IDS-Water - White Paper
A Practical Guide to Sampling and Testing Filter Media
F. B. Leopold Co, Inc.
F. B. Leopold Co, Inc.
F. B. Leopold Co, Inc.
The water filtration industry has generally accepted the American Water Works Association (AWWA) B100 Standard for Filtering Material as the law for describing filtration materials used in municipal water filters. As the standard states “It is not a specification.” It does “…describe minimum requirements…” but does “…not contain all of the engineering and administrative information normally contained in specifications. The AWWA standards usually contain options that must be evaluated by the user of the standard… The use of AWWA standards is entirely voluntary. AWWA standards are intended to represent a consensus of the water supply industry that the product described will provide satisfactory service.” However, some specifications are written so that certain parameters or operations are to be performed in accordance with the AWWA B100 standard when those parameters or operations are not fully described in the standard. One such operation is the sampling of semi-bulk containers on the jobsite. Furthermore, some operations are performed in total disregard to the standard that can cause errors such as certain special requirements for testing filtering materials that are contained in the B100 Standard.
Section 5.3 states “If filter materials testing is not witnessed at the shipping point by the purchaser, the material should be tested at the jobsite.” We recommend that filtering materials be tested at the point of manufacture, as most manufacturers are equipped to do the sampling and testing as a function of their quality control program. Furthermore, most manufacturers are thoroughly familiar with the B100 Standard requirements for sampling and testing. In fact some larger municipalities make periodic inspections of filter material manufacturers to qualify the manufacturer as an acceptable supplier. When filtering material is produced, they then witness the sampling and identify the sampled material using tags or their distinctive mark. After testing acceptance, the material is shipped directly to the jobsite with no further testing required. This procedure decreases turn around time for the project and eliminates all of the challenges associated with jobsite sampling and testing that we will now discuss.
This paper is being written to provide practical guidance in sampling filtering materials and to highlight the special provisions of the AWWA B100 standard that are in place of or supersede the ASTM standards. This need has arisen from field experiences where filtering materials are specified to be sampled on site and tested by geologic testing facilities that sometimes are not experienced in filtering material testing. In particular, Section 5: Verification in the B100 Standard states that sampling semi-bulk containers “across the cross section of the material being loaded” at the production site is recommended. It also recommends that “If filter materials testing is not witnessed at the shipping point by the purchaser, the material should be tested at the jobsite. The material shall be sampled in accordance with ASTM D 75….” However, ASTM D 75 Standard Practice for Sampling Aggregates does not provide a procedure for sampling semi-bulk containers on the jobsite which is being more frequently specified. ASTM D 75 provides four methods of sampling which are Sampling from a Flowing Aggregate Stream (Bins or Belt Discharge), Sampling from the Conveyor Belt, Sampling from Stockpiles or Transportation Units, and Sampling from Roadway (Bases and Subbases) none of which are suitable for sampling semi-bulk containers. In fact ASTM D 75 does state “Avoid sampling coarse aggregate or mixed coarse and fine aggregate from stockpiles or transportation units whenever possible, particularly when the sampling is done for determining aggregate properties that may be dependent upon the grading of the sample.” ASTM D 75 also states “Sampling is equally important as the testing, and the sampler shall use every precaution to obtain samples that will show the nature and conditions of the materials which they represent.” In other words, the sample must be representative of the material and in all cases should be drawn from the entire cross section of the transportation unit as materials tend to segregate while in transit.
Since the B100 Standard is widely used as an arbiter of filtering material, it should be noted that the “foreword is for information purposes only and is not a part of AWWA B100.” The B100 is not a law and to our knowledge has not been accepted as a law. However a good understanding and knowledge of the specific requirements of the standard are necessary prior to specifying or modifying the standard in contract documents and prior to implementing the standard in the field.
The key in developing sound sampling programs for filtration media is strict adherence to the Standards of AWWA B100 and the referenced ASTM Procedures. Whether testing is required as part of an on-site quality assurance program, or the result of project specifications, the sampling program must satisfy several procedural requirements. Poor sampling practices yield unrepresentative test reports that can consume an enormous amount of time from project personnel. Valuable resources are often spent analyzing results that became invalid the moment the sampling plan failed to meet the sampling requirements of the B100 Standard. The solution isn’t to hire an independent testing lab to assume the responsibility of sampling and testing. There are many cases in which laboratory and project personnel didn’t fully understand the complications of adhering to proper procedures. The process of media sampling and testing, including analyzing the results is time consuming, often involving people from several different companies. Inexperience leads to re-sampling and re-testing that inflates project costs. Hastily grabsampling a bag or two of each media type doesn’t constitute a sound sampling program. If the sampling portion fails to satisfy the standards of AWWA B100 the testing also fails to satisfy the standards and they become invalid.
To help gain an understanding into media sampling and testing, worksheet (FBL-B100-96) presented in Figure 1 was developed to ensure the sampling and testing plan addresses basic AWWA requirements. The bottom section of the worksheet can be used as a reference guide in determining the sampling and testing procedures referenced in the B100 Standard (the worksheet and reference guide are only applicable to AWWA B100-96 edition). Contractors, Engineers, Plant Owners, and Media Manufacturer must understand these requirements, and demand that sampling and testing procedures are properly addressed before sampling begins. At the end of the process each participant must agree the samples and test results are representative. Without a consensus media quality issues cannot be resolved. From a manufacturing mindset, if the media is produced per specification and in accordance with the B100 Standard, the media must be sampled and tested on-site using procedures that likewise fulfill the requirements of the B100 Standard. The concept is to create a level playing field where media quality is evaluated using approved standard practices. Once on-site sampling and testing has been complete, per the guidelines of AWWA and ASTM, the results are valid and final. Sometimes it takes several attempts to get it right, with each attempt becoming a costly learning experience. Hopefully the attached worksheet will help reduce some of the re-sampling, re-testing, and re-analyzing that happens everyday.
ON-SITE SAMPLING AND TESTING WORKSHEET
The first task undertaken in developing any on-site sampling program is an inventory of all the media that requires testing. It is best to include as many types and sizes as possible during each round of sampling, as this will reduce the amount of nonproductive time spent by project and laboratory personnel. It is more effective to cover as much sampling as possible each round than it is to set-up and reassemble personnel for additional sampling at a later time. In addition, some test procedures may take several days from preparation to completion, making sample turnover time an important factor. This is especially true when installation schedules require results on a quick turn basis.
Inventory data must be complete and accurate, indicating exactly how many bags of each media type and size are present along with bag weights. When project specifications require anthracite, sand, and gravel to be tested on-site, it will be necessary to number each bag of anthracite beginning at #1 until all the bags of anthracite of that specific size have been numbered. Likewise, each bag of sand and gravel of similar size will be numbered, (beginning at #1) until all the bags of sand and gravel of that size are numbered. This not only makes it easier to confirm inventory numbers, but it will also aid in determining which bags will be randomly sampled. After inventory is complete the sampling program can begin to be developed. The following sections refer to the Worksheet FBL-B100-96 in Figure 1.
Section 1: Type of Media
Determine the types of filter media or filter gravel that require testing. AWWA B100-96 covers five different types of materials, anthracite, silica sand, high-density sand (classified as filter media), silica gravel, and high-density gravel (classified as filter gravel). The worksheet is limited to these types of materials only.
Section 2: Media Size
From the inventory list indicate the size of media and/or gravel to be tested. For filter media the size is usually the effective size range and uniformity coefficient, i.e., ES 0.95 mm x 1.05 mm and UC < 1.30, while gravel is sized using upper and lower size limits, i.e., 3” x 7”.
Section 3: # of Bags in Lot
From the inventory list indicate the total number of bags of each specific type and size of media requiring testing. This information is critically important in developing the sampling program, as it determines the MINIMUM number of bags of a specific type and size media that must be sampled. These individual samples will ultimately be combined to produce a single composite sample.
Section 4: # of Bags to Sample
Sampling programs usually fail the requirements of AWWA by not sampling the proper number of bags per lot size. Grab-sampling a few media bags is often the norm when it comes to on-site sampling. Although it is much easier, many people fail to understand the impact of taking this approach. Grab sampling works both ways, it not only gives invalid unrepresentative test results that may indicate the media fails to meet project specifications, but it can also yield invalid unrepresentative results that indicate the media meets the specifications. The composite sample must be representative of the lot, which can only be accomplished by being comprised of a minimum number of sub-samples taken from the lot.
Valid sampling programs comply with AWWA B100-96, Section 5.2, TABLE 4, Sampling of bagged media, which determines the minimum number of bags required to be sampled per the lot of media specific to type and size. Table 4 is reproduced in Figure 2 of this paper. It is a minimum number, and occasionally will require additional bags to be included in the sampling scheme in order to achieve the minimum weight limit for composite samples, (discussed later). In using Table 4, if a truckload of anthracite, i.e., E.S. 0.95 mm x 1.05 mm UC < 1.30, is comprised of 18 superbags @ 2500# each, a minimum of five (5) superbags would need to be randomly sampled and combined to produce one (1) composite sample representing the entire truckload of material. If the same truck were delivering a load of anthracite in cubic foot bags @ 50# each, the number of bags to be sampled increases to eighty (80), (since the total number of bags on the truck is 900). A word of advice about sampling and testing cubic foot bags, perform the sampling and testing at the media manufacturers’ facility during production, it is much less messy and the integrity of the bags is not compromised.
Section 5: Individual Bag #s to Sample
Once the number of bags to sample has been determined it will be necessary to identify the actual bag numbers to sample. During the process of taking inventory it was suggested that each bag specific to a type and size be numbered starting at 1, these numbers will be used to ensure that superbag selection is truly random. Picking five (5) bag numbers arbitrarily doesn't make them random; some bias exists.
A method of determining random bag numbers for sampling can be found in ASTM D 3665, Practice for Random Sampling of Construction Materials, referenced in ASTM D 75 Standard Practice for Sampling Aggregates. Both of these ASTM procedures along with ASTM C 702 Standard Practice for Reducing Samples of Aggregate to Testing Size are required reading for any personnel involved with sampling, testing, or review.
To determine which bags to sample refer to ASTM D 3665 and find TABLE 1, Table of Random Numbers reproduced in Figure 3 of this paper. Earlier it was determined that a minimum of five (5) superbags required sampling from the truckload of anthracite superbags. Laying both pages of TABLE 1 on a flat surface, randomly, with a pointer, place a mark in the body of the table. The number directly under the mark is read, for this example the number is 0.280 indicating that the first bag # will be determined by a number in row # 28. The same procedure is repeated, randomly placing another mark on the table and record that number, for this example the number is 0.799, this indicates that the bag # we are looking for is in column #7. The number in column 7, row 28, of Table 1 of ASTM D 3665 is 0.660. The process is repeated, row and column, until at least five (5) numbers have been determined, (sometimes it is better to pick a few additional numbers just in case problems of duplicate bag numbers arise later). The five random numbers are each then multiplied by the number of bags (n) on the truck, (or in the lot). Since the number of bags are 18, we multiply 18 X 0.660 and the bag # we are after is 11.88 rounded to bag #12. Completing the process our bag #’s are 12, 15, 8, 10, and 4. Should the same bag # be repeated twice, disregard the number and use TABLE 1 to determine an additional number if you haven’t determined extras. Although the process may seem complicated, it is easier after reading the whole of ASTM D 3665; (the process of establishing bag numbers takes less than 5 minutes from start to finish).
The act of sampling begins after the bags have been identified and located. There are a variety of sampling methods used in recovering samples from bagged filter media which will be described later. Regardless of the method used, each bag must be sampled using the same method and procedures; all samples will be weighted equally. When sampling tubes are used it is important to recover the sample from a cross-section of each bag when a sample is recovered. Individual samples should be tagged and bagged until it is determined that the combined weight of the individual samples can produce a composite sample of sufficient size to fulfill the requirements of AWWA.
Section 6: Sample ID #
The composite sample must meet the minimum weight limits defined in AWWA B100-96 Section 5.2, Table 3: Minimum size of composite sample as reproduced in Figure 4. Occasionally the weight of the samples removed from the superbags, when combined into the composite sample will fail to meet the limit. If each sample removed from the anthracite superbags in the example weighted 1 pound, the composite sample would weigh 5 pounds. Although it may be of adequate size for testing purposes, it fails to meet a requirement of B100. In the example Table 3 indicates that the minimum size of the composite sample is 10 lbs. (4.5 kg) since the maximum size of particles in the sample are smaller than 3/8”. The weight of the composite sample must be increased by an additional 5 lbs., either by randomly determining additional bags to sample, or by doubling the size of the samples recovered from each of the original five bags. Again, care must be taken to ensure that individual sample size is roughly the same; big differences can skew test results.
Once the individual samples have been combined to form the composite sample, and the composite meets the weight requirements of AWWA B100-96 TABLE 3, the sample can be reduced. Although sampling anthracite and sand generally produces composite samples of manageable size, quite often and especially with gravels, the composite sample will need to be reduced for handling purposes. When reduction is needed it is accomplished by one of the methods recognized in ASTM C 702, Standard Practice for Reducing Samples of Aggregate to Testing Size. The characteristic determining how the composite sample will be reduced is the moisture condition of the sample. If the sample has visible moisture on the surface it is reduced using the cone and quartering method of reduction, if the composite is dry, it is reduced using a mechanical splitter or riffle so as not to crush individual grains. When reducing samples by the cone and quartering method make sure the sample is handled in a manner that will not cause the sample to crush during the repeated quartering process. A good practice when reducing composite samples to testing size is to create splits of the sample that can be archived for later use if problems occur during testing.
When the composite sample has been reduced to a manageable size it should be placed in a clean, airtight container and sealed to prevent contamination. The container must be affixed with a sample ID number, initial weight, and the signature of the person(s) responsible for verifying the sampling program. Include as much information in the sample ID number as possible and make sure that the sampling information is forwarded to the testing laboratory along with the sample.
Some materials and transportation containers are not easily sampled. For instance, cubic foot bags can be opened, sampled and resealed. One method is to insert a small grain thief into the bag and reseal the bag using tape. However, this does compromise the integrity of the bag and subsequent handling may affect the sealing method. Also, the grain thief is not suitable for gravel material in cubic foot sacks. These samples may be reduced using a mechanical splitter also known as a riffler or the cone and quarter method; both referenced in ASTM C 702. The cone and quarter method involves placing the material on a hard clean surface, mixing the material by forming cones at least three times, flattening the material, dividing the material into quarters, bagging two opposite quarters, and repeating until the appropriate sample size has been obtained. All of the preceding methods are logistically difficult at the jobsite due to the number of bags involved.
Dry anthracite and granular activated carbon have been successfully sampled from semibulk containers using a brass seed sampler. The brass seed sampler actually consists of two nested tubes, which contain slotted openings. One end of each tube is pointed and the other end is open. The inside tube is rotated so that the slots are not aligned and the sampler is inserted into the semi-bulk container to refusal but not through the bottom or sides of the container. The inside tube is rotated so that the slots align and both tube openings align to allow the sample to flow into the sampler. The inside tube is again rotated so that the slots are not aligned and the sampler is removed from the container. The sample is poured from the open end of the sampler into the sample storage container. Care must be exercised to not force the sampler into the container being sampled especially with a hammer nor to force rotating the tubes as this may cause attrition of the material being sampled. Figure 5 presents a semi-bulk container of anthracite being sampled using a brass seed sampler along with samplers in the open and closed positions.
The brass sampler has not been found to be suitable for sampling silica gravel, silica sand, high-density gravel and high-density sand in semi-bulk containers. Generally it is not possible to insert the brass sampler into these materials without applying substantial force and the tubes tend to bind both on opening and closing as these materials jam the tubes. Also, some of the larger gravel will not flow through the slotted openings. Alternative methods for sampling these materials in semi-bulk containers must be devised as the B100 and the ASTM standards do not describe any sampling techniques. Four methods have been used and have met with varying success. They are, in decreasing order of accuracy:
Figure 6 presents the apparatus and equipment necessary to obtain a flowing stream from
a semi-bulk container of material. It consists of a support structure, a hopper large enough
to contain one semi-bulk container, a flow control valve such as this slide gate, a method to
support the semi-bulk container to be filled such as a fork lift, and sampling devices such
as pans or buckets. The container to be sampled is deposited into the hopper and the
same container is positioned below the slide gate. On cue the slide gate is opened and
samples are taken through the flowing stream. The samples are deposited into another
In any event, as the ASTM D 75 states, it is the responsibility of the sampler to obtain a representative sample of the material to be tested. No matter how precise the tests are performed on the sample obtained, if the sample is not representative, the test results will be in error. Conversely, even though the sample is representative, if the tests are not performed correctly the results can still be in error.
To ensure reliable, repeatable results the B100 Standard bases most of the filtering material tests on existing ASTM standard tests. However, in some cases the B100 Standard makes significant modifications that can effect the results if the technician performing the testing is unaware. Prior to procuring the services of a testing laboratory, the testing technician should be asked whether they are experienced in AWWA B100 filtering material testing. If not, they should be cautioned that not all of the tests follow the ASTM standards exactly. If the testing facility does not have a copy of the B100 standard, they should be instructed to obtain a copy and review it before the samples arrive. In some cases, additional testing equipment may need to be procured. Usually they will have to obtain sieves in order to complete a proper B100 sieve stack. The following is a discussion of each of the major Test Procedures described in Section 5.3 of the B100 Standard:
5.3.1 Acid Solubility
The acid solubility test is unique to the B100 Standard and the test in its entirety is described in the standard. Prior editions of the B100 Standard required the hydrochloric acid concentration to be 20% and the sample had to stand for 24 hours, but the most recent edition uses a 1:1 ratio and a time limit of 30 minutes after effervescence ceases. This has decreased the time necessary to perform the test.
5.3.2 Gravel Shape
Again the gravel shape test is unique to the B100 standard and is described in its entirety in the standard. Of all the B100 tests, this is one of the most subjective. It requires the observer to estimate and identify various characteristics. The first is fractured faces which are “defined as a surface surrounded by sharp edges, such as those produced by crushing, that occupy more than approximately 10 percent of the total surface area of the particle. This is intended to exclude a surface with small nicks and chips from classification as a fractured face.” Section 220.127.116.11.3 allows no more than 25% by dry weight to have more than one fractured face. This definition not only requires the observer to estimate what constitutes 10% of the total area, but also whether the face is surrounded by sharp edges. Some flat surfaces that do not have sharp edges have been erroneously identified as fractured faces.
The other gravel shape test is shape determination. The shape is defined as “the ratio of the longest axis to the shortest axis of the circumscribing rectangular prism for a piece of gravel shall be determined using a caliper or a proportional divider.” Less subjective, the observer has the use of a mechanical device and only has to determine the circumscribing prism.
5.3.3 Specific Gravity
The B100 utilizes both the ASTM C 127 Standard Test Method for Specific Gravity and Adsorption of Coarse Aggregate and the ASTM C 128 Standard Test Method for Specific Gravity and Absorption of Fine Aggregate. It also allows the use of the Noble Large Aggregate Test for silica gravel, but the test is unique to the standard and requires special equipment which most laboratories may not have. However, it is less complicated and faster to perform than the ASTM C 127. Currently, the B100 Committee is considering a similar alternative test to be performed on anthracite if it can be shown to be as accurate and repeatable as the Noble Large Aggregate Test was shown for gravel.
The ASTM C 127 specific gravity test for silica gravel is reported as a saturated surface dry specific gravity. Inherent with this test and the ASTM C 128 is the determination as to when the material has been dried, usually using towels to remove the excess moisture, to a saturated surface dry condition. With experience most technicians are able to provide repeatable results. Another aspect of the ASTM C 127 test is that it is intended for material retained on a No. 4 (4.75 mm) sieve. Since many gravel gradations include a barrier layer that is 1/8” x No.12 (3.175 mm x 1.70 mm) and a support layer of ¼” x 1/8” (6.35 mm x 3.175 mm) these tests need to be handled differently. The 1/8” x No. 12 should be tested in accordance with ASTM C 128 and the ¼” x 1/8” should be split between ASTM C 127 for the larger fraction and ASTM C 128 for the smaller fraction.
All other materials including silica sand are tested using the ASTM C 128 and reported as apparent specific gravity. The procedure empirically determines the specific gravity of an aggregate or media on a bulk saturated surface dry basis, and the absorption value in the bulk saturated surface dry state. These two values are then used to mathematically determine the specific gravity of the media in both an apparent and bulk basis. The ASTM C 128 states that apparent specific gravity “pertains to the relative density of the solid materials making up the constituent particles not including the pore space within the particle that is accessible to water.” It represents the specific gravity of the solid dense anthracite, less the pore space. The pore space within anthracite, determined by absorption, is as much as eight times higher than that of silica or garnet materials. These higher absorption values widen the difference in specific gravity between the apparent and bulk saturated surface dry basis. In the case of anthracite a more accurate or true value of specific gravity may be bulk (saturated surface dry), since it represents the specific gravity of the media particles when the pore spaces are saturated with water, as they are when in a filter.
5.3.4 Sieve Analysis
The most specified and possibly the most controversial test is the sieve analysis also known as gradation analysis. This test is performed in accordance with the ASTM C 136 Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates with significant modifications stipulated by the B100. The most significant modifications are the sample size, machine-shaking times, sieve calibration, and sieve stack. The purpose of the sieve analysis for fine filtering material such silica sand, garnet sand, and anthracite is to produce a plot of the results on semi-log paper to obtain the effective size and uniformity coefficient of the tested material. For a further explanation of these criteria, refer to the B100 Standard. The purpose of the sieve analysis for gravel is to obtain the percent over and under size of the tested material. For instance the percent greater than ¾” and the percent less than ½” for a ¾” x ½” gravel.
The minimum sample size is listed in Table 6 (Minimum sample size for sieve analysis) of the B100 Standard and reproduced as Figure 7 in this paper. However, for anthracite sieve analysis we find that using much more than the minimum sample size causes too much anthracite to accumulate on individual sieves and can cause blinding of the sieve. If the sieve blinds it may not allow all of the smaller particles to pass leading to erroneous results. The reason for this is the difference in bulk density between silica / high-density material and anthracite. The bulk density of anthracite is 50 pounds /cubic foot versus 100 pounds / cubic foot for silica materials and up to 300 pounds /cubic foot for high-density material. That means that the same weight sample of anthracite has twice the volume of the same weight of a silica material. We recommend that the anthracite sample size for sieving be no more than 150 grams. This anthracite sample size limit is also being considered in the next update of the B100 Standard.
A significant modification of the ASTM C 136 is the required machine shaking times. The B100 Standard states “Generally, sieves require machine shaking times of 10 min. ± 0.5 min. for sand and gravel and 5 min. ± 0.5 min. for anthracite.” This is in contrast to the ASTM C 136 requirement of “Continue sieving for a sufficient period and in such manner that, after completion 1 mass % of the residue on any individual sieve will pass that sieve during 1 min. of continuous hand sieving…” Most testing laboratories are unaware of this B100 Standard requirement and excessive sieving can reduce the particle sizes. The B100 Standard states that “Care shall be taken to avoid breaking anthracite particles when sieving.”
Calibration of the sieves used can be critical in obtaining accurate results. The B100 requires that “All standard sieves used for testing filter materials shall conform to the tolerances required in ASTM E 11 Standard Specification for Wire Cloth and Sieves for Testing Purposes. If compliance to specifications arise when nominal sieve openings are used, standard reference materials (glass spheres) certified by the National Bureau of Standards shall be used in accordance with their calibration procedure to accurately determine the effective opening size of each sieve.” In fact when sieving anthracite, silica sand and high density sand, most laboratories use glass sphere calibrated sieves because the effective size tolerances of these filter materials are 0.001 mm or 0.00004 inches. That is the difference between a filtering material being accepted or rejected. If glass sphere calibration is not available, the B100 allows replotting the data “using both the maximum and minimum permissible variation of average opening from the standard sieve designation as shown in Table 1, column 4 of ASTM E 11” also reproduced in Appendix B of the B100 Standard.
Most testing laboratories will list the effective sieve size as derived from the nominal sieve size opening. The effective sieve size is the average sieve opening determined by glass sphere calibration. The nominal size opening is the nominal dimension listed in ASTM E 11, Table 1. If the reported sieve opening varies from the nominal sieve opening, the sieves usually have been glass sphere calibrated. If the nominal sieve size is listed, it is usually assumed that the sieves are not glass sphere calibrated, but still comply with the ASTM E 11 tolerances. Sieves that do not meet the ASTM E 11 tolerances must not be used. Laboratories and manufacturers experienced in B100 filtering material testing will usually maintain a separate set of glass sphere calibrated sieves that are used exclusively for testing filtering material and maintain other sieves for their other work. If in doubt ask the testing laboratory if their sieves are glass sphere calibrated. If they are not glass sphere calibrated, questions of calibration are relegated to replotting the results using the maximum and minimum permissible variation of average opening from the ASTM E 11, Table 1 standard sieve designation.
The B100 Standard requires that “To avoid excessive interpolation when determining the effective size… and the D60… the sieves used on a particular sieve analysis shall have openings such that the ratio between adjacent sizes is the fourth root of 2, or 1.1892.” In other words, the sieves should be in consecutive order as listed in the B100 Appendix, i.e. No. 10, No. 8, No. 6, No. 5, etc. Skipping one sieve size can drastically effect the effective size and uniformity coefficient that will provide erroneous results especially with the tolerances described above. It is always prudent to check with the testing laboratory prior to delivering the samples to determine if they have on site calibrated, consecutive sieve sizes in the range of the material being tested and to prevent delay of receiving accurate results. The other stipulation by the B100 is that “The sieves shall be chosen so that the nominal opening of only one sieve is smaller than the smallest effective size so that the greatest range of particle size distribution can be measured in one standard nest of six sieves.” This is to ensure that the sieve stack is limited to the opening sizes of most importance to the gradation plot.
18.104.22.168 Mohs’ scale of hardness.
The B100 states that “No standard test method has been found; however, all commercial laboratories follow the same procedure.” This statement is very ambiguous and makes this a very subjective test. The B100 Committee will be reviewing a test method to make it more objective. Most testers use various types of known hardness geologic, flat faced rocks, to determine the hardness of filtering materials. Usually samples of the filtering material are mounted on sticks. The mounted samples are used to scratch the rock samples of increasing hardness, using a firm, consistent pressure, until the filtering material can not longer scratch the rock samples. The last scratched geologic rock of say 75% of the sample filtering material is then used as the final Moh number. For instance if a majority of the filtering material last scratches a 3.0 Moh hardness calcite, the material is said to have a Moh hardness greater than 3.0. The experience of the tester is important in selecting the samples, determining the edge of the sample to be used, maintaining a consistent pressure and knowing when the sample is scratching the geologic rock or when the sample is crushing under pressure. Moh’s hardness is truly a subjective test with no present standard for guidance.
The AWWA B100 Standard for Filtering Material is the standard recognized throughout the
water industry and the world as the document that best describes quality filtering material.
More often than not the B100 is specified in water engineering documents as the minimum
requirements for supplying filtering material. Furthermore, some engineering documents
while specifying the minimum B100 standards modify these either by making them more
stringent or by adding additional requirements. It is recommended that when modifying the
B100 requirements, one should be intimately familiar with the basis of the standard as well
We recommend that if at all possible sampling and testing of filtering material should be witnessed at the point of manufacture. Most manufacturers have the facilities to sample and test the materials on site and are experienced in the AWWA B100 requirements. If it is not possible to witness the sampling and testing at the point of manufacture, the manufacturer’s sampling and testing is not adequate, or the filtering materials must be sampled and tested on site, a detailed sampling and testing program must be organized. The program must adhere to the B100 requirements as outlined before.
Sampling a filtering material is a critical process that could effect the overall results of a testing program. Care must be exercised that the sample is representative of the material being sampled. Some materials such as anthracite and GAC are readily sampled using a brass sampler, but most silica and high density material is difficult to sample and adequate methods are not described in the standards. Prior to specifying or implementing a sampling program, the actual logistics must be adequately planned to obtain representative samples or the samples produced could be inaccurate.
Prior to obtaining the samples, testing laboratories should be interviewed before one is retained. During the interview it should be determined if the laboratory is experienced in AWWA B100 testing. If a laboratory that is experienced in AWWA B100 testing can not be found, the selected laboratory should procure the appropriate standards and thoroughly familiarize themselves with the modifications contained in the B100 Standard. After receiving a test report on filtering materials the result should be checked to determine if the proper B100 procedures were followed. If not, the samples should be re-tested using B100 methods. If there is any doubt about the results received, both the sampling and testing should be investigated to ensure that a representative sample of the filtering material was obtained and that AWWA B100 methods were used. If after an investigation of the methods used confirms that all procedures were in compliance with all standards, including the AWWA B100 and the appropriate ASTM standards, only then can the material be considered either compliant or not compliant with the AWWA B100 Standard for Filtering Material.
“ASTM C 127 Standard Test Method for Specific Gravity and Adsorption of Coarse Aggregate,” American Society for Testing and Materials
“ASTM C 128 Standard Test Method for Specific Gravity and Adsorption of Fine Aggregate,” American Society for Testing and Materials
“ASTM C 136 Standard Test Method for Sieve Analysis of Fine and Coarse Aggregate,” American Society for Testing and Materials
“ASTM C 702 Standard Practice for Reducing Samples of Aggregate to Testing Size,” American Society for Testing and Materials
“ASTM D 75 Standard Practice for Sampling Aggregate,” American Society for Testing and Materials
“ASTM D 3665 Practice for Random Sampling of Construction Materials,” American Society for Testing and Materials
“ASTM E 11 Standard Specification for Wire Cloth and Sieves for Testing Purposes,” American Society for Testing and Materials “Standard for Filtering Material, B100-96,” American Water Works Association