Due to the wide variety of filter media, filter designs, suspension properties, condi- tions for separation and cost, selection of the optimum filter medium is complex.
Filter media selection should be guided by the following rule: a filter medium must incorporate a maximum size of pores while at the same time providing a sufficiently pure filtrate. Fulfilment of this rule invokes difficulties because the increase or decrease in pore size acts in opposite ways on the filtration rate and solids retention capacity.
The difficulty becomes accentuated by several other requirements that cannot be achieved through the selection of a single filter me&um. Therefore, selection is often reduced to determining the most reasonable compromise between different, mutually contradictory requirements as applied to the filter 'medium at a specified set of filtration conditions. Because of this, some problems should be solved before final medium selection. For example, should attempts be made to increase filtration rate or filtrate purity? Is cost or medium life more important? In some cases a relatively more expensive filter medium, such as a synthetic cloth, is only suitable
SELECTING TfIE RIGHT FILTER MEDIA 149
under certain filtration conditions, which practically eliminates any cost consideration in the selection process.
Thus, the choice may only be made after consideration of all requirements. It is, however, not practical to analyze and compare each requirement with the hope of logically deducing the best choice. There is, unfortunately, no generalized formula for selection that is independent of the details of the intended application. Each cake requires study of the specific considerations, which are determined by the details or the separation process.
One can to outline a general approach for medium selection along with a test sequence applicable to a large group of frlter media of the same type. There are three methods of filter media tests: laboratory- or bench-scale pilot-unit, and plant tests. The laboratory-scale test is especially rapid and economical, but the results obtained are often not entirely reliable and should only be considered preliminary.
Pilot-unit tests provide results that approach plant data. The most reliable results are often obtained from plant trials.
Different filter media, regardless of the specific application, are distinguished by a number of properties. The principal properties of interest are the permeability of the medium relative to a pure liquid, its retention capacity relative to solid particles of known size and the pore size distribution. These properties are examined in a laboratory environment and are critical for comparing different filter media.
The permeability relative to a pure liquid, usually water, may be determined with the help of different devices that operate on the principle of measurement of filtrate volume obtained over a definite time interval at known pressure drop and filtration area. The permeability is usually expressed in terms of the hydraulic resistance of the filter medium. This value is found from:
When the cake thickness is 0, we may write the equation as:
Note that:
150 WATER AND WASEWATER TREATMENT TECHNOLOGIES
where Apt = pressure difference accounting for the hydrostatic pressure of a liquid column at its flow through the filter medium, supporting structure and device channels, and Ap, = same pressure drop when the flow of liquid is through the supporting structure and device channels
Analytical determination of the hydraulic resistance of the medium is difficult.
However, for the simplest filter medium structures, certain empirical relationships are available to estimate hydraulic resistance. The relationship of hydraulic resistance of a cloth of monofilament fiber versus fiber diameter and cloth porosity can be based on a fixed-bed model.
In evaluation and selection of a filter medium, one should account for the fact that hydraulic resistance increases gradually with time. In particular, the relationship between cloth resistance and the number of filter cycles is defined by:
R = RineMV
The retentivity relative to solid particles (e.g., spherical particles of polystyrene of definite size) is found from experiments determining the amount of these particles in the suspension to be filtered before and after the filter media. The retentivity K is determined as follows: where g', g" =amounts of solid particles in liquid sample before and after the medium, respectively.
The pore sizes distribution, as well as the average pore size, is determined by the
"bubble" method. The filter medium to be investigated is located over a supporting device under a liquid surface that completely wets the medium material. Air is introduced to the lower surface of the medium. Its pressure is gradually increased, resulting in the formation of single chains of bubbles. This corresponds to air passages through the largest-diameter pores. As pressure is increased, the number of bubble chains increases due to air passing through the smaller pores. In many cases a critical pressure is achieved where the liquid begins to "boil." This means that the filter medium under investigation is characterized by sufficiently uniform pores. If there is no "boiling," the filter medium has pores of widely different sizes.
The pore size through which air passes is calculated from known relations. For those pores whose cross section may be assumed close to a triangle, the determining size should be the diameter of a circle that may be inscribed inside the triangle.
For orientation in cloth selection for a given process, the followmg information is
SELJXTING ”HE RIGHT FILTER MEDIA 151
essential: filtration objectives (obtaining cake, filtrate or both), and complete data (if possible) on the properties of solid particles (size, shape and density), liquid (acid, alkali or neutral, temperature, viscosity, and density), suspension (ratio of solids to liquid, particle aggregation and viscosity), and cake (specific resistance, compressibility, crystalline, friable, plastic, sticky or slimy). Also, the required capacity must be known as well as what constitutes the driving force for the process (e.g., gravity force, vacuum or pressure). Based on such information, an appropriate cloth that is resistant to chemical, thermal and mechanical aggression may be selected. In selecting a cloth based on specific mechanical properties, the process driving force and filter type must be accounted for. The filter design may determine one or more of the following characteristics of the filter cloth: tensile strength, stability in bending, stability in abrasion, and/or ability of taking the form of a filter-supporting structure. Tensile strength is important, for example, in belt filters. Bending stability is important in applications of metallic woven cloths or synthetic monofilament cloths. If the cloth is subjected to abrasion, then glass cloth cannot be used even though it has good tensile strength.
From the viewpoint of accommodation to the filter-supporting structure, some cloths cannot be used, even though the filtering characteristics are excellent. For rotary drum filters, for example, the cloth is pressed onto the drum by the
“caulking” method, which uses cords that pass over the drum. In this case, the closely woven cloths manufactured from monofilament polyethylene or polypropylene fiber are less desirable than more flexible cloths of polyfilament fibers or staple cloths.
Depending on the type of filter device, additional requirements may be made of the cloth. For example, in a plate-and-frame press, the sealing properties of cloths are very important. In this case, synthetic cloths are more applicable staple cloths, followed by polyfilament and monofilament cloths. In leaf filters operating under vacuum and pressure, the cloth is pulled up onto rigid frames. Since the size of a cloth changes when in contact with the suspension, it should be pretreated to minimize shrinkage.
In selecting cloths made from synthetic materials, one must account for the fact that staple cloths provide a good retentivity of solid particles due to the short hairs on their surface. However, cake removal is often difficult from these cloths - more than from cloths of polyfilament and, especially, monofilament fibers. The type of fiber weave and pore size determine the degree of retentivity and permeability. The objective of the process, and the properties of particles, suspension and cake should be accounted for. The cloth selected in this manner should be confirmed or corrected by laboratory tests. Such tests can be performed on a single filter. These tests, however, provide no information on progressive pore plugging and cloth wear. However, they do provide indications of expected filtrate pureness, capacity and final cake wetness.
A single-plate filter consists of a hollow flat plate, one side of which is covered by
152 WATER AND WASTEWATER TREATMENT TECHNOLOGIES
cloth. The unit is connected to a vacuum source and submerged into the suspension (filtration), then suspended in air to remove filtrate, or irrigated by a dispersed liquid (washing). The filter cloth is directed downward or upward or located vertically, depending on the type of filter that is being modeled in the study.
The following is a recommended sequence of tests that can assist in cloth selection for continuous vacuum filters. If the cycle consists of only two operations (filtration and dewatering), tests should be conducted to determine the suspension weight concentration after 60 sec of filtration and 120 seconds of dewatering. The cake thickness should be measured and the cake should be removed to determine the weight of wet cake and the amount of liqrud in it. The weight of filtrate and its purity are also determined. If the cake is poorly removed by the device, it is advisable to increase the dewatering time, vacuum or both. If the cake is poorly removed after an operating regime change, it should be tested with another cloth.
If the cake is removed satisfactorily, filtration time should be decreased under increased or decreased vacuum. Note that compressible cakes sometimes plug pores faster at higher vacuum. After the filtration test for a certain filter cycle (which is based on the type of the filter being modeled), the suspension‘s properties should be examined. Based on the assumed cycle, a new filtration test should be conducted and the characteristics of the process noted. Capacity (N/m’-hr), filtration rate (m3/m2-hr) and cake wetness can then be evaluated. Also, if possible, the air rate and dewatering time should be computed. The results of the first two or three tests should not be taken into consideration because they cannot exactly characterize the properties of the cloth. A minimum of four or five tests is generally needed to achieve reproducible results of the filtration rate and cake wetness to within 3 to 5 % . When the cycle consists of filtration, washing and dewatering, the tests are considered principally in the same manner. The economic aspects of cloth selection should be considered after complete determination of cloth characteristics.
RECOMMENDED RESOURCES FOR THE READER
Check out these references on pretreatment. They can all be found on the EPA Website (www. epa. gov) .
1. Application and Use of the Regulatory Definition of Significant Noncompliance for Industrial Users, September 9, 1991 (Memorandum) CERCLA Site Discharges to POTWs Treatability Manual, August 1990 540/2-90-007 ERIC: W570; NTIS: PB91-921269 Disk PB91-507236.
CERCLA Site Discharges to POTWs: CERCLA Site Sampling Program - DetailedData Report, May 1990 540/2-90-008 ERIC: W515; NTIS: PB91- 92 1270.
ERIC: W986; NTIS: PB95-201786.
2.
3.
SELECTING THE RIGHT FILTER MEDIA 153
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
CERCLA Site Discharges to POTWs: Guidance Manual, August 1990 Environmental Regulations and Technology: The National Pretreatment Program, July 1986 625/10-86-005 ERIC: W350; NTIS: PB90-246521.
Federal Guidelines: State and Local Pretreatment Programs Appendices 1-7 -Volume 2, January 1977430/9-76-017B ERIC: W185; NTIS: PB-266782.
Federal Guidelines: State and Local Pretreatment Programs Volume 1, Federal Guidelines: State and Local Pretreatment Programs Appendix 8 - Volume 3, January 1977 430/9-76-017C ERIC: W186; NTIS: PB-266783.
Guidance for Reporting and Evaluating POTW Noncompliance with Pretreatment Implementation Requirements, September 1987 ERIC: W304;
Guidance Manual for Conducting RCRA Facility Assessments at Publicly Owned Treatment Works, September 1987 ERIC: W830; NTIS: PB95- 157715.
Guidance Manual for Control of Slug Loadings to POTWs, September 1988 Guidance Manual for POTW Pretreatment Probam Development, October Guidance Manual for Preventing Interference at POTWs, September 1987 Guidance Manual for the Control of Wastes Hauled to Publicly Owned Treatment Works, September 1999 833/B-98-003 NSCEP: 833B-98-003;
Guidance Manual for the Identification of Hazardous Wastes Delivered to Publicly Owned Treatment Works by Truck, Rail, or Dedicated Pipe, June Guidance Manual for the Use of Production Based Pretreatment Standards and the Combined Wastestream Formula, September 1985 8331B-85-201 Guidance Manual on the Development and Implementation of Local Discharge Limitations Under the Pretreatment Program - Volume 1, November 1987 ERIC: W025; NTIS: PB95-157707.
Guidance Manual on the Development and Implementation of Local Discharge Limitations Under the Pretreatment Program - Volume 2 - Appendices, November 1987 ERIC: W026; NTIS: PB95-157699.
Guidance Manual on the Development and Implementation of Local 540/G-90-005 ERIC: W150; NTIS: PB90-27453 1.
January 1977 430/9-76-017A ERIC: U041; NTIS: PB-266781.
NTIS: PB95-157764.
ERIC: W111; NTIS: PB93-202745.
1983 833/B-83-100 ERIC: W639 ; NTIS: PB93-186112.
833/B-87-201 NSCEP: 833B-87-201; ERIC: W106 ; NTIS: PB92-117969.
WRC: 833/B-98-003; ERIC: C281. NTIS: PB2000-102387.
1987 833B-87-100 ERIC: W202; NTIS: PB92-149251.
NSCEP: 833/B-85-201; ERIC: U095 ; NTIS: PB92-232024.
154 WATER AND WASTEWATEX TREATMEW TECHNOMGIES
20, 21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
Discharge Limitations Under the Pretreatment Program, December 1987 Guidance to POTWs for Enforcement of Categorical Standards, November 5, 1984, Memorandum ERIC: W296; NTIS: PB95-157673.
Guidance to Protect POTW Workers from Toxic and Reactive Gases and Vapors, June 1992 812/B-92-001 NSCEP: 812/B-92-001; ERIC: W115;
Guide to Discharging CERCLA Aque~us Wastes to Publicly Owned Treatment Works (FQTWs), March 1991 NTIS: PB91-219364 Handbook for Monitoring Industrial Wastewater, August 1973 625/6-73-002 ERIC: W318;
Industrial User Inspection and Sampling Manual for POTWs (Diskette Version and Printed Appendices), April 1994 ERIC: W493; NTIS: PB96- 502646.
Industrial User Permitting Guidance Manual, September 1989 833/R-89-001 Pretreatment Bulletin No. 3, November 6,1987 ERIC: W859; NTIS: PB95- 1594 14.
Pretreatment Bulletin No. 4, November 6,1987 ERIC: W860 ; NTIS: PB95- 159406.
Pretreatment Bulletin No. 6, June 1989 ERIC: W861 ; NTIS: PB95-159398.
Pretreatment Compliance Inspection and Audit Manual for Approval Authorities, July 1986 833/B-86-100 NSCEP: 833/B-86-100; WRC: 833/B- Pretreatment of Industrial Wastes: Joint Municipal and Industrial Seminar, 1978 6294-78-012 ERIC: W662 Procedures Manual for Reviewing a POTW Pretreatment Program Submission, October 1983 833/B-83-200 ERIC:
Treatability Manual: Volume I - Treatability Data, September 1981 Revised 600/2-82-001A ERIC: W754 Treatability Manual: Volume 11 - Industrial Descriptions, September 1981 Revised 600/2-82-001B ERIC: W755 Treatability Manual: Volume III - Technologies for Control/Removal of Pollutants 600/2-82-001C ERIC: W756 Treatability Manual: Volume IV - Cost Estimates, April 1983 Revised 600/2-82-001D ERIC: W757 Treatability Manual: Volume V - Summary, January 1983, Change 2 600/2- 82-001E ERIC: W753 U.S. EPA Pretreatment Compliance Monitoring and Enforcement System Version 3.0: User's Guide, Final, September 1992 833/B-87-202 ERIC: W107; NTIS: PB92-129188.
NTIS: PB92-173236.
NTIS: PB-259146.
ERIC: W109; NTIS: PB92-123017.
86-100; ERIC: W277; NTIS: PB90-183625.
W137; NTIS: PB93-209880.
831/F-92-001; NSCEP: 831/F-92-001; ERIC: W269; NTIS: PB94-118577.
SELECIWG "€E RIGHT FILTER MEDIA 155
Check out the following Web sites for more general information:
31.
32.
33.
34.
35.
36.
37.
38.
39.
Provides water chemicals and equipment for potable, waste and process water in industrial, municipal and mining treatment systems.URL:
http: //www . tramfloc .corn.
Tramfloc, Inc. - Home Page - Text Version - flocculant, coagulants.
Provides water chemicals and equipment for potable, waste and process water in industrial, municipal and mining treatment systems. URL:
http://www,tramfloc.com/indext.html.
Water and Wastewater Treatment Plant Operators Occupational Outlook Handbook; URL: http://stats.bls .gov/oco/ocos229.htm.
Control and Optimization of Wastewater Treatment Plants, Department of Systems.. .Research progress in wastewater treatment at Uppsala University.
URL: http: //www . sysconm. selResearchlwaste.htm1.
Wastewater Treatment and Metal Finishing Equipment . . . Your complete source of new and reconditioned industrial wastewater treatment, metal finishing, and biological treatment systems and equipment. URL: http://wmi- inc . corn.
Water and Wastewater Research and Co-operation Directory - European Centres of ... The Water and Wastewater Directory is a data source of more than 750 European organisations from 3 1 countries. It offers an easy search for experts.. .URL: http://www.metra-martech.com.
Environmental Dynamics - Worldwide Wastewater Treatment Systems Wastewater : Environmental Dynamics, Inc. wastewater treatment.
Biological wastewater treatment and advanced technology aeration-mixing systems, URL: http: //www .wastewater .corn.
Wastewater Treatment Engineering; Bureau of Land & Water Quality Last update: 03/12/01. Wastewater Treatment Engineering, Technical Assistance and Pollution Prevention. Waste Treatment; Go to the following web site:
http://janus . state.me.us/dep/blwq/engin. htm.
International provider of water purification and wastewater treatment solution
. . . Waterlink is a provider of water purification and wastewater treatment solutions, carbon systems, separations technology, aeration systems, solids.. .URL: http://www.waterlink.com.
QUESTIONS FOR THINKING AND DISCUSSING
1. What are the characteristics of a good filter medium? Make a list of several commercially available products and determine whether or not they meet your criteria.
156 WATER AND WASTEWATER TREATMENT TECHNOLOGIES
2.
3.
4.
5 . 6.
7.
8 .
9.
10.
11.
12.
13.
14.
15.
16.
What is the % open area of the weave of a cloth filter, where the opening is on the average 50 microns and the thread diameter is 85 mills.
What is the micron rating of a 50 X 250 Dutch weave filter cloth?
Select a cloth filter for a filter press application in which the water is both alkaline and has a high content of solvents. The operating temperature could reach as high as 200' F on excursions.
Define the terms mesh, wire and opening.
Select a bag filter that must operate at temperatures above 300" F and handle concentrations of organic acids in the wastewater.
What are some of the factors the impact on the adsorptivity of materials like carbon and diatomaceous earth?
Discuss how carbon adsorption works and how it can be used in water treatment applications. Give some specific examples where this technology is used to remove specific contaminants.
Develop a general classification system for dry bulk chemical additives and filter aids based on ease of feeding to a filtering machine.
Conduct a lab test to measure the bulk densities of several fiiter aids. If you don't have the chemicals, then describe how you would do the tests and what specific measurements and calculations are needed.
The pore volume of a material is 30 % and its particle density is 1.3 g/cm3.
Calculate the skeletal density of the material.
What is the effect of moisture on the flowability and feeding characteristics of dry bulk fiiter aids?
Explain the terms angle of repose, angle of internal friction, and angle of slide. Why are these important to dry bulk chemical handling?
What is permeability and how can it be expressed as an engineering parameter.?
Describe the method for estimating pore size distribution.
Develop a checklist of itemdissues you should follow when selecting a filter media.
Chapter 5