Filtration Processes

Introduction

Filtration is a process that removes particles from suspension in water. Removal takes place by a number of mechanisms that include straining, flocculation, sedimentation and surface capture. Filters can be categorised by the main method of capture, i.e. exclusion of particles at the surface of the filter media i.e. straining, or deposition within the media i.e. in-depth filtration.

Strainers generally consist of a simple thin physical barrier made from metal or plastic. In water treatment they tend to be used at the inlet to the treatment system to exclude large objects (e.g. leaves, fish, and coarse detritus). These may be manually or mechanically scraped bar screens. The spacing between the bars ranges from 1 to 10 cm. Intake screens can have much smaller spacing created by closely spaced plates or even fine metal fabric. The latter are usually intended to remove fine silt and especially algae and are referred to as microstrainers.

Filters, as commonly understood in water treatment generally consist of a medium within which it is intended most of the particles in the water will be captured. Such filters might be manufactured as disposable cartridge filters, which can be suitable for domestic (i.e. point-of-use treatment) and small-scale industrial applications. Larger forms of cartridge filters exist which can be cleaned. One version is precoat filtration in which a porous support surface is given a sacrificial coating of diatomaceous earth, or other suitable material, each time the filter has been cleaned. Additionally, a small amount of the diatomaceous earth is applied continuously during filtration. However, in most cases, filters used in municipal water treatment contain sand or another appropriate granular material (e.g. anthracite, crushed glass or other ceramic material, or another relatively inert mineral) as the filter medium. Filtration using such filters is often referred to as in-depth granular media filtration.

Granular media filters are used in either of two distinct ways which are commonly called slow-sand filtration and rapid gravity or pressure filtration. When the filters are used as the final means of particle removal from the water, then the filters may need to be preceded by another stage of solid-liquid separation (clarification) such as sedimentation (Sedimentation Processes), dissolved-air flotation  (Flotation Processes) or possibly a preliminary stage of filtration.

Other processes take place in vessels similar to those used for granular media filtration, and in some respects the processes do have similarities with filtration but filtration is not their sole or primary purpose. Therefore, such processes are not considered further in this article. Examples include vessels filled with granular activated carbon for removal of dissolved organic substances, and vessels filled with ion exchange resin for removal of inorganic and organic ions. There are applications of filters that whilst filtration (removal of particles) does take place a secondary process is intended to also occur, e.g. iron and manganese removal, and arsenic removal.

Strainers

There is a vast variety of strainers with respect to how the straining is carried out, with and by what (Purchas, 1971). The straining part might be made of metal or other inert material e.g. plastic, cotton or a ceramic. If metal, it could be simply a perforated sheet, a grid of rods, a stack of discs or woven wire. If plastic, it could be a grid, woven or simply a fused felt. In cartridge filters the usually disposable cartridge might simply consist of a porous and non-compressible material or be cord wound on a cylindrical support. Cartridge filters find application generally in small scale applications such as for domestic point-of-use water treatment.

Only a few types of strainers are likely to find application in municipal water treatment. Some require manual cleaning others are cleaned mechanically and even automatically when the pressure drop across them reaches a specific value. A water treatment works might have a simple bar strainer at its inlet to keep out logs, large fish and swimming animals. Next there might be a fine strainer with its aperture small enough to exclude all but the smallest of fish, leaves, clumps of algae etc . Generally, this strainer would have to be automatically cleaned. Where algae might be a distinct problem then the bar strainer might have closely spaced bars and be automatically cleaned followed by a microstrainer.

One particular type of mechanical strainer has found limited application in smaller municipal water treatment works. The straining medium is a bundle of fibres. In filtration mode the bundle is twisted tight. In the wash mode the bundle is untwisted and the trapped detritus removed by reversing the flow of water.

Precoat Filters

In precoat filtration a thin layer of an inert medium is laid down on a support structure to provide a porous straining surface. The precoat layer might be created with loose fibres or powders (Purchas, 1971). A small quantity of the precoat or other similar material might be added continuously during filtration such that some in-depth filtration also then takes place. When resistance to flow becomes too great then the accumulated detritus and inert medium are discharged and the cycle repeated. In most instances the precoat material is used just once and is not recovered and recycled.

Precoat filtration is unlikely to be used in conjunction with coagulation and therefore its application in municipal water treatment is very limited.

Slow Sand Filters

In slow sand filtration the rate of filtration is intentionally slow with use of sand that is smaller than sand used in rapid sand filters, so that particles are not driven far into the bed of sand held within the filter shell. The principal mechanisms taking place in slow sand filters is accumulation of a layer of debris on the surface of the filter (straining) and capture within about the top 20 cm of the sand. This debris is allowed to develop biological activity which contributes to the treatment of the water passing through it. This biologically active layer is often called the ‘schmutzdecke’. Because the filtration rate is relatively slow the resistance to flow through slow sand filters develops slowly and may take up to 3 months before it becomes unacceptable. Because filtration rate is slow a large area for filtration is needed. Consequently, the large filters are cleaned by removing the schmutzdecke with about 5 cm of sand usually by mechanical means. Eventually the depth of sand remaining becomes too shallow and the remaining sand is removed, cleaned and replaced with additional clean sand back to the original starting depth.

Slow sand filtration was the main method of filtration of potable water before rapid sand filtration was developed. Although it has a large footprint, many slow sand filters are still used. Developments to make them more cost effective have included:

• Sand removal, washing and replacement have been mechanised as much as possible.
• The need for sand removal has been made as predictable as possible so that the equipment and labour is efficiently utilised.
• Filtration rates have been increased as much as possible to improve the economics and contribute to predictability of need for sand removal.
• Pre-treatment, including raw water storage and management, is applied to reduce the impact of solids in suspension and contribute to predictability.
• Granular activated carbon has been used in some filters to replace the lower part of the sand to help with removal of pesticides, taste and odour and other trace organic substances that the biological mechanism does not deal with effectively.

There are two important requirements for slow sand filters to function properly. Firstly, the water entering the filters must not contain any disinfectant or other chemical that might interrupt the biological activity of the schmutzdecke. Secondly, if pre-treatment is carried out with coagulation then most of the resulting floc particles must be removed as part of the pre-treatment, otherwise the floc will accelerate the rate at which resistance to flow through the filter develops.

Rapid Gravity and Pressure Filters

In-depth granular media filtration can be carried out under gravity (rapid gravity filtration) or under pressure (pressure filtration). The basic mechanisms of particle removal are fundamentally the same in both gravity and pressure modes. The principal differences between the two modes are likely to be hydraulic, notably distribution of flow between filters and control of flow through individual filters.

The filter media is usually sand, but other relatively inert material can be used, but the choice depends on costs and what other objectives there might be. In some cases, part of the sand might be replaced with anthracite. The lower density of the anthracite allows a larger grain size to be used such that after backwash the larger anthracite sits on top of the smaller sand. In this way filtration takes place through first a larger and then a smaller media to help make better use of filter bed depth. 

The principal mechanism of in-depth filtration is surface capture. The area of media available for surface capture depends on both media depth and size. Depth and size also govern the space available for storage of captured detritus. Grain shape of the filter media also affects capture and storage, in that angular particles are preferable to rounded particles.  The choice of size has to take account of how quickly the medium might become blocked by captured detritus and the ease with which it can be backwashed. Regardless of the choice of media material, size tends to be limited to the range 0.5 to 2.0 mm. The greatest application of in-depth filtration in municipal water treatment is after coagulation, perhaps also with prior clarification. The choice of coagulation chemistry, its application and any clarification, govern the nature and quantity of the particles to be removed by the filtration, which in turn affect the choice of filter media, depth and filtration rate.

In potable water treatment, in-depth filtration is often the last, and sometimes the only, physical barrier to particles. Therefore the performance reliability of the filters is important in ensuring the quality of the water on completion of treatment complies with the standards. The standards defined by the relevant regulations have become substantially more rigorous as they have developed over the past 50 years. Reliability of exclusion of Cryptosporodium oocysts has been of particular concern.

The bed of granular filter media is cleaned by applying backwash. This generally involves: draining down the water until its upper surface is at about the same level as the top of the media, loosening the bed with air (air scour), applying water upwash at a rate great enough to just fluidise the functional part of the bed of filter media, allow a short interval for the media to settle, and starting to refill the filter with water from above the bed whilst opening the outlet so that filtration starts slowly. A more rigorous backwash can be achieved if the water upwash is started at a reduced rate whilst the air scour is occurring (combined air-water wash). Older filter installations sometimes have other features like mechanical rakes or surface flush that operate during upwash. The viscosity of water depends on water temperature. Therefore, it is important that the rate of upwash takes account of water temperature to ensure the filter media is fluidised.

It is usual to have at least four filters, so that the filtration can continue whilst one filter is backwashed. Large treatment works have many more than four in a group, and possibly two or more independent groups of filters.

Problems with operating in-depth filters include:

• Loss of media during backwash,
• Ineffective backwashing resulting in mud-binding of the media and its associated symptoms.
• Short filter runs due to either rapid rate of headloss or early breakthrough of particles.

These are usually indicators of the likes of incorrect upwash rate, problems with the underdrain system, excessive dosing of polyelectrolyte, presence of filter-blocking algae, inappropriate choice of either or both filter media size and depth, or simply either or both inadequate prior coagulation and clarification. Trouble-shooting should also check to what extent distribution of flow between filters in a bank or group is equitable or not.

Novel Forms of Granular Media Filters

There are a number of relatively novel forms of granular media filters. Each is a 'horse for a course' having its specific set of advantages and disadvantages and therefore relative appropriateness for certan applications.

Upflow filters

In normal in-depth granular media filtration the flow of water is down through the filter bed, except during backwashing. Upflow of water during filtration is possible; it offers an advantage but also poses problems. With backwashing of the filter media, normally the media is encouraged to stratify with the largest and densest material towards the bottom of the filter bed and the smallest and lightest towards the top. This means that in downward filtration, the filtration is progressively through increasingly larger media, unless the media is tightly graded before installation. This contradicts the ideal bed geometry of filtration through progressively smaller media. It follows that that one way of avoiding this situation is to filter upwards. Upward filtration allows the capacity of the media to collect and store solids to be exploited better. However, as the filter bed accumulates deposit and the resistance to flow through it increases the bed progressively becomes more likely to be hydraulically disrupted. Two approaches have been used to restrict this hydraulic disruption. The Immedium filter uses a simple metal grid about 15 cm below the top of the bed to help keep the bed compacted. The Biflow filter applies downflow filtration to the top of the bed to keep the lower part with upflow filtration compacted.

A reservation for the use of upflow filters as the final stage of solids removal in potable water treatment is that backwash flow is in the same direction of filtration. Another reservation is that filter breakthrough can happen suddenly. Consequently, upflow filters are more likely to be found in applications where protection of treated water quality  does not have to be as rigorous as required for potable water treatment, although they might be appropriate to use as a clarification stage prior to normal in-depth filtration.

a.       Immedium filters

The Immedium filter was developed in the Netherlands in the 1960’s.  The key feature is the use of a simple metal grid across the filter bed about 15 cm below the top of the sand. The grid delays the onset of breakthrough of particles in the water. The grid helps to maintain compaction of the sand and delays the start of localised penetration of flow as the water finds paths of least resistance through the sand. A point is reached when the flow through such a low resistance path is too great for particles to be removed and is great enough to fluidise the sand in the upper part of the flow path. This can be observed at the upper surface of the bed by the appearance of ‘blow holes’.

b.      Biflow filters

The Biflow filter was developed as an alternative to the Immedium filter.  As the name implies, flow for filtration is in two directions. The larger proportion of flow is upwards from the base of the filter bed, whilst the smaller proportion is downwards from the top of the filter bed. The two flows meet a short way down the bed where there is an outlet grid across the bed. When the filter needs washing both flows are stopped and air scour applied for a few minutes before water upwash is carried out to wash out the detritus. Combined air and water upwash can be carried out only if the filter has been designed for this.

c.       Buoyant media filters

Whilst in Immedium and Biflow filters the filter sand is kept compacted, in buoyant media filters the media is chosen to be buoyant and is retained in the filter by a straining mesh above the media. The media is selected to have a low density and accordingly is usually a plastic. During the filtration mode the media is in a compacted state under the retaining mesh. When the media needs to be washed to clean out the captured detritus, the upflow rate is reduced to release the compaction and air is bubbled up through the bed. Buoyant media filters have been used in water treatment as a clarification stage prior to normal filtration

d.      Moving bed filters

All the granular media filters described above have flow through for filtration stopped whilst they are backwashed. In a moving bed filter, the filter media is constantly moving so that filtration is not interrupted for the sand to be backwashed. The sand in the filtration zone slowly moves downward due to its own weight against the upflow of the water being filtered. In the conical base of the filter the sand is hydraulically carried into a vertical tube up through the centre of the filter bed. As the sand is carried up through the tube the filtered deposits are released. At the top of the tube above the filter bed the sand settles out from the wash water and feeds back to the top of the filter bed whilst the dirty wash water is kept separate from the filtered water emerging from the top of the filter.  In order that the proportion of water lost in the wash stream is kept small, moving bed units should be operated close to design capacity.

Cell filters

There is a maximum size to which a normal filter can be built if the whole of the filter bed is to be backwashed at the same time. If the filter bed can be backwashed in sections then the filter shell can be larger. A bed can be backwashed in sections by having the filter bed divided by walls from the filter floor to just above the bed so that a hood can be placed over the section to be backwashed. The hood is mounted on a gantry that runs on rails along the top of the main side walls of the filter. This approach results in reduced civil engineering costs but greater mechanical engineering costs, compared to a larger number of filters of equivalent total filtration area. The operational reliability of a cell filter depends much on the functioning of the gantry and hood system and how effectively the hood seals with the walls of a cell.

Automatic backwash filters

As deposits accumulate in a filter bed the resistance to flow through the bed increases.  Flow can be kept constant by having an outlet valve that is progressively opened and provide less resistance to flow through it to compensate for the increased resistance to flow through the bed. In this way the level (head) of water above the media remains relatively constant. Alternatively, the flow to the filter is kept constant and the flow through the filter remains relatively constant with the level of water above the bed increasing. If the filter is contained in a deep shell then the increasing level of water can be used to prime a siphon. When the level reaches a predetermined level the siphon is activated and is used to draw water up through the filter to cause backwash. A risk is that the upwash rate of water may be inadequate for effective backwashing. However, the design lends itself to package plant and situations where quality and quantity of particles to be removed remains relatively constant.  The design is unlikely to be suitable for potable water treatment.

Horizontal and radial filters

a.       Horizontal filters

Instead of the flow of water being up or down through a filter bed, it can be horizontally across through the bed. If the filter bed is contained in a rectangular tank then the filtration rate remains constant along the length (inlet to outlet) of the filter. The filter can be backwashed hydraulically as required.  It would be necessary for the main filter material to be as uniform in size as possible so that there is not a distinct bias through the depth due to stratification of the media by size by the backwashing, or the backwash is arranged to keep the media mixed. A horizontal filter could be split into two or more sections each with a different size media, with a vertical mesh between each to keep the different size media separated. The backwash of each section would need to take account of this.

Horizontal filters have been used filled with gravel (pebbles) of selected sizes in third world situations for use as clarifiers. Because the size of the gravel precludes normal backwashing, they filters are routinely cleaned by draining and hosing and occasionally by removing the gravel for washing.

b.      Radial filters

A radial filter is a horizontal filter but with increasing width of filter bed in the direction of flow. The ultimate shape of the filter bed is annular in cross-section with flow from the centre to the periphery. The rate of filtration decreases as the water progresses through the filter media so allowing progressively more efficient removal of particles.

Membrane Filters

Historically cloth has been used to filter water. In microstraining the  water is filtered through fabric made from finely woven wire. In both these cases the cloth or fabric is a kind of membrane, albeit a coarse one. Modern technology allows manufacture of membranes from synthetic materials, to be less than about 1mm thick and be semi-permeable.  Being semi-permeable means that the membrane is selective in what submicron-size particles can and cannot pass through it that is in the feed stream. During operation, permeable components in the water pass through the membrane with the water whilst impermeable submicron-size components are retained on the feed side. Consequently, the product stream is relatively free of the impermeable components and the waste stream is rich in impermeable components. Flow of water through such a semi-permeable membrane is achieved by pressure, usually produced by pumping.

There are four categories of membranes loosely defined by the types of materials rejected, operating pressure and nominal pore size. The categorisation of pore size is approximate since, for example a high-end UF membrane can have similar permeability to a low-end NF membrane:

• Microfiltration (MF)  - approx 0.1 µm pores: impermeable to particles, algae, animalcules and bacteria
• Ultrafiltration (UF) – approx 0.01 µm pores: impermeable to small colloids and viruses
• Nanofiltration (NF) – approx 0.001 µm pores: impermeable to dissolved organic matter (DOM) and divalent ions
• Reverse osmosis (RO) – effectively non-porous: impermeable to monovalent ions

The predominant mechanism in MF and UF is straining, or simple size exclusion. In NF and RO separation of dissolved species involves mass transfer, a process of diffusion that depends on concentration, pressure and rate of flow through the membrane (flux).  Consequently, membrane filtration usually refers to MF and UF but not NF and RO, whilst NF is usually considered to be a form of RO.

The thickness of membranes means that they have to be formatted in a way that provides structural strength, so they will not collapse because of the pressure difference across them, provide a large area for filtration but are compact and can be cleaned effectively. They are generally structured as thin tubes (hollow fibres) or as a coiled sheet. A coil is a sandwich of the semi-permeable membrane, a separating mesh, a thin sheet of impermeable material and a second layer of thin mesh. The layers of mesh provide the channels for flow to the inlet and from the outlet side of the membrane.

It is usual to include a preliminary stage of treatment before membrane filtration to protect the membrane from being fouled too rapidly by excluded material, although there are also ways to operate membrane filters to slow the rate of fouling of the membrane before having to apply a cleaning process. The routine, and frequent, cleaning process is flushing to remove the accumulated detritus on the feed side. However, over time there is a slow loss in membrane performance that can only be recovered by chemical cleaning.

Membrane filtration (MF, UF and low end NF) have become relatively common in potable water treatment, such as for removal of colour from otherwise relatively good quality water so avoiding complexities associated with coagulation, and for reliable exclusion of Cryptosporidium.

References

Tobiason J.E., Cleasby J.L.,Logsdon G.S. and O'Melia C.R. (2010) Granular Media Filtration, Chapt.10 in Water Quality & Treatment, 6th Edtn., AWWA & McGrawHill.

MWH (2005) Water Treatment Principles and Design (2nd Edtn.), Wiley

Purchas D.B. (1971) Industrial Filtration of Liquids (2nd Edtn), Leonard Hill, UK

Stuetz R. (2009) Principles of Water and Wastewater Treatment Processes, IWA

Related Publications

Hexavalent Chromium Removal Using Anion Exchange and Reduction With Coagulation and Filtration - M McGuire, N Blute, G Qin, P Kavounas, D Froelich, L Fong 
Publication Date: Apr 2008 - ISBN - 9781843396208