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.Derek B Purchas and Ken Sutherland, Handbook of Filter Media (2nd Edition), Elsevier Advanced Technology (2002). 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.Shukla, A. A. and Kandula, J. R., 2008, Harvest and recovery of monoclonal antibodies from large-scale mammalian cell culture. BioPharm International, May 2008, p. 34-45. Designs available 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.Kenneth S Sutherland, 2008. Filters and Filtration Handbook, Fifth Edition. 5th Edition. Elsevier Science. 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.Mervyn Smyth, 2011. Solar Energy in the Winemaking Industry (Green Energy and Technology). 2011 Edition. Springer. Food and Beverage – fruit juices, soft drinks
Chemicals –Manufacturing paints, organic solvents, ink
Petroleum – Wax, kerosene
Winery, cosmetics Thick cartridgeT. Christopher Dickenson, 1998. Filters and Filtration Handbook, Fourth Edition.4th Edition. Elsevier Science. 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 Irwin M. Hutten, 2007. Handbook of Nonwoven Filter Media. 1 Edition. Elsevier Science. Deep bed (sand filter) 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 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
Applications of depth filters and advantages
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.
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.Ben Aim R., Shanoun A., Visvanathan C., and Vigneswaran S. (1993). New filtration media and their use in water treatment. Proceedings, World Filtration Congress, Nagoya, 273–276 In sewage treatment plants, the majority of suspended solids and other contaminants 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. 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 . 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%).Thomas P.O’brien, Large-scale, Single use of depth filtration systems for mammalian cell culture clarification, 2012
Limitations of depth filtrations over competitive processes Besides Depth Filtration, a number of membrane filtration methods are also used for different industrial applications such as Reverse Osmosis, nano-filtration and Microfiltration.Syed A. Hashsham, Dead End Membrane Filtration, Laboratory Feasibility Studies in Environmental Engineering, 2006 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,MEMBRANE FILTRATION GUIDANCE MANUAL, United States Environmental Protection Agency, 2005 hence a combination of the two processes can provide a suitable filtration system, which can be adapted to many applications.
Assessment of main process characteristics 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.
Treated flow rate
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. 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.Trussell, R.R. 2004, Deep Bed Filters and High Rate Service, lecture edn, California Nevada Section-American Water Works Association, Sacramento.
Backwashing in depth filters Source:
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.Armenante, P. Depth (or deep bed) filtration, lecture edn, New Jersey Institute of Technology, New Jersey. 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. 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.
Separation efficiencies
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.Lekang, O. 2013, "Depth filtration: Granular medium filters" in Aquaculture Engineering, 2nd edn, Wiley-Blackwell, West Sussex, pp. 58-59-60.
Filter media
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. Increasing the media diameter and filter rate however results in degradation of effluent turbidity. 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. Sand, magnetite, coke and anthracite are the most commonly used particle mediums in industry particularly to their wide availability.
Table Process/Design Characteristics of Monomedium Filter Beds for Wastewater treatment (Deep Bed): Characteristic Parameter Range Commonly employed parameter values Media type: Sand Media Depth (cm) 90-180120 Effective Size (mm) 2-32.5 Filtration Rate m/h 5-2412 Media Type: Anthracite Media Depth (cm) 90-215150 Effective Size (mm) 2-42.75 Filtration Rate m/h5-2412 Table Design Parameters for Pressure Depth Filters:
Media Effective Size (mm) Filtration Rate m/h0.3525-350.5540-500.7555-700.9570-90
Design heuristics
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.
Particle retention and filter media
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.