Filtration principles
Filtration is the process of separating suspended particulate matter from water.
Although industrial uses vary greatly, all filtration equipment bypasses the solution or suspension via a porous membrane or medium, where the solid particles are held on the medium’s surface or within the medium’s pores while the fluid, referred to as the filtrate, passes through.
The operation is generally carried out for one or both of the following reasons.
It can be used to recover valuable products (either suspended solids or fluid) or to filter the liquid stream, hence enhancing product quality or both.
Adsorption, chromatography, operations requiring the flow of suspensions through packed columns, ion exchange and numerous reactor engineering applications are all examples of processes that rely on filtration.
Types of filtration
Filtration is classified into the following three types:
Depth filtration
a) Slow sand filtration
b) Rapid porous and compressible medium filtration
c) Intermittent porous medium filtration
d) Recirculating porous medium filtration
Surface filtration
a) Laboratory filters used for TSS test
b) Diatomaceous earth filtration
c) Cloth or screen filtration
Membrane filtration
Depth filtration
In this method, the removal of suspended particulate material from the liquid slurry is done by passing the liquid through a filter bed composed of a granular or compressible filter medium.
Depth filtration is a solid/liquid separation method that involves passing a dilute suspension or effluent through a packed bed of sand, anthracite, or other granular media.
This technology is almost universally employed in the treatment of surface waters for potable water supply and it is also frequently used successfully as a tertiary treatment for wastewater.
Failure of depth filtration affects the other downstream processes significantly and most of the time results in overall plant failure.
The duration of a filter run is limited by a variety of factors, including available head, effluent quality and flow requirements.
A filter’s head loss and removal efficiency are complex effects of suspension characteristics (particle size distribution and concentration, particle surface chemistry and solution chemistry), filter design parameters (media size, type and depth) and operation conditions.
Surface filtration
Surface filtration involves the removal of suspended material in a liquid by mechanical sieving.
This method passes the liquid through a thin septum (i.e., filter material).
Filter septum materials have included woven metal textiles, cloth fabrics of various weaves and a range of synthetic materials.
Membrane filtration
Membrane filtration is a separate procedure that divides a feed stream into two parts: a permeate that contains the material flowing through the membranes and a retentate that contains the species that is left behind.
Membrane filtration can be further characterized based on the size range of penetrating species, rejection processes, driving forces used, chemical structure and composition of membranes, and construction geometry.
The most important types of membrane filtration are pressure-driven processes including microfiltration (MF), ultrafiltration (UF), nanofiltration (NF and reverse osmosis (RO).
Involved mechanisms in filtration Processes
Table 1 lists the mechanisms involved in the filtration process.
Straining has been recognized as the primary mechanism at work in the removal of suspended solids during the filtration of biological treatment operations’ settling secondary effluent.
Other mechanisms including impaction, interception and adhesion are also operative even though their effects are small and mostly masked by the straining action.
Table.1 Mechanisms involved in the filtration processes
| Mechanism/
phenomenon |
Description |
| Straining
a) Mechanical b) Chance contact |
Particles larger than the pore space of the filtering medium are strained out mechanically.
Particles smaller than the pore space are trapped within the filter by chance contact |
| Sedimentation | Particles settle on the filtering medium within the filter |
| Impaction | Heavy particles do not follow the flow streamlines |
| Interception | Particles get removed during contact with the surface of the filtering medium |
| Adhesion | Particles become attached to the surface of the filtering medium as they pass through |
| Flocculation | It can occur within the interstices of the filter medium |
| Chemical adsorption
a) Bonding b) Chemical interaction |
Once a particle has been brought in contact with the surface of the filtering medium or with other particles, either one of these mechanisms, chemical or physical adsorption or both, may occur. |
| Physical adsorption
a) Electrostatic forces b) Electrokinetic forces c) Van der Waals forces |
|
| Biological growth | Biological growth within the filter lowers pore volume and improves particle removal using any of the previous removal processes. |
Filtration in wastewater treatment
In a very general sense, there are two types of wastewater flow – municipal and industrial.
Although the composition of municipal wastewater varies, certain qualities allow filtering equipment to be easily selected and specified.
When dealing with industrial wastewater streams, this is not always the case.
The contents and features of industrial wastewater vary significantly and these flows can be extremely different even within single industry sectors.
While filtering is a physical process, it is dependent on and critical to chemical treatment techniques such as preconditioning, buffering and filter aid conditioning.
To ensure that a properly-designed filtration system is used, these chemical treatment methods, as well as the filtration equipment itself, must be correctly defined.
Filtration equipment selection can be difficult due to the large variations in suspension properties, as well as the sensitivity of suspension and cake qualities to different process conditions and the range of filtering equipment available.
As a result, generalities in selection criteria are sparse; however, there are some suggestions applicable to specific groups of filtration applications.
When working with polydisperse suspensions, one example is selecting a filter with a flow orientation parallel to gravity.
This arrangement is preferable to an up-flow design because larger particles tend to settle first on the filter medium, preventing holes within the medium structure from clogging.
A further recommendation, depending on the application, is not to increase the pressure difference to increase the filtration rate.
For example, if the cake is very compressible, increasing pressure will result in considerable increases in specific cake resistance.
We may generalize the selection procedure so that three rules apply to all filtration problems
1. The aims of a filtration process should be defined.
2. Physical and/or chemical pretreatment solutions should be considered for the desired application based on their availability, affordability, ease of implementation and ability to offer optimal filterability.
3. The final filtering equipment selection should be based on the ability to achieve all application requirements within budgetary restrictions.
When using these general criteria, keep the intended application in mind.
Filtration can be used at many stages in wastewater treatment applications.
It can be used as a pretreatment process, to remove coarse, gritty debris from the waste stream.
This is a preconditioning procedure for wastewaters that will be treated chemically and physically farther downstream.
Filtration may also be used as a preparatory step for the following operation.
Later processes may include solids drying or burning, concentration, or direct use of the filtrate.
Filtration equipment must be chosen with the ability to give the best feed material to the following phase in mind.
Where grinding procedures are not used, dry, thin, porous, flaky cakes are most suited for drying.
In such instances, the cake will not ball up and will dry quickly.
A clean, concentrated filtrate frequently improves downstream treatment, as the filter can be controlled to boost the efficiency of downstream equipment without compromising its efficiency.
Filtration may also be used as the last stage of treatment in the process.
This is typically known as a polishing process.
Filtration can be used in the pretreatment, polishing and even as an intermediary stage in the wastewater treatment process.
The selection of filtration equipment is determined by the specific operation that the equipment must accomplish.
References
[1] Nicholas P. Cheremisinoff, Handbook of Water and Wastewater Treatment Technologies, Butterworth-Heinemann,2002, p 62.
[2] Metcalf & Eddy, Tchobanoglous, G., Burton, F. L., Stensel, H. D. “Wastewater engineering: treatment and reuse/Metcalf & Eddy, Inc.”, Tata McGraw-Hill, 2003.
[3] Cushing, R., Lawler, D. Depth filtration: a fundamental investigation through three-dimensional trajectory analysis. Environmental Science and Technology, 1998, 32, 3793- 3801.
[4] Spellman, F. R. “Handbook of water and wastewater treatment plant operations”, CRC Press, 2nd edition, 2009
[5] Mallevialle, J., Kendall, P. E. and Wiesner, M. R. “Water treatment membrane processes”, McGraw-Hill, New York, 1996.
[6] Zhou, H., Smith, D. W. Advanced water and wastewater treatment technologies. Canadian Journal of Civil Engineering, 2001, 28, 49-66.
[7] Nicholas P. Cheremisinoff, Handbook of Water and Wastewater Treatment Technologies, Butterworth-Heinemann,2002, p 78.
