What is surface filtration?

In the surface filtration, also called refuse filtration, the molecules do not penetrate in the filter, but they are retained by its surface. The filter pores must be smaller than the solid particles to remove.

Filter materials can be permeable or semi – permeable, prepared to retain all or only a part of the particles suspended in the fluid. The filter can by made of paper, fabric or other membranes kinds. In the first phase of the filtration, if in the suspension there are also particles smaller than the filter pores, they pass through the filter, while the particles bigger than the filter pores remain attached to the surface. If the flux is perpendicular to the surface, a part of the retained particles obstructs the filter pores, gradually developing a material layer, called cake layer. At the beginning the cake layer contributes to increase filter's efficiency. After this phase the filter is constituted by the cake layer, while the filter is only a support. Gradually the filter surface is filled up of particles, which deform, establishing an obstacle to the flux, up to it is under acceptable levels. At this point the filter must be cleaned, to continue with the filtration. The cake layer's resistance depends on the porosity and the thickness of the particles deposit, which can vary over time. Sometimes there can be come devices, called precoat o pre panel, disposed as layers on the filter to reduce the compression on the cake layer and to increase the permeability. The precoat is usually composed by a cellulose layer, fossil flour and a portion of the inlet fluid or water.

The filter cartridges are one of the most adaptable kinds of filltration. Their functioning is based on the surface filtration.
The filter cartridges have a central perforated nucleus, around which the filter material layers are wound. The inlet fluid is pumped in the central nucleus and it passes through the filter barriers. The described structure is inserted in a plastic or metal cylindrical container, able to sustain the fluid pressure. The container has several purposes: to bring the water to the filter, to collect the filtered solution, to keep the filter in the right position to avoid the water to carry out a reduced path around it and to maintain an adequate pressure in the system. The filter cartridges are different from each other in lenght, diameter, material and structure. The length and the diameter must be chosen on the basis of the volume to filter and the available space. The most used filter materials for the cartridges are cellulose, il polyester and polypropylene and their different combinations. Sometimes the filter can be composed by a material that has not filtering properties, but it is arranged to form pores or it is drilled. The smaller the pore of the filter material are, the greater is the pressure that must be used for the filtration. The structure can be of two different kinds: wound, in which the wires, made of filter material, are wound around the central nucleus and folded, in which the sheets, made of filter material, have several folds and are attached to the central nucleus. This last structure gives to the cartridge more filter surface, that grantees an efficient separation of the solid phase.
Filter cartridges can remove particles with dimension from 0.5 mm up to 0.1 μm or also smaller if membranes are used. In the cartridges the use of the precoat or anticoagulants is not recommended because the flux is tangential and their functioning is not based on the development of the cake layer, but on the effective dimensions of the filter pores. So those devices would only increase the pressure lost in the filter, causing the necessity of a more frequent replace of the cartridges.

Architectures of the filter cartridges plants

Filter cartridges can be installed in different schemes on the basis of the flux, the fluid viscosity and the required filtration efficiency. Those different architectures are: a single cartridge, multiple cartridges or in series. A system composed by a single filter cartridge is quite rare in the industrial framework, it's usually used in domestic context or where there are small quantities and an excellent inlet water's quality. In a system composed by multiple cartridges there are several filter cartridges connected with each others in a sequence in which the inlet water passes through filters with pores progressively smaller to obtain a filtered water with an excellent quality. In a system in series the different filter cartridges are grouped together in a single container, to guarantee a bigger flux in the system and a longer life time for every cartridge .

Filter cartridges' applications

The filter cartridges are usually employed for:

• pre filtration for membranes filtration plants (for example reverse osmosis plants);
• surface water filtration to remove microorganisms, in particular oocysts of Cryptosporidium and cysts of Giardia;
• turbidity decrease;
• metals remove.

Benefits and limitations of filter cartridges

The limitations of the use of filter cartridges are mainly two: inlet water's quality and plant's dimensions. If the inlet water is characterised by big amounts of colloids, algae or high turbidity levels, those devices are not recommanded. They are not suitable for elevated flux plants too. The benefits of the use of the filter cartridges are the contained costs, a low necessity of interventions of the operators and the facility of the operations. In effect the filter cartridges are simple to use and to replace in case of necessity.

One of the most common cleaning methods is the back – flushing: the water passes through the filter in a direction contrary to the normal flow to remove all the components. However this method is not so efficient. Once the cleaned open area of the screen (the sum of open pores) reaches the same area (e.g. square inches) as the inlet and outlet flanges, the velocities are equal throughout the filter system. Since this also implies that energies are equal, there is no energy differential across the screen, therefore; no additional screen cleaning can occur. Since most of the filter have open areas equal to 5 to 10 times the cross-sectional area of the inlet and outlet flanges, the differential pressure across the screen will measure zero after a short back-flush cycle but only a small portion of the screen is really clean. Consequently the time intervals between a back flushing and the next one are very short and only a part of the filter is available for the filtration. Another problem of this method is that it requires the filter to be taken off-line during the cleaning phase.

Focused back-flushing or suction scanning is the process in which a suction force is developed reversing the flow through a small portion of the filter surface. The differential pressure between the positive working pressure of the system and the atmospheric pressure creates a very high velocity water stream travelling backward through this small portion of the screen area, thereby; pulling the debris of the cake layer off the screen. This small area is then slowly moved across the screen surface to progressively clean the entire filter. During the cleaning cycle, there is no interruption of flow downstream of the filter.

One of the main problems of the surface filtration is the tendency to block quickly and the frequent necessity of the filter to be cleaned or replaced. The auto cleaning filters are equipped with a cleaning system with automatic or semi automatic control, that activates when the procedure must be done.

Auto cleaning filters' applications

Auto cleaning filters can be mainly used for:

• removal of suspended solids from the inlet water in treatment systems and in the distribution networks;
• pre treatment for reverse osmosis plants and other desalination systems.