Depth filters are filters that use a porous filtration medium to retain particles throughout the medium, rather than just on the surface of the medium. Depth filtration, typified by multiple porous layers with depth, is used to capture the solid contaminants from the liquid phase. [1] These filters are commonly used when the fluid to be filtered contains a high load of particles because, relative to other types of filters, they can retain a large mass of particles before becoming clogged. [2]
Various designs have been implemented to ensure feasible processes whilst retaining the main objective of depth filters.
Design | Characteristic | Number of cycles | Dimensions | Industrial applications |
---|---|---|---|---|
Pads and panels (cassettes) | Thick sheets or thinner sheets folded filter material shaped into rectangular a shape. Packed into a rectangular frame with a dividing wall. [3] | Only used for 1–2 cleaning cycles | Available in 400, 1600, or 3600 cm2 with a flow 75 L/h over each pad and can increase to 130 L/h by polishing filtration. [4] | Food and Beverage – fruit juices, soft drinks Chemicals –Manufacturing paints, organic solvents, ink Petroleum – Wax, kerosene Winery, [4] cosmetics |
Thick cartridge [5] | A single piece of the filter material wound around a perforated cylinder, made of metal or stiff plastic, where the fluid or gas with solute flows inside the cylinder. | Once the filter medium has reached its maximum solute load the cartridge is discarded. Backwashing can make the filter complete more cleaning cycles | Home water and pool filters Industrial separations for hydrocarbon fuels [6] | |
Deep bed (sand filter) [3] | The filter medium has the solution on top and utilizes gravity to filter particles. It is the oldest and simplest method of filtration. | Multiple filtering cycles and is usually cleaned by flow reversal. | Types of deep bed
| Potable water, polishing following wastewater treatment, pre-treatment for desalination |
Lenticular [4] | Stacked disc design - mechanical compression seals (seven seals per eight-cell filter) between plastic “knife edges” and filter media. | 300 or 400 mm disc diameter | Fermented products, cannabis oil filtration |
The use of deep bed sand filters as the final step in municipal potable water treatment has increased significantly over the past decade, with its application ranging from clarification and processing of drinking water to wastewater treatment plants where the wastewater is required to be polished before being discharged. [1]
The main deep bed filtration processes currently used are direct filtration and contact-flocculation filtration. Direct filtration involves a short period of pre flocculation stage followed by the filtration process. [7] In sewage treatment plants, the majority of suspended solids and other contaminates are successfully removed after the primary and secondary treatment stages. To remove the remaining solids and organic compounds from the wastewater stream, direct filtration method is utilised with prior flocculation. As the contaminant separation process takes place in the filter medium, factors such as flocculation time, filtration velocity and flocculent dosage are required to be monitored regularly, as they can directly affect the flocculent size produced. This is vital to the process in order to prevent potential clogging or bioclogging of the filter bed.
The advantages associated with this process include the ability to produce large flocculent, which can then be filtered. The other advantage of the depth filtration method is the flexibility in the choice of filter arrangement, which allows high solid storage capacities to be obtained, while keeping the energy consumption rate within an acceptable range. [1] The downside of using direct filtration is that microbes are able to grow within the channels of the filter and hence reproduce throughout long operating runs. This reproduction of organisms within the filter matrix can result in the contamination of the filtrate.
Depth filtration is also widely used for the clarification of cell culture clarification. The cell culture systems can contain yeast, bacterial and other contaminant cells and hence, an efficient clarification stage is vital to separate the cells and other colloidal matter to produce a particle free cell system [9]. Most depth filters used in pharmaceutical processes such as cell system harvesting are composed of cellulose fibres and filter aids. The direct flow design in depth filters provides a financially suitable solution by trapping the contaminants within the filter channel while ensuring the maximum recovery rate of the product. The other advantages of this system include its low power costs, since the pumps utilised in depth filters require minimal power input due to the small pressure within the system. Depth filtration is also flexible in terms of being able to scale up or down the system while outputting a high rate of yield (>95%). [8]
Besides Depth Filtration, a number of membrane filtration methods are also used for different industrial applications such as Reverse Osmosis, nano-filtration and Microfiltration. [9] These use the same principle, rejecting contaminants larger than the filter size. The main distinguishing feature amongst them is their effective pore size. For example, Microfiltration operates by allowing large particles to pass through the filter media, whilst Reverse Osmosis rejects all the particles except very small species. Most membrane filters can be utilized for final filtration whilst depth filters tend to be more effective when used in clarifying applications, [10] hence a combination of the two processes can provide a suitable filtration system, which can be adapted to many applications.
Process Characteristics such as filtration rate and filter media are important design considerations and greatly impact filter performance, as a result continuous monitoring and assessment is necessary to ensure greater control over the process quality.
The flow rate is defined as the ratio of the driving force over the filter resistance. The two conventional types of depth filter designs: the rapid and slow filters operate with velocities of 5–15 m/h and 0.1-0.2 m/h respectively; whereas pressurised sand filters have design flow rates of 238 L/min. [11] During operation the filter rate decreases due to increasing filter resistance as particulates get lodged within the media. The rate of filtration affects the rate of clogging with high filter rates causing faster build up. Pilot tests demonstrate that the higher the filter rate the lower the filter area whilst increasing filter rate reduces the time to breakthrough, reduces the time to head loss (increases head loss) and results in shorter runs and lower optimum depths. They also demonstrate that higher filter rates can be achieved by using larger diameter media and increased media depth. High filtration rates depend on media design with the highest filtration rate design in service at 13.5gpm/ft2. [11]
Source: [12]
Backwashing is an important operation employed to remove filtered solids as this build up causes resistance to filtration to increase with time. Backwashing involves inverting the direction of liquid flow while using clean liquid. [13] This process is employed for times in the range of 5–15 minutes with typical flow rates per unit area in the range of 6.8- 13.6 L/m2.s. [13] Most designs typically employ backwashing once per day of operation. The operation of depth filters is inherently cyclic due to the necessity of solids removal build up during the process, as such two or more units are typically used so that backwashing does not interfere with the filtration. Effective backwashing occurs when the filter medium is fluidized. Fluidization flow rates generally fall in the range of 20-50 gpm/ft2. [13]
Rates of removal for pressurized sand filters with media typically in the range of 0.3- 0.5 mm have been reported to be at 95 of particles as small as 6 μm with media size of 0.3 mm and 95% removal rate of particles as small as 15 μm for media size of 0.5 mm. [14]
There is a variety of filter media that can be employed in depth filter processes, the most common being sand. Choice of filter media has effects on filter rate, turbidity and filter surface area. Clean bed head loss (pressure drop) is sensitive to media diameter where increasing media diameter results in a longer time to design head loss. [11] Increasing the media diameter and filter rate however results in degradation of effluent turbidity. [13] To compensate, media depth can be increased to reduce the effects on effluent turbidity. The max value of media depth used in designs so far for high rate filtration is 100 in, whilst the maximum media size used in pilots is 2mm in diameter. [11] Sand, magnetite, coke and anthracite are the most commonly used particle mediums in industry particularly to their wide availability.
Table [1] Process/Design Characteristics of Monomedium Filter Beds for Wastewater treatment (Deep Bed): [13]
Characteristic | Parameter Range | Commonly employed parameter values |
---|---|---|
Media type: Sand | ||
Media Depth (cm) | 90-180 | 120 |
Effective Size (mm) | 2-3 | 2.5 |
Filtration Rate m/h | 5-24 | 12 |
Media Type: Anthracite | ||
Media Depth (cm) | 90-215 | 150 |
Effective Size (mm) | 2-4 | 2.75 |
Filtration Rate m/h | 5-24 | 12 |
Table [2] Design Parameters for Pressure Depth Filters: [13]
Media Effective Size (mm) | Filtration Rate m/h |
---|---|
0.35 | 25-35 |
0.55 | 40-50 |
0.75 | 55-70 |
0.95 | 70-90 |
Depth filtration may be used in pre-treatment, removing suspended particles from a carrying fluid intended to be used as a feed stream or in the context of clarification where particulates are removed to purify a product stream.
Several heuristics are adopted into the design of depth filters in order to ensure consistent operation throughout the life of the filter.
The relationship between retention and particles size is not a step function. Larger particles are easily retained by the filter media; however particulates that are within the intermediate range between the nominal particle and waste components are harder to preserve and as a result are often lost as a waste component.
To maximize the retention passage for a range of particle sizes, filter media is layered in a manner such that sections with a higher pore size are closer to the inlet stream, capturing particles of a larger size. Pore sizes decrease as it approaches the outlet stream. By adopting this method, the filter media caters for a wider range of particle sizes, resulting in greater control of retention and extending the life of the filter. [15]
Filter selection is reliant on a number of variables such load, duration, shape, size and distribution of the substance desired to be filtered. Ideally if the medium is too large, filtrate will be of a poor quality as it will fail to collect particulates within its matrix. Conversely if the medium is very small, solids will accumulate on the surface of the cartridge causing close to immediate blockages. In regards to the shape using grains that are round in shape have the tendency to erode due to the pressure the inlet stream may possess on the system, whereas grains that are flat (may increase surface area) however may float out of the system during backwash. Particles which are high on Moh's scale of hardness and have a relatively large specific gravity are often recommended to be used as particle media. The softer and lighter the material is, the more susceptible it is to erosion and fluidization. Thus particles such as silica and sand are often used as they are affordable however are resistant to the high flows of the incoming fluid. The uniformity coefficient is a measure of the uniformity of the material used within the filter. It is a ratio of a sieve pore that allows 60% of the material through in comparison to a pore size that allows 10% of material through. The closer the ratio is to one, means the closer the particles are in size. An ideal system would have a coefficient between 1.3 and 1.5 and must not exceed 1.7. Anything less than 1.3 is an indication that it is unnecessary to the system and may result in higher costs without providing any additional form of optimization. Beyond 1.5 indicates that the system may experience a greater pressure drop and as mentioned may result in clogging, seeping of waste flow and reduced filtration rate. [16] As a guideline it is recommended that the smallest particles used within depth filters should be placed at least 150 mm from the outlet stream to prevent fluidization. [16]
Depth filters are operated as dead end filters with the velocity of the inlet streams crucial to the performance of the filter. High velocity inlet streams with relatively large particulates will cause possible clogging and wearing down of the filter media. This will cause an increase in pressure drop of the system. In situations where the filter media is clogged and pressure drop continually increases it is common that waste particles and streams may seep through the zones within the cartridge and pass through the outlet stream resulting in no purification
To minimize the effects of clogging and particle build-up a back flushing system must accommodate approximately 1-5% of the bulk flow as back flush, operating at approximately 6-8 bar. Beyond this range particulates may become fragmented making them difficult to be removed from the system, and potentially cause fluidization of the system. [14]
The main purpose of a depth filter is to act as a clarifier, separating suspended solids from a bulk flow liquid stream and as a result is employed within the final stage of a separation process. By convention, depth filters consist of a single outlet stream of a purified liquid retaining the waste particles within its system. Due to its length it has greater residue holding capabilities than standard filters. In terms of a waste stream, often the outlet stream may be recycled into a subsequent filter in order to ensure that the stream is free from particulates. A waste stream may also be produced when cleaning the filter media as the water passes in the opposite direction residue caught within the filter media or media particles that have been displaced may emerge from the unit before it is adequately disposed. [12]
With the ongoing advancements in process technologies, depth filters have been modified to improve its feasibility within a range of industrial sectors.
Design | Characteristic | Improvement | Industry |
---|---|---|---|
Pod Lenticular | Filtration is achieved by forces, such as gravity and water pressure, acting on the knife edge seals compressing the filter material and filtering the liquid |
| Pharmaceutical sector-separation of cellular organisms from liquid. |
Continuous deep-bed filters | Applying rapid sand filtration and having the dirty solid with the filter material being captured. A jet of air carries the filter medium with solid into a wash zone above the filter and is separated. The cleaned filter material is then added back into the deep-bed filter. | Water and Solid flows are counter current, therefore increasing the solid removal | Water treatment- improved separation techniques during pre-treatment |
Filtration is a physical separation process that separates solid matter and fluid from a mixture using a filter medium that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter medium are described as oversize and the fluid that passes through is called the filtrate. Oversize particles may form a filter cake on top of the filter and may also block the filter lattice, preventing the fluid phase from crossing the filter, known as blinding. The size of the largest particles that can successfully pass through a filter is called the effective pore size of that filter. The separation of solid and fluid is imperfect; solids will be contaminated with some fluid and filtrate will contain fine particles. Filtration occurs both in nature and in engineered systems; there are biological, geological, and industrial forms.
Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for human consumption, but water purification may also be carried out for a variety of other purposes, including medical, pharmacological, chemical, and industrial applications. The history of water purification includes a wide variety of methods. The methods used include physical processes such as filtration, sedimentation, and distillation; biological processes such as slow sand filters or biologically active carbon; chemical processes such as flocculation and chlorination; and the use of electromagnetic radiation such as ultraviolet light.
Microfiltration is a type of physical filtration process where a contaminated fluid is passed through a special pore-sized membrane filter to separate microorganisms and suspended particles from process liquid. It is commonly used in conjunction with various other separation processes such as ultrafiltration and reverse osmosis to provide a product stream which is free of undesired contaminants.
A media filter is a type of filter that uses a bed of sand, peat, shredded tires, foam, crushed glass, geo-textile fabric, anthracite, crushed granite or other material to filter water for drinking, swimming pools, aquaculture, irrigation, stormwater management, oil and gas operations, and other applications.
In the oil industry, waterflooding or water injection is where water is injected into the oil reservoir, to maintain the pressure, or to drive oil towards the wells, and thereby increase production. Water injection wells may be located on- and offshore, to increase oil recovery from an existing reservoir.
Total suspended solids (TSS) is the dry-weight of suspended particles, that are not dissolved, in a sample of water that can be trapped by a filter that is analyzed using a filtration apparatus known as sintered glass crucible. TSS is a water quality parameter used to assess the quality of a specimen of any type of water or water body, ocean water for example, or wastewater after treatment in a wastewater treatment plant. It is listed as a conventional pollutant in the U.S. Clean Water Act. Total dissolved solids is another parameter acquired through a separate analysis which is also used to determine water quality based on the total substances that are fully dissolved within the water, rather than undissolved suspended particles.
Sand filters are used as a step in the water treatment process of water purification.
A particulate air filter is a device composed of fibrous, or porous materials which removes particulates such as smoke, dust, pollen, mold, viruses and bacteria from the air. Filters containing an adsorbent or catalyst such as charcoal (carbon) may also remove odors and gaseous pollutants such as volatile organic compounds or ozone. Air filters are used in applications where air quality is important, notably in building ventilation systems and in engines.
In terms of water treatment, including water purification and sewage treatment, backwashing refers to pumping water backwards through the filters media, sometimes including intermittent use of compressed air during the process. Backwashing is a form of preventive maintenance so that the filter media can be reused. In water treatment plants, backwashing can be an automated process that is run by local programmable logic controllers (PLCs). The backwash cycle is triggered after a set time interval, when the filter effluent turbidity is greater than a treatment guideline or when the differential pressure across the filter exceeds a set value.
A dust collector is a system used to enhance the quality of air released from industrial and commercial processes by collecting dust and other impurities from air or gas. Designed to handle high-volume dust loads, a dust collector system consists of a blower, dust filter, a filter-cleaning system, and a dust receptacle or dust removal system. It is distinguished from air purifiers, which use disposable filters to remove dust.
The rapid sand filter or rapid gravity filter is a type of filter used in water purification and is commonly used in municipal drinking water facilities as part of a multiple-stage treatment system. These systems are complex and expensive to operate and maintain, and therefore less suitable for small communities and developing nations.
An oil filter is a filter designed to remove contaminants from engine oil, transmission oil, lubricating oil, or hydraulic oil. Their chief use is in internal-combustion engines for motor vehicles, powered aircraft, railway locomotives, ships and boats, and static engines such as generators and pumps. Other vehicle hydraulic systems, such as those in automatic transmissions and power steering, are often equipped with an oil filter. Gas turbine engines, such as those on jet aircraft, also require the use of oil filters. Oil filters are used in many different types of hydraulic machinery. The oil industry itself employs filters for oil production, oil pumping, and oil recycling. Modern engine oil filters tend to be "full-flow" (inline) or "bypass".
The physical process of sedimentation has applications in water treatment, whereby gravity acts to remove suspended solids from water. Solid particles entrained by the turbulence of moving water may be removed naturally by sedimentation in the still water of lakes and oceans. Settling basins are ponds constructed for the purpose of removing entrained solids by sedimentation. Clarifiers are tanks built with mechanical means for continuous removal of solids being deposited by sedimentation; however, clarification does not remove dissolved solids.
In chemical engineering, biochemical engineering and protein purification, cross-flow filtration is a type of filtration. Cross-flow filtration is different from dead-end filtration in which the feed is passed through a membrane or bed, the solids being trapped in the filter and the filtrate being released at the other end. Cross-flow filtration gets its name because the majority of the feed flow travels tangentially across the surface of the filter, rather than into the filter. The principal advantage of this is that the filter cake is substantially washed away during the filtration process, increasing the length of time that a filter unit can be operational. It can be a continuous process, unlike batch-wise dead-end filtration.
A lamella clarifier or inclined plate settler (IPS) is a type of clarifier designed to remove particulates from liquids.
Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and/or thickening. Inside the clarifier, solid contaminants will settle down to the bottom of the tank where it is collected by a scraper mechanism. Concentrated impurities, discharged from the bottom of the tank, are known as sludge, while the particles that float to the surface of the liquid are called scum.
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Biological membranes include cell membranes ; nuclear membranes, which cover a cell nucleus; and tissue membranes, such as mucosae and serosae. Synthetic membranes are made by humans for use in laboratories and industry.
Gravity filtration is a method of filtering impurities from solutions by using gravity to pull liquid through a filter. The two main kinds of filtration used in laboratories are gravity and vacuum/suction. Gravity filtration is often used in chemical laboratories to filter precipitates from precipitation reactions as well as drying agents, inadmissible side items, or remaining reactants. While it can also be used to separate out strong products, vacuum filtration is more commonly used for this purpose.
Pile Cloth Media Filtration is a mechanical process for the separation of organic and inorganic solids from liquids. It belongs to the processes of surface filtration and cake filtration where, in addition to the sieve effect, real filtration effects occur over the depth of the pile layer. Pile Cloth Media Filtration represents a branch of cloth filtration processes and is used for water and wastewater treatment in medium and large scale. In Pile Cloth Media Filtration, three-dimensional textile fabrics are used as filter media. During the filter cleaning of the pile layer the filtration process continues and is not interrupted.
Diatomaceous earth (DE) filtration is a special filtration process that removes particles from liquids as it passes through a layer of fossilized remains of microscopic water organism called diatoms. These diatoms are mined from diatomite deposits which are located along the Earth's surface as they have accumulated in sediment of open and moving bodies of water. Obtained diatomaceous earth is then purified using acid leaching or liquid-liquid extraction in order for it to be used in any form of application. The process of D.E. filtration is composed of three main stages: pre-coating, body feed, and cleaning.