The most effective method of liquid filtration is the use of a filter press, which means that the material to be filtered releases the liquid as a result of the physical action of the press. The heart of the filter press is the filter sheet, on the quality of which the success of the whole process depends. Let's see what you need to know about it!

The structure of filter presses is quite simple: the filter plates create a closed chamber whose function is to trap the solid materials in a liquid. The filter material, which can be paper or even a simple piece of cloth, is fixed to the frame of the filter plate. Drainage elements are built into the filter sheets to support the filter material and assist the filtrate to escape.

Fundamentals of choosing the right filter sheet

Care should be taken when choosing a filter sheet. The following considerations should be considered when making your decision:

• chemical resistance,
• differential pressure resistance,
• filter fabric support,
• drainage capacity,
• treatment, maintenance.

Types of filter sheets

The thickness of the filter sheets should be chosen according to the unique circumstances: from an average of 400 mm x 400 mm to 2000 mm x 2000 mm, the filter sheets range in size, and the filter cake thickness ranges from 15 mm to 50 mm.

The filter presses with membrane filter sheets allow the filter cake to be mechanically pressed back so that more liquid can be squeezed out to increase the dry matter content. The filter sheets are available with both built-in fixed membranes and replaceable membranes. Filter sheets are also available in drip-tight and gas-tight versions. 

Filter sheets are therefore fully customisable, depending on the purpose, the material to be filtered, etc.

Chambered filter sheet vs. membrane filter sheet

The two main technologies for filter presses are chamber and membrane solutions. The technology used determines which filter sheet is required.

Chambered filter sheets

Chamber filter sheets are single-component and therefore more cost-effective. They can withstand filtration pressures up to 50 bar and can withstand high temperatures. They are particularly stable due to their reinforced edges. The filter surface itself consists of a piston structure supporting the filter cloth. This guarantees high filtration performance.

The membrane filter sheets

There are several types of filter sheets used in membrane filters. The basic welded membrane sheets are used for filtration with homogeneous chamber filling and standard parameters. Some versions can be used in empty conditions or at a low level without damaging the membrane due to compression. There are also replaceable membrane sheets and those that are resistant to corrosion.

Learn more about our modern filtration solutions! We offer a wide range of filter sheets for chamber and membrane filter presses, with many different geometries and functions up to 30 bar pressure.

The purpose of the industrial liquid filtration process is to separate liquids from solid materials. In this article, we summarise the elements of the liquid filtration process and its main types.

Liquid Filtration Process 

The filtration process is about the separation of liquids from solid materials. This is done by passing the filtering liquid through the filter medium, which retains the solids. The filtered liquid is called the filtrate, and a pressure difference occurs during the process, called filter resistance. Solids are therefore retained in the filter medium and form the filter bed.

Two types of liquid filtration processes are distinguished: static filtration and dynamic filtration. Static filtration creates a pressure difference between the media, which leads to the separation itself. It is worth noting that, depending on the design of the filtration equipment, a distinction must be made between surface, layer, spatial and depth filtration. The other variant is dynamic filtration, i.e. membrane filtration, which allows cross-flow filtration. The filtrate is therefore absorbed perpendicular to the direction of flow.

Elements of the liquid filtration process

The liquid filtration process is a complex procedure. The first step is the loading of the material, during which the material to be filtered is fed into the filtering machine. This is usually done by a pump, but in some cases, gravity is used.

The next stage is pre-pressing, which is negligible in most filtration processes. Similarly, washing is not always obligatory. 

In the post-pressing stage, high-pressure air or water is injected into the filtration sheets, causing the sheet to expand and compress the accumulated sludge slurry. In the case of post-compression, the upper limit is on average 16 bar, so that a drier sludge slurry can be produced. This step is only important for machines that have a membrane flap.

During the blow-down, high-pressure air is introduced through the loading pipe into the closed slurry stack, which removes the remaining filtrate and contributes to the drying of the slurry. Finally, during the flake discharge, the sludge flakes are removed from the filtering machine and the flake is moved.

Applications of liquid filtration

In industrial production, lubricating fluids, oils and coolants are most filtered. In addition to manufacturing, the chemical, pharmaceutical and food industries are major users of filtration equipment. In their case, the material to be filtered is typically a process fluid.

But the application of filtration technologies does not stop there. They are used for water pipe filtration, filtration of lakes, rivers, well-water, heating systems and filtration of water from heat exchangers. In an industrial environment, liquid filtration processes are used to filter open and closed cooling water. In power plants, they are used to remove solids from condensate water, while in agriculture they are used to filter thermal and irrigation water.

Choosing the right liquid filter

To choose the right liquid filter, it is important to consider the yield, temperature, pressure, viscosity, particle size and volume of the liquid to be filtered.

Particle size is a key factor in filter selection. Microfiltration is when the particle size is between 1-0.1µm, such as bacteria. Ultrafiltration, when the size ranges from 0.1-0.001µm, is used to filter out viruses and in wastewater treatment. We talk about nanofiltration when the particle size is 10-1 nm, and reverse osmosis below 1 nm, for example for conductivity reduction and desalination.

Makrofilt offers customized filtration systems for all sectors. Find out more and contact us!

Sludge is the outcome that remains after the wastewater treatment process. This waste usually contains pollutants, which means that it must be disposed of instantly. In almost all cases, sludge dewatering is part of the treatment process, as it reduces the volume and mass of the material and makes treatment and transport much easier and faster. Let's look at the possible solutions for sludge dewatering.

Sludge dewatering process

Based on the design of the press plates, there are chamber filter presses and membrane filter presses, which are both excellent for sludge dewatering and can also be used in processes where the slurry is to be recovered. In these machines, the filter plates form the chambers on the surface of which the filter cloth is placed. The sludge enters the structure through the holes connecting the chambers, by using a dosing pump. 

The liquid passes through the filters, which trap larger particles, leaving the solids in the chamber. As the dry matter content of the sludge increases, the permeability of the chambers decreases. When a certain pressure level is reached, automatic loading stops, and as water escapes through the holes and consequently the pressure decreases, the machine starts functioning again. The process ends when the sludge remaining in the chambers has reached a sufficient dry matter content. The waste is discharged at this point and the whole process can restart.

What technologies can be used to carry out sludge dewatering?

Sludge dewatering can be carried out using several technologies. We have collected what you need to know about them.

Centrifuge

The advantage of centrifuges is that they can achieve a very dry matter outcome and can handle even larger volumes. However, due to the high cost of the technology itself, these machines could not flood the market. 

Belt filter press

Belt filter presses are often found in residential wastewater treatment plants. These machines are capable of dewatering huge volumes of material and are also capable of operating continuously. However, it is important to know that these are complex structures and therefore require uninterrupted supervision and a lot of washing water to operate.

Chamber filter press

Chamber filter presses are typically used where the aim is to reduce the dry matter content of the sludge as much as possible while reducing its mass. These machines can perform well in the drying process, but their capacity is affected by the chamber volume. For sludge created by the galvanising, painting and engineering industries, chamber filter presses are highly recommended.

Screw press

The screw press can replace belt presses and centrifuges in certain situations. A new technology, the screw press is easy to operate, quiet and requires low-level maintenance.

The press types have different dewatering performances. The efficiency of the screw press and the centrifuge is about the same. The belt filter press has slightly lower efficiency compared to them. Overall, the chamber press has the highest efficiency. The centrifuge has a higher noise level, while the other presses are much quieter during operation. Centrifuges and belt filter presses have more wear parts and a shorter lifetime than screw and chamber presses. 

The sludge dewatering process can therefore be carried out using several technologies. There is no single best solution, and specific circumstances must always be considered.

Makrofilt offers sludge dewatering services to a wide range of industries, including pharmaceuticals, chemicals, brewing, paints, food, galvanising, ceramics and municipal wastewater treatment.

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The filter cloth plays a very important role in the operation of filter presses. In both chamber filter presses and frame filter presses, it is the textile-based filter cloth that separates the different materials, based on the particle size.

The filtration process

When initiating the filtration process, the first step is the loading of the material itself, the sludge, into the equipment. If necessary, pre-filtration is carried out, followed by washing and post-pressing. This involves the introduction of high-pressure air or water into the membranes of the filter press, which causes the plate to expand and compress the sludge collected in the chambers of the filter sheets.

A high-pressure blow-off removes the filtrate collected in the filter sheet bundles and the layers of the channels. This is followed by draining, which means that the excess water, the unwanted liquid, is removed from the system using a pump. The complexity of the process depends on the situation, if it is a simple filtration, pre-pressing, washing and post-pressing can be omitted.

What do you need to know about filter cloths?

The filter cloth's role in this process is the filtration itself. The properties and parameters of the filter cloth determine fundamentally which elements it filters out, i.e. what becomes sludge and what leaves the filter.

The filter cloth is held in place by the filter sheet, which is shaped to allow the formation of sludge deposits and has holes to help the material flow away. The filter sheets form the filter sheet bundles, which are called head sheets, end sheets and intermediate sheets, depending on where the filter sheet is located.

Filter sheets can also be grouped by function. These include chamber flaps, membrane flaps, wash flaps and other special solutions. The usability of filter sheets is determined by various parameters, such as their dimensions, the location of the drainage channels, the chamber volume, the maximum pressure, the dimensions of the filter surface and the quality of the material.

The filter cloth thus creates a filter surface on both sides of the press plate, and these materials can therefore be produced in both fold-over and fold-through versions.

What are filter cloths made of?

Filter cloths are typically made from woven industrial fabrics, which can be polypropylene-, polyester-, cotton-, polyamide- or fluoropolymer-based.

The properties of the filter cloth are determined by the material itself. For example, the fibre structure, which can be mono- or multifilament, the fibre thickness, the weaving method, the production technology, the density, the pattern, the surface design and finally the breathability is essential. To obtain a satisfactory final filtration result, the above parameters must be combined. 

Filtration fabrics can also be used in specific situations. If, for example, extremely high temperatures or extreme PH are to be expected, a fabric made of heat-resistant fibres should be chosen as the material for the cloth. All fabrics are prepared to withstand high temperatures and are treated with a thermofixing process to ensure that the size of the canvas does not change.

It is also essential that the flaking process is carried out with the appropriate efficiency. To achieve this, the filter fabrics are smoothed and calendered, i.e. pressed between two rollers, but it is also possible to weave monofilament fibres between the multifilament fibres.

Makrofilt also offers a solution for filter cloths: filter cloths in flow meters, filter cloths tailored to narrow plates, and customised filter cloths.

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