Filtration

 Chapter 9 

FILTRATION

INTRODUCTION

• Filtration is a unique unit mechanical or physical process of separating suspended and colloidal particles from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. It is a process that removes particles from suspension in water. Removal takes place by a number of mechanisms that include straining, flocculation, coagulation, sedimentation and surface capture. Filters can be categorized by the main method of capture, for example, exclusion of particles at the surface of the filter media (straining), or deposition within the media (in-depth filtration).

• When the proportion of solids in a liquid is less, the term clarification is used. It is a common operation which is widely employed in the production of sterile products, bulk drugs, and in liquid oral formulation. The suspension of solid and liquid to be filtered is known as the “slurry”. The porous medium used to retain the solids is described as the filter medium; the accumulation of solids on the filter is referred to as the filter cake, while the clear liquid passing through the filter is the filtrate. The porous medium used to retain the solids is known as filter medium. The accumulated solids on the filter are referred as filter cake and the clear liquid passing through the filter is filtrate. The pores of the filter medium are smaller than the size of particles to be separated. Therefore, solids are trapped on the surface of the filter medium. After a particular point of time, the resistance offered by the filter cake is high that stops the filtration.

• Filtration is also used to describe some biological processes, especially in water treatment and sewage treatment in which undesirable constituents are removed by absorption into a biological film grown on or in the filter medium as in slow sand filtration.  

Water Filtration Process: 

• During filtration in a conventional down-flow depth filter, wastewater containing suspended matter is applied to the top of the filter bed. As the water passes through the filter bed, the suspended matter in the wastewater is removed by a variety of removal mechanisms. With passage of time, as material accumulates within the interstices of the granular medium, the head-loss through the filter starts to build up beyond the initial value. After some period of time, the operating head-loss or effluent turbidity reaches a predetermined head loss or turbidity value. The filter must be cleaned (backwashed) to remove the material (suspended solids) that has accumulated within the granular filter bed. 

• Backwashing is accomplished by reversing the flow through the filter. A sufficient flow of wash water is applied until the granular filtering medium is fluidized (expanded), causing the particles of the filtering medium to abrade against each other

Classification of Filtration Process: 

Depth filtration:

• Slow sand filtration
• Rapid porous and compressible medium filtration. 
• Intermittent porous medium filtration.
• Recirculating porous medium filtration

 Surface filtration: 

• Laboratory filters used for TSS test.
• Diatomaceous earth filtration.
• Cloth or screen filtration. 

Membrane flirtation:

Mechanisms of Filtration:

• The mechanisms whereby particles are retained by the filter medium are of significance only in the early stages of liquid filtration. Once a preliminary layer of particles has been deposited, the filtration is affected by the filter cake. The filter medium serves only as a support

• Straining: The simplest filtration procedure is “straining”, in which, like sieving, the pores are smaller than the particles. These particles are retained on the filter medium as a bed. 

• Entanglement: If the filter medium consists of a cloth with a nap or a porous felt, then particles become entangled in the mass of fibers. Usually, the particles are smaller than the pores, so that it is possible that impingement is involved. 

• Attractive forces: In certain circumstances, particles may collect on a filter medium as a result of attractive forces. The ultimate in this method is the electrostatic precipitator, where large potential differences are used to remove the particles from air streams. In practice, the filtration process may combine various mechanisms, but the solids removal is effected normally by a straining mechanism once the first complete layers of solids has begun to form the cake on the filter medium. 

OBJECTIVES 

The main learning objective of filtration is to separate solids from liquid or gas medium. The other objectives include:

• To eliminate the contaminant particles so as to recover dispersing fluid.
• To recover solid particles by eliminating the dispersing fluid.
• To produce high-quality solvents and solids. 
• To purify air and pharmaceutically useful gases by removing particulate matter. 
• To sterilize thermolabile parenteral products. 

APPLICATIONS

• Water Purification: Purification of water for pharmaceutical use is the most important application of filtration. Water used in pharmaceutical industries for various applications has to comply with different standards specified in official pharmacopoeias. These standards are namely, purified water, Water For Injection and Sterile Water for Injection. To produce water complying with these standards potable water is required initially to be filtered. Filtration is commonly used for the removal of residual biological floc in settled effluents from secondary treatment before disinfection to remove residual precipitates from the metal salt or lime precipitation or phosphate

• Pharmaceutical Industry: Filtration, as a physical operation is very important in chemistry for the separation of materials of different chemical composition. This is one of the most important techniques used by chemists to purify compounds. Various enzymes, amino acids, antibiotics, pharmaceutical intermediates, bulk drugs, medicine, blood products, antibiotics, calcium phytate, Chinese inositol, growth derived sand, organic phosphorus, glucoamylase etc. are purified by using filtration. Filtration may be simultaneously combined with other unit operations to process the feed stream, as in the bio-filter, which is a combined filter and biological digestion device. 

• Biopharmaceutical Industry: The separation process of filtration is widely used in the biopharmaceutical industry to remove contaminants such salts, particulate matter, micro-organisms etc. from liquids, air, and gases.

• Chemical Industry: Filtration is used for separation and purification of dyes, pesticides, silicic acid, glycerin, white carbon, sodium carbonate, additives, basic chemicals, chemical filler, pigment, white alumina, manganese, caustic soda, soda ash, and alkali salts mud, saponins, and graphite etc.

• Food Industry: Wine filter press, yeast, fruit juice filter press, edible oil, vegetable oil, soy sauce, sugar mills, rice wine, white wine, fruit juices, soft drinks, beer, yeast, citric acid, vegetable protein, plant density sweetener, glucose etc are filtered to remove unwanted matter.

• Environmental Engineering: Filtration is used for separation and purification of chemical wastewater, mining wastewater, domestic wastewater, leather wastewater and salt mud wastewater. 

• Clay Industry: Filtration is used for separation and purification of kaolin, bentonite, activated clay, clay and electronic ceramics cla

• Heating, ventilation and air conditioning (HVAC): Filtration is used for obtaining high quality air from poor quality air that contain micro-organisms and particulate matter such as dust and smoke; volatile organic compounds and allergens responsible for respiratory illnesses and allergy, asthma and sick-building symptoms. Ideally, these triggers are eliminated or reduced significantly by the air filters in a building's HVAC system. Advances in air filtration have led to the development of systems that provide superior air with reduced energy costs and help commercial and institutional buildings to achieve green-building milestones.

• Making new food and beverage products: Bacteria and spoilage organisms in milk are easily removed by micro filters with pore sizes ranging from 0.1 to 20 microns. Ultra-filtration units with pores ranging from 0.01-0.2 microns have been shown to affect the appearance and sensory properties of fluid milk because of the protein molecules that can be retained and then added back.

• Making new food and beverage products: Bacteria and spoilage organisms in milk are easily removed by micro filters with pore sizes ranging from 0.1 to 20 microns. Ultra-filtration units with pores ranging from 0.01-0.2 microns have been shown to affect the appearance and sensory properties of fluid milk because of the protein molecules that can be retained and then added back.

THEORIES OF FILTERATION 

• The flow of any liquid through any porous medium offers a resistance to its flow. The rate of filtration in such cases is expressed.

• The net driving force in filtration is pressure above the medium minus pressure below the medium. The resistance offered by filter medium is not constant over the period of filtration as it goes on increasing with time due to particle deposition on filter medium. Rate is expressed as volume of filtrate per unit time as dv/dt.  

• Depending on dispersing (fluid) medium filtration theory is divided in two parts.

Gas Filtration Theory     

• Gas filtration includes filtration of aerosol and lyosol. Membrane filters and nucleopore filters are used for gas filtration which works on three mechanisms.

• Diffusion deposition: In this mechanism the path followed by individual small particles do not coincide with the streamlines of the fluid because of Brownian motion. With decreasing particle size the intensity of Brownian motion increases and thus as a consequence, the intensity of diffusion deposition also increases. 

• Direct interception: This mechanism involves finite size particles. These particles are intercepted as they approach the collecting surface to a distance equal to its radius. A special case of this mechanism is the so-called ‘sieve effect’, or ‘sieve mechanism’. 

• Inertial deposition: The presence of a body in the flowing fluid results in a curvature of the streamlines in the neighborhood of the body. Because of their inertia, the individual particles do not follow the curved streamlines but are projected against the body and may deposit there. It is obvious that the intensity of this mechanism increases with increasing particle size and velocity of flow. 

• Gravitational deposition: Individual particles have a certain sedimentation velocity due to gravity. As a consequence, the particles deviate from the streamlines of the fluid and, owing to this deviation; the particles may touch a fiber. 

• Electrostatic deposition: Both the particles and the fibers in the filter may carry electric charges. Deposition of particles on the fibers may take place because of the forces acting between charges or induced forces.

Liquid Filtration Theory 

• The term filtration covers all processes in which a liquid containing suspended solid is freed of some or the entire solid when the suspension is drawn through a porous medium. Filtration is of two types namely, ‘Cake filtration’ where the proportion of solids in suspension is large and most of the particles are collected in the filter cake which can subsequently be detached from the medium and ‘Deep bed filtration’ where the proportion of solids is very small. For example, in water filtration, the particles are often considered as smaller than the pores of the filter medium and penetrate at a considerable depth for being captured. 

Kozeny-Carman Equation: 

• In filtration because the particles forming the cake are small and the flow through bed is slow, streamline conditions are almost invariably obtained, and therefore at any instant it may be represented.

Darcy’s Equation: 

• When using Poiseuille equation for filtration it is considered that capillaries in filter medium are highly irregular and non-uniform. In order to approximate the flow rate, the height of cake is taken as length of capillaries and a correction factor is introduced for the radius of capillaries. This makes flow rate more simplified and is expressed as

• The permeability coefficient (K) in depends on porosity, specific surface area and compressibility of cake. This coefficient may be defined as the flow rate of a liquid of unit viscosity across unit area of cake with unit thickness under the unit pressure gradient.

FACTORS INFLUENCING FILTRATION

The most important factors on which the filtration depends are

• Proportion of solids in the slurry. 
• Properties of the liquid namely density, viscosity, corrosiveness etc. 
• Properties of the solid such as particle shape, size, size distribution and compressibility.
• The properties of filter medium especially resistance and initial layers of cake formed. 

The following are the factors that influences rate of filtration:

• Permeability Coefficient: According to Darcy’s equation permeability coefficient (K) represents the resistance of both the filter medium and the filter cake. As the thickness of the cake increase, the rate of filtration decreases. Also, the surface area of the particles, the porosity of the cake, and rigidity or compressibility of the particles could also affect the permeability of the cake. 

• Area of Filter Medium: The total volume of filtrate flowing through the filter is proportional to the area of the filter. The area can be increased by using larger filters. For example, in the rotary drum filter, the continuous removal of the filter cake gives an infinite area for filtration.

• Pressure Drop: The rate of filtration is proportional to the pressure difference across both the filter medium and filter cake. The pressure drop can be achieved in a number of ways:

• Gravity: A pressure difference could be obtained by maintaining a sufficient head of slurry above the filter medium. The pressure developed will depend on the density of the slurry

• Vacuum: The pressure below the filter medium may be reduced below atmospheric pressure by connecting the filtrate receiver to a vacuum pump and creating a pressure difference across the filter. 

• Pressure: The simplest method is to pump the slurry onto the filter under pressure.

• Centrifugal force: Application of higher centrifugal force on slurry increases rate of filtration.

• Viscosity of Filtrate: It is considered that an increase in the viscosity of the filtrate increases the resistance of flow, so that the rate of filtration is inversely proportional to the viscosity of the fluid. This problem can be overcome by two methods:

• Temperature: The rate of filtration may be increased by raising the temperature of the liquid, which lowers its viscosity. However, it is not practicable if thermolabile materials are involved or if the filtrate is volatile. 

• Temperature: The rate of filtration may be increased by raising the temperature of the liquid, which lowers its viscosity. However, it is not practicable if thermolabile materials are involved or if the filtrate is volatile. 

• Thickness of Filter Cake: The rate of flow of the filtrate through the filter cake is inversely proportional to thickness of the cake. Preliminary decantation may be useful to decrease the amount of the solids that ultimately help to increase filtration rate

• Particle Size of Solids: Generally the larger the particle size the higher the filtration rate (Kg/m2 /h). A small average particle size, a narrow distribution range has a high filtration rate. 

• Ratio of Slimes to Coarser Particles: The slimes (or extreme fines) in a slurry affect filtration rates to a vastly greater extent than their percentage. Particularly, difficult slurry is one that contains relatively coarse particles and a number of very fine or slimy particles with little or no intermediate size.  

• Flocculation/Dispersion of Fine Solids: Flocculation is generally desirable for slurries of fine solids which are in a dispersed state and generally filter poorly. The wide variety of polyelectrolyte flocculants provides space for a substantial improvement in filtration rates. Effective use of flocculants, especially polyelectrolytes, on moderately high concentration slurry requires strong agitation to get good solids-flocculant contact. A minimum of further agitation and minimum aging are important. Some slurry may be so viscous as to create filtering problems and a dispersant may be a better way to gain fluidity than dilution.

• Slurry Age: Sometimes processes involve detention times which provide a conditioning effect modifying filter performance. Samples stored for longer time for testing involve a risk that excessive aging may have some effect on filtration. 

• Agitation Speed: Some slurry, particularly with a wide particle size range tends to classify in the test slurry container or the filter tube. Increasing the agitation speed (or stirring) to a point that the coarse and fine particles are always thoroughly mixed may be desirable. A too high speed could limit cake thickness; prevent coarser particles from forming in the cake or cause delicate flocs to break down. 

• Cycle Time: Cycle time of the filter drum speed is generally expressed in seconds or minutes per revolution. Generally, the faster the drum speed the higher the output. However, under these conditions, the cake is thinner and sometimes wetter, so discharge may deteriorate. At all times, a dischargeable cake must be produced. 

• Cake Compression: Cake compression is normally achieved as an adjunct to the filtration step to reduce cake moistures of compressible cakes. 

• Type of Filter Medium: Filtering characteristics of fabrics depend mostly on the type of yarn and weave. Yarns can be mono-filament, multi-filament, spun from staple fiber, or a combination of the latter two. A high twist can make a multi-filament perform more like a mono-filament. Permeability and porosity are prime qualities in cloth selection. The Frazier permeability rating, expressed as cfm/sq ft, is a measure of air flow at one-half inch water pressure through a dry cloth, and is comparable to per cent open area. Porosity and particle retention may not be accurately indicated by permeability; there being no direct measure of porosity. \

• Filter Cloth Condition: Cloth conditioning refers to the reduction of pore size or open area due to entrapment of fine solids in the interstices. 

•  Filter Aids: Filter aids like diatomaceous earth, perlite, powdered coal, and fly-ash or paper pulp may be added to the slurry to increase its filtration rate and cake porosity.  Q\

• Solid Concentration in Slurry: In general, the greater the percentage of suspended solids in a given slurry, the higher the cake filter rate in Kg/m2 /h and the lower the filter rate in m3 /m2 /h. Where maximum solids capacity is desired it is advisable to consider thickening the slurry by gravity. In some applications involving thickening with sludge recycle, particle size is actually increased and both cake and filtrate rates can increase. 

• Filter Thickening: Filter thickening normally occurs in a continuous filter rotating in a tank containing slurry wherein the solids in the filter tank increase in concentration and shift to a coarser size distribution. While an equilibrium concentration and size distribution is usually obtained, it may sometimes be necessary to dilute the pulp. 

•  Slurry pH: The slurry pH and particle dispersion are closely related. The change in pH can be one of the most effective methods to achieve flocculation and thus the improved filterability. 

FILTER AID 

• The “Filter Aids” is a group of inert materials that can be used in filtration pretreatment. Usually, the resistance to flow due to the filter medium itself is very low, but will increase as a layer of solids builds-up, blocking the pores of the medium and forming a solid cake. The filter aid is used to prevent the medium from becoming blocked and to form an open, porous cake, so reducing the resistance to flow of the filtrate.

• There are two objectives related to the addition of filter aids. One is to form a layer of second medium which protects the basic medium of the system. This is commonly referred to as “precoat”. The second objective of filter aids is to improve the flow rate by decreasing cake compressibility and increasing cake permeability. This type of usage is termed as “admix” or “body feed”. Filtration without filter aid, with precoat, and with precoat and body feed is shown. The particles must be inert, insoluble, incompressible, and irregular shaped. The common filter aids are diatomaceous earth (Kieselguhr, DE), perlite, cellulose and others

• 
  Diatomaceous earth: Diatomaceous earth (DE) is the skeleton of ancient diatoms. They are mined from ancient seabed, processed, and classified to make different grade of filter aids. DE is the most commonly used filter aid today. However, the crystalline type DE is a suspicious carcinogen and inhalation needs to be avoided during handling. There are different grades of commercial DE. A finer grade may be employed to increase the clarity of filtrate. The smaller the filter aid particle size, the smaller the process particles can be removed. However, the filtration rate is lower. There is always a balance between initial filtrate clarity and filtration rate. The particle size capture by various filter aids may also vary because of liquid viscosity, surface charge etc

• Perlite: Perlite is another important mineral filter aid. It is a particular variety of naturally occurring glassy volcanic rock, characterized by onion like, splintery breakage planes. After crushing and heating, this rock expands in an explosive fashion to about ten times its original volume. Diatomaceous earth and perlite are silica-based minerals. 

• Perlite: Perlite is another important mineral filter aid. It is a particular variety of naturally occurring glassy volcanic rock, characterized by onion like, splintery breakage planes. After crushing and heating, this rock expands in an explosive fashion to about ten times its original volume. Diatomaceous earth and perlite are silica-based minerals. 

Problems in Filtration

There are three major types of filter problems. They can be caused by chemical treatment before the filter, control of filter flow rate, and backwashing of filters.

• Chemical addition before filtration: Coagulation and flocculation stages of the water treatment must be monitored continuously. Adjustments in the amount of coagulant added must be made frequently to prevent the filter from becoming overloaded with suspended material. This overload may cause the filter to prematurely reach its maximum head loss. If there is early turbidity breakthrough in the filter effluent, more coagulant may have to be added to the coagulation process. There may be a need for better mixing during the coagulation or the addition of more filter aid. If there is a rapid increase in filter head loss, too much coagulant may be clogging the filter. Less coagulant or less filter aid should be used.

• Control of filtration flow rate: When a filter is subjected to rapid changes in flow rate it causes filter media to be dirtier. When a feed flow changes, the filter flow also has to change to run the process smoothly. If there is an increase in feed rate, the flow rate should be increased gradually to reduce the impact on the filter. Addition of filter aids may also reduce the impact on the filter medium. This problem can be avoided by keeping one filter in reserve to accept this additional flow. Many plants are not operated continuously, and the start-up at the beginning causes a gush to the filter(s). The filters should be backwashed before putting them back into operation or operated to waste until the effluent meets the standards. 

• Backwashing of Filters: Backwashing of the filters is the single most important operation in the maintenance of the filters. If the filter is not backwashed effectively, problems may occur that may be impossible to correct without totally replacing the filter media. These problems could be caused by improper backwashing procedures.

FILTER MEDIA

• The surface upon which solids are deposited on or in a filter is called the “Filter medium”. Filters are used in several different industries, including pharmaceutical, food and beverage, cosmetic and chemical. Without efficient filters, the end products would be of poor quality and the safety element would not be there. The types of filter media and their characteristics are given.        

  Properties of Ideal Filter Medium

• It must be capable of delivering a clear filtrate at a suitable production rate. 
• It must withstand the mechanical stresses without rupturing or being compressed.
• No chemical or physical interactions with the components of the filtrate should occur.
• It must retain the solids without plugging at the start of filtration. 
• For sterile filtration the pore size must not exceed the dimension of bacteria or spores. 

Classification of Filter Media

The list of some of the main types of filter media that are used to keep products clear, consistent and uncontaminated is given below.

• Woven filters: Woven filters include wire screens and fabrics of cotton, wool, and nylon. Wire screens, for example, stainless steel is durable, resistance to plugging and easily cleaned. Cotton is a common filter, however, Nylon is superior for pharmaceutical use, since it is unaffected by mold, fungus or bacteria and has negligible absorption properties. Bag filter is common examples of this type. Bag filters come in several different varieties and are best suited for situations where large solids must be removed from a liquid. In this scenario, the liquid would flow through the bag and the solids would get caught inside. Bag filters may be made from a variety of different materials and are usually available in different lengths and micron ratings to suit whatever job you need it for.

• Non-woven filters: Filter paper is a common example of non-woven filter medium since it offers controlled porosity, limited absorption characteristic, and low cost. A panel filter is non-woven type filter. It is industrial filter that may come in a variety of different shapes and sizes, depending on the application. In the majority of cases, panel filters are plain and are found inside ventilation or air conditioning units.

• Membrane filters: Membrane filters are basic tools for micro-filtration, useful in the preparation of sterile solutions. These filters are made by casting of various esters of cellulose, or from nylon, Teflon, polyvinyl chloride. The filter is a thin membrane with millions of pores per square centimeter of filter surface. A cartridge filter is cylindrical in shape, but occasionally it looks flat. Cartridge filters use a barrier/sift method to remove sediment and harmful solids from liquid. Some cartridge filters are designed to remove microscopic elements, and some are designed to stop larger particles from getting into the finished product. Membrane filters are cartridge units and are economical and available in pore size of 100 µm to even less than 0.2 µm. They can be either surface cartridges or depth type cartridges. Surface cartridges are corrugated, and resin treated papers and used in hydraulic lines. 

• For example, ceramic cartridges and porcelain filter candles. They can be reused after cleaning. Depth type cartridges are made-up of cotton, asbestos or cellulose. These are disposable items, since cleaning is not feasible.

•  Porous plates: These include perforated metal or rubber plates, natural porous materials such as stone, porcelain or ceramics, and sintered glass. Sintered glass, sintered metal, earthenware and porous plastics are used for fabrication. These are used for its convenience and effectiveness.

• Hydraulic Filters: Hydraulic filters are industrial filters that are designed to purify petroleum-based liquids such as oils. These types of filters are often found in a different hydraulic system to prevent a breakdown caused by oil impurities. Hydraulic filters can also be used to purify water-based liquids.

• Strainers: Strainers often become part of the manufacturing process when the process contains solids that are too large to be caught up in other types of filter media. Strainers consist of baskets that can be removed and cleaned out regularly to prevent them from being stucked inside. Some strainers can be cleaned without interrupting the system, while others cause some disruption each time when they are cleaned. 

• Air Filters: Air filters are an important part of most industrial processes, as they remove dust, dirt and other particles from the air. Most air filters consist of a mesh that catches the particles when air is forced through. If the air is to be protected from gases and odors as well as particles, a high efficiency particulate air filter is used

•  Gas Filters: Gas filters are a type of industrial filters that helps to remove contaminants or particulates from a dry or liquid gas stream. Depending on the situation, the contaminants may be solid or liquid. Some of the different elements and accessories for gas filters include gas filter separators, coalescers and gas scrubbers. 

   

EQUIPMENTS USED IN FILTRATION

• Filtration equipment is used to filter, thicken, or clarify a mixture of different elements. There are several different ways to classify equipment. 

Types of Equipements 

Below is the description of various types of filtrations equipment: 

• Sedimentation equipment: This equipment uses a gravitational force or chemical process to cause particles to settle to the bottom. Equipment that uses flocculation and gravity sedimentation are included in this category

• Flocculation: Flocculation is the formation of a cake or aggregate, usually through a chemical process, although a magnetic field may be used for particles containing iron. Flocculation is an important process in the treatment of wastewater. 

• Gravity sedimentation: Gravity sedimentation is used to reduce the solids concentration of the material to be processed. It can be either a clarification or a thickening process. In gravity sedimentation, the heavier particles sink to the bottom under the force of gravity. The rate of settling varies depending on the difference in density between the liquid phase and the solids and the size of the solid particles.

• Gravity filtration equipment: Gravity filtration equipment uses the hydrostatic pressure of a pre-filter column above the filter surface to generate the flow of the filtrate. Common products include bag filters, gravity Nutsches, and sand filters. Bag filters are used mainly as collection equipment. They use bag-shaped woven-fabric or felt filters. Bag filters are not recommended for process filtration. A gravity Nitsche is a tank with a perforated or porous false bottom. They may or may not have a separate filter medium. The hydrostatic head of the slurry in the tank provides the filtration driving force. The sand filter is the most common type of gravity filters used. It is constructed of a tank containing layers of gravel and sand or pulverized anthracite. The size of the bed particle decreases from bottom to top of the bed. This granular bed forms the filter media. Sand filters are clarifying equipment that forms a cake on the surface. They are used almost exclusively for water conditioning. 

• Vacuum filtration equipment: Vacuum filtration is a category of liquid-solid separation equipment and this type of filtration equipment has many different types. Vacuum filters are available in batch (vacuum Nutsches and vacuum leaf filters) and continuous (drum filters, disk filters and horizontal filters) operating cycles. Continuous vacuum filters are widely used in the process industry. The three main classes of continuous vacuum filters are drum, disk, and horizontal filters. All of these vacuum filters have the following common features:

• For example, disc filters, horizontal belt filters, rotary drum filters, rotary drum pre-coat filters, table filters, tilting pan filters, tray filters, and vacuum Nutsches. 

• Pressure filtration equipment: Pressure filters operate at super atmospheric pressures at the filtering surface. The media is fed to the machine by diaphragm, plunger, screw and centrifugal pumps, blow cases and streams from pressure reactors. Most pressure filters are batch or semi-continuous machines. Rotary drum pressure filters have continuous operating cycles and are more expensive and less flexible than batch machines. For example, automatic pressure filters, candle filters, filter presses, horizontal plate pressure filters, polishing filters, and vertical pressure leaf filters. 

• Thickening equipment: Thickening equipments (Thickeners) are used to separate solids from liquids by means of gravity sedimentation. Most thickeners are larger, continuous operation pieces of equipment. They are used for heavy duty applications such as coal, iron ore taconites, copper pyrite, phosphates, and other beneficiation processes. Thickeners are a category of industrial filtration equipment that includes conventional thickeners, high-rate thickeners, lamella thickeners, and tray thickeners 

• Clarifying equipment: Clarifying equipment (Clarifiers) encompass conventional clarifiers, sludge-basket clarifiers, suction clarifiers, and reverse osmosis (RO) clarifiers. The primary end product of clarifiers is a clarified liquid. They are virtually identical in design to thickeners but have a lighter duty drive mechanism. They are generally used for industrial and residential waste. For example, conventional clarifiers, reverse osmosis equipment, sludge-blanket, suction, and other clarifiers.

• Centrifugal separators: Centrifugal separators uses centrifugal force to separate solid particles from a liquid solution. They include both centrifuges and hydro cyclones. A centrifuge is a device for separating particles from a solution according to their size, shape, density, viscosity of the medium and rotor speed. A hydro cyclone uses centrifugal force to separate particulate elements of different sizes, shapes, and densities

Plate and Frame Filter Press 

Principle: 

• The mechanism of this filter is surface filtration. The slurry enters the frame by pressure and flows through the filter medium. The filtrate is collected on the plates and sent to the outlet. A number of frames and plates are used so that surface area increases and consequently large volumes of slurry can be processed simultaneously with or without washing\

Construction: 

• The construction of a plate and frame filter press is shown. The filter press is made of two types of units, plates and frames.

The components of this filter are as follows

• Frame: It maintains the slurry reservoir and has inlet (eye) for slurry.
• Filter medium: It is for solid retention. 
• Plate: This equipment along with section-supports the filter medium, receives the filtrate and has outlet (eye).

Assembly of plate and frame filter press. 

• Plate and frame are usually made of aluminum alloy. Sometimes these are also coated for protection against corrosive chemicals and made suitable for steam sterilization. Frame contains an open space inside wherein the slurry reservoir is maintained for filtration and an inlet to receive the slurry. The plate has a studded or grooved surface to support the filter cloth and an outlet. The filter medium (usually cloth) is fitted between plate and frame. Frames of different thicknesses are available and frame with optimum thickness is chosen. Selection is mainly based upon the thickness of the cake formed during filtration. Plate, filter medium, frame, filter medium and plate are arranged in the sequence as shown in Fig. 9.3 and clamped in a supporting structure. A number of plates and frames are used to have a large surface area. These filtration units are operated in parallel. Channels for the slurry inlet and filtrate outlet can be arranged by fitting inlet to the plates and frames, these join together to form a channel. In some types, only one inlet channel is formed, while each plate is having individual outlets controlled by valves.

Working:

• The working of the frame and plate process can be described in two steps, namely filtration and washing of the cake (if desirable). The working of a plate and frame press is shown in Fig. 9.4. Slurry enters the frame from the feed channel and passes through the filter medium on to the surface of the plate. The solids form a filter cake and remain in the frame. The thickness of the cake is usually half the frame thickness as filtration occurs on each side of the frame. Thus, two filter cakes are formed, which meet eventually in the centre of the frame. The optimum thickness of filter cake for any slurry depends on the solid content in the slurry and the resistance of the filter cake. The filtrate drains between the projections on the surface of the plate and escapes from the outlet. With the time resistance of the cake increases and the filtration rate decreases. It is advisable to stop the process at a certain point rather than continuing at very low flow rates. On completion of filtration the press is emptied and the cycle is restarted.

• Washing: Generally washing of the filter cake is necessary and for that the ordinary plate and frame press is not suitable. The cake built-up at the centre in the frame brings flow to a standstill. Thus, water washing of cake using the same channels used for the filtrate is inefficient. A modification of the plate and frame press is another option. In modified form an additional channel is included for washing. In half the wash plate there is a connection from the wash water channel to the surface of the plate.

The steps of the filtration are as follows:

• Slurry enters the plates and filtration proceeds in the ordinary way until the frames are filled with cake. 
• To wash the filter cake, the outlets of the washing plates are closed. 
• Wash water is pumped into the washing channel. The water enters through the inlets on to the surface of the washing plates. 
• Water passes through the filter cloth and enters frame which contains the cake. The water washes the cake, passes through the filter cloth and enters the plate down the surface. 

Finally washed water escapes through the outlet of that plate.

• Thus, provision of special washing plates makes it possible for the wash-water to flow over the entire surface of washing plate that makes the uniform resistance of cake to the flow and the entire cake is washed efficiently. The water wash is efficient only when the frames are full with filter cake. If the cake does not fill the frame completely the wash water causes the cake to break on the washing plate side of the frame and the washing will be less effective. It is essential to allow the frames to become completely filled with the cake. This helps in emptying the frames as well as in washing the cake correctly.

Special Provisions: 

• A provision of glass tube (sight glass) to observe contamination can be made at the outlet on each plate. This permits the inspection of quality of the filtrate. The filtrate goes through the control valve to an outlet channel. The filtration process from each plate can be seen. In the case of a broken cloth, the faulty plate can be isolated, and filtration can be continued with one plate less. A provision of filter sheets composed of asbestos and cellulose capable of retaining bacteria can be used for sterile filtration with whole filter press and filter medium previously sterilized. Usually, steam is passed through the assembled unit for sterilization. The examples include collection of precipitated antitoxin, removal of precipitated proteins from insulin liquors and removal of cell broth from the fermentation medium. Heating/cooling coils can be incorporated in the press for the filtration of viscous liquids. \

Advantages:

• Simple in operation, convenient in maintenance, and safe in operation with multiple safety devices. Variety of materials such as cast iron (for handling common substances), bronze (for smaller units), stainless steel (to avoid contamination), hard rubber or plastics (to avoid metal) and wood (for lightness) can be used. 

• It provides a large filtering area in a relatively small floor space. 

• It is versatile equipment whose capacity can be varied according to the thickness and the number of frames used. Surface area can be increased by employing chambers up to 60. 

• This press has sturdy construction that makes use of pressure difference of 2000 kilopascals to be used. 

•  Cake washing is very efficient.

• Operation and maintenance are low as it has no moving parts, and the filter cloths can be easily renewable. 

• The presence of all external joints makes plate to be isolated in case of if any leaks and the contamination of the filtrate can be avoided. 

• It produces dry cake in the form of slab. 

Applications: 

• Filter presses are used in a huge variety of different applications, from dewatering of slurries to purification. At the same time, filter press technology is widely established for ultrafine dewatering as well as filtrate recovery. 

• Solid-liquid separation in alcohol, chemical, metallurgy, pharmaceutical, light industry, coal mining, foodstuff, textile, environmental protection, energy source and other industries. 

Disadvantages:

• It is a batch filter so there is a good deal of ‘down-time’, which is non-productive.

• The filter press is expensive. The emptying time, the labor involved and the wear and tear of the cloth results in high-cost process. 

• The operation is critical as the frames should be full otherwise inefficient washing results in difficulty to remove cake

• The filter press is used for slurries containing less than 5% solids. So high costs make it imperative that this filter press is used for expensive materials such as collection of precipitated antitoxin and removal of precipitated proteins from insulin liquors.

Filter Leaf

• The filter leaf is the simplest form of filter consisting of a frame that encloses a drainage screen or grooved plate and the whole unit being covered with filter cloth. The outlet for the filtrate connects to the inside of the frame. The frame used may be of circular, square or rectangular shape. There are two types of filter leaf namely; vertical and horizontal.

Construction:

• The filter leaf is a versatile piece of equipment. The slurry is pumped under pressure into a vessel that is fitted with a stack of vertical leaves that serve as filter elements. Each leaf has a centrally located neck at its bottom which is inserted into a manifold that collects the filtrate. The leaf is constructed with ribs on both sides to allow free flow of filtrate towards the neck and is covered with coarse mesh screens that support the finer woven metal screens or filter cloth that retain the cake,(enlarged section). 

• The space between the leaves may vary from 30-100 mm depending on the cake formation properties and the ability of the vacuum to hold a thick and heavy cake to the vertical leaf surface. The space is set by the filtrate necks of the leaves at the bottom end and with spacers at the top end brackets. For fast filtering slurries the space may be doubled by removing every second plate so consequently the cake space doubles but the filtration area is reduced to half. 

The Vessel:     

• There are two types of vessel configuration namely vertical vessels and horizontal vessels. The leaves inside horizontal tanks may be positioned either along the tank axis or perpendicular to the axis. The leaves are graduated to fit to the circular contour of the tank to reduce the slurry heel volume that surrounds the leaves. The vessels are fitted with highly secured cake discharge openings to ensure safe sealing of the tank under pressure. For wet cakes the vessel normally has a small outlet fitted with a valve while for dry cakes the opening is large, and the closure locks up electrically or hydraulically with a bayonet wedge. The head cover of vertical vessels is often pivoted to swung away to allow the upwards removal of the leaves in the stack. In vertical leaf filter cakes depart easily while in horizontal leaf filter it is necessary to incorporate means to assist discharge. For such cakes that do not discharge readily a special mechanism that vibrates the entire stack such as oscillating high impact jet headers is provided. The headers also serve to wash the filtering medium and dislodge particles that clog the metal screen or cloth. The largest leaf filters in horizontal vessels have a filtration area of 300 m2 and in vertical vessels is 100 m2 , both designed for an operating pressure of 6 bars. 

Selection Criteria:  

• Vertical leaf filters are best selected when minimum floor space for large filtration areas is required. If, the liquids are volatile and may not be subjected to vacuum, there is a risk of environmental hazard from toxic, flammable or volatile cakes, high filtrate clarity is required for polishing applications, handling saturated brines that require elevated temperatures the tank may be steam jacketed and the cake may be discharged either dry or as a thickened slurry. They should be selected with care if, the cake is thick and heavy, and the pressure is not sufficient to hold it on the leaf. Coarse mesh screens are used, and the filtration step must be preceded with a precoat to retain cakes with fine particles. The precoating with a thin layer of diatomite or perlite is not a simple operation and should be avoided whenever possible.     

Working: 

• The operation of a vertical pressure leaf filter is Laboure intensive and requires a complex manipulation of valves, so installations are in most cases fully automated. 

• Precoating: The precoating stage is done when the contaminants are gelatinous and sticky and the precoat layer forms a barrier that avoids cloth blinding. Likewise, the interface between the precoat and the cloth departs readily so the cake discharges leaving a clean cloth. In addition, it is done when a clear filtrate is required immediately after the filtration cycle commences otherwise recirculation must be employed until a clear filtrate is obtained. 

• Filtration: Once the precoating stage is completed the process slurry is pumped into the filter, the forming cake is retained on the leaves and the filtrate flows to further processing. When the solids are fine and slow to filter, a filter-aid is added to the feed slurry in order to enhance cake permeability. The addition of filter-aid    increases the solids concentration in the feed, so it occupies additional volume between the leaves and increases the amount of cake for disposal. Similarly, for all those applications when the cake is the product, precoat and filter-aid may not be used since they mix and discharge together with the cake

• Heel Removal: Once the filtration cycle is completed air or gas is blown into the vessel and the slurry heel that surrounds the leaves is pushed and displaced downwards until it reaches the lowest part of the leaf stack. At this point the remaining heel slurry is evacuated back to the feed tank by a special dip pipe that is located at the very bottom of the vessel so that the vessel is empty from slurry.

• Cake Drying: The air then continues to pass through the cake until the captive moisture is reduced to a minimum and the cake is in practical terms considered to be dry.

• Cake Discharge: At this stage the air pressure is released, the cake outlet is opened and the leaf stack is vibrated to discharge the cake. The cake outlet opening must be interlocked with a pressure sensor to avoid opening under pressure. On some filters the cloth or mesh screen may be backwashed with water after cake discharge to dislodge and remove any cake residue that adhered to the medium

Advantages

• The cloth or woven mesh screens that cover the leaves of horizontal tanks may be accessed easily once the stack is pulled out of the vessel. 

• If there is cake bridges between the leaves manually washing of the medium with high impact jets is possible. 

• Filter leaf is mechanically simple since there are no complex components in it.

• The method has the advantage that the slurry can be filtered from any vessel and the cake can be washed simply by immersing the filter, in a vessel of water. 

• A number of leaves can be connected to provide a larger area for filtration. 

• Labour costs for operating the filter are comparatively moderate. 

• The special feature of the leaf filter is the high efficiency of washing.

Disadvantages:

• High headroom is required for dismantling the leaves on vertical vessels. 

• Large floor space is required for discharging the cake on horizontal vessels. 

• The emptying of the vessel in between cake filtration, washing and drying requires close monitoring of the pressure inside the vessel to ensure that the cake holds on to the candles. 

• The operation of a vertical pressure leaf filter is labor intensive and requires a complex manipulation of valves. 

Applications: 

• The vertical pressure leaf filter is used for the polishing slurries with very low solid content of 1- 5% or for cake filtration with a solid concentration of 20 - 25%. (ii) The vertical leaf filters are suitable for handling flammable, toxic and corrosive materials since they are autoclaved and designed for hazardous environments when high pressure and safe operation are required. (iii) The vertical leaf filters may be readily jacketed for applications whenever hot or cold temperatures are to be maintained.

Rotary Filter

• In large scale filtration when continuous operation is desirable, and it is necessary to filter slurries containing a high proportion of solids. Rotary drum filter, patented in 1872, is one of the oldest filters used in the industrial liquid-solids separation. A rotary drum filter is one which is continuous in operation and has a system to remove the cake formed and in addition it can handle concentrated slurries too. It offers a wide range of industrial processing applications and provides a flexible application of dewatering, washing and/or clarification. 

Construction: 

• The rotary drum filter usually consists of 16 - 20 filter units. It has the shape of longitudinal segments of the periphery of a cylinder, . Each filter unit is rectangular in shape with a curved profile and are joined to form a drum. Each unit has a perforated metal surface to the outer part of the drum and is covered with filter cloth. Appropriate connections are again made from each unit through a rotating valve at the center of the drum. Rotary filters may be up to 2 m in diameter and 3.5 m in length, giving areas of the order of 20 m2 . The drum is suspended on an axial over a trough containing liquid/solids slurry with approximately 50-80% of the screen area immersed in the slurry. One form is the rotary disc filter, in which the sector shaped filter leaves form a disc with the outlet from the peach leaf connected to the vacuum system, compressed air, and the appropriate receivers, in the correct sequence, by means of special rotating valve. 

• The rotary drum filter usually consists of 16 - 20 filter units. It has the shape of longitudinal segments of the periphery of a cylinder. Each filter unit is rectangular in shape with a curved profile and are joined to form a drum. Each unit has a perforated metal surface to the outer part of the drum and is covered with filter cloth. Appropriate connections are again made from each unit through a rotating valve at the canter1 of the drum. Rotary filters may be up to 2 m in diameter and 3.5 m in length, giving areas of the order of 20 m2 . The drum is suspended on an axial over a trough containing liquid/solids slurry with approximately 50-80% of the screen area immersed in the slurry. One form is the rotary disc filter, in which the sector shaped filter leaves form a disc with the outlet from the peach leaf connected to the vacuum system, compressed air, and the appropriate receivers, in the correct sequence, by means of special rotating valve. 

• The rotary drum filter usually consists of 16 - 20 filter units. It has the shape of longitudinal segments of the periphery of a cylinder, Fech filter unit is rectangular in shape with a curved profile and are joined to form a drum. Each unit has a perforated metal surface to the outer part of the drum and is covered with filter cloth. Appropriate connections are again made from each unit through a rotating valve at the Centre of the drum. Rotary filters may be up to 2 m in diameter and 3.5 m in length, giving areas of the order of 20 m2 . The drum is suspended on an axial over a trough containing liquid/solids slurry with approximately 50-80% of the screen area immersed in the slurry. One form is the rotary disc filter, in which the sector shaped filter leaves form a disc with the outlet from the each leaf connected to the vacuum system, compressed air, and the appropriate receivers, in the correct sequence, by means of special rotating valve.   

Working:

• The basic operating zones in rotary filter are given. As the drum rotates into and out of the trough, the slurry is sucked on the surface of the cloth and rotated out of the liquid/solid's suspension as a cake. When the cake is rotating out, it is dewatered in the drying zone. 

• The cake is dry because the vacuum drum is continuously sucking the cake and taking the water out of it. At the final step of the separation, the cake is discharged as solid product and the drum rotates continuously to another separation cycle. In case of string discharge filter the strings of filter lift the filter cake of the filter medium, and the cake is broken by the sharp bend, over the discharge roller so that it is easily collected while the strings return to the drum. 

Advantages: 

• The rotary filter is automatic and continuous in operation with very low labor costs.
• These filters have a large filter area and thus the high output capacity.
• Variation of the speed of rotation enables the cake thickness to be controlled, for example, for solids that form an impermeable cake, the thickness may be limited to less than 5 mm and for coarse solids forming a porous cake the thickness may be 100 mm or more. 

Disadvantages:

• This filter has ancillary components such as vacuum pumps, vacuum receivers and traps, slurry pumps and agitators with many moving parts it is a complex piece of equipment.
• The cake tends to crack due to the air drawn through by the vacuum system so that washing and drying are not efficient. 
• Being a vacuum filter, the pressure difference is limited to 1 bar and hot filtrates may boil.
•  The rotary filter is suitable only for straight forward slurries, being less satisfactory if the solids formed an impermeable cake or will not separate cleanly from the cloth. 

Applications:

• The rotary filter is most suitable for continuous operation on large quantities of slurry. 
• It is used in the collection of calcium carbonate, magnesium carbonate and starch.
• It is especially useful for filtering the fermentation liquor in the manufacture of antibiotics where the mold is difficult to filter by ordinary methods because it forms a felt-like cake. 
• It is useful for the slurry that contains considerable amount of solids (15-30%)

Membrane Filters 

• A membrane is a thin layer of semi-permeable material that separates substances when a driving force is applied across the membrane. It works on the principle of physical separation. These are used for removal of bacteria, micro-organisms, particulates, and natural organic material, which can impart color, tastes, and odors to water and react with disinfectants to form disinfection byproducts. Advancements are made in membrane production and module design with the view to reduce capital and operating costs. The membrane processes include:

1. Microfiltration 
2. Ultrafiltration 
3. Nanofiltration
4. Reverse osmosis

Construction: 

• Membrane filters are plastic membranes based on cellulose acetate, cellulose nitrate or mixed cellulose esters with pore sizes in the micron or submicron range. They are very thin about 120 µ and must be handled carefully. They act like a sieve trapping particulate matter on their surface. Several grades of filters are available with pore sizes ranging from 0.010 ± 0.002 µ to 5.0 ± 1.2 µ. Type codes VF and SM are given by Millipore Filter Corp. for these two extreme ranges respectively. 

• Filters with pore sizes from 0.010 to 0.10 µ can remove virus particles from water or air. Filters with pore sizes from 0.30 to 0.65 µ are employed for removing bacteria. Filters with the larger pore sizes, viz. 0.8, 1.2 and 3.0 to 5.0 µ are employed, for example, in aerosol, radio activity and particle sizing applications. Membrane filters are manufactured as flat sheet stock or as hollow fibers and formed into several different types of membrane modules. Module construction involves potting or sealing the membrane material into an assembly, such as with hollow-fiber module. These types of modules are designed for long-term use over the course of a number of years. Spiral-wound modules are also manufactured for Longterm use. 

Working: 

• The membrane separation process is based on the presence of semi-permeable membranes. The principle is membrane acts as a very specific filter that will let water flow through, while it catches suspended solids and other substances. During use membrane filters are supported on a rigid base of perforated metal, plastic or coarse sintered glass as in the case of fibrous pad filters. If the solution to be filtered contains a considerable quantity of suspended matter, preliminary filtration through a suitable depth filter avoids clogging of the membrane filter during sterile filtration. They are brittle when dry and can be stored indefinitely in the dry state but are fairly tough when wet. 
• Microfiltration (MF): Microfiltration is defined as a membrane separation process using membranes with a pore size of approximately 0.03 to 10 µ, a molecular weight cut-off (MWCO) of greater than 10,00,000 Daltons and a relatively low feed water operating pressure of approximately 100 to 400 kPa (15 to 60 psi). Sand, silt, clays, Giardia lamblia and Cryptosporidium cysts, algae, and some bacterial species are removed by MF. This filter is not an absolute barrier to viruses but when used in combination with disinfectant it appears to control them. There is a growing emphasis on limiting the concentrations and number of chemicals that are applied during water treatment. By physically removing the pathogens, membrane filtration can significantly reduce chemical addition, such as chlorination. It can also be used to remove natural synthetic organic matter to reduce fouling potential. The pretreatment helps to increase removal of organic material. MF can also be used as a pretreatment to reverse osmosis (RO) or Nano-filtrations (NF) to reduce bad odor as well as to remove hardness from ground water.

• Ultrafiltration (UF): The ultra-filters have a pore size of approximately 0.002 to 0.1 µ, with a MWCO of approximately 10,000 to 100,000 Daltons, and an operating pressure of approximately 200 to 700 kPa (30 to 100 psi). It removes all microbiological species such as partial removal of bacteria, as well as holmic materials and some viruses but is not an absolute barrier to viruses. Disinfection can provide a second barrier to contamination and is therefore recommended. The primary advantages of low-pressure UF membrane processes are: 

• No need for chemicals (coagulants, flocculants, disinfectants, pH adjustment). 
• Size-exclusive filtration. 
• Constant quality of the treated water in terms of particle and microbial removal. 
• The process and plant are compact. 
• Simple and automatic, however, fouling can cause difficulties in membrane technology for water treatment. 
•  Nanofiltration: Nanofiltration membranes have a nominal pore size of approximately 0.001 µ and a MWCO of 1,000 to 100,000 Daltons. Pushing liquid through these very small size membrane pores requires a higher operation pressure. Operating pressures are usually near to 600 kPa and can be as high as 1,000 kPa. NF can apparently remove all cysts, bacteria, viruses, and hemic materials. They provide an excellent protection from disinfection by-product (DBP) formation (if the disinfectant residual is added) and 

• remove alkalinity. NF also removes hardness from water, which accounts for NF membranes sometimes being called “softening membranes.” Hard water treated by NF needs pretreatment to avoid precipitation of hardness ions on the membrane. 

Reverse Osmosis (RO):

• Reverse osmosis can effectively remove nearly all inorganic contaminants from water. It can effectively remove radium, natural organic substances, pesticides, cysts, bacteria and viruses. It is particularly effective when used in series with multiple units. Disinfection is also recommended to ensure the safety of water. Advantages of this filter include removal of nearly all contaminant ions and most dissolved non-ions. It is relatively insensitive to flow and total dissolved solids. It operates immediately without any minimum break-in period. It is possible to concentrate effluents with low solid. It is capable of removing bacteria and particles. The operational simplicity and automation features allow for less supervision. 

Limitations: 

• High capital and operating costs. 
• Managing the wastewater (brine solution) is a potential problem. 
• High level of pretreatment is required in some cases. 
• Membranes are prone to fouling.
• Produces the most wastewater at between 25-50 % of the feed. 

Advantages: 

• Filtration rate is rapid. 
• They are disposable and hence no cross contamination take place. 
• No bacterial growth through the filter takes place during prolonged filtration. 
• Adsorption is negligible they yield no fibers or alkali into the filtrate. 

Disadvantages:

• Ordinary types are less resistant to solvents like chloroform.
• They may clog though rarely. 

Applications:

• It allows the isolation and categorization of micro-organisms. 

• It is used in removal of ammonium ions from potable water.

• Dairy industry: MF is a valuable part in the manufacture of dairy ingredients. It has applications in milk, whey and clarified cheese brine.

• Starch and sweetener industry: It can be used to increase in the performance of the products for example, clarification of corn syrups such as dextrose and fructose, the concentration of rinse water from starch, the enrichment of dextrose, the dehydrogenation of dextrose syrup and the division/concentration of maceration water.

• Sugar industry: MF can be used to clarify unprocessed juice without using primary clarifiers. It can be used to clarify, divide and concentrate various sugar solutions in the production process. 

• Chemical industry: MF can be used in many chemical processes to desalinate defaulter and purify dyes, pigments and optical brighteners, to clean the wastewater and rinse water currents, for the concentration and dehydration of minerals such as kaolin clay, titanium dioxide and calcium carbonate, the clarification of caustic agents and the production of polymers or the recuperation of metals. 

• Pharmaceutical industry: It is used in the harvesting of cells or the recuperation of biomass in the process of fermentation during manufacturing of antibiotics. MF improves production as well as reduces labor and maintenance costs. It is used in the industrial production lines for enzymes in concentrating them prior to other processes. 

Cartridge Filters 

• Cartridge filters are fabric or polymer-based filters designed primarily to remove particulate material from fluids. They are usually rigid or semi-rigid and manufactured by affixing the fabric or polymer to a central core. Pre-formed cartridge filters of all sizes are one of the simplest and most commonly found means of removing particulate material from water supplies. 

Principle: 

• The main principle of this filter is physical filtration. These systems work by pushing water from the reservoir into the cylinder. A cylinder collects the larger debris and the secondary filter catches anything that the first one may have missed. The water passes through the polyester filters, and dirt gets stuck on the screen allowing for clean water to pass by. The liquid to be filtered is imported and clear liquid flows to the discharge port. Drainage inlet filter size, structure and size of the filter depends on the design flow and the filter medium characteristics of the liquid.

Construction: 

• It is designed to remove solid particulate material from the water. These may be constructed in a number of different ways and from various materials, including pleated paper or fiber, plastics such as polypropylene or nylon, cellulose, membrane, fiber glass, ceramics etc. It is characterized by a composite structure of the filter cartridge, saving the volume of the cylinder. It is easy to clean and maintain and can be designed to a pressure up to 5 M.Pa and 280 °C temperature. The shell is made of carbon steel or stainless steel. Filter cartridge housings consists of single cartridges up to hundreds of cartridges

• The housing is sized based on the flow rate, viscosity of liquid, filtration efficiency required, and allowable clean differential pressure drop. Cartridge filter is used in chemical industry to filter 50 – 150 µ particles of the media. These filters are rated by the maximum size of particle they allow to pass through. A “nominal” rating means that a stated percentage of pores in the filter are smaller than a specified size. An “absolute” rating states the maximum pore size that may be found anywhere on the filter. These filters are often installed in series in descending order of pore size to maximize removal efficiency and protect downstream filters or other treatment processes.

Working:    

• When the liquid through the cylinder enters into the basket, the solid impurity particles are blocked in the filter basket, and clean fluid through the filter basket is discharged from the filter outlet. For cleaning, the bottom of the head plung is unscrewed to drain the fluid after removing flange cover. After cleaning the components are reloaded. Therefore, the use of maintenance is very convenient. Solid material suspended in the water gets trapped on the cartridge filter. The filter is rated to remove particles of a certain size. A typical choice would be a 20 µ filter followed by a 5 and / or 1 µ filter, but the exact choice depends on the quality of the supply and the substance(s) that need to be removed. The filter should be clearly marked with its size rating. 

Advantages: 

• Cartridge filters are relatively cheap, compact and requiring minimal maintenance other than changing the cartridge. 

• These filters are disposable and easily replaceable.

• Cartridge filter systems are modular and can theoretically accommodate any flow rate by increasing or decreasing the number of filter or filter arrays. 

• Being cheaper they are suitable in their use in smaller water systems. 

• These filters are of high quality with reasonable structure. 

• They have higher flow rates.

• They are easy to maintain and assemble with high filter accuracy. 

• They require low maintenance and oversight. 

Disadvantages: 

• As cartridge filters are not backwashed, they are simply replaced once they become dirty or block.
• The use of coagulants or a pre-coat with a cartridge filter is not usually recommended.
•  Cartridge filtration for water treatment is limited by two sources namely, water quality and system size.

Applications:

• It is used in the chemical and petrochemical production of weak corrosive materials, such as: water, oil, ammonia, hydrocarbons and so on. 

• In the chemical production of corrosive materials, such as: caustic soda, soda ash, concentrated sulphury acid, carbonic acid, uranic acid and so on.      

• In the case of the cooling of low-temperature materials, such as: liquid methane, liquid ammonia, liquid oxygen and various refrigerants.

• In food production of the health requirements materials, such as: beer, beverages, dairy products, syrup and so on. 

• In pharmaceutical production of tissue culture media, enzymes, aqueous solutions, chemical and reagent purification, immunological and cosmetic products. 

• It can be used to remove general debris such as leaves or sediment, or chemical precipitates that have been encouraged to form in the water (for example by oxidation) to enable their removal. 

Meta Filter

• Meta filter is the commonest filter used in the pharmaceutical industry.

Principle:

• Meta filter is a type of pressure filter. Pressure filters feed the product to the filter at a pressure greater than that which would arise from gravity alone. Meta filter functions as a strainer (service filtration) for the separation of particles. The metal rings contain semicircular projections, which are arranged as a nest to form channels on the edges. This channel offers resistance (strainer) to the flow of solids (forced particles). The clear liquid is collected into a receiver from the top.

Construction: 

• The meta filter, in its simplest form, consists of a grooved drainage rod on which is packed a series of metal rings. These rings are usually of stainless steel and are about 15 mm inside diameter, 22 mm outer diameter and 0.8 mm in thickness. These rings have a number of semicircular projections on one surface and when they are packed on the rod, the opening between the rings is about 0.2 mm. The height of the projections and the shape of the section of the ring are such as that when the rings are packed together, all the same way up, and tightened on the drainage rod with a nut, channels are formed that taper from about 250 µm down to 25 µm. One or more of these packs is mounted in a vessel, and the filter may be operated by pumping in the slurry under pressure or, occasionally, by the application of reduced pressure to the outlet side. 

• In this form, the meta filter can be used as a strainer for coarse particles, but for finer particles a bed of a suitable material such kieselguhr is first built up. The pack of rings, therefore, serves essentially as a base on which the true filter medium is supported. 

Working: 

• Meta filters are placed in a vessel and may be operated by pumping the slurry under pressure or occasionally by the application of reduced pressure to the outlet side. The slurry passes through the channels formed on the edges between the rings. The clear liquid rises-up and collected from the outlet into the receiver. When vacuum is applied, liquid flows from outside to inside. In this form a metafilter can only be used as strainer for coarse particles. But for separation of finer particles, a bed of suitable materials such as Kieselguhr is used. In this way the pack of rings acts as a base on which the true filter medium is supported. 

Advantages:

• The Meta filter is sturdy having enough strength to handle high pressures with no danger of bursting the filter medium.
• As it has no filter medium as such, the running costs are low and are very economical.
• This filter provides excellent resistance to corrosion and avoid contamination of the most sensitive product.
• It is sustainable to filter off very fine particles. 
• It can be used to sterilize some liquids by filtration.  
• It also can be used to remove larger particles simply by building up a bed of coarse substances, or even by using the Meta filter candle itself if the particles are sufficiently large. 
• Removal of the cake is effectively carried out by back flushing with water.

Applications: 

• The small surface of the meta filter restricts the amount of the solids that can be collected. This, together with the ability to separate very fine particles, means that the meta filter is used almost exclusively for clarification purposes. 

• The strength of the meta filter permits the use of high pressures (15 bars) making it suitable for viscous liquids.

• It can be constructed with appropriate corrosion resistant materials.

• Specific examples of pharmaceutical uses include the clarification of syrups, elixirs, injection solutions, and of products such as insulin liquors.