Coliform and E. Coli Testing
(1st of 3 articles)
by Tim Loftus
The fecal
coliform group and E. coli (which is one specie of bacteria in
the fecal group) are used as indicator organisms to test the
effectiveness of effluent disinfection in a wastewater treatment plant.
While these organisms are generally harmless to us, they do live under the same
conditions that human pathogens live. Since we cannot test for every pathogen in
the effluent, we test for easily detectable indicator organisms. The assumption
is that if we kill the indicator organisms then we most likely kill the
pathogens during effluent disinfection.
Testing
requirements for the fecal coliform group or specifically for E. coli is
determined by your individual state governing body. The sampling frequency and
limits will be outlined in your NPDES permit.
The most common
method of testing for the fecal coliform group and for E. coli is the
membrane filtration technique. A sample of treated wastewater is filtered
through a membrane filter with a pore size of 0.45 microns. The filter is placed
on a specific broth designed to allow colonies of the indicator organisms to
grow. After an incubation period the colonies are counted and the results are
calculated. Details on how to make the broth, how to recognize a colony, and
incubation temperature requirements for your indicator organisms are outlined in
the test method specified in your NPDES permit. While there are many differences
in testing for these indicator organisms, there are a number of aspects common
to both procedures that every laboratory should address. These are outlined
below.
There is no DMR-QA
study on the fecal coliform group or E. coli bacterial tests. Because of
this, your quality assurance (QA) program determines the validity of these
bacterial tests for reporting purposes. If you have good QA, then most likely
your results will be valid. (Note that one company I know of, Environmental
Resource Associates, does have fecal coliform and E. coli quantitative
standards. These are not used by regulating authorities as blind checks at this
time. However, you can use these for in-house QC samples.)
There are many
requirements and recommendations for a good QA program. The following list shows
the most common aspects of a program that will help validate your numbers to
your regulating authority.
Training:
all personnel performing bacteriological testing should be trained for the
proper sampling, testing, interpreting results, and specific hygiene
requirements for these analyses.
Standard
Operating Procedures (SOPs): All directions on performing the analyses,
equipment and chemicals needed, what is required for QC, and how to interpret
results should be clearly spelled out in a written procedure.
Cleanliness and
Sterilization of Work Area and Equipment: All areas of bacteriological
testing (benches, floor, walls) should be cleaned with a disinfectant. All
sample bottles, equipment, and chemicals should be autoclaved (the most common
form of sterilization) for a minimum of 15 minutes at 121 C and 15 psi. Use heat
tape to show these have been sterilized. Spore strips can be used to check the
effectiveness of sterilization.
Record Keeping:
Record daily the temperatures of any incubator ovens and water baths (using a
NIST traceable thermometer, of course!), autoclave temperature/pressure/time,
spore strip results, chemical batch dates, sampling times, dilutions, and
anything else that relates to the analyses.
Producing valid
results is the most important aspect of being a lab analyst. Since
bacteriological testing has a whole set of different challenges than physical
and chemical testing, it’s important to incorporate all measures available to
assure reliable and accurate results. The second article in this series will
address general sampling, QC, and sample dilutions. A third article will review
calculating the geometric mean for NPDES reporting purposes.
The information in
this article is based on general test methods that are used throughout the New
England area for NPDES monitoring of the fecal coliform group and for E. coli.
As usual, check your federal, state, and local regulations. You may have
additional regulations or reporting requirements that you must meet.
If you have any questions, suggestions, or comments, please contact NEWEA Lab
Practices Committee Chair Phyllis Arnold Rand at (207) 782-0917 (rcrand@gwi.net)
or Tim Loftus at (508) 949-3865 (timloftus@email.msn.com).
