Aerial view of the Warren, Maine lagoon system. Photo courtesy of Woodard and Curran.

Lagoon Systems In Maine 

Systems In Maine

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Mars Hill Wastewater Lagoon System - Mars Hill  Maine. Photo Courtesy of Wright-Pierce Engineers.
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Coliform and E. coli Testing (3rd of 3 articles)  

Tim Loftus

       The first article in this series emphasized that the ways of validating bacteriological testing differ from those methods used to validate chemical or physical analyses. The second article was an overview of federally required techniques and procedures needed to produce reportable results.  This article will cover the requirements for reading the results and for calculating the geometric mean for NPDES reporting.

 Reading the results:

       After incubating, count the colonies under 10X to 15X magnification. Itís important to report only the results that meet certain requirements. Often background colonies will grow on the membrane filter (will be a different color than the target organism). The total of the fecal coliform colonies and the background should not exceed 200 colonies per filter. If it does, consider that dilution to be invalid. Note that the E.coli test methods are not specific on this point.

       Report only the fecal coliform results from membrane filters that have between 20 and 60 colonies or the E. coli colonies that have between 20 to 80 colonies per filter. Disregard the other dilutions. Donít forget to factor all dilutions back up to a 100 mL sample volume. The following example shows how the result of a fecal coliform analysis is to be determined:


Sample Dilution Colony Count per Filter Background Colonies Dilution Factor Calculation Result
25 ml to 100 ml 15 67 4 4 x 15 60 col / 100 ml
50 ml to 10 ml 34 110 2 2 x 34 68 col / 100 ml
No dilution 100ml 55 190 1 1 x 55 55 col / 100ml

Report on your NPDES DMR a fecal coliform value of 68 col/100 mL.

 Hereís the reason: The first dilution (25 mL to 100 mL) does not produce a fecal colony count of 20 to 60 colonies per membrane filter (it produces only 15). Therefore it doesnít meet the counting requirements of the test. The third sample (no dilution) does produce enough colonies on the filter (55), but the total of fecal and background colonies is too high (55 + 190 >200). Likewise, this dilution does not meet the reporting requirements. Only the second dilution (50 mL to 100 mL) produces a reportable result.

    There are a few exceptions to this. If your effluent disinfection system is working well, you may not have any, or very few, fecal coliform or E. coli bacteria in the sample. No matter what dilution you use, you will get a very low result. Choose the dilution closest to the requirements and report that result as an estimate.

Conversely, if your disinfection system isnít working properly, even a highly diluted sample will exceed the reporting requirement limits. While the actual number of colonies may be inaccurate, it is obviously an NPDES discharge violation. The results should be reported as TNTC (Too Numerous To Count).

 Calculating the Geometric Mean:

    The geometric mean is a calculation to determine an average when the set of numbers covers a wide range. Results of bacteriological testing often cover such a large range.  The easiest way to calculate the geometric mean is to use a scientific calculator.  The first step in the calculation involves converting all your daily coliform results to log values. Add up these log values and divide by the number of samples. For example: The daily coliform results during the week were 25 col/100 mL, 285, 15, and 460.

(log 25 + log 285 + log 15 + log 460)/4 = (1.3979 +  2.4584 + 1.1760 + 2.6627)/4 = 1.9238.

The second step is to take this value and find the antilog. This will be the 10x button on your calculator. Press the button for 10x  then type in 1.9238. Press ďenterĒ and the display will show 84. Report 84 colonies/100 mL as the weekly geometric mean. Monthly geometric means are done the same way, except you will be using many more data points.

     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 207-782-0917  ( or Tim Loftus at (508) 949-3865 (




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