vvEPA
United States
Environmental Protection
Agency
Method 1681: Fecal Coliforms in
Sewage Sludge (Biosolids) by Multiple-
Tube Fermentation using A-1 medium
July 2006
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U.S. Environmental Protection Agency
Office of Water (4303T)
1200 Pennsylvania Avenue, NW
Washington, DC 20460
EPA-821-R-06-013
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Acknowledgments
This report was prepared by the DynCorp/CSC Biology Studies Group under the direction of Robin K.
Oshiro, of the Office of Science and Technology's Engineering and Analysis Division within the U.S.
Environmental Protection Agency (EPA's) Office of Water. This report was prepared in support of a
project lead by Mark C. Meckes of the EPA Office of Research and Development's National Risk
Management Research Laboratory (NRMRL).
The contributions of the following persons and organizations to this study are gratefully acknowledged:
Referee Laboratory
• EPA Office of Research and Development, National Risk Management Research Lab: Mark C.
Meckes and Karen M. White
Volunteer Participant Laboratories
• American Interplex: John Overbey and Lizbeth Huggins
• BioVir Laboratories: Rick Danielson and Jim Truscott
• City of Los Angeles Bureau of Sanitation Environmental Monitoring Division: Farhana
Mohamed and Zora Bahariance
• County Sanitation Districts of Los Angeles County: Shawn Thompson and Julie Millenbach
Environmental Associates: Susan Boutros and John Chandler
• Hampton Roads Sanitation District: Anna Rule and Bob Maunz
King County Environmental Laboratory: Greg Ma and Bobbie Anderson
• Hoosier Microbiological Laboratories: Don Hendrickson, Keri Nixon, Katy Bilger, and Lindsey
Shelton
• Massachusetts Water Resources Authority: Steve Rhode and Mariya Gofshteyn
• Milwaukee Metropolitan Sewerage District: Jeff MacDonald and Tim O'Neill
• University of Iowa Hygienic Laboratory: Nancy Hall and Cathy Lord
Utah Department of Health: Sanwat Chaudhuri and Devon Cole
The following facilities provided biosolid matrices for the study:
• Compost Facility, Columbus, OH: Angela Bianco
Wastewater Treatment Facility, Sturgeon Bay, WI: Todd Maurina
• Wastewater Treatment Facility, Fairfield, OH: Drew Young
Wastewater Treatment Facility, Mason, OH: Ernie Stickler
• N-Viro Treatment Facility, Toledo, OH: Cindy Drill
Media Photographs
Mark C. Meckes, NRMRL, US EPA
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Disclaimer
The Engineering and Analysis Division, of the Office of Science and Technology, has reviewed and
approved this report for publication. Neither the United States Government nor any of its employees,
contractors, or their employees make any warranty, expressed or implied, or assumes any legal liability or
responsibility for any third party's use of or the results of such use of any information, apparatus, product,
or process discussed in this report, or represents that its use by such party would not infringe on privately
owned rights. Mention of trade names or commercial products does not constitute endorsement or
recommendation for use.
Questions concerning this method or its application should be addressed to:
Robin K. Oshiro
Engineering and Analysis Division (4303T)
U.S. EPA Office of Water, Office of Science and Technology
1200 Pennsylvania Avenue, NW
Washington, DC 20460
oshiro.robin@epa.gov
202-566-1075
202-566-1053 (facsimile)
IV
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Table of Contents
1.0 Scope and Application 1
2.0 Summary of Method 2
3.0 Definitions 2
4.0 Interferences 3
5.0 Safety 3
6.0 Equipment and Supplies 3
7.0 Reagents and Standards 4
8.0 Sample Collection, Handling, and Storage 8
9.0 Quality Control 10
10.0 Equipment Calibration and Standardization 14
11.0 Sample Preparation 14
12.0 A-l Procedure 21
13.0 Verification 23
14.0 Data Analysis and Calculations 23
15.0 Sample Spiking Procedure 30
16.0 Method Performance 36
17.0 Pollution Prevention 37
18.0 Waste Management 37
19.0 References 38
20.0 Figures 38
21.0 Glossary 44
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Method 1681: Fecal Conforms in Sewage Sludge (Biosolids) by
Multiple-Tube Fermentation using A-1 medium
July 2006
1.0 Scope and Application
1.1 This method describes multiple-tube fermentation procedures [also called the most probable
number (MPN) procedure] for the detection and enumeration of fecal coliform bacteria in
biosolids. These methods use culture-specific media and elevated temperature to isolate and
enumerate fecal coliform organisms. Fecal coliform bacteria, including Escherichia coll (E. coll),
are commonly found in the feces of humans and other warm-blooded animals, and indicate the
potential presence of other bacterial and viral pathogens.
1.2 This method is adapted from method 9221E in Standard Methods for the Examination of Water
and Wastewater, 20th Edition, for the determination of fecal coliform bacteria in a variety of
matrices (Reference 19.1).
1.3 This method is designed to meet the survey and monitoring requirements of the U.S.
Environmental Protection Agency (EPA) in regulating the use and disposal of biosolids under 40
CFR Part 503. Subpart D of the 503 regulations protects public health and the environment
through requirements designed to reduce the potential for contact with disease-bearing
microorganisms (pathogens) in biosolids applied to land or placed on a surface disposal site.
1.4 Fecal coliform density is expected to correlate with the probability of pathogens present and
document process performance (vector attraction reduction).
1.5 This method may be used to determine the density of fecal coliform bacteria in Class A and Class
B biosolids to satisfy the pathogen reduction requirements of Subpart D of Part 503. A biosolid
sample is classified as Class A if it contains a fecal coliform density below 1,000 MPN/g of total
solids (dry weight basis). A biosolid sample is classified as Class B if the geometric mean fecal
coliform density is less than 2 x 106 MPN/g of total solids (dry weight basis).
1.6 To satisfy the pathogen reduction monitoring alternatives for Class B biosolids, seven samples of
treated biosolids are collected at the time of use or disposal and the geometric mean fecal
coliform bacterial density of these samples is confirmed not to exceed 2 x 106 MPN/g of total
solids (dry weight basis). Although the Part 503 regulation does not specify the total number of
samples for Class A biosolids, it is recommended that a sampling event extend over two weeks,
and that at least seven samples be tested to confirm that the mean bacterial density of the samples
is below 1,000 MPN/g of total solids (dry weight basis). The analysis of seven samples increases
the method precision by reducing the standard error caused by inherent variations in biosolid
quality.
1.7 The presence of fecal coliforms may be determined in both Class A and Class B biosolids using
the MPN procedure.
1.8 Any modification of the method beyond those expressly permitted is subject to the application
and approval of alternative test procedures under 40 CFR Parts 136.4 and 136.5.
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Method 1681
1.9 Method 1681 was submitted to interlaboratory validation in Class A and Class B biosolid
matrices. A summary of method performance results from this validation study are provided in
Section 16.0. A comprehensive evaluation of the study results is presented in the validation study
report (Reference 19.2). For method application please referto Title 40 Code of Federal
Regulations Part 136 (40 CFRPart 136).
Note: Based on the high false positive rates observed for Method 1681 in some matrices, EPA
recommends that laboratories conduct their own matrix-specific comparisons to determine the
most appropriate method (1680 or 1681).
2.0 Summary of Method
2.1 Fecal coliform densities of biosolids may be determined by the MPN procedure.
2.2 MPN procedure (Class A and B)
Method 1681 provides for the enumeration of fecal coliforms in Class A and Class B biosolids
using the most probable number (MPN) procedure. In Method 1681, A-1 medium is used as a
direct, single step test.
2.2.1 Summary of the A-l procedure (see Figure 1 in Section 20.0) [see Sections 11.0 and
12.0 for explanation]
2.2.1.1 A minimum of four sample dilutions are required, while five or more are
preferred. Each sample dilution is inoculated into five test tubes
containing A-l medium and inverted vials.
2.2.1.2 Sample tubes are incubated in a waterbath or jacketed incubator at
35°C ± 0.5°C for 3 hours, then transferred to a waterbath at 44.5°C ±
0.2°C. After 21 ± 2 hours, tubes are examined for growth and gas
production. Gas production in 24 ± 2 hours or less is a positive reaction
indicating the presence of fecal coliforms.
2.2.1.3 Results of the MPN procedure using A-l medium are reported in terms
of the most probable number (MPN) / g calculated from the number of
positive A-l culture tubes and percent total solids (dry weight basis).
3.0 Definitions
3.1 Fecal coliform bacteria are gram-negative, non-spore-forming rods that are found in the intestines
and feces of humans and other warm-blooded animals. The predominant fecal coliform is E. coll.
In this method, fecal coliforms are those bacteria that ferment lactose and produce gas within 24 ±
2 hours in A-l broth after incubation at 44.5°C ± 0.2°C. Since coliforms from other sources often
cannot produce gas under these conditions, this criterion is used to define the fecal component of
the coliform group.
3.2 Class A biosolids contain a fecal coliform density below 1,000 MPN/g of total solids (dry weight
basis).
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Method 1681
3.3 Class B biosolids contain a geometric mean fecal coliform density of less than 2 x 106 MPN/g of
total solids (dry weight basis).
3.4 Definitions for other terms are given in the glossary at the end of the method.
4.0 Interferences
4.1 MPN procedure: Since the MPN tables are based on a Poisson distribution, if the sample is not
adequately mixed to ensure equal bacterial cell distribution before portions are removed, the
MPN value will be a misrepresentation of the bacterial density.
5.0 Safety
5.1 The analyst must observe normal safety procedures required in a microbiology laboratory while
preparing, using, and disposing of media, cultures, reagents, and materials, and while operating
sterilization equipment.
5.2 Field and laboratory staff collecting and analyzing environmental samples are under some risk of
exposure to pathogenic microorganisms. Staff should apply safety procedures used for pathogens
to handle all samples.
5.3 Mouth-pipetting is prohibited.
6.0 Equipment and Supplies
6.1 Sterile plastic bags, 1-gallon
6.2 Sterile plastic or glass jars with lids, 1-L
6.3 Sterile auger
6.4 Sterile scoops (do not use curved scoops)
6.5 Ice chest
6.6 Wet ice
6.7 Ice packs, blue ice
6.8 Bubble wrap
6.9 Sterile trowels
6.10 Sterile aluminum foil or kraft paper
6.11 Sterile container, such as a stainless steel or plastic bucket suitable for sample collection
6.12 Flat shovel
6.13 Dilution bottles, borosilicate glass, screw cap, marked at 99 mL or screw cap, borosilicate glass or
plastic tubes marked at 9 mL
6.14 Tubes, 16 x 150 mm, borosilicate glass, with loose-fitting aluminum, stainless steel or
autoclavable caps
6.15 Durham tubes or vials, 10 x 75 mm, borosilicate glass
6.16 Tubes, 16 x 100 mm, screw cap, borosilicate glass, with autoclavable plastic caps
3 July 2006
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Method 1681
6.17 Test tube racks to hold sterile culture tubes
6.18 Pipet container, stainless steel, aluminum or borosilicate glass, for glass pipets
6.19 Pipets, sterile, T.D. bacteriological or Mohr, glass or plastic, wide-tip of appropriate volume
6.20 Pipet bulbs, or automatic pipettor
6.21 Platinum wire inoculation loops, at least 3 mm diameter in suitable holders; or sterile plastic
loops
6.22 Sterile disposable applicator sticks
6.23 Bunsen burner or alcohol burner
6.24 Cornwall syringe, sterile, to deliver at least 5 mL
6.25 Media dispensing pump
6.26 Incubator, water- or air-jacketed, humidity-controlled, microbiological type to hold temperature
at35.0°C±0.5°C
6.27 Gable covered waterbath, with circulating system to maintain temperature of 44.5°C ± 0.2°C
Water level should be maintained above the media in immersed tubes
6.28 Plastic sterile petri dishes, microbiological grade, 15 mm x 100 mm
6.29 Erlenmeyer flasks, 1-L and 2-L
6.30 Stir bar
6.31 Stir plate
6.32 Sterile blender jars and base
6.33 Waterbath maintained at 50°C for tempering agar
6.34 Balance, analytical balance capable of weighing 0.1 mg
6.35 Thermometer, checked against a National Institute of Standards and Technology (NIST) certified
thermometer, or one that meets the requirements of NIST Monograph SP 250-23
6.36 Latex gloves for handling samples
6.37 pH meter
6.38 Vortex mixer
6.39 Flasks, borosilicate glass, screw-cap, 250-2000 mL volume
6.40 Graduated cylinders, 100- to 1000-mL, covered with aluminum foil or kraft paper and sterilized
6.41 Beakers, glass or plastic, assorted sizes
6.42 Steel pan of water, 30" x 26" x 10"
6.43 Autoclave or steam sterilizer capable of achieving 121°C [15 Ib pressure per square inch (PSI)]
for 15 minutes
6.44 Crucible or aluminum evaporating dish
6.45 Drying oven maintained at 103°C - 105°C for tempering agar
7.0 Reagents and Standards
7.1 Reagent-grade chemicals shall be used in all tests. Unless otherwise indicated, reagents shall
conform to the specifications of the Committee on Analytical Reagents of the American Chemical
Society (Reference 19.3). The agar used in preparation of culture media must be of
microbiological grade.
July 2006 4
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Method 1681
7.2 Whenever possible, use commercial dehydrated culture media.
7.3 Purity of reagent water: Reagent-grade water conforming to specifications in Standard Methods
for the Examination of Water and Wastewater (latest edition approved by EPA in 40 CFR Part
136 or 141, as applicable), Section 9020 (Reference 19.1).
7.4 Phosphate buffered dilution water
7.4.1 Composition of stock phosphate buffer solution:
Monopotassium phosphate (KH2PO4) 34.0 g
Reagent-grade water 500.0 mL
Preparation: Dissolve KH2PO4 in 500 mL reagent-grade water. Adjust the pH of the
solution to 7.2 with 1 N NaOH, and bring the volume to 1 L with reagent-grade water.
Sterilize by filtration or autoclave at 121°C (15 PSI) for 15 minutes.
7.4.2 Preparation of stock magnesium chloride (MgCl2) solution: Add 38 g anhydrous MgCl2
or 81.1 g magnesium chloride hexahydrate (MgCl2 • 6H2O) to 1 L reagent-grade water.
Sterilize by filtration or autoclave at 121°C (15 PSI) for 15 minutes.
7.4.3 After sterilization, store the stock solutions in the refrigerator until used. If evidence of
mold or other contamination appears, the affected stock solution should be discarded and
a fresh solution should be prepared.
7.4.4 Working phosphate buffered dilution water: Mix 1.25 mL of the stock phosphate buffer
and 5 mL of the MgCl2 stock per liter of reagent-grade water. Dispense in appropriate
amounts for dilutions and/or for use as rinse buffer. Autoclave at 121 °C (15 PSI) for 15
minutes. Final pH should be 7.0 ± 0.2. The amount of time in the autoclave must be
adjusted for the volume of buffer in the containers and the size of the load.
Note: When test tube racks containing 9.0 mL sterile dilution water are prepared, they are
placed into an autoclavable pan with a small amount of water to contain breakage and
minimize evaporation from the tubes.
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Method 1681
7.5 Heart infusion agar (HIA)
7.5.1 Composition:
Beef heart, infusion from 500 g 10. Og
Bacto tryptose 10.Og
Sodium chloride 5.0g
Bacto agar 15.0g
Reagent-grade water l.OL
7.5.2 Add reagents to 1 L of reagent-grade water, mix thoroughly, and heat to dissolve. Adjust
pH to 7.4 ± 0.2 with 1.0 N hydrochloric acid or 1.0 N sodium hydroxide. Stir well and
autoclave at 121°C (15 PSI) for 15 minutes. Pour into 15 x 100 mm sterile petri plates.
Let the media warm to room temperature prior to inoculation. Other general growth
media may be used for quality assurance (QA) (Section 9.0) purposes.
7.6 A-l medium
7.6.1 Composition:
Lactose 5.0g
Tryptone 20.0 g
Sodium chloride 5.0g
Salicin 0.5 g
Triton® X-100 1.0 mL
Reagent-grade water l.OL
7.6.2 For single strength (IX) A-l, add reagents to 1 L of reagent-grade water, mix thoroughly,
heat to dissolve and add 1.0 mL of Triton® X-100. Adjust pH to 6.9 ± 0.1 by addition of
1.0 N hydrochloric acid or 1.0 N sodium hydroxide, if necessary. Prior to sterilization,
dispense 10 mL into 16 x 150 mm test tubes with inverted vials. Make sure there is
enough medium to cover the inverted vial at least halfway after sterilization. Close with
metal or autoclavable plastic caps. Sterilize by autoclaving at 121°C (15 PSI) for 10
minutes. Ignore formation of precipitate. Media should fill inverted tubes leaving no air
spaces.
7.6.3 For double strength (2X) A-l, prepare as in Section 7.6.2 but use 500 mL of reagent-
grade water instead of 1 L. Note: 2X A-l is necessary for 10-mL inoculations, to ensure
that the 10 mL inoculation volume does not excessively dilute the media.
July 2006
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Method 1681
7.7 LTB medium
7.7.1 Composition:
Tryptose 20.0 g
Lactose 5.0 g
Dipotassium phosphate (K2HPO4) 2.75 g
Monopotassium phosphate (KH2PO4) 2.75 g
Sodium chloride (NaCl) 5.0 g
Sodium lauryl sulfate 0.1 g
Reagent-grade water 1.0 L
7.7.2 Add reagents to 1 L of reagent-grade water, mix thoroughly, and heat to dissolve. Adjust
pH to 6.8 ± 0.2 with 1.0 N hydrochloric acid or 1.0 N sodium hydroxide, if necessary.
Prior to sterilization, dispense medium into appropriate tubes/bottles. Close tubes with
metal or autoclavable plastic caps and autoclave at 121°C (15 PSI) for 15 minutes. After
cooling, the medium should fill the inverted vials completely, leaving no air space.
7.8 Positive controls
7.8.1 Obtain a stock culture of E. coll (e.g., ATCC # 25922) as apositive control for A-l.
Note: ATCC recommends that no more than 5 transfers be made before returning to the
original culture. This will minimize the chance of contamination during transfers and
genetic shift of the culture. One suggestion is to make your own frozen seed stock upon
receipt of the organism that can be used for future work. For additional information go to
http://www.atcc.org.
7.9 Negative controls
7.9.1 Obtain a stock culture of Enterobacter aerogenes (e.g., ATCC # 13048) as a negative
control for A-l.
7.10 The storage times for prepared media used in this method are provided in Table 1.
Table 1. Storage Times for Prepared Media
Media
Agar in loose-cap tubes
Agar in tightly closed screw-cap tubes
Broth (A-1 )
Poured agar plates (should be stored inverted)
Large volume of agar in tightly closed screw-cap flask or bottle
Storage Time
2 weeks
3 months
7 days
2 weeks
3 months
(Note'. If media is refrigerated, remove from refrigerator 1-1.5 hours prior to inoculation, so that it reaches room
temperature prior to use.)
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Method 1681
7.11 Milorganite® (CAS 8049-99-8) or equivalent
Milorganite® (heat-dried Class A biosolid) is produced by Milwaukee Metropolitan Sewerage
District. It is available in many home gardening centers.
Obtain Milorganite® as the reference matrix for initial precision and recovery (IPR) and ongoing
precision and recovery (OPR) analyses. Milorganite® is used as the reference matrix because it
is easily accessible, inexpensive, generally does not contain the analyte of interest, and is of
consistent quality.
8.0 Sample Collection, Handling, and Storage
8.1 The most appropriate location for biosolid sample collection is the point prior to leaving the
wastewater treatment plant. Samples may be taken from pipes, conveyor belts, bins, compost
heaps, drying beds, and stockpiles.
8.2 Collect samples in sterile, non-toxic glass or plastic containers with leak-proof lids. All sampling
containers and equipment must be clean and sterile.
8.3 Equipment and container cleaning procedure
8.3.1 Wash apparatus with laboratory-grade detergent and water
8.3.2 Rinse with tap water
8.3.3 Rinse with 10% HC1 acid wash
8.3.4 Rinse with distilled water
8.3.5 Allow to air dry
8.3.6 Cover with foil and autoclave for 15 minutes at 121°C (15 PSI)
8.4 Digester biosolids sampling procedure
8.4.1 Collect digester biosolids sample from the discharge pipe.
8.4.2 Purge the discharge pipe of old biosolids and warm to the digester temperature by
allowing biosolids to flow through the pipe into a container or waste collection device.
8.4.3 Position a 1-gal. sterile bag under the flow so that only the sample touches the inside of
the bag. Fill the bag, leaving 0.5 inches of head space in the bag for gas production.
Leaving head room is extremely important when taking samples of biosolids that have
been anaerobically digested.
8.5 Procedure for sampling conveyor belt biosolid output
8.5.1 Using a sterile scoop, transfer the pressed biosolids directly from the conveyer into a
sterile container, without mixing or transferring to another area.
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Method 1681
8.5.2 Pack sample into sterile container. Leaving additional head space is not as important as
in Section 8.4 because there is less gas formation.
8.6 Procedure for sampling from a bin, drying bed, truck bed, or similar container
8.6.1 Remove surface material (upper six inches) and set it aside. Divide the underlying
material to be sampled into four quadrants.
8.6.2 Use a scoop or core the sample if material is deep.
8.6.3 Take a sample from each of the quadrants and combine in a sterile container.
8.6.4 After all the samples have been taken, pour the contents of the container out onto a sterile
surface and mix by folding the sample back onto itself several times.
8.6.5 Reduce the sample size by "coning and quartering". Divide the container contents into
four even piles. If sample size is still too large, divide each quarter into quarters and
discard half. Put into a glass or plastic sampling container.
8.6.6 An alternate method to "coning and quartering" is to randomly take a flat shovel full of
biosolids from the contents of the container that has been placed on a sterile surface and
put samples into a sampling container. (Curved scoops have been shown to favor a
certain size particle and should not be used.)
8.7 Record the following in your log book:
8.7.1 Facility name and location
8.7.2 Date
8.7.3 Arrival time
8.7.4 Name of facility and contact
8.8 Record the following onto sample container and in log book when known:
8.8.1 Sample number
8.8.2 Date and time
8.8.3 Sampler name
8.8.4 Sample location
8.8.5 Parameters (e.g., type of analysis, field measurements- pH and temperature)
8.8.6 Volume
8.8.7 Observations
8.9 Ensure that the chain-of-custody form is filled out.
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Method 1681
8.10 Sample handling: Maintain bacteriological samples at <10°C during transit to the laboratory. Do
not allow the sample to freeze. Use insulated containers to ensure proper maintenance of storage
temperature. Sample bottles should be placed inside waterproof bags, excess air purged, and bags
sealed to ensure that bottles remain dry during transit or storage. Refrigerate samples upon
arrival in the laboratory and analyze as soon as possible after collection. Bring samples to room
temperature before analysis.
8.11 Holding time and temperature limitations: For fecal coliform samples for sewage sludge
(biosolids) only, the holding time is extended to 24 hours for the following sample types using
either EPA Method 1680 (LTB-EC) or 1681 (A-l): Class A composted, Class B aerobically
digested, and Class B anaerobically digested. All other matrices should be analyzed within 8
hours of sample collection, 6 hour maximum transport and 2 hours for sample processing.
Note: Adherence to sample handling procedures and holding time limits is critical to the
production of valid data. Sample results will be considered invalid if these conditions are not
met.
9.0 Quality Control
9.1 Each laboratory that uses this method is required to operate a formal quality assurance (QA)
program. The minimum requirements of this program consist of an initial demonstration of
laboratory capability through the analysis of positive and negative control samples and blanks
(Sections 9.6 and 9.7). Laboratory performance is compared to the performance criteria specified
in Section 16.0 to determine whether the results of the analyses meet the performance
characteristics of the method. Specific quality control (QC) requirements for Method 1681 are
provided below. General recommendations on QA and QC for facilities, personnel, and
laboratory equipment, instrumentation, and supplies used in microbiological analyses are
provided in Reference 19.4.
9.2 The minimum analytical QC requirements for the analysis of samples using Method 1681 include
an initial demonstration of laboratory capability through performance of the initial precision and
recovery (IPR) analyses (Section 9.3), ongoing demonstration of laboratory capability through
performance of the ongoing precision and recovery (OPR) analysis (Section 9.4) and matrix spike
(MS) analysis (Section 9.5, disinfected wastewater only), and the routine analysis of positive and
negative controls (Section 9.6), method blanks (Section 9.7), and media sterility checks (Section
9.8). For the IPR, OPR and MS analyses, it is necessary to spike samples with
laboratory-prepared spiking suspensions as described in Section 15.0.
9.3 Initial precision and recovery (IPR): The IPR analyses are used to demonstrate acceptable
method performance (recovery and precision) and should be performed by each laboratory before
the method is used for monitoring field samples. EPA recommends but does not require that an
IPR be performed by each analyst. IPR samples should be accompanied by an acceptable method
blank (Section 9.7) and appropriate media sterility checks (Section 9.8). The IPR analyses are
performed as follows:
9.3.1 Prepare four, 30-g samples of Milorganite® and spike each sample with E. coll ATCC
# 25922 according to the spiking procedure in Section 15.0. Process and analyze each
IPR sample according to the procedures in Sections 11.0 and 12.0 and calculate the fecal
coliform MPN/g dry weight according to Section 14.0.
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Method 1681
9.3.2 Calculate the percent recovery (R) for each IPR sample using the appropriate equations in
Section 15.7.
9.3.3 Using the percent recoveries of the four analyses, calculate the mean percent recovery
and the relative standard deviation (RSD) of the recoveries. The RSD is the standard
deviation divided by the mean, multiplied by 100.
9.3.4 Compare the mean recovery and RSD with the corresponding IPR criteria in Table 2,
below. If the mean and RSD for recovery of fecal coliforms meet acceptance criteria,
system performance is acceptable and analysis of field samples may begin. If the mean
recovery or the RSD fall outside of the required range for recovery, system performance
is unacceptable. In this event, identify the problem by evaluating each step of the
analytical process, media, reagents, and controls, correct the problem and repeat the IPR
analyses.
Table 2. Initial and Ongoing Precision and Recovery (IPR and OPR) Acceptance Criteria
Performance test
Initial precision and recovery (IPR)
Mean percent recovery
Precision (as maximum relative standard deviation)
Ongoing precision and recovery (OPR) as percent recovery
A-1 acceptance criteria
1%-312%
96%
1%-371%
9.4 Ongoing precision and recovery (OPR): To demonstrate ongoing control of the analytical
system, the laboratory should routinely process and analyze spiked Milorganite® samples. The
laboratory should analyze one OPR sample after every 20 field and matrix spike samples or one
per week that samples are analyzed, whichever occurs more frequently. OPR samples must be
accompanied by an acceptable method blank (Section 9.7) and appropriate media sterility checks
(Section 9.8). The OPR analysis is performed as follows:
9.4.1 Spike a 30-g sample of Milorganite® with E. coli ATCC # 25922 according to the
spiking procedure in Section 15.0. Process and analyze each OPR sample according to
the procedures in Sections 11.0 and 12.0 and calculate the number of fecal coliform
MPN/g dry weight according to Section 14.0.
9.4.2 Calculate the percent recovery (R) for the OPR sample using the appropriate equations
in Section 15.7.
9.4.3 Compare the OPR result (percent recovery) with the corresponding OPR recovery
criteria in Table 2, above. If the OPR result meets the acceptance criteria for recovery,
method performance is acceptable and analysis of field samples may continue. If the
OPR result falls outside of the acceptance criteria, system performance is unacceptable.
In this event, identify the problem by evaluating each step of the analytical process
(media, reagents, and controls), correct the problem and repeat the OPR analysis.
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Method 1681
9.4.4 As part of the laboratory QA program, results for OPR and IPR samples should be
charted and updated records maintained in order to monitor ongoing method
performance. The laboratory should also develop a statement of accuracy for Method
1681 by calculating the average percent recovery (R) and the standard deviation of the
percent recovery (sr). Express the accuracy as a recovery interval from R - 2sr to R + 2sr.
9.5 Matrix spikes (MS): MS analysis are performed to determine the effect of a particular matrix on
fecal coliform recoveries. The laboratory should analyze one MS sample when biosolid samples
are first received from a source from which the laboratory has not previously analyzed samples.
Subsequently, 5% of field samples (1 per 20) from a given biosolids source should include a MS
sample. MS samples must be accompanied by the analysis of an unspiked field sample
sequentially collected from the same sampling site, an acceptable method blank (Section 9.7), and
appropriate media sterility checks (Section 9.8). When possible, MS analyses should also be
accompanied by an OPR sample (Section 9.4). The MS analysis is performed as follows:
9.5.1 Prepare two, 30-g field samples that were sequentially collected from the same site.
One sample will remain unspiked and will be analyzed to determine the background or
ambient concentration of fecal coliforms for calculating MS recoveries (Section 9.5.3).
The other sample will serve as the MS sample and will be spiked with E. coli ATCC #
25922 according to the spiking procedure in Section 15.0.
9.5.2 Select dilutions based on previous analytical results or anticipated levels of fecal
coliforms in the field sample in order to accurately estimate fecal coliform density.
Neither above or below the detection limit of the method. Section 12.0 includes possible
dilution schemes for both Class A and Class B biosolids.
9.5.3 Spike the MS sample with a laboratory-prepared suspension as described in Section 15.0.
Process and analyze the unspiked and spiked field samples according to the procedures in
Sections 11.0 and 12.0.
9.5.4 For the MS sample, calculate the fecal coliform MPN/g dry weight according to Section
14.0 and adjust the density (MPN/g dry weight) based on the ambient concentration of
fecal coliforms observed in the unspiked matrix sample.
9.5.5 Calculate the percent recovery (R) for the MS sample (adjusted based on ambient fecal
coliform in the unspiked sample) using the appropriate equations in Section 15.7.
9.5.6 Compare the MS result (percent recovery) with the appropriate method performance
criteria in Table 3. If the MS recovery meets the acceptance criteria, system performance
is acceptable and analysis of field samples from this biosolid source may continue. If the
MS recovery is unacceptable and the OPR sample result associated with this batch of
samples is acceptable, a matrix interference may be causing the poor results. If the MS
recovery is unacceptable, all associated field data should be flagged.
July 2006 12
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Method 1681
Table 3. Matrix Spike Precision and Recovery Acceptance Criteria
Performance test
Class A
Class B
Class A
•
•
Class B
•
•
Biosolids: Matrix spike (MS)
MS percent recovery
Biosolids: Matrix spike (MS)
MS percent recovery
Biosolids: Matrix spike, matrix spike duplicate (MS/MSD)
Percent recovery for MS/MSD
Precision (as maximum relative percent difference of MS/MSD)
Biosolids: Matrix spike, matrix spike duplicate (MS/MSD)
Percent recovery for MS/MSD
Precision (as maximum relative percent difference of MS/MSD)
A-1 acceptance criteria
2% -541%
>0%-6172%
2% -541%
182%
>0%-6172%
184%
9.5.7 Laboratories should record and maintain a control chart comparing MS recoveries for all
matrices to batch-specific and cumulative OPR sample results analyzed using Method
1681. These comparisons should help laboratories recognize matrix effects on method
recovery and may also help to recognize inconsistent or sporadic matrix effects from a
particular source.
9.6 Culture Controls
9.6.1 Negative controls: The laboratory should analyze negative controls to ensure that the A-1
is performing properly. Negative controls should be analyzed whenever a new batch of
media or reagents is used. On an ongoing basis, the laboratory should perform a negative
control every day that samples are analyzed.
9.6.1.1 Negative controls are conducted by inoculating A-1 with a known
negative fecal coliform species (e.g., Enterobacter aerogenes ATCC #
13048) and analyzing as described in Section 12.0. Viability of the
negative controls should be demonstrated using a non-selective media
(e.g., nutrient agar or tryptic soy agar).
9.6.1.2 If a negative control fails to exhibit the appropriate response, check
and/or replace the associated media or reagents, and/or the negative
control, and reanalyze the appropriate negative control.
9.6.2 Positive controls: The laboratory should analyze positive controls to ensure that the A-1
is performing properly. Positive controls should be analyzed whenever a new batch of
media or reagents is used. On an ongoing basis, the laboratory should perform a positive
control every day that samples are analyzed. An OPR sample (Section 9.4) may take the
place of a positive control.
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Method 1681
9.6.2.1 Positive controls are conducted by inoculating A-l with a known
positive fecal coliform species (e.g., E. coll ATCC # 25922) and
analyzing as described in Section 12.0.
9.6.2.2 If the positive control fails to exhibit the appropriate response, check
and/or replace the associated media or reagents, and/or the positive
control, and reanalyze the appropriate positive control.
9.7 Method blank. Test a 20-mL sterile dilution water sample in the analytical scheme to verify the
sterility of equipment, materials, and supplies. Absence of growth indicates freedom of
contamination from the target organism. On an ongoing basis, the laboratory should perform a
method blank every day that samples are analyzed.
9.8 Media sterility check. To test sterility of media, subject a representative portion of each
batch to incubation at 44.5°C ± 0.2°C for 24 ± 2 hours and observe for growth. With respect to
media, a batch is defined as one tube/plate out of 50 in each lot or one tube/plate if the lot
contains less than 50 tubes/plates. Absence of growth indicates media sterility. On an ongoing
basis, the laboratory should perform a media sterility check every day that samples are analyzed.
10.0 Equipment Calibration and Standardization
10.1 Check temperatures in incubators/waterbaths twice daily, with the readings separated by at least 4
hours, to ensure operation is within stated limits of the method and record daily measurements in
the incubator log book.
10.2 Check temperatures in refrigerators/freezers at least once daily to ensure operation is within stated
limits of the method. Record daily measurements in refrigerator/freezer log book.
10.3 Calibrate thermometers and incubators at least annually against an NIST certified thermometer or
one that meets requirements of NIST Monograph SP 250-23 (Reference 19.1). Check mercury
columns for breaks.
10.4 Calibrate the pH meter prior to each use period with the two standards (pH 4.0, 7.0, and 10.0)
closest to the range being tested.
10.5 Calibrate top-loading balances monthly with reference weights of ASTM Class 2.
11.0 Sample Preparation
11.1 Homogenization
Sample homogenization procedures are based on whether the sample is a liquid or a solid. If
sample is alkaline-stabilized (liquid or solid), adjust the pH as described in Section 11.1.3. Liquid
samples are generally defined as samples containing <7% total solids (dry weight).
11.1.1 Liquid samples: Homogenize 300 mL of sample in a sterile blender on high speed for one
to two minutes. Adjust the pH to 7.0-7.5 by adding 1.0 N hydrochloric acid or 1.0 N
sodium hydroxide, if necessary. This is the "homogenized" sample. When adjusting the
pH do not exceed the homogenized sample volume by greater than 5% (15 mL).
July 2006 14
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Method 1681
11.1.2 Solid samples: Weigh out 30.0 ± 0.1 g of well-mixed sample in a sterile dish.
Whenever possible, the sample tested should contain all materials that will be included in
the biosolid. For example, if wood chips are part of the biosolid compost, some mixing
or grinding may be needed to achieve homogeneity before testing. Large pieces of wood
that are not easily ground may be discarded before homogenizing. Transfer the sample to
a sterile blender. Alternatively, the sample may be weighed directly into the sterile
blender jar. Use 270 mL of sterile dilution water (Section 7.4) to rinse any remaining
sample into the blender. Cover and blend on high speed for one minute. This is the
"homogenized" sample. A volume of 1.0-mL of the "homogenized" sample contains 10"1
g of the original sample. Adjust the pHto 7.0-7.5 by adding 1.0 N hydrochloric acid or
1.0 N sodium hydroxide, if necessary.
Note: Do not suspend bacteria in dilution water for more than 30 minutes at room
temperature. Chill on wet ice or at 4°C ± 1°C to slow replication between spiking
samples.
11.1.3 Alkaline-stabilized: The alkaline-stabilized biosolid samples generally have a pH
of approximately 12. Prior to analysis, the alkaline-stabilized biosolid sample must be
neutralized to a pH of approximately 7.5. Do not add lab-prepared spikes to the samples
prior to pH adjustment.
11.1.3.1 Adjustment of pH should be done in a fume hood. Prior to adjusting the
pH of the sample, calibrate/standardize the pH meter with pH buffers 7.0
and 10.0. Weigh out 30 g of sample into a sterile 600 mL beaker, add
250 mL of sterile buffered dilution water and a sterile magnetic stir bar.
Place beaker on a mixing plate, insert pH probe into mixture, begin
stirring, and take an initial pH reading. To minimize the amount of
volume added to each sample, pH should be adjusted using ION HC1.
Note: The addition of the 10 N HC1 will produce fumes, do not be
alarmed. The addition of the acid should be done incrementally to
ensure that the pH does not drop instantaneously below 5.0. It is
recommended that the pH adjustment be completed within 10-15 minutes
and monitored for an additional 15 minutes to ensure that the sample is
able to maintain a constant pH of around 7.5. Pour pH adjusted sample
into blender jar, use the remaining sterile buffered dilution water (15 mL)
to rinse the beaker twice and pour rinse water into the blender jar.
11.2 Dilution and inoculation
Biosolid samples analyzed for fecal coliforms using this method may require dilution prior to
analysis. An ideal sample volume will yield results that accurately estimate fecal coliform
density. Because fecal coliform concentrations in undiluted samples could easily exceed the
analytical range of this procedure, the laboratory must follow the dilution and inoculation
schemes in Section 11.2.1 (liquid) or 11.2.2 (solid), if necessary additional dilutions may be
analyzed to ensure results obtained are not censored (less-than or greater-than) values. Although
other dilution and inoculation schemes may be used, the first transfer from the "homogenized"
sample should always be 11 mL of homogenized sample to 99 mL dilution water or 10 mL of
homogenized sample to 90 mL dilution water. This will ensure that a sufficient amount of the
original biosolid sample is transferred at the beginning of the dilution scheme.
15 July 2006
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Method 1681
Note: Do not suspend bacteria in dilution water for more than 30 minutes at room temperature.
For some transfers, it may be convenient to use a sterile, wide-mouth pipette, capable of
transferring particulate matter. If samples are being spiked, a maximum of 1 hour may elapse
between initial unspiked sample homogenization and analysis of spiked samples.
11.2.1 Class A liquid samples: Four series of five tubes each will contain 1.0, 10"1, 10"2, and 10"3
mL of the original sample. See Figure 2 in Section 20.0 for an overview of this dilution
and inoculation scheme. Note: For spiked samples, four series of five tubes each will be
used for the analysis with 10"2, 10"3, 10"4, and 10"5 mL of the original sample.
11.2.1.1 Dilution
(A) Use a sterile pipette to transfer 11.0 mL of "homogenized" sample
(Section 11.1.1) to 99 mL of sterile dilution water (Section 7.4), cap, and
mix by vigorously shaking the bottle a minimum of 25 times. This is
dilution "A." A 1.0-mL of dilution "A" contains 10"1 mL of the original
sample.
(B) Use a sterile pipette to transfer 11.0 mL of dilution "A" to 99 mL of
sterile dilution water, and mix as before. This is dilution "B." One mL
of dilution "B" is 10"2 mL of the original sample.
(C) Use a sterile pipette to transfer 11.0 mL of dilution "B" to 99 mL of
sterile dilution water, and mix as before. This is dilution "C." One mL
of dilution "C" is 10"3 mL of the original sample.
(D) Additional dilutions for analysis of spiked samples:
• Use a sterile pipette to transfer 11.0 mL of dilution "C" to 99 mL
of sterile dilution water, and mix as before. This is dilution "D."
One mL of dilution "D" is 10"4 mL of the original sample.
• Use a sterile pipette to transfer 11.0 mL of dilution "D" to 99 mL
of sterile dilution water, and mix as before. This is dilution "E."
One mL of dilution "E" is 10"5 mL of the original sample.
11.2.1.2 Inoculation
(A) Use a sterile pipette to inoculate each of the first series of five tubes with
1.0 mL of the original "homogenized" sample per tube (unspiked
samples only).
(B) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "A" (unspiked samples only). This is 10"1 mL of the original
sample.
(C) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "B" (unspiked or spiked samples). This is 10"2 mL of the
original sample.
July 2006 16
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Method 1681
(D) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "C" (unspiked or spiked samples). This is 10~3 mL of the
original sample.
(E) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "D" (spiked samples). This is 10"4 mL of the original sample.
(F) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "E" (spiked samples). This is 10"5 mL of the original sample.
11.2.1.3 Repeat steps 11.2.3.1 and 11.2.3.2 for the remaining Class A samples.
When inoculations are complete, go to Section 12.3.4 to continue the A-
1 method.
11.2.2 Class A solid samples: For unspiked samples, four series of five tubes will be used for the
analysis with 1.0, 10"1, 10"2, and 10"3 g of the original sample. The first series of tubes
must contain 2X media. See Figure 5 in Section 20.0 for a summary of this dilution and
inoculation scheme. Note: For spiked samples, four series of five tubes each will be used
for the analysis with 10"2, 10"3, 10"4, and 10"5 g of the original sample.
11.2.2.1 Dilution
(A) A 1.0-mL volume of the "homogenized" sample (Section 11.1.2)
contains 10"1 g of the original sample.
(B) Use a sterile pipette to transfer 11.0 mL of the blender contents to 99 mL
of sterile dilution water (Section 7.4) and shake vigorously a minimum of
25 times. This is dilution "A." One mL of dilution "A" contains 10"2 g
of the original sample.
(C) Use a sterile pipette to transfer 11.0 mL of dilution "A" to 99 mL of
sterile dilution water and mix as before. This is dilution "B." One mL of
dilution "B" contains 10"3 g of the original sample.
(D) Additional dilutions for analysis of spiked samples.:
• Use a sterile pipette to transfer 11.0 mL of dilution "B" to 99 mL
of sterile dilution water, and mix as before. This is dilution "C."
One mL of dilution "C" contains 10"4 g of the original sample.
• Use a sterile pipette to transfer 11.0 mL of dilution "C" to 99 mL
of sterile dilution water and mix as before. This is dilution "D."
One mL of dilution "D" contains 10"5 g of the original sample.
17 July 2006
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Method 1681
11.2.2.2 Inoculation
(A) Use a sterile pipette to inoculate each of the first series of five tubes with
10.0 mL of the "homogenized" sample (unspiked samples only). This
series of tubes must contain 2X media. This is 1.0 g of the original
sample. Since test tubes with inverted vials are being used, shaking is
not practical. Solids that will not separate easily and/or may float should
be submerged into the broth with a sterile loop.
(B) Use a sterile pipette to inoculate each of five tubes with 1 mL of the
"homogenized" mixture (unspiked samples only). This is 10"1 g of the
original sample.
(C) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "A" (unspiked or spiked samples). This is 10"2 g of the original
sample.
(D) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "B" (unspiked or spiked samples). This is 10"3 g of the original
sample.
(E) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "C" (spiked samples). This is 10"4 g of the original sample.
(F) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "D" (spiked samples). This is 10"5 g of the original sample.
11.2.2.3 Repeat Section 11.2.4.1 and 11.2.4.2 for remaining Class A samples.
When inoculations are complete, go to Section 12.3.4 to continue the A-
1 method.
11.2.3 Class B liquid samples: For unspiked samples, four series of five tubes each will
be used for the analysis with 10"3, 10"4, 10"5, and 10"6 mL of the original sample
(additional dilutions may be analyzed as necessary). See Figure 2 in Section 20.0 for a
summary of this dilution and inoculation scheme. Note: For spiked samples, five series
of five tubes each will be used for the analysis with 10'5, lO'6, 10'7, 10'8, and 10'9 mL of
the original sample.
11.2.3.1 Dilution
(A) Use a sterile pipette to transfer 11.0 mL of homogenized sample (from
Section 11.1.1) to 99 mL of sterile dilution water (Section 7.4), cap, and
mix by vigorously shaking the bottle a minimum of 25 times. This is
dilution "A." One mL of dilution "A" is 10"1 mL of the original sample.
(B) Use a sterile pipette to transfer 11.0 mL of dilution "A" to 99 mL of
sterile dilution water, and mix as before. This is dilution "B." One mL
of dilution "B" is 10"2 mL of the original sample.
July 2006 18
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Method 1681
(C) Use a sterile pipette to transfer 11.0 mL of dilution "B" to 99 mL of
sterile dilution water, and mix as before. This is dilution "C." One mL of
dilution "C" is 10~3 mL of the original sample.
(D) Use a sterile pipette to transfer 11.0 mL of dilution "C" to 99 mL of
sterile dilution water, and mix as before. This is dilution "D." One mL
of dilution "D" is 10"4 mL of the original sample.
(E) Use a sterile pipette to transfer 11.0 mL of dilution "D" to 99 mL of
sterile dilution water, and mix as before. This is dilution "E." One mL
of dilution "E" is 10"5 mL of the original sample.
(F) Use a sterile pipette to transfer 11.0 mL of dilution "E" to 99 mL of
sterile dilution water, and mix as before. This is dilution "F." One mL
of dilution "F" is 10"6 mL of the original sample.
(G) Additional dilutions for analysis of spiked samples:
Use a sterile pipette to transfer 11.0 mL of dilution "F" to 99 mL
of sterile dilution water, and mix as before. This is dilution "G."
One mL of dilution "G" is 10"7 mL of the original sample.
Use a sterile pipette to transfer 11.0 mL of dilution "G" to 99 mL
of sterile dilution water, and mix as before. This is dilution "H."
One mL of dilution "H" is 10"8 mL of the original sample.
Use a sterile pipette to transfer 11.0 mL of dilution "H" to 99 mL
of sterile dilution water, and mix as before. This is dilution "I."
One mL of dilution "I" is 10"9 mL of the original sample
11.2.3.2 Inoculation
(A) Use a sterile pipette to inoculate each of the first series of five tubes with
1.0 mL of dilution "C" (unspiked samples only). This is 10"3 mL of the
original sample.
(B) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "D" (unspiked samples only). This is 10"4 mL of the original
sample.
(C) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "E" (unspiked or spiked samples). This is 10"5 mL of the
original sample.
(D) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "F" (unspiked or spiked samples). This is 10"6 mL of the
original sample.
(E) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "G" (spiked samples). This is 10"7 mL of the original sample.
19 July 2006
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Method 1681
(F) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "H" (spiked samples). This is 10~8 mL of the original sample.
(G) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "I" (spiked samples). This is 10"9 mL of the original sample.
11.2.3.3 Repeat Section 11.2.1.1 and 11.2.1.2 for each remaining Class B sample.
When inoculations are complete, proceed to Section 12.3.4 to continue
the A-l method.
11.2.4 Class B solid samples: For unspiked samples, four series of five tubes each will
contain 10"3, 10"4, 10"5, and 10"6 g of the original sample (additional dilutions may be
analyzed as necessary). See Figure 3 in Section 20.0 for a summary of this dilution and
inoculation scheme. Note: For spiked samples, five series of five tubes each will be used
for the analysis with 10'5, 10'6, 10'7, 10'8, and 10'9 g of the original sample.
11.2.4.1 Dilution
(A) A volume of 1.0-mL of the "homogenized" sample (Section 11.1.2)
contains 10"1 g of the original sample.
(B) Use a sterile pipette to transfer 11.0 mL of the blender contents to 99 mL
of sterile dilution water (Section 7.4) and shake vigorously a minimum of
25 times. This is dilution "A." One mL of dilution "A" contains 10"2 g
of the original sample.
(C) Use a sterile pipette to transfer 11.0 mL of dilution "A" to 99 mL of
sterile dilution water, and mix as before. This is dilution "B." One mL
of dilution "B"contains 10"3 g of the original sample.
(D) Use a sterile pipette to transfer 11.0 mL of dilution "B" to 99 mL of
sterile dilution water, and mix as before. This is dilution "C." One mL
of dilution "C" contains 10"4 g of the original sample.
(E) Use a sterile pipette to transfer 11.0 mL of dilution "C" to 99 mL of
sterile dilution water and mix as before. This is dilution "D." One mL
of dilution "D" contains 10"5 g of the original sample.
(F) Use a sterile pipette to transfer 11.0 mL of dilution "D" to 99 mL of
sterile dilution water and mix as before. This is dilution "E." One mL of
dilution "E" contains 10"6 g of the original sample.
(G) Additional dilutions for analysis of spiked samples:
• Use a sterile pipette to transfer 11.0 mL of dilution "E" to 99 mL
of sterile dilution water, and mix as before. This is dilution "F."
One mL of dilution "F" is 10"7 g of the original sample.
July 2006 20
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Method 1681
• Use a sterile pipette to transfer 11.0 mL of dilution "F" to 99 mL
of sterile dilution water, and mix as before. This is dilution "G."
One mL of dilution "G" is 10~8 g of the original sample.
• Use a sterile pipette to transfer 11.0 mL of dilution "G" to 99 mL
of sterile dilution water, and mix as before. This is dilution "H."
One mL of dilution "H" is 10"9 g of the original sample.
11.2.4.2 Inoculation
(A) Use a sterile pipette to inoculate each of the first series of five tubes with
1.0 mL of dilution "B" (unspiked samples only). This is 10"3 g of the
original sample.
(B) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "C" (unspiked samples only). This is 10"4 g of the original
sample.
(C) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "D" (unspiked or spiked samples). This is 10"5 g of the original
sample.
(D) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "E" (unspiked or spiked samples). This is 10"6 g of the original
sample.
(E) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "F" (spiked samples). This is 10"7 g of the original sample.
(F) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "G" (spiked samples). This is 10"8 g of the original sample.
(G) Use a sterile pipette to inoculate each of five tubes with 1.0 mL of
dilution "H" (spiked samples). This is 10"9 g of the original sample.
11.2.4.3 When inoculations are complete, go to Section 12.3.4 to continue the
A-l method.
12.0 A-1 Procedure
12.1 In this protocol, A-l medium is used to determine fecal coliform densities in Class A and B
biosolid samples. Analysis of seven samples collected at the time of disposal using this
procedure will satisfy the requirements of the monitoring alternative for demonstrating pathogen
reduction in both Class A and Class B biosolids. In Method 1681, A-l is used as a direct, single
step test. Precision of the test increases with increasing numbers of replicates per sample tested.
For an overview of the MPN procedure, refer to Figure 1 in Section 20.0.
12.2 Since sample fecal coliform densities are expected to be variable, it is recommended that at least
seven biosolid samples be analyzed using this method. The geometric mean fecal coliform
density of the seven biosolids samples should not exceed 2 x 106 MPN/g of total solids (dry
21 July 2006
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Method 1681
weight basis) to qualify as Class B biosolids. Although there is not a specific number of samples
required for Class A biosolids, it is recommended that a sampling event extend over two weeks
and that at least seven samples be collected and determined to be below 1,000 MPN/g of total
solids (dry weight basis) to qualify as Class A biosolids.
12.3 A-l procedure
12.3.1 Prepare A-l broth tubes as directed in Section 7.6. Note: If media is refrigerated, remove
from refrigerator 1-1.5 hours prior to inoculation, so that it reaches room temperature
prior to use.
12.3.2 For each sample, arrange test tubes in four or five rows of five tubes each (Section 11.2).
Use 10 mL of 2X A-l broth for 10 mL inoculations. Clearly label each row of tubes to
identify the sample and dilution volume to be inoculated. Note: 2X A-l is needed for 10-
mL inoculations, to ensure that the 10-mL inoculation volume does not excessively dilute
the A-l.
12.3.3 Dilute and inoculate samples depending on the matrix (i.e., Class A solid, Class B liquid),
as described in Section 11.2.
12.3.4 Incubate inoculated A-l tubes at 35°C ± 0.5°C for 3 hours ± 15 minutes.
12.3.5 Transfer A-l tubes to a waterbath at 44.5°C ± 0.2°C and incubate for an additional 21 ± 2
hours. Maintain water level above the media in immersed tubes. Total incubation time
should not exceed 24 ± 2 hours.
12.3.6 After incubation, remove tubes from the waterbath, swirl each tube gently, and examine
for growth and gas production. Gas production with growth is considered a positive fecal
coliform reaction. Please note that for the A-l procedure, any evolution of gas is
considered a positive result (See Photo 1). Collection of gas in the durham tube is not
necessary. Note: The presence of gas in the absence of growth is usually due to
mishandling or improper shaking of the tubes after inoculation.
12.3.7 Record positive and negative A-l results and calculate MPN/g total solids (dry weight)
from the number of positive A-l broth tubes as described in Section 14.0.
Photo 1. Fecal coliforms produce gas in A-l medium.
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Method 1681
12.4 Total solids determination
12.4.1 Determination of percent dry weight - When sample results are to be calculated on a dry
weight basis, a second portion of sample should be weighed at the same time as the
portion used for analytical determination.
WARNING: The drying oven should be contained in a hood or be vented. Significant
laboratory contamination may result from drying a heavily contaminated sample.
12.4.2 Immediately after weighing the sample for microbiological examination, weigh 10-30 g
of the sample into a tarred crucible or aluminum evaporating dish. Dry this aliquot
overnight at 103°C to 105°C. Allow to cool in a desiccator before weighing. Calculate
the % dry weight as follows:
g dry sample
% dry weight = x 100
g sample
13.0 Verification
13.1 Individual biochemical tests including oxidase, citrate, lactose fermentation, indole, ONPG
hydroloysis, methyl red, and Voges-Proskauer may be used to verify positive and negative
results.
13.2 Alternately, commercially available multi-test identification systems may be used to verify
positive and negative results. Such identification systems for Enterobacteriaceae must include
lactose fermentation, o-nitrophenyl-p-D-galactopyranoside (ONPG), and cytochrome oxidase test
reactions.
Note: Due to the high false positive and negative rates, it is recommended that analysts be
required to submit all positive and negative tubes with growth to verification for at least one
biosolid sample for each type of biosolid that the analyst normally evaluates on a monthly basis.
This should increase analyst proficiency in using these procedures.
14.0 Data Analysis and Calculations
The estimated density of fecal coliform bacteria, based on the direct test with A-l, is calculated in
terms of most probable number (MPN). Due to the extreme variability in the solid content of
biosolids, fecal coliform results from biosolid samples are reported as MPN/g total solids (dry
weight basis). MPN/g total solids (dry weight) is calculated in three steps (Sections 14.1, 14.2,
and 14.3):
Selection of significant dilutions
Calculation of MPN/mL (wet weight)
Conversion to MPN/g total solids (dry weight)
The calculation of geometric means is provided in Section 14.4.
23 July 2006
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Method 1681
14.1 Step 1: Select Significant Dilutions
A dilution refers to the mL (liquid samples) or g (solid samples) of original sample that was
inoculated into each series of tubes. For example, with Class B solid samples (Section 11.2.4),
four, five-tube dilutions are used, with 10~3, 10~4, 10~5, and 10~6 g of the original sample in each
tube. Only three of the four dilution series will be used to estimate the MPN. The three selected
dilutions are called significant dilutions and are selected according to the following criteria.
Examples of significant dilution selections are provided in Table 5, below. For these examples,
the numerator represents the number of positive tubes per sample dilution series and the
denominator represents the total number of tubes inoculated per dilution series.
14.1.1 Choose the highest dilution (the most dilute, with the least amount of sample) giving
positive results in all five tubes inoculated and the two succeeding higher (more dilute)
dilutions (For Table 5, Example A, 10~4 is higher/more dilute than 10~3.)
14.1.2 If the lowest dilution (least dilute) tested has less than five tubes with positive results,
select it and the two next succeeding higher dilutions (Table 5, Examples B and C).
14.1.3 When a positive result occurs in a dilution higher (more dilute) than the three significant
dilutions selected according to the rules above, change the selection to the lowest dilution
(least dilute) that has less than five positive results and the next two higher dilutions
(more dilute) (Table 5, Example D).
14.1.4 When the selection rules above have left unselected any higher dilutions (more dilute)
with positive results, add those higher-dilution positive results to the results for the
highest selected dilution (Table 5, Example E).
14.1.5 If there were not enough higher dilutions tested to select three dilutions, then select the
next lower dilution (Table 5, Example F).
14.2 Step 2: Calculate MPN / mL (wet weight)
14.2.1 Obtain the MPN index value from Table 4 using the number of positive tubes in the three
significant dilutions series and calculate MPN/mL using the following equation. The
95% confidence limits may also be obtained from Table 4.
Note: The example calculated numbers provided in the tables below have been rounded at the end
of each step. If your laboratory recalculates the examples using a spreadsheet and rounds only
after the final calculation, values may be slightly different.
MPN Index from Table 4
MPN/mL =
Largest volume tested in the dilution series used for MPN determination
July 2006 24
-------
Method 1681
14.2.2 When using MPN tables other than those provided in this method (e.g. Table 9221:IV,
Standard Methods for Water and Wastewater, Reference 19.1), additional
steps/calculations are required to determine final reporting value of MPN/g dry weight.
For example, Table 9221:IV MPN index is per 100 mL (MPN/100 mL); which will need
to be converted to MPN/mL . In addition, the MPN index must be multiplied by a factor
of 10 when using the largest volume tested in the dilution series used for MPN
determination.
10 x MPN Index from Table 9221 :VI
MPN/100mL =
Largest volume tested in the dilution series used for MPN determination
MPN/100mL
MPN/mL =
100
25 July 2006
-------
Method 1681
Table 4. MPN Index and 95% Confidence Limits for Various Combinations of Positive Results
When Five Tubes are Used per Dilution
Combination of
Positives
0-0-0
0-0-1
0-0-2
0-0-3
0-0-4
0-0-5
0-1-0
0-1-1
0-1-2
0-1-3
0-1-4
0-1-5
0-2-0
0-2-1
0-2-2
0-2-3
0-2-4
0-2-5
0-3-0
0-3-1
0-3-2
0-3-3
0-3-4
0-3-5
0-4-0
0-4-1
0-4-2
0-4-3
0-4-4
0-4-5
0-5-0
0-5-1
0-5-2
0-5-3
0-5-4
0-5-5
1-0-0
1-0-1
1-0-2
1-0-3
1-0-4
1-0-5
1-1-0
1-1-1
1-1-2
1-1-3
1-1-4
1-1-5
1-2-0
1-2-1
1-2-2
1-2-3
1-2-4
1-7-5
MPN Index
ml
O.1803
0.18
0.36
0.54
0.72
0.91
0.18
0.36
0.55
0.73
0.91
1.10
0.37
0.55
0.74
0.92
1.11
1.29
0.56
0.74
0.93
1.12
1.30
1.49
0.75
0.94
1.12
1.31
1.50
1.69
0.94
1.13
1.33
1.52
1.71
1.90
0.20
0.40
0.60
0.81
1.01
1.22
0.40
0.61
0.81
1.02
1.23
1.44
0.61
0.82
1.03
1.24
1.46
1 67
95% Confidence Limits
Lower Uccer
0.03
0.03
0.03
0.08
0.15
0.03
0.03
0.03
0.08
0.15
0.23
0.03
0.03
0.08
0.15
0.23
0.31
0.03
0.09
0.16
0.23
0.31
0.39
0.09
0.16
0.24
0.32
0.40
0.48
0.16
0.24
0.32
0.40
0.48
0.56
0.03
0.03
0.03
0.11
0.19
0.28
0.03
0.03
0.11
0.19
0.28
0.37
0.03
0.12
0.20
0.29
0.38
047
0.63
1.01
1.37
1.74
2.12
0.63
1.01
1.38
1.75
2.14
2.56
1.02
1.39
1.76
2.15
2.58
3.07
1.40
1.77
2.17
2.60
3.10
3.72
1.79
2.19
2.63
3.13
3.77
4.62
2.21
2.65
3.17
3.82
4.70
5.63
0.68
1.08
1.49
1.91
2.36
2.87
1.09
1.50
1.92
2.38
2.90
3.54
1.51
1.94
2.40
2.93
3.59
451
Combination of
Positives
1-3-0
1-3-1
1-3-2
1-3-3
1-3-4
1-3-5
1-4-0
1-4-1
1-4-2
1-4-3
1-4-4
1-4-5
1-5-0
1-5-1
1-5-2
1-5-3
1-5-4
1-5-5
2-0-0
2-0-1
2-0-2
2-0-3
2-0-4
2-0-5
2-1-0
2-1-1
2-1-2
2-1-3
2-1-4
2-1-5
2-2-0
2-2-1
2-2-2
2-2-3
2-2-4
2-2-5
2-3-0
2-3-1
2-3-2
2-3-3
2-3-4
2-3-5
2-4-0
2-4-1
2-4-2
2-4-3
2-4-4
2-4-5
2-5-0
2-5-1
2-5-2
2-5-3
2-5-4
7-5-5
MPN Index
mL
0.83
1.04
1.25
1.47
1.69
1.91
1.05
1.27
1.48
1.70
1.93
2.15
1.28
1.50
1.72
1.95
2.17
2.40
0.45
0.68
0.91
1.15
1.39
1.64
0.68
0.92
1.16
1.41
1.66
1.92
0.93
1.18
1.43
1.68
1.94
2.21
1.19
1.44
1.70
1.97
2.23
2.51
1.46
1.72
1.99
2.26
2.54
2.82
1.74
2.01
2.29
2.57
2.86
3 15
95% Confidence Limits
Lower Uccer
0.12
0.20
0.29
0.38
0.48
0.57
0.21
0.30
0.39
0.48
0.58
0.67
0.30
0.40
0.49
0.58
0.68
0.77
0.03
0.06
0.15
0.25
0.35
0.46
0.06
0.15
0.25
0.36
0.46
0.57
0.16
0.26
0.36
0.47
0.58
0.69
0.26
0.37
0.48
0.59
0.70
0.82
0.38
0.49
0.60
0.72
0.83
0.94
0.50
0.61
0.73
0.84
0.95
1 07
1.96
2.43
2.96
3.64
4.60
5.66
2.45
3.00
3.70
4.68
5.75
6.57
3.03
3.75
4.77
5.83
6.64
7.31
1.19
1.64
2.13
2.69
3.38
4.37
1.66
2.16
2.72
3.43
4.47
5.71
2.18
2.76
3.49
4.56
5.81
6.75
2.79
3.55
4.67
5.91
6.83
7.59
3.61
4.77
6.00
6.92
7.68
8.36
4.88
6.10
7.00
7.76
8.45
9 10
a Table was developed using the MPN calculator developed by Albert Klee (Reference 19.5)
July 2006
26
-------
Method 1681
Table 4. MPN Index and 95% Confidence Limits for Various Combinations of Positive Results
When Five Tubes are Used per Dilution (cont.)a
Combination of
Positives
3-0-0
3-0-1
3-0-2
3-0-3
3-0-4
3-0-5
3-1-0
3-1-1
3-1-2
3-1-3
3-1-4
3-1-5
3-2-0
3-2-1
3-2-2
3-2-3
3-2-4
3-2-5
3-3-0
3-3-1
3-3-2
3-3-3
3-3-4
3-3-5
3-4-0
3-4-1
3-4-2
3-4-3
3-4-4
3-4-5
3-5-0
3-5-1
3-5-2
3-5-3
3-5-4
3-5-5
4-0-0
4-0-1
4-0-2
4-0-3
4-0-4
4-0-5
4-1-0
4-1-1
4-1-2
4-1-3
4-1-4
4-1-5
4-2-0
4-2-1
4-2-2
4-2-3
4-2-4
4-7-5
MPN Index
ml
0.79
1.06
1.35
1.65
1.96
2.29
1.07
1.37
1.67
1.99
2.32
2.67
1.38
1.70
2.02
2.36
2.71
3.08
1.72
2.05
2.40
2.76
3.13
3.52
2.09
2.44
2.81
3.19
3.58
3.99
2.48
2.86
3.25
3.65
4.07
4.50
1.30
1.66
2.07
2.53
3.02
3.55
1.69
2.12
2.58
3.10
3.65
4.25
2.16
2.64
3.17
3.75
4.38
504
95% Confidence
Lower
0.10
0.21
0.33
0.46
0.59
0.73
0.22
0.34
0.47
0.60
0.74
0.88
0.35
0.48
0.62
0.76
0.90
1.04
0.49
0.63
0.77
0.92
1.06
1.20
0.64
0.79
0.93
1.08
1.23
1.37
0.80
0.95
1.10
1.25
1.40
1.54
0.31
0.46
0.64
0.82
1.02
1.21
0.48
0.66
0.85
1.05
1.25
1.45
0.67
0.87
1.08
1.29
1.50
1 71
Limits
Upper
1.88
2.46
3.23
4.40
5.89
6.99
2.50
3.29
4.52
6.01
7.10
8.00
3.35
4.64
6.13
7.20
8.10
8.94
4.77
6.24
7.31
8.21
9.06
9.89
6.36
7.42
8.33
9.18
10.02
10.86
7.53
8.44
9.31
10.17
11.03
11.89
3.11
4.45
6.31
7.64
8.81
9.96
4.60
6.46
7.79
8.98
10.16
11.38
6.61
7.94
9.15
10.37
11.64
1797
Combination of
Positives
4-3-0
4-3-1
4-3-2
4-3-3
4-3-4
4-3-5
4-4-0
4-4-1
4-4-2
4-4-3
4-4-4
4-4-5
4-5-0
4-5-1
4-5-2
4-5-3
4-5-4
4-5-5
5-0-0
5-0-1
5-0-2
5-0-3
5-0-4
5-0-5
5-1-0
5-1-1
5-1-2
5-1-3
5-1-4
5-1-5
5-2-0
5-2-1
5-2-2
5-2-3
5-2-4
5-2-5
5-3-0
5-3-1
5-3-2
5-3-3
5-3-4
5-3-5
5-4-0
5-4-1
5-4-2
5-4-3
5-4-4
5-4-5
5-5-0
5-5-1
5-5-2
5-5-3
5-5-4
5-5-5
MPN Index
mL
2.71
3.26
3.86
4.51
5.21
5.93
3.35
3.98
4.66
5.39
6.15
6.93
4.11
4.83
5.59
6.39
7.22
8.06
2.40
3.14
4.27
5.78
7.59
9.53
3.29
4.56
6.31
8.39
10.62
12.93
4.93
7.00
9.44
12.05
14.79
17.67
7.92
10.86
14.06
17.50
21.22
25.27
12.99
17.24
22.12
27.81
34.54
42.56
23.98
34.77
54.22
91.78
160.90
>16090
95% Confidence Limits
Lower Upper
0.90
1.11
1.32
1.54
1.76
1.96
1.14
1.37
1.59
1.81
2.02
2.23
1.41
1.64
1.87
2.09
2.30
2.50
0.76
1.06
1.46
1.92
2.39
1.65
1.12
1.56
2.07
2.57
3.04
3.04
1.67
2.24
2.80
3.31
3.81
5.03
2.47
3.08
3.68
4.34
5.29
8.14
3.48
4.29
5.63
8.82
11.59
14.37
7.62
11.72
17.91
26.72
38.37
8.09
9.34
10.60
11.92
13.31
14.77
9.53
10.84
12.23
13.68
15.21
16.81
11.11
12.56
14.09
15.70
17.39
19.16
7.63
9.08
11.42
14.46
18.16
22.34
9.40
12.02
15.53
19.85
24.85
30.90
12.76
16.94
22.13
28.43
37.14
52.30
18.86
25.44
34.45
51.31
67.98
79.71
31.08
49.75
70.87
86.00
101.10
118.00
76.29
101.60
1 41 .90
220.10
410.30
1 Table was developed using the MPN calculator developed by Albert Klee (Reference 19.5)
27
July 2006
-------
Method 1681
Examples of MPN / mL calculations are provided in Table 5.
Table 5. Examples of Significant Dilution Selection and Calculation of MPN / mL'
Example
(liquid
or solid)
A
B
C
D
E
F
io-3
mL or g
5/5
4/5
0/5
5/5
4/5
5/5
io-4
mL or g
5/5
5/5
1/5
3/5
4/5
5/5
1Q-5
mL or g
3/5
1/5
0/5
1/5
0/5
5/5
io-6
mL or g
0/5
0/5
0/5
1/5
1/5
2/5
Step 1 :
Significant
Dilutions
5-3-0
4-5-1
0-1-0
3-1-1
4-4-1
5-5-2
Step 2:
(MPN from Table 2 / largest sig. dilution)
= MPN / mL wet weight
(7.92 / ID'4) = 79,200 MPN / mL
79,000 MPN /mL
(4.8 3/1 0-3) = 4830 MPN / mL
4800 MPN / mL
(0.18/10-3)= 180 MPN /mL
(1.37/10-4)= 13,700 MPN / mL
14,000 MPN /mL
(3.98 /ID'3) = 3980 MPN / mL
4000 MPN / mL
(54.22 / ID'4) = 542,200 MPN / mL
540,000 MPN /mL
a Significant dilutions are underlined and largest significant dilutions highlighted
14.3 Step 3: Convert to MPN / g total solids (dry weight)
For analysis and calculation of percent total solids, see Section 12.4.
For the conversion to MPN/g total solids (dry weight), we assume that,
MPN/mL wet weight = MPN/g wet weight.
Therefore, we may convert to MPN/g total solids (dry weight) using the following
equation:
MPN / g (dry weight) =
MPN / mL (wet weight) from step 2
percent total solids (expressed as a decimal)
Examples of the conversion to MPN/g (dry weight) are provided in Table 6.
July 2006
28
-------
Method 1681
Table 6. Examples of Conversion to MPN / g Total Solids (Dry Weight), Continuing From Step 2
in Table 5.
Example
(liquid or solid)
A
B
C
D
E
F
Total
Solids
4%
60%
56%
22%
18%
43%
Step 3:
(MPN / ml_ wet weight from step 2) / percent total solids = MPN /g dry weight
79,000 / 0.04 = 1,975,000 = 2.0 x 106 MPN / g dry weight
4800 / 0.6 = 8000 = 8.0 x 103 MPN / g dry weight
180 / 0.56 = 321 = 3.2 x 102 MPN / g dry weight
14,000 / 0.22 = 63,636 = 6.4 x 104 MPN / g dry weight
4,000/0.18 = 22,222 = 2.2 x 104 MPN / g dry weight
540,000 / 0.43 = 1 ,255,814 = 1 .3 x 106 MPN / g dry weight
14.4 Calculation of geometric mean
To satisfy pathogen reduction requirements for Class B biosolids in Subpart D of Part 503, seven
biosolid samples are collected and the geometric mean density of fecal coliforms is calculated.
The geometric mean is calculated by:
converting each sample's MPN fecal coliforms / g (dry weight) to the Iog10 value,
• averaging the Iog10 values, and
taking the antilog of the mean Iog10 value.
An example is provided in Table 7.
Table 7. Calculation of Geometric Mean Fecal Coliform Density for Biosolid Samples
Sample No.
1
2
3
4
5
6
7
MPN Fecal coliforms / g (dry weight)
600,000 = 6.0 X105
4,200,000 = 4.2 X106
1,700,000= 1.7X106
1,400,000= 1.4X106
400,000 = 4.0 X105
1,100,000= 1.1 X106
510,000 = 5.1 X105
'og10
5.78
6.62
6.23
6.15
5.60
6.04
5.71
Mean of Iog10 values = (5.78 + 6.62 + 6.23 + 6.15 + 5.60 + 6.04 + 5.71 ) / 7 = 6.02
Antilog of 6.02 = 1,047,128 = 1.0x 106 geometric mean MPN of fecal coliforms /g (dry weight)
29
July 2006
-------
Method 1681
15.0 Sample Spiking Procedure
15.1 Method 1681 QC requirements (Section 9.0) include the preparation and analysis of spiked
reference (Milorganite®) samples in order to monitor initial and ongoing method performance.
For the IPR (Section 9.3) and OPR (Section 9.4) analyses it is necessary to spike samples with
laboratory-prepared spiking suspensions. Section 15.0 is arranged in the following order
preparation of the E. coll spiking suspension (Section 15.2), spiking suspension dilution (Section
15.3), spiking suspension enumeration (Section 15.4), Class A sample spiking (Section 15.5),
Class B sample spiking (Section 15.6), and calculation of spiked E. coll percent recovery (Section
15.7).
15.2 Preparation of E. coli Spiking Suspension (Class A or B)
15.2.1 Stock Culture. Prepare a stock culture by inoculating a heart infusion agar (HIA) slant
[or other non-selective media (e.g., Tryptic Soy Agar)] with Escherichia coli ATCC #
25922 and incubating at 35°C ± 3°C for 20 ± 4 hours. This stock culture may be stored
in the dark at room temperature for up to 30 days.
15.2.2 1% Laurvl Tryptose Broth (LTBV Prepare a 1% solution of LTB by combining 99 mL of
sterile phosphate buffered dilution water and 1 mL of sterile single strength lauryl
tryptose broth in a sterile screw cap bottle or re-sealable dilution water container. Shake
to mix.
15.2.3 Spiking Suspension (Undiluted). From the stock culture of E. coli ATCC # 25922,
transfer a small loopful of growth to the 1% LTB solution and vigorously shake a
minimum of 25 times. Incubate at 35°C ± 3°C for 20 ± 4 hours. The resulting spiking
suspension contains approximately 1.0 x 107 to 1.0 x 10s E. coli colony forming units
(CPU) per mL. This is referred to as the "undiluted spiking suspension."
15.3 Spiking Suspension Dilution
15.3.1 Mix the spiking suspension by vigorously shaking the bottle a minimum of 25 times.
Use a sterile pipette to transfer 1.0 mL of the undiluted spiking suspension to 99 mL of
sterile dilution water (Section 7.4), cap, and mix by vigorously shaking the bottle a
minimum of 25 times. This is spiking suspension dilution "A." A 1.0-mL volume of
dilution "A" is 10"2 mL of the original undiluted spiking suspension.
15.3.2 Use a sterile pipette to transfer 11.0 mL of spiking suspension dilution "A" to 99 mL of
sterile dilution water, cap, and mix by vigorously shaking the bottle a minimum of 25
times. This is spiking suspension dilution "B." A 1.0-mL volume of dilution "B" is 10"3
mL of the original undiluted spiking suspension.
15.3.3 Use a sterile pipette to transfer 11.0 mL of spiking suspension dilution "B" to 99 mL
of sterile dilution water, cap, and mix by vigorously shaking the bottle a minimum of 25
times. This is spiking suspension dilution "C." A 1.0-mL volume of dilution "C" is 10"4
mL of the original undiluted spiking suspension.
15.3.4 Use a sterile pipette to transfer 11.0 mL of spiking suspension dilution "C" to 99 mL
of sterile dilution water, cap, and mix by vigorously shaking the bottle a minimum of 25
times. This is spiking suspension dilution "D." A 1.0-mL volume of dilution "D" is 10"5
mL of the original undiluted spiking suspension.
July 2006 30
-------
Method 1681
15.3.5 Use a sterile pipette to transfer 11.0 mL of spiking suspension dilution "D" to 99 mL
of sterile dilution water, cap, and mix by vigorously shaking the bottle a minimum of 25
times. This is spiking suspension dilution "E." A 1.0-mL volume of dilution "E" is 10~6
mL of the original undiluted spiking suspension.
15.4 Spiking Suspension Enumeration
15.4.1 Prepare heart infusion agar (HIA) (Section 7.5), add 10 - 15 mL of HIA per 100 x 15
mm petri dish, and allow to solidify. Ensure that agar surface is dry. Note: To ensure
that the agar surface is dry prior to use, plates should be made several days in advance
and stored inverted at room temperature or dried using a laminar-flow hood.
15.4.2 Each of the following will be conducted in triplicate, resulting in the evaluation of
nine spread plates:
• Pipet 0.1 mL of dilution "C" onto surface of pre-dried HIA plate [10~5 mL
(0.00001) of the original spiking suspension].
Pipet 0.1 mL of dilution "D" onto surface of pre-dried HIA plate [10~6 mL
(0.000001) of the original spiking suspension].
• Pipet 0.1 mL of dilution "E" onto surface of pre-dried HIA plate [10~7 mL
(0.0000001) of the original spiking suspension].
15.4.3 For each spread plate, using a sterile bent glass rod or spreader, distribute inoculum
over surface of medium by rotating the dish by hand or on a turntable.
15.4.4 Allow inoculum to absorb into the medium completely.
15.4.5 Invert plates and incubate at 35°C ± 0.5°C for 24 ± 4 hours.
15.4.6 Count and record number of colonies per plate. Refer to Section 15.7 for calculation of
spiking suspension concentration.
15.5 Class A Biosolid Sample Spiking
Homogenize the unspiked Class A biosolid sample (Section 11.1). For the enumeration of fecal
coliforms in the unspiked sample, dilute and inoculate according to Section 11.2. After the
unspiked sample has been diluted and media inoculated, spike the biosolid sample as indicated
below.
Since the objective of spiking the biosolid sample is to establish percent recovery, it is necessary
to determine the number of E. coli in the undiluted spiking suspension. Instructions for spiking
suspension enumeration are provided below.
15.5.1 Liquid Samples: Since the unspiked, homogenized sample was analyzed by the A-l
procedure and a dilution series was prepared (Section 11.2), 284 mL of the original 300
mL of unspiked, homogenized sample remains. To spike the sample, add 1.0 mL of
spiking suspension dilution "B" (from Section 15.3.2) for every 100-mL of unspiked
homogenized sample remaining, cover, and blend on high speed for 1-2 minutes. This
is the "spiked, homogenized" sample. The volume (mL) of undiluted spiking suspension
added to each mL of the spiked biosolid sample is 1.0 x 10~5 mL per mL [(2.8 mL x 10~3
31 July 2006
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Method 1681
mL) / 284 mL of biosolid], which is referred to as Vsplkedperumtblosollds below. Proceed to
Section 11.2.3 (dilution and inoculation).
15.5.2 Solid Samples: Since the unspiked, homogenized sample was analyzed by the A-l
procedure and a dilution series was prepared (Section 11.2), 234 mL of the original 300
mL of unspiked, homogenized sample remains. To spike the sample, add 1.0 mL of
spiking suspension dilution "B" (from Section 15.3.2) for every 100-mL of unspiked
homogenized sample remaining, cover, and blend on high speed for 1-2 minutes. This
is the "spiked, homogenized" sample. The volume (mL) of undiluted spiking suspension
added to each g (wet weight) of the spiked biosolid sample is 1.0 x 10"4 mL per g [(2.3
mL x 10"3 mL) / 23.4 g of biosolid], which is referred to as Vsplkedpermitblosollds below.
Proceed to Section 11.2.4 (dilution and inoculation).
15.6 Class B Biosolid Sample Spiking
Homogenize the unspiked Class B biosolid sample (Section 11.1). For the enumeration of fecal
coliforms in the unspiked sample, dilute and inoculate according to Section 11.2. After the
unspiked sample has been diluted and media inoculated, spike the biosolid sample as indicated
below.
Since the objective of spiking the biosolid sample is to establish percent recovery, it is necessary
to determine the number of E. coll in the undiluted spiking suspension. Instructions for spiking
suspension enumeration are provided below.
Since a dilution series was prepared from the unspiked, homogenized sample (Section 11.2); 289
mL of the original 300 mL of unspiked, homogenized sample remains. To spike the sample, add
1.0 mL of well-mixed undiluted spiking suspension, for every 100-mL of unspiked homogenized
sample remaining after unspiked sample evaluation and dilution preparation, cover, and blend on
high speed for 1 - 2 minutes. This is the "spiked, homogenized" sample.
Note: The volumes of undiluted spiking suspensions added per mL or g (wet weight) of the
spiked Class B biosolids are different from those for the Class A biosolids, since different
volumes of the unspiked, homogenized sample remains.
15.6.1 Liquid Samples: The volume (mL) of undiluted spiking suspension added to each mL of
the spiked biosolid sample is 1.0 x 10~2 mL [(2.9 mL spiking suspension) / 289 mL of
biosolid], which is referred to as Vsplkedpermitblosollds below. Proceed to Section 11.2.1
(dilution and inoculation).
15.6.2 Solid Samples: The volume (mL) of undiluted spiking suspension added to each g (wet
weight) of the spiked biosolid sample is 1.0 x 10"1 mL per g [(2.9 mL spiking suspension)
/ 28.9 g of biosolids], which is referred to as Vsplkedperumtblosollds below. Proceed to Section
11.2.2 (dilution and inoculation).
15.7 Calculation of Spiked E. coli Percent Recovery
Spiked E. coli percent recovery will be conducted in four steps as indicated below.
Note: The example calculated numbers provided in the tables below have been rounded at the end
of each step. If your laboratory recalculates the examples using a spreadsheet and rounds only
after the final calculation (Step 4), the percent recoveries may be slightly different.
July 2006 32
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Method 1681
15.7.1 Step 1: Calculate Concentration of E. coli (CFU / mL) in Undiluted Spiking
Suspension
15.7.1.1 The number of E. coli CFU / mL in the spiking suspension will be
calculated using all plates yielding counts within the ideal range of 30 to
300 CFU per plate.
15.7.1.2 If the number of colonies exceeds the upper range (i.e. >300) or if the
colonies are not discrete, results should be recorded as "too numerous to
count" (TNTC).
15.7.1.3 Calculate the concentration of E. coli (CFU / mL) in the undiluted
spiking suspension according to the following equation. Example
calculations are provided in Table 8, below.
EC
undiluted spike
CFU2
CFU,,
V + V
V \ T y 2
Where:
-*-'*-' undiluted spike
CFU
V
E. coli CFU / mL in undiluted spiking suspension
number of colony forming units from HIA plates
yielding counts within the ideal range of 30 to 300 CFU
per plate
volume of undiluted sample in each HIA plate yielding
counts within the ideal range of 30 to 300 CFU per plate
number of plates with counts within the ideal range of 30
to 3 00 CFU per plate
Table 8. Example Calculations of E. coli Spiking Suspension Concentration
Examples
Example 1
Example 2
CFU / plate (triplicate analyses) from
HIA plates
KT5 mL plates
275, 250, 301
TNTC, TNTC,
TNTC
10"6 mL plates
30, 10, 5
TNTC, 299, TNTC
10~7 mL plates
0, 0, 0
12, 109, 32
E. coli CFU / mL in undiluted
spiking suspension
(EC undiluted spike)
(275+250+30) /(10-5+10-5+10-6) =
5557 (2.1 x10-5) = 26,428,571 =
2.6x107CFU/mL
(299+1 09+32) /(10-6+10-7+10-7) =
440 / (1 .2 x ID'6) =366,666,667 =
3.7x108CFU/mL
' EC undNuted spike is calculated using all plates yielding counts within the ideal range of 30 to 300 CFU per plate
33
July 2006
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Method 1681
15.7.2 Step 2: Calculate Spiked E. coli [CFU / mL or g (wet weight)]
15.7.2.1 The volume of undiluted spiking suspension per unit (mL or g) of spiked
biosolid samples (Vsplkedpermitblosollds) is provided in Table 9.
Table 9. Volume of Undiluted Spiking Suspension per Unit (mL or g) of Spiked Biosolid Samples
I "spiked per unit biosolids/
Description of spiked sample
Class A liquid
Class A solid
Class B liquid
Class B solid
''spiked per unit biosolids
1 .0 X 1 0"5 mL per mL of biosolids
1 .0 x 1 0"4 mL per g of biosolids (wet weight)
1 .0 X 1 0"2 mL per mL of biosolids
1 .0 x 1 0~1 mL per g of biosolids (wet weight)
15.7.2.2 Calculate concentration of spiked E. coli (wet weight) in biosolid sample
according to the following equation.
ECy
wetweight
- (Etymdiluted spike) X (^spiked per unit biosolids)
Where:
Spiked ECwetweight
FT =
-LjV-'undiluted spike
^ spiked per unit biosolids
Examples are provided in Table 10.
Number of spiked E. coli CFU per mL or g of biosolid
(wet weight)
E. coli CFU / mL in undiluted spiking suspension
mL of undiluted spiking suspension per mL or g of
spiked biosolid
July 2006
34
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Method 1681
Table 10.
Example Calculations of Spiked EC,
wet weight
FP
"-0 undiluted spike
2.6x107CFU/mL
3.7x108CFU/mL
V spiked
Class A liquid: 1.0 X 10~5 ml_ per ml_ of
biosolids
Class A solid: 1 .0 x 1 0"4 ml_ per g of
biosolids (wet weight)
Class B liquid: 1.0 x 10"2 mL per mL of
biosolids
Class B solid: 1 .0 x 1 0"1 mL per g of
biosolids (wet weight)
Class A liquid: 1.0 X 10~5 mL per mL of
biosolids
Class A solid: 1 .0 x 1 0"4 mL per g of
biosolids (wet weight)
Class B liquid: 1 .0 x 1 0"2 mL per mL of
biosolids
Class B solid: 1 .0 x 1 0"1 mL per g of
biosolids (wet weight)
Spiked ECwet weiqht
(2.6 x 107 CPU / mL) x (1.0 x lO'5 mL / mL) =
2.6x102CFU/mL
(2.6 x 107 CPU / mL) x (1.0 x 10'4 mL / g) = 2.6
x 1 03 CPU /g (wet weight)
(2.6 x 107 CPU / mL) x (1 .0 x 10'2 mL / mL) =
2.6x105CFU/mL
(2.6x107CFU/mL)x(1.0x10-1 mL/g) =
2.6 x 106 CPU /g (wet weight)
(3.7 x 108 CPU / mL) x (1.0 x lO'5 mL / mL) =
3.7x103CFU/mL
(3.7 x 108 CPU / mL) x (1.0 x 10'4 mL / g) =
3.7 x 104 CPU /g (wet weight)
(3.7 x 108 CPU / mL) x (1 .0 x 10'2 mL / mL) =
3.7x106CFU/mL
(3.7x108CFU/mL)x(1.0x10-1 mL/g) =
3.7 x 107 CPU /g (wet weight)
15.7.3 Step 3: Convert to "True" Spiked E. coli CFU / g Total Solids (dry weight)
Convert to "true" spiked CFU / g total solids (dry weight) as indicated in Section 14.3
using the E. coli mL or g (wet weight) from Section 15.7.2 as the numerator in the
equation. Examples are provided in Table 11.
Table 11. Examples of Conversion to "True" Spiked E. coli CFU / g Total Solids (Dry Weight)
Example Total Solids
Class A liquid: 9%
Class A solid: 82%
Class B liquid: 4%
Class B solid: 23%
Class A liquid: 7%
Class A solid: 88%
Class B liquid: 3%
Class B solid: 40%
(CFU / mL or g) / percent total solids
=True spiked E. coli CFU / g dry weight
2.6 x 1 02 / 0.09 = 2889 = 2.9 x 1 03 CFU / g dry weight
2.6 x 1 03 / 0.82 = 31 71 = 3.2 x 1 03 CFU / g dry weight
2.6 x 105 / 0.04 = 6,500,000 = 6.5 x 106 CFU / g dry weight
2.6 x 1 06 / 0.23 = 1 1 ,304,348 = 1 .1 x 107 CFU / g dry weight
3.7 x 103 / 0.07 = 52,857 = 5.3 x 104 CFU / g dry weight
3.7 x 104 / 0.88 = 42,045 = 4.2 x 104 CFU / g dry weight
3.7 x 106 / 0.03 = 123,333,333 = 1.2 x 108 CFU / g dry
3.7 x 107 / 0.40 = 92,500,000 = 9.3 x 107 CFU / g dry weight
35
July 2006
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Method 1681
15.7.4 Step 4: Calculate Percent Recovery
15.7.4.1 Calculate percent recovery (R) using the following equation:
R= 100 x
T
Where:
R
Ns
Nu
T
15.7.4.2
Percent recovery
Fecal coliform MPN / g (dry weight) in the spiked sample
Fecal coliform MPN / g (dry weight) in the unspiked sample
True spiked E. coll CPU / g (dry weight) in spiked sample
Example percent recovery calculations are provided in Table 12.
Table 12. Example Percent Recovery Calculations
Matrix
Class A liquid
Class A solid
Class B liquid
Class B solid
Ns
2.5 x103
3.9 x103
1.4x108
8.3 x106
Nu
1.5x101
5.0 x102
1.7x106
8.0 x105
T
2.9 x103
3.2 x103
1.2x108
1.1 x107
Percent recovery (R)
100x[(2.5x103)-(1.5x101)]/2.9x103 =
86%
100 x [(3.9 x 103) - (5.0 x 102)] / 3.2 x 103 =
107%
100x[(1.4x108)-(1.7x106)]/1.2x108 =
115%
100 x [(8.3 x 106) - (8.0 x 105)] / 1.1 x 107 =
68%
16.0 Method Performance
16.1 Interlaboratory validation of Method 1681
16.1.1 Twelve volunteer laboratories and a referee laboratory participated in the U.S.
Environmental Protection Agency's (EPA's) interlaboratory validation study of EPA
Method 1681. The purposes of the study were to characterize method performance
across multiple laboratories and multiple biosolid matrices and to develop quantitative
quality control (QC) acceptance criteria. A detailed description of the of the study and
results are provided in the validation study report (Reference 19.2). Results submitted by
laboratories were validated using a standardized data review process to confirm that
results were generated in accordance with study-specific instructions and the October
2002 Draft version of EPA Method 1681.
16.1.2 Recovery - Method 1681 mean recoveries of fecal coliforms from Class A matrices,
compost, and thermophilically digested biosolids, spiked with laboratory-prepared spikes
were 91% and 71%, respectively. Median recoveries of fecal coliforms from compost,
thermophilically digested biosolids, spiked with laboratory-prepared spikes were 46%
and 52% , respectively. For Milorganite® (a heat-dried, Class A biosolid that was used
July 2006
36
-------
Method 1681
as the reference matrix) samples spiked with laboratory spiking suspensions, the mean
recovery was 53%, with a median percent recovery of 29%. Mean recoveries of fecal
coliforms from Class B matrices, aerobically and anaerobically digested biosolids, spiked
with laboratory-prepared spikes were 660% and 140%, respectively. Median recoveries
of fecal coliforms from aerobically and anaerobically digested biosolids, spiked with
laboratory-prepared spikes were 64% and 22%, respectively.
16.1.3 Precision - Method 1681 was characterized by high variability. For percent recovery, the
overall relative standard deviations (RSDs) ranged from 81% to 250% for Class A
matrices and from 210% to 340% for Class B matrices for EPA-prepared spikes. For
ambient (unspiked) fecal coliform results, the overall RSDs ranged from 3 7% to 300%
for Class A matrices and from 100% to 170% for Class B matrices.
16.1.4 False positive rates - Method 1681 false positive rates were relatively high for both
thermophilically digested (54%) and composted (73%) matrices. False positive rates for
Milorganite® could not be accurately assessed during the study because no positive tubes
were observed during the study. False positive rates for aerobically digested and
anaerobically digested samples were relatively low compared to Class A rates and ranged
from 5% to 9%.
16.1.5 False negative rates - Method 1681 false negative rates for Class A were relatively low
compared to Class B rates and ranged from 0% to 9%. False negative rates for
aerobically digested and anaerobically digested matrices were 23% and 18%,
respectively.
17.0 Pollution Prevention
17.1 The solutions and reagents used in this method pose little threat to the environment when
recycled and managed properly.
17.2 Solutions and reagents should be prepared in volumes consistent with laboratory use to minimize
the volume of expired materials to be disposed.
18.0 Waste Management
18.1 The laboratory is responsible for complying with all Federal, State, and local regulations
governing waste management, particularly hazardous waste identification rules and land disposal
restrictions, and for protecting the air, water, and land by minimizing and controlling all releases
from fume hoods and bench operations. Compliance with all sewage discharge permits and
regulations is also required. An overview of requirements can be found in Environmental
Management Guide for Small Laboratories (EPA 233-B-98-001).
18.2 Samples, reference materials, and equipment known or suspected to have viable bacteria or viral
contamination must be sterilized prior to disposal.
18.3 For further information on waste management, consult The Waste Management Manual for
Laboratory Personnel and Less Is Better: Laboratory Chemical Management for Waste
Reduction, both available from the American Chemical Society's Department of Government
Relations and Science Policy, 1155 16th Street NW, Washington, DC 20036.
37 July 2006
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Method 1681
19.0 References
19.1 American Public Health Association, American Water Works Association, and Water
Environment Federation. 1995. Standard Methods for Water and Wastewater. 20th Edition.
Sections: 9020, 9221, 9222.
19.2 USEPA. 2004. Results of the Interlaboratory Validation of EPA Method 1681 (A-l)for Fecal
Coliforms in Biosolids. EPA-821-R-04-009. December 2004.
19.3 American Chemical Society (ACS). 2000. Reagent Chemicals, American Chemical Society
Specifications. American Chemical Society, New York. For suggestions of the testing of reagents
not listed by the American Chemical Society, see AnalaR Standards for Laboratory Chemicals,
BDH, Poole, Dorset, UK and the United States Pharmacopeia.
19.4 Bordner, R., J.A. Winter, and P.V. Scarpino (eds.). 1978. Microbiological Methods for
Monitoring the Environment, Water and Wastes. EPA-600/8-78-017. Office of Research and
Development. USEPA.
19.5 Klee, A. J. 1993. A computer program for the determination of the most probable number and its
confidence limits. Journal of Microbiological Methods. 18:91-98.
20.0 Figures
20.1 The following pages contain flow charts of dilution and inoculation schemes (Section 11.0) and
for the procedures (Section 12.0). Schemes for dilution and inoculation are dependent on Class
(A or B) and matrix (solid or liquid).
July 2006 38
-------
FIGURE 1. MULTIPLE TUBE FERMENTATION PROCEDURE
A-1 Method
Inoculate A-1 and incubate at
35°C ± 0.5°C for 3 hr
Transfer to water bath at
44.5°C ± 0.2°C for additional 21 ± 2 hr
Failure to produce gas within
24 ± 2 hr:
NEGATIVE fecal coliform
Growth and gas production
within 24 ± 2 hr:
POSITIVE fecal coliform
-------
FIGURE 2. CLASS A LIQUID SAMPLE DILUTION AND INOCULATION SCHEME
Class A
liquid sample
"Homogenized sample"
Delivery Volume
-11.0ml-
99 ml of sterile
buffered dilution water
"A"
(ID'1)
-11.0ml-
99 ml of sterile
buffered dilution water
"B"
(ID'2)
-11.0ml-
99 ml of sterile
buffered dilution water
"C"
(ID'3)
1.0ml
1.0ml
1.0ml
1.0ml
o
3
-o_
CD
CO' CD
<§• co
Q) K)
8
3
"
O 1)
^. ^~
(Q CD
g- CO
Q)^ CO
CO
Q)
3
•D.
CD
q
3
i—
2, w
O CD
eg' jj>'
ID
Q)^ -^
I
3
CD"
-------
FIGURE 3. CLASS A SOLID SAMPLE DILUTION AND INOCULATION SCHEME
Class A
solid sample
Delivery Volume
Rinse sample into blender
with 270 ml of sterile
buffered dilution water
"Homogenized Sample"
(ID'1)
10.0mL
1.0mL
-11.0ml-
b
_. CO
= 0
^ ~i (D
t02 s>'
_ g »
= U)
31
CD"
p
(Q
o
?»
<°s-
3 U)
5L K>
I
CD"
99 ml of sterile
buffered dilution water
"A"
(ID'2)
-11.0mL-
99 ml of sterile
buffered dilution water
"B"
(ID'3)
1.0ml
1.0ml
(Q
O
O
<9. CD'
3 tfl
5L co
&
•D.
0>
(Q
O
' W
(D
-------
FIGURE 4. CLASS B LIQUID SAMPLE DILUTION AND INOCULATION SCHEME
Class B
liquid
sample
"Homogenized sample"
—HOrnL-
Delivery Volume
gg.omLof
sterile
buffered
dilution
water
"C"
(ID'3)
1.0 mL
-HOmL-
99.0 mL of
sterile
buffered
dilution
water
"D"
(ID'4)
1.0 mL
1- oi
Q) NJ
-H.OmL-
99.0 mL of
sterile
buffered
dilution
water
"E"
(ID'5)
lOmL
y. en
O
-------
FIGURE 5. CLASS B SOLID SAMPLE DILUTION AND INOCULATION SCHEME
Class B
solid
sample
—30 g-
Rinse sample into blender
with 270 ml_ of sterile
buffered dilution water
"Homogenized Sample"
(10-1)
Delivery Volume
1.0 mL
-11.0 ml_-
99.0 mLof
sterile
buffered
dilution
water
"C"
(10-4)
1.0 mL
-11.0 ml_-
1.0mL
-------
Method 1681
21.0 Glossary
The definitions and purposes are specific to this method but have been conformed to common
usage as much as possible.
21.1 Units of weight and measure and their abbreviations
21.1.1 Symbols
°C degrees Celsius
< less than
> greater than
% percent
± plus or minus
21.1.2 Alphabetical characters
EPA United States Environmental Protection Agency
EC Escherichia coli
g gram
L liter
LTB lauryl tryptose broth
mg milligram
mL milliliter
mm millimeter
MPN most probable number
NIST National Institute of Standards and Technology
QA quality assurance
QC quality control
TD to deliver
21.2 Definitions, acronyms, and abbreviations (in alphabetical order):
Analyte—The microorganism tested for by this method. The analytes in this method are fecal
coliforms.
Enrichment—A non-selective culture media for enhanced growth.
Liquid samples—Generally defined as samples containing <7% total solids (dry weight).
May—This action, activity, or procedural step is neither required nor prohibited.
May not—This action, activity, or procedural step is prohibited.
Method blank—An aliquot of sterile reagent water that is treated exactly as a sample including
exposure to all glassware, equipment, media, procedures that are used with samples. The method
blank is used to verify the sterility of equipment, materials, and supplies.
July 2006 44
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Method 1681
Most probable number method (MPN)—A statistical determination of the number of bacteria per
weight or volume of sample. It is based on the fact that the greater the number of bacteria in a
sample, the more dilution is needed to reduce the density to the point at which no bacteria are left
to grow in a dilution series.
Must—This action, activity, or procedural step is required.
Negative control—A control culture that, when analyzed exactly like a field sample, will produce
a known negative result for a given type of media.
Positive control—A control culture that, when analyzed exactly like a field sample, will produce
a known positive result for a given type of media.
Preparation blank—See Method blank.
Selective media—A culture media designed to suppress the growth of unwanted microorganisms
and encourage the growth of desired ones.
Should—This action, activity, or procedural step is suggested but not required.
Solid samples—Generally, samples containing >7 % total solids (dry weight).
45 July 2006
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