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.
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.
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.