Sewage backup could lead to sewage contamination of the occupied spaces.
Exposure to sewage contamination increases the risk of contracting diseases of the digestive system and other related illnesses. Potential disease causing organisms in sewage contamination include Escherichia coli, Salmonella, and Shigella.
Testing for all possible disease causing microorganisms (pathogens) after sewage backup could be expensive, tedious and time consuming.
Therefore, the detection of indicator bacteria is more practical than direct pathogen detection because the former are considered to be normal, non-pathogenic intestinal inhabitants that are present in feces in much higher numbers than pathogenic microorganisms. The preferred indicator bacteria for sewage contamination are total coliforms, fecal coliforms and Enterococcus.
Sources of sewage contamination may include raw sewage backup, severe flooding and leaking sewer lines or septic tanks. To determine the potential health risks from sewage contamination swab samples could be collected and tested for total coliforms, fecal coliforms and Enterococcus.
Sampling Procedure for total coliforms, fecal coliforms and Enterococcus
Sampling for indicator organisms can be performed before and after clean up. Sampling after cleanup is recommended to determine if the cleanup was successful.
- Wear suitable gloves
- Using a measuring tape, measure an area of approximately 100 cm2 of the surface suspected to be contaminated with sewage.
- Collect a swab sample by removing a sterile, rayon (non-cotton) swab from a sterile tube. If the surface to be tested is dry, moisten the swab by inserting it into the tube which contains a sponge soaked with sterile buffer.
- Swab the selected surface by rolling the swab back and forth across the surface with several horizontal strokes, then several vertical strokes.
- After sampling, return the swab to the sterile tube (with the sponge) and label the sample
- Collect a control sample from a non-contaminated area by repeating steps 2 to 5.
- On the chain of custody clearly indicate what indicator organisms you want tested, i.e., total coliforms, fecal coliforms, E. Coli and/or Enterococcus. The section on results interpretation (see below) will help you decide what to test for. Bacterial testing is time sensitive so samples should be sent to the laboratory within 24 hours of sample collection if possible and should be shipped with an ice pack.
How to interpret positive sewage results
Positive sewage results should be interpreted with caution since not all coliforms are of fecal origin.
Coliforms are common in the intestines of animals including human beings and hence they end up in sewage. They are also present in the environment as part of the natural microflora. These non-fecal coliforms can potentially cause false positive results when testing environmental samples.
Therefore positive tests for total coliforms do not necessarily indicate sewage contamination. For recreational waters, total coliforms are no longer recommended as an indicator. For drinking water, total coliforms are still the standard test because their presence indicates contamination of a water supply by an outside source.
The term “fecal coliform” is rather misleading since not all bacteria found in this group are of fecal origin. Fecal coliforms include bacteria such as E. coli, Klebsiella, Enterobacter, and Citrobacter.
With the exception of E. coli, these bacteria could also be associated with plants. Therefore, while the fecal coliform test is more specific than the total coliform test in indicating potential sewage contamination, this test is also subject to giving false negatives.
Escherichia coli (E. coli)
As mentioned above the E. coli belong to the fecal coliform group. Presence of E. coli is a reliable indication of fecal or sewage contamination. However, E. coli has also been isolated from soil suggesting that it may not be 100% reliable indicator of fecal or sewage contamination.
Enterococci are a subgroup within the fecal streptococcus group. Enterococci are typically more human-specific than the larger fecal streptococcus group. EPA recommends enterococci as the best indicator of health risk. They are rarer than the coliforms in the environment and are always present in the feces of warm-blooded animals.
What Indicator Bacteria Should You Test For?
From the above discussion it’s clear that no single indicator organism on it’s own is 100% reliable to demonstrated sewage or fecal contamination. Which bacteria you test for depends on what you want to know.
Do you want to know whether the contaminated surface poses a health risk? If the answer to this question is yes, the best indicators of health risk E. coli and enterococci. Fecal coliforms as a group are a poor indicator of the risk of digestive system illness.
Some experts therefore recommend use of 3 or more indicators, i.e., total coliforms, fecal coliforms, E. coli and/or enterococci.
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