Evaluation of Septic Tank and Subsurface Wetland for
Evaluation of Septic Tank and Subsurface Wetland for
Evaluation of Septic Tank and Subsurface Wetland for
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Appendix 7 – Thomas’ Most Probable Number Sample Calculation <strong>for</strong> Coli<strong>for</strong>m<br />
When the number <strong>of</strong> positive fermentation tubes does not fit into the normal distribution<br />
table available in the St<strong>and</strong>ard Methods text (APHA), a calculation using Thomas’<br />
<strong>for</strong>mula is used to determine MPN as follows:<br />
MPN = (# positive tubes x 100) (ml in neg. tubes x total ml)<br />
MPN determination <strong>for</strong> November 16 sample PS1<br />
Range # positive Sample ml in neg. total ml<br />
Thomas'<br />
<strong>for</strong>mula<br />
Tubes size (ml) Tubes MPN<br />
10M 5 0.0001 0 0.0005<br />
CM 2 0.00001 0.00003 0.00005<br />
MM 3 0.000001 0.000002 0.000005<br />
Total 10 0.000032 0.000555 7,503,753<br />
MPN = (10 x 100) (0.000032 x 0.000555) = 7,503,753<br />
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