Sewage Testing Methods after Remediation for Fecal Contamination

swab sewage testing
Sewage Testing of Presence/Absence (P/A) for Indicator Organisms vs. ATP Measurement

Indoor Environmental Technologies (IET) uses two sewage testing methods for contamination in Post-Remediation Verification inspections. The goal is to determine whether remediation has been successful and the areas tested have been returned to a sanitary condition:


  • Swab testing for presence/absence of indicator sewage organisms or groups of organisms.
  • Testing for adenosine tri-phosphate (ATP) concentrations using a luminometer (light meter).

IET performs sewage remediation protocols to determine extent of contamination in a home, along with sewage testing services after remediation.  These services are provided in the Tampa Bay area and Florida. If in need of an inspection, please contact us today.

Swab sewage testing for three indicator organisms

Sewage contains hundreds or thousands of different organisms. Sewage testing for all of them is not practical.  Therefore, the most common tests measure for the presence or absence (P/A), or in some cases quantification, of groups of organisms that are almost universally found in sewage, using the groups as an indicator or surrogate for fecal contamination in general.  These organisms are not necessarily themselves among those most likely to cause serious health effects.

The sewage testing method IET most often uses tests for the presence or absence (P/A) of three groups of viable (living) bacteria:  1) fecal coliform bacteria, 2) Enterococcus bacteria, and 3) Escherichia coli (E. coli) bacteria.

Fecal Coliform Bacteria

Fecal coliform bacteria

Fecal coliform bacteria

This bacteria include all those coliform (rod-shaped) bacteria that are limited primarily to the intestines of warm-blooded animals, thus consisting of many genera.  Till recently, the P/A test was normally for Total Coliforms, a much larger group that includes many environmental bacteria, so it was prone to false positives.  Now that the P/A test is only for Fecal Coliforms, this tendency is much reduced.  However, Fecal Coliforms as a group still include some non-fecal bacteria. So, there is still some chance of a false positive result.

Enterococcus bacteria

Enterococcus bacteria

Enterococcus Bacteria

This bacteria are a single large genus of cocci (roundish bacteria) that are almost entirely limited to intestinal tracts.  For this reason, the presence or absence of Enterococcus is more definitive of fecal contamination, or its absence, than the test for Fecal Coliforms.  False positives are unlikely.


E. coli Bacteria

e. coli

E. coli is one of the most common bacteria in animal intestines. It is one of the species included in the fecal coliform group, as shown in the image to left. We often find samples negative for E. coli even when they are positive for fecal coliforms and/or Enterococcus.  This apparently is because E. coli is somewhat more fragile and becomes non-viable more quickly once outside an intestinal tract.  As with Enterococcus, false positives are unlikely.

Presence of Bacteria Explained

The presence of one or more of these bacteria types is a good indicator of fecal contamination, though other explanations for their presence in the environment are still possible.  For instance, small children, and pets are often not as sanitary as they should be and their activities can introduce these bacteria types onto surfaces.  Also, as noted above, P/A testing for fecal coliforms is somewhat more prone to false positives than the other two tests.  However, there is no obvious reason why fecal contaminants of any variety would be present on top of a shelf if they had not become aerosolized because of a sewage backup event.

The P/A test consists of swabbing a 4” x 4” area, then submitting the swab to a laboratory for analysis.  A sample can be heavily soiled and the test will still be negative if it does not contain these specific organisms.  The test is very sensitive, purportedly able to detect even a single organism.   Turnaround time is normally 48 to 72 hours from the sample being taken.

The P/A test reacts to only viable organisms. Thereby, the results can come back negative even for a not particularly clean surface if any of these organisms that may be present are no longer viable.  This test cannot measure for other potentially problematic constituents of fecal contamination, such as bacterial endotoxins, allergens, etc.

Pros of the P/A test method

  • Directly measures fecal contamination with viable fecal bacteria.
  • The only way to determine an accurate scope of work on initial investigation.
  • Can be used to determine whether fecal contamination became aerosolized and how far it may have spread.

Cons of the P/A test method

  • Up to 10x more expensive per sample, so fewer locations are likely to be sampled, increasing the potential for sampling results to not accurately indicate conditions.
  • Three to four day turn-around time (TAT) from sample being taken. This can significantly increase cost and/or disruption on a project, especially if there are successive failures to pass “clearance.”
  • Results must be read 24 to 48 hours after the testing process begins. This means that sampling cannot be done towards the end of the work-week unless the homeowner is willing to pay a hefty charge for the lab technician to come into the lab over the weekend.

Sewage testing for adenosine tri-phosphate (ATP) concentrations using a luminometer (light meter)

ATP is an enzyme found in all living and recently dead things, or, to put it another way, in all organic material or debris.  For the ATP testing method, IET swabs a 4” x 4” section to be tested with a special swab, then inserts it into a meter that measures the amount of light given off by the swab.  The swab contains a chemical based on the chemistry of fireflies that fluoresces when mixed with ATP, the amount of light produced being proportionate to the concentration of ATP, which is then expressed in Relative Light Units (RLU) by the meter.

The criterion that IET uses to determine whether a surface is sanitary using the ATP method is that it must measure 15 RLU or less.  15 RLU is a standard routinely used in the food industry to determine whether a food preparation surface, such as a cutting board, is adequately sanitary for its use.  A surface this clean will, of course, support very few fecal contaminants.

Important Points about ATP testing

  • ATP testing cannot be used to determine whether a surface is contaminated. Most surfaces in any structure that has not been recently cleaned and sanitized will exceed the 15 RLU cutoff. This is regardless of whether there has been a sewage contaminating event or not. This means that ATP testing cannot be used to accurately determine a scope of work on the initial investigation. It is only used to determine if remediation was effective in a Post-Remediation Verification inspection.  This also means that a surface should normally be tested, if possible, immediately after cleaning. This is to prevent cross-contamination that might produce a false positive.
  • The ATP method measures organic soil or debris, NOT fecal contamination as such, as the other method discussed here does. A surface that is not immaculately clean will not pass the test, even if no fecal organisms are present.  This also means that porous or semi-porous surfaces, such as unfinished wood or concrete, are likely to be more difficult to get clean to pass the test.
  • Since the goal is absolute cleanliness, simply killing organisms, even when the process is 100% effective, will not necessarily bring conditions down below 15 RLU. This means the surface can have zero fecal organisms. This means it would pass the presence/absence test, but can still fail the ATP test.  Conversely, a soiled surface would fail the ATP test but would pass the P/A test if there were no fecal bacteria present.
  • Different manufacturers of ATP test equipment may use a different scale for their RLUs. Results from one brand are not necessarily identical to those from another.

Pros of the ATP test method

  • Relatively low cost. This allows for a more statistically valid number of locations to be sampled at reasonable cost.
  • Since the ATP test is for cleanliness, it ensures that a surface passing the test contains no significant amounts of organic debris. Thereby, other potentially problematic constituents of sewage contamination, such as endotoxins and allergens, will not be present in significant quantity.
  • On-site real-time results. This means that if a surface fails the designated cleanliness criteria, it can be cleaned and sanitized again, then immediately retested.  This can significantly speed the Post-Remediation Verification process.

Cons of the ATP test method

  • Does not measure directly for fecal organisms.
  • In IET’s opinion, the ATP test cannot accurately determine the needed scope of work during initial investigation. Attempts have been made, but IET has seen no scientific evidence that ATP sewage testing provide valid results.
  • In some cases, the validity of this test for determining a sanitary condition may be challenging for homeowners to understand.

Note:  Getting a sewage-contaminated environment clean and sanitary enough to pass either the P/A or the ATP test method is inherently challenging.  The remediation procedures for sewage remediation are quite different from those appropriate for effective mold remediation. Also, the appropriate procedures may vary depending on the test method that will be used for “clearance.”

Post-Remediation Sewage Testing for Verification

IET performs sewage post testing to document that the structure has been returned to a sanitary condition before reconstruction begins. We provide sewage testing throughout the Tampa Bay area including Clearwater, St. Pete, Sarasota and Lakeland.

Feel free to contact us today!

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