, each with its own specific characteristics and applications. Some of the most common types are described below:

  • Plate and frame filter presses: This is the most traditional type. It consists of a series of alternating plates and frames, with a filter media between them. The solids accumulate in the chambers formed between the plates, while the water passes through the filter media and is evacuated. This type of filter press is versatile and can be adapted to different types of sludge.
  • Filter presses with post press membranes: In this type of filter press, elastic membranes are added to each plate. These membranes allow a higher compression of the sludge cake, as it is hydraulically or pneumatically inflated, resulting in higher dewatering efficiency.
  • Pressed belt filters: Unlike plate and frame filter presses, these filters use a continuous filter belt that moves through a system of tensioned rollers. The sludge is pumped over the belt, where water is filtered through it, leaving the solids trapped. This type of filter is particularly suitable for large volumes of sludge and offers high dewatering capacity, but obtains lower dewatering values than the plate and frame filter. The concentration of the dried sludge can be increased by adding specific reagents and with the previous arrangement of draining tables.
  • Vertical or tower filter press: This type of filter press is similar to the plate and frame type, but instead of stacking the plates horizontally, they are arranged vertically in a tower. This allows for greater space utilization and higher dewatering capacity in a reduced area. Tower filter presses are ideal for installations with space limitations.

Recessed chamber filters: The plate and frame filter press uses a series of filter plates and frames, while the recessed chamber filter press uses filter plates to form recessed chambers. The plate and frame design allows the fabric to be easily covered, while the recessed chamber design requires the fabric to be firmly attached to the plates. The choice between these two designs depends on specific application requirements, filtration capacity and other operational considerations.

6. The DAF - filter press assembly

In a conventional wastewater treatment plant, the sludge separated by a settler will have a concentration varying between 0.6 and 1.5 % DM, depending on whether it is a secondary or primary sludge. This sludge concentration is too low to be dried in a filter press, so the installation of an intermediate thickener is necessary.

A thickener is a large and costly piece of equipment that requires a tank, mechanical systems and suitable transfer pumps. The sludge thickened in this equipment usually reaches concentrations of 3% to 4% DM, which is then considered suitable for treatment by mechanical drying.

This sludge is stored in an accumulation tank and introduced into the filter press by means of a pneumatic pump, after the addition of flocculant and coagulant reagents, if necessary.

If instead of sedimentation clarification equipment, a dissolved air flotation (DAF) system is installed, the intermediate thickening system is dispensable, since the sludge obtained by DAF has between 4% and 5% DM. A DAF system offers the following advantages:

  • Space saving.
  • The installation of a thickener is not necessary.
  • Higher efficiency in the separation of suspended solids.
  • A better quality clarified water is obtained.

If we make an economic balance, there is no great difference in investment and consumption between the two systems, since the DAF requires additional equipment for the pressurization and depressurization of the air provided.

Flow diagram of DAF + filter press system for sludge separation and drying

7. Conclusion

The solid wastes generated during wastewater treatment must be extracted, treated and dehydrated to reduce their volume to the maximum, reaching at least a level of reduction at which they can be accepted by an authorized landfill. For this purpose, there are several technologies, among which mechanical drying processes stand out, and among them, drying by means of filter presses, since it provides a high degree of dryness at a low energy and reagent cost.

An important point to take into account is that the sludge to be treated must arrive conditioned to the filter press, by adding the appropriate coagulant and flocculant reagents, and at a minimum concentration that makes the process viable (3% - 5%).

Sludge from sedimentation clarification systems rarely reaches these concentrations, so intermediate thickening treatment is required.

An effective alternative, which saves space and costs, and achieves a greater reduction of pollutants in wastewater, is dissolved air flotation (DAF), in which Sigmadaf has proven experience.

The sludge extracted from the DAF already has the minimum concentration necessary to be directly introduced into the filter press for dewatering.

Bibliography and consultations

What Is The Difference Between A Plate And Frame Filter Press And A Recessed Chamber Filter Press? - Kintek Solution (

Filter Press, the best alternative to sludge dewatering - AS Filtración (

Filter press: features, parts, how it works, uses (

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