There are many ways to classify filters:
1. Filtration types. Depth, surface, and screen are general filter types.
2. Driving force. Flow through the filter can be induced by pumps (pressure), centrifugal force, or gravity.
3. Function. The goal of the filtration process is either retention of the dry solid when the filter cake is of value or disposal of the filter cake when process liquid is of value.
4. Operating cycle. The cycle of operation can be batch mode or continuous.
5. Nature of the solid. The accumulation of solids within a filter matrix can be either deformable (compressible) or rigid (incompressible).
The classification of filters is not exclusive and the distinction between them is arbitrary. Here the characterization of filters will be based on the type of filtration, the characteristic generally used in utility and service water filtration systems.
Interceptors, strainers, and filters are all devices used to reduce (or remove) and retain suspended solids. Other separation processes, such as sedimentation and centrifugation, that are used to treat large quantities of water or for dewatering, are outside the scope of this book. Design and selection criteria for specific contaminant removal are provided where appropriate in various other chapters discussing individual systems.
The distinction between filtration and water purification is arbitrary. Methods such as membrane and membrane exchange filtration that removes ions, allows preferential passage of specific substances, and does not conform to the previous definition of filtration are considered water purification methods and are discussed in Chap. 4.
GENERAL
Feed water, raw water, and source water are various ways of referring to a solution whose components are intended to be separated. Filtration is the process used for separation and retention of suspended and colloidal particles by mechanical capture and adsorption from fluids by passage through a porous medium. Mechanical capture physically prevents a contaminant particle from passing through a barrier with openings (pores). Adsorption is the attraction to and adhering of a particle to the surface of the filter medium. Adsorption can occur even if the pore is larger than the particle. This attraction is due to a variety of surface chemical forces between the particle and filter medium.
The mechanical properties of the particles suspended in the water stream must also be considered. At one extreme are solid, undeformable particles such as sand or quartz, and at the other extreme are gelatinous or deformable materials such as synthetic colloids and bacteria. Because they can deform, they are more likely than hard particles of the same size to pass through a filter.
FILTER CATEGORIES AND DEFINITIONS
Screen, surface, and depth filtration are the three broad categories of the filtering process.
A screen filter is best thought of as a single, thin layer of a material that has a symmetrical arrangement of openings or passages called pores. These pores trap all particles larger than the pore size on the surface of the filter. This process is called sieving, or size exclusion, and is the classic filtration method. Sieving can also be referred to as screening or straining. Screen filtration is essentially absolute because any particle larger than the pore size cannot pass through. Another mechanical capture mechanism, called bridging, occurs as particles captured by direct interception form a particle mat, or bridge, across the filter medium. By partially blocking the filter pores, this bridge or filter cake may produce a smaller filter pore structure that will aid in particle capture. Examples of screen filters are woven metal, nylon, and Dacron mesh. Cast polymeric membranes are used where the smallest size pores are required for sub micron and macromolecular separations.
A surface filter is thicker than the screen filter and constructed from thick or multiple layers of filter media, often glass or polymeric fibers. When the water passes through a surface filter, particles larger than the spaces within the fiber matrix are retained, primarily on the surface. Smaller particles are trapped within the matrix, giving this type of filter the properties of both a screen and depth filter.
A depth filter relies on the density and thickness of the layers to mechanically trap the particles, and it will retain relatively large quantities of them. Depth filtration occurs on the surface and throughout all or part of the filter medium as the water passes through a complex network of flow channels. The particles are retained by random adsorption and mechanical entrapment. Depth filters can be of two types, granular and preformed. Preformed depth filters are composed of fibrous or sintered materials that have a random pore structure. Granular depth filters have either a graded or consistent density of granular media and typically are long in length. Graded granular filters have layers of media that become progressively denser through the matrix as water flows through them. Constant density granular filters’ have the same size filter media or openings throughout the matrix.
A filter that is hydrophilic is one that has an affinity for water; it can be wetted with almost any liquid. A hydrophobic filter is one that cannot be wetted by an aqueous solution. Some filter materials may leach substances into the fluid as it is processed, thereby affecting its purity. Such substances, called extrac tables, can be minimized by preflushing. There is a test for plastics conforming to USP class VI that is used to ensure that there will be no adverse reaction of body fluids to extractables from filter housing or media materials.
The molecular weight of any compound is measured in daltons. Some filter media measure passage through the filter by molecular weight for separation of one compound from another.
1. Filtration types. Depth, surface, and screen are general filter types.
2. Driving force. Flow through the filter can be induced by pumps (pressure), centrifugal force, or gravity.
3. Function. The goal of the filtration process is either retention of the dry solid when the filter cake is of value or disposal of the filter cake when process liquid is of value.
4. Operating cycle. The cycle of operation can be batch mode or continuous.
5. Nature of the solid. The accumulation of solids within a filter matrix can be either deformable (compressible) or rigid (incompressible).
The classification of filters is not exclusive and the distinction between them is arbitrary. Here the characterization of filters will be based on the type of filtration, the characteristic generally used in utility and service water filtration systems.
Interceptors, strainers, and filters are all devices used to reduce (or remove) and retain suspended solids. Other separation processes, such as sedimentation and centrifugation, that are used to treat large quantities of water or for dewatering, are outside the scope of this book. Design and selection criteria for specific contaminant removal are provided where appropriate in various other chapters discussing individual systems.
The distinction between filtration and water purification is arbitrary. Methods such as membrane and membrane exchange filtration that removes ions, allows preferential passage of specific substances, and does not conform to the previous definition of filtration are considered water purification methods and are discussed in Chap. 4.
GENERAL
Feed water, raw water, and source water are various ways of referring to a solution whose components are intended to be separated. Filtration is the process used for separation and retention of suspended and colloidal particles by mechanical capture and adsorption from fluids by passage through a porous medium. Mechanical capture physically prevents a contaminant particle from passing through a barrier with openings (pores). Adsorption is the attraction to and adhering of a particle to the surface of the filter medium. Adsorption can occur even if the pore is larger than the particle. This attraction is due to a variety of surface chemical forces between the particle and filter medium.
The mechanical properties of the particles suspended in the water stream must also be considered. At one extreme are solid, undeformable particles such as sand or quartz, and at the other extreme are gelatinous or deformable materials such as synthetic colloids and bacteria. Because they can deform, they are more likely than hard particles of the same size to pass through a filter.
FILTER CATEGORIES AND DEFINITIONS
Screen, surface, and depth filtration are the three broad categories of the filtering process.
A screen filter is best thought of as a single, thin layer of a material that has a symmetrical arrangement of openings or passages called pores. These pores trap all particles larger than the pore size on the surface of the filter. This process is called sieving, or size exclusion, and is the classic filtration method. Sieving can also be referred to as screening or straining. Screen filtration is essentially absolute because any particle larger than the pore size cannot pass through. Another mechanical capture mechanism, called bridging, occurs as particles captured by direct interception form a particle mat, or bridge, across the filter medium. By partially blocking the filter pores, this bridge or filter cake may produce a smaller filter pore structure that will aid in particle capture. Examples of screen filters are woven metal, nylon, and Dacron mesh. Cast polymeric membranes are used where the smallest size pores are required for sub micron and macromolecular separations.
A surface filter is thicker than the screen filter and constructed from thick or multiple layers of filter media, often glass or polymeric fibers. When the water passes through a surface filter, particles larger than the spaces within the fiber matrix are retained, primarily on the surface. Smaller particles are trapped within the matrix, giving this type of filter the properties of both a screen and depth filter.
A depth filter relies on the density and thickness of the layers to mechanically trap the particles, and it will retain relatively large quantities of them. Depth filtration occurs on the surface and throughout all or part of the filter medium as the water passes through a complex network of flow channels. The particles are retained by random adsorption and mechanical entrapment. Depth filters can be of two types, granular and preformed. Preformed depth filters are composed of fibrous or sintered materials that have a random pore structure. Granular depth filters have either a graded or consistent density of granular media and typically are long in length. Graded granular filters have layers of media that become progressively denser through the matrix as water flows through them. Constant density granular filters’ have the same size filter media or openings throughout the matrix.
A filter that is hydrophilic is one that has an affinity for water; it can be wetted with almost any liquid. A hydrophobic filter is one that cannot be wetted by an aqueous solution. Some filter materials may leach substances into the fluid as it is processed, thereby affecting its purity. Such substances, called extrac tables, can be minimized by preflushing. There is a test for plastics conforming to USP class VI that is used to ensure that there will be no adverse reaction of body fluids to extractables from filter housing or media materials.
The molecular weight of any compound is measured in daltons. Some filter media measure passage through the filter by molecular weight for separation of one compound from another.
FILTER RATINGS
Filters and strainers are rated in several ways. Absolute and nominal ratings are based on the size particle the filter is expected to capture and retain. Particles are measured in micrometers (microns), which is 1 / 1,000,000 m (1 / 25,000 in) and abbreviated m. This rating is a single number called the micrometer (micron) rating. The micron rating of a filter or strainer can be absolute or nominal. These ratings are often misunderstood and this is an area of confusion in the filtration industry. Another method is called the beta rating, which is based on actual particle counts of different particle sizes of both the influent and effluent liquid stream. The beta rating is considered the most accurate rating measurement of a filter. Refer to Table 3.1 for the relationship between beta value and percent removal efficiency.
Efficiency is a measure of particle removal. It indicates what percent of particles above a certain size will be retained. For absolute rated filters, the rated pore size indicates 100 percent removal and is based on the log reduction values associated with bacterial retention testing. Because the pore size of some filters is not well defined, it is not possible to assign those filters an absolute rating. Instead they are given a nominal pore rating, which indicates the particle size above which a predictable percentage of particulates will be retained. As an example, a nominally rated 1.0- m depth filter will remove 90 to 95 percent of all particles 1.0 m or larger. For a surface filter of the same rating, the efficiency would be 99.99 percent.
An absolute micron rating indicates the smallest size particle that the filter will capture; no particles of that diameter or larger will pass through the filter. The absolute rating generally depends on sieving, since the capture of particles by adsorption is never assured. Since absolute ratings are generally unrealistic for most services, nominal ratings are the most common method used to rate filters. One exception is in pharmaceutical service, where absolute ratings are required to assure that all particulates of a certain size are removed.
Nominal ratings allow the filter rating to consider particles retained by adsorption. The nominal rating has no generally accepted definition in the industry, and there are no industry standards. As defined by ANSI, the nominal rating is an arbitrary micrometer value indicated by the filter manufacturer. Due to its lack of reproducibility, this rating is depreciated. The ambiguity of this rating method makes it difficult to achieve reliable and consistent results. Many manufacturers use different methods to rate their filters, for example, expressing the results gravimetrically, which does not represent the particle size and number in the effluent stream. Some have specific test conditions that do not represent the actual conditions for which the filters will be used. These test conditions may use fine or coarse particles such as AC test dust, latex beads, carbon fines, or bacteria. The nominal rating can be used as a guideline, provided that the micron rating includes the percent removal efficiency rating of that micron size.
Void volume of preformed fibrous media is the ratio of pore area to the fiber diameter of the filter media. If all other factors are equal, the medium with the greatest void volume will have the longest life and lowest initial clean pressure drop per unit thickness. Factors such as strength, compressibility of the fiber material under pressure (which reduces void volume), cost, and compatibility of the media with the water contaminants being removed should all be considered when selecting a filter for a specific application.
Filters and strainers are rated in several ways. Absolute and nominal ratings are based on the size particle the filter is expected to capture and retain. Particles are measured in micrometers (microns), which is 1 / 1,000,000 m (1 / 25,000 in) and abbreviated m. This rating is a single number called the micrometer (micron) rating. The micron rating of a filter or strainer can be absolute or nominal. These ratings are often misunderstood and this is an area of confusion in the filtration industry. Another method is called the beta rating, which is based on actual particle counts of different particle sizes of both the influent and effluent liquid stream. The beta rating is considered the most accurate rating measurement of a filter. Refer to Table 3.1 for the relationship between beta value and percent removal efficiency.
Efficiency is a measure of particle removal. It indicates what percent of particles above a certain size will be retained. For absolute rated filters, the rated pore size indicates 100 percent removal and is based on the log reduction values associated with bacterial retention testing. Because the pore size of some filters is not well defined, it is not possible to assign those filters an absolute rating. Instead they are given a nominal pore rating, which indicates the particle size above which a predictable percentage of particulates will be retained. As an example, a nominally rated 1.0- m depth filter will remove 90 to 95 percent of all particles 1.0 m or larger. For a surface filter of the same rating, the efficiency would be 99.99 percent.
An absolute micron rating indicates the smallest size particle that the filter will capture; no particles of that diameter or larger will pass through the filter. The absolute rating generally depends on sieving, since the capture of particles by adsorption is never assured. Since absolute ratings are generally unrealistic for most services, nominal ratings are the most common method used to rate filters. One exception is in pharmaceutical service, where absolute ratings are required to assure that all particulates of a certain size are removed.
Nominal ratings allow the filter rating to consider particles retained by adsorption. The nominal rating has no generally accepted definition in the industry, and there are no industry standards. As defined by ANSI, the nominal rating is an arbitrary micrometer value indicated by the filter manufacturer. Due to its lack of reproducibility, this rating is depreciated. The ambiguity of this rating method makes it difficult to achieve reliable and consistent results. Many manufacturers use different methods to rate their filters, for example, expressing the results gravimetrically, which does not represent the particle size and number in the effluent stream. Some have specific test conditions that do not represent the actual conditions for which the filters will be used. These test conditions may use fine or coarse particles such as AC test dust, latex beads, carbon fines, or bacteria. The nominal rating can be used as a guideline, provided that the micron rating includes the percent removal efficiency rating of that micron size.
Void volume of preformed fibrous media is the ratio of pore area to the fiber diameter of the filter media. If all other factors are equal, the medium with the greatest void volume will have the longest life and lowest initial clean pressure drop per unit thickness. Factors such as strength, compressibility of the fiber material under pressure (which reduces void volume), cost, and compatibility of the media with the water contaminants being removed should all be considered when selecting a filter for a specific application.
MEMBRANE FILTER TESTING
An important feature of a membrane filtration system is its ability to be tested before and after filtration runs. Testing can detect a damaged membrane, ineffective seals, or a system leak that may result in passage of contaminants that the filter is designed to trap. These tests are commonly called integrity tests. Testing before and after a run will ensure that the entire system is intact, thereby validating the process. Prior to testing, cleaning to remove large-scale contamination (and sterilizing the filter and apparatus if necessary to ensure elimination of microbial contamination) is required.
The type of test selected is dependent on the specific filter chosen. However, if the previous history of a specific filter is not available, the only accurate method of testing the filter is to place it in service and run an on-site fouling and compatibility test.
Air Permeability Test
An air permeability test is normally used to test wound cartridges. It is a simple, nondestructive test that correlates well with filter performance and it is considered more revealing than micron rating.
Bubble Point Test
Membrane filters have discrete, uniform passages from one side of the membrane to the other which, in effect, are fine uniform capillaries. The bubble point test is based on the fact that a liquid is held in these capillary-like structures by surface tension and the minimum pressure required to force this liquid out of the capillary space is a measure of the capillary diameter. The pressure required is inversely proportional to the largest pore size. After the filter is wetted, air pressure upstream of the filter is increased.
There are two widely used variations of the bubble point test. The first is the visual test. For this variation, the downstream side is watched for the appearance of bubbles, which indicate that the air is passing through the capillaries. The pres- sure that produces a steady, continuous stream of bubbles is the bubble point pres- sure. The second variation is the monitored method, where a pressure drop will occur as the gas begins to flow through the filter.
It is not necessary to determine the exact pressure of a given filter to prove its integrity. If the pressure exceeds the minimum point determined by the manufacturer of the filter, its integrity is assured. The bubble point test is also used to test the integrity of the filter cartridge.
Diffusion Test
In a high volume system where a large volume of water must be displaced before bubbles can be detected, a diffusion test should be performed instead of the bubble point test. This test is based on the fact that in a wetted membrane filter under pressure, air flows through the water-filled pores of the filter at a differential pres- sure below the bubble point pressure by a diffusion process following Fick’s law. In small filters, the flow of air is very slow. But in a large filter it is significant and can be measured to perform a sensitive filter integrity test. In a wetted filter, a constant air pressure is applied at approximately 80 percent of the bubble point pressure established for that particular filter.
There are two widely used variations of the diffusion test. The first is the forward flow test, which relies on direct measurement of the diffusive gas flow rate. This flow rate is measured either by instruments placed in the gas flow upstream of the filter or by calculating the volume of airflow according to the rate of flow of dis- placed water downstream of the filter. The second method, called the pressure decay method, calculates the loss of diffusion gas pressure from a known volume of gas over a period of time.
Water Breakthrough Test
Similar to the bubble test except that water is used instead of air. Water pressure is increased on the upstream side of the filter, and the pressure that results in a steady stream of water downstream of the filter is recorded. The breakthrough pressure must be correlated to empirical data on contaminant retention from the manufacturer in order to be a valid test.
Water Intrusion Test
Also called the water pressure integrity test, this is often used for hydrophobic filters (which resist wetting by water). This test requires that the filter be wetted by an alcohol / water mixture. Water pressure is applied upstream of the filter and the pressure decay is measured. Care must be taken to test with water from the same source because of variations in surface tension. When used as a vent filter, the membrane must be dried before being placed back in service.
An important feature of a membrane filtration system is its ability to be tested before and after filtration runs. Testing can detect a damaged membrane, ineffective seals, or a system leak that may result in passage of contaminants that the filter is designed to trap. These tests are commonly called integrity tests. Testing before and after a run will ensure that the entire system is intact, thereby validating the process. Prior to testing, cleaning to remove large-scale contamination (and sterilizing the filter and apparatus if necessary to ensure elimination of microbial contamination) is required.
The type of test selected is dependent on the specific filter chosen. However, if the previous history of a specific filter is not available, the only accurate method of testing the filter is to place it in service and run an on-site fouling and compatibility test.
Air Permeability Test
An air permeability test is normally used to test wound cartridges. It is a simple, nondestructive test that correlates well with filter performance and it is considered more revealing than micron rating.
Bubble Point Test
Membrane filters have discrete, uniform passages from one side of the membrane to the other which, in effect, are fine uniform capillaries. The bubble point test is based on the fact that a liquid is held in these capillary-like structures by surface tension and the minimum pressure required to force this liquid out of the capillary space is a measure of the capillary diameter. The pressure required is inversely proportional to the largest pore size. After the filter is wetted, air pressure upstream of the filter is increased.
There are two widely used variations of the bubble point test. The first is the visual test. For this variation, the downstream side is watched for the appearance of bubbles, which indicate that the air is passing through the capillaries. The pres- sure that produces a steady, continuous stream of bubbles is the bubble point pres- sure. The second variation is the monitored method, where a pressure drop will occur as the gas begins to flow through the filter.
It is not necessary to determine the exact pressure of a given filter to prove its integrity. If the pressure exceeds the minimum point determined by the manufacturer of the filter, its integrity is assured. The bubble point test is also used to test the integrity of the filter cartridge.
Diffusion Test
In a high volume system where a large volume of water must be displaced before bubbles can be detected, a diffusion test should be performed instead of the bubble point test. This test is based on the fact that in a wetted membrane filter under pressure, air flows through the water-filled pores of the filter at a differential pres- sure below the bubble point pressure by a diffusion process following Fick’s law. In small filters, the flow of air is very slow. But in a large filter it is significant and can be measured to perform a sensitive filter integrity test. In a wetted filter, a constant air pressure is applied at approximately 80 percent of the bubble point pressure established for that particular filter.
There are two widely used variations of the diffusion test. The first is the forward flow test, which relies on direct measurement of the diffusive gas flow rate. This flow rate is measured either by instruments placed in the gas flow upstream of the filter or by calculating the volume of airflow according to the rate of flow of dis- placed water downstream of the filter. The second method, called the pressure decay method, calculates the loss of diffusion gas pressure from a known volume of gas over a period of time.
Water Breakthrough Test
Similar to the bubble test except that water is used instead of air. Water pressure is increased on the upstream side of the filter, and the pressure that results in a steady stream of water downstream of the filter is recorded. The breakthrough pressure must be correlated to empirical data on contaminant retention from the manufacturer in order to be a valid test.
Water Intrusion Test
Also called the water pressure integrity test, this is often used for hydrophobic filters (which resist wetting by water). This test requires that the filter be wetted by an alcohol / water mixture. Water pressure is applied upstream of the filter and the pressure decay is measured. Care must be taken to test with water from the same source because of variations in surface tension. When used as a vent filter, the membrane must be dried before being placed back in service.