What is filtration?
Filtration is a solid-liquid separation process that is used in Water treatment systems to decrease the concentration of suspended solids (particles) found in it. There are several types of filters and each of them has its application depending on the size and concentration of particles to be retained. In the Figure 1 shows the different types of filtration and the typical size of particles they retain.
Figure 1. Comparison of water filtration processes and size ranges of the particles they retain (Weiner, 2012)
As for the concentration of particles, in a filtration line, you can have several filters in series to retain particles of the same size. This is because there are filters for high solids loads and others for lower loads. So, why choose a filter to retain low concentrations of solids?
It’s all about efficiency. The filters that are used when we have high loads of solids, have the peculiarity of having cleaning mechanisms that are activated when the filter is saturated. Among them are the mesh filters, disc filters and granular media filters. However, most of these filters allow a certain percentage of particles equal or higher than the micronage they claim to retain to pass through. The amount of particles that leak into the effluent depends on each manufacturer’s design. There are applications where particles cannot be allowed to pass, so other types of filters are used. These are chosen according to their filtration efficiency and the size of particles they are intended to retain. Among them are: bag filters, membranes and cartridge filters. As they do not have a cleaning mechanism, once these filters are saturated, they generally have to be discarded, so it is not viable to filter currents with high solid loads with this type of technology.
Are you interested in knowing a specific method of separation in filtration?
Keep reading the following articles:
Reverse osmosis (RO) is a process in which the flow rate through a semipermeable membrane is reduced and a pushing force greater than the osmotic pressure is exerted in the opposite direction to the osmosis process (Figure 1). In this way it is possible to separate the substances found in the water on one side of the membrane (concentrate) and on the other side a dilute solution low in dissolved solids (permeate) is obtained.
Nanofiltration is a filtration process using a nanoporous membrane that is used in waters with low total dissolved solids. The purpose is to remove the polyvalent ions, as well as to disinfect it by retaining organic matter.
It is a separation process based on a porous membrane with openings between 0.01 and 0.1 microns.
Ultrafiltration membranes are more closed compared to microfiltration, but more open than nanofiltration and Reverse osmosis ROThe membranes work with low pressure, which results in lower operating costs. In addition, they are very effective as a pretreatment for Reverse osmosis RO, and they also have a backwash system, which gives them a longer life.
Microfiltration is a filtration process using a microporous medium that retains suspended solids in a fluid. The pore size of the membrane ranges from 0.1 to 1 micron or microns.
A filter medium is defined as: “any material which, under specified operating conditions, is permeable to one or more components of a mixture, solution or suspension and impervious to the remaining components” (Purchas and Sutherland, 2002). The main role of a filter media is to cause the separation of particles from a fluid with minimum energy consumption.
There are several types of filtering materials, these have the characteristic of having or forming pores (empty spaces), such as:
Meshes and fabrics woven from metals, polymers or natural fibers such as cotton.
The Non-woven materials (Non-woven are an accumulation of fibers adhered to each other, by chemical agents (resins or adhesives) or physical means (such as temperature and pressure). Some examples are thermo-adhesive fibers or foamed polymers. These materials generally create more complex porosities, thus complicating the passage of solids through them.
Membranes are permeable materials. They are usually polymers and ceramics, which according to the method of manufacture of the membrane, various structures can be obtained, such as those shown below:
Granular materials are used as filter media or support systems in deep bed filters. In packed columns these form intra-articular spaces, the size of which depends on the shape and size of the granular medium. Below is a picture of a silica sand bed approach.
Surface and depth filtration.
The filter media retains particles in two ways. The first is when the particles are larger than the pores of the filter media, these are deposited on the surface of the media forming a cake (Figure 2), this phenomenon is known as surface filtration.
Figure 2. Surface filter. (Donaldson)
When particles are smaller than the pores of the filter media, retention occurs in the internal structure or depth of the material. This is achieved by the attraction of electrostatic charges between the material and the contaminant, and by the tortuous path that solids have to travel within the depth of the filter media. However, not all particles are retained in the internal structure; some manage to pass into the effluent. This phenomenon is called depth filtration (Figure 3).
Figure 3. Depth and pore filter not fixed. (Donaldson)
- Wakeman (2005), Richard J., Solid/liquid separation: principles of industrial filtration. Elsevier.
- (1999). Water Quality &Treatment a Handbook of Community Water Supplies. McGraw-Hill.