Fecal Coliform Bacteria Counts: What They Really Mean About Water Quality
Alternative measurements of bacteriological water quality can help you understand general and fecal coliform bacteria counts and relate them to the real world. Understanding water quality can help you design and use systems better.
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What the heck does it mean that there are "52 MPN fecal coliforms/100ml of water?
Is it good to drink? To wash dishes in? To bath in? Irrigate with?
The average person, or even engineers and scientists who don't have a public health or microbiology background wouldn't have a clue.
In fact, these units are so obscure that even people who work with them every day for years and make important decisions based on test results often have little sense of how to relate contamination either to cause or effect in a quantitative way.
- If a kid sneaks a swim in an un-chlorinated 50,000 gallon drinking water tank without wiping their butt, what is the likely level of contamination?
- If there is a bear poop bleeding into the edge of a swiftly flowing river (250 gallons per minute), how likely are you to get sick from drinking the water?
- If you irrigate your fruit trees with kitchen sink water, how likely is a kid to get sick from eating the dirt under the trees?
Most public health professionals would say that there would be a hazard. But they would be hard pressed to come up with an assessment of the size of the hazard that was accurate within a factor of a hundred. Many would be off by a factor of ten thousand. (Read on and then see how these questions are worked out below).
Further obscuring the picture, almost all standards are expressed in terms of concentration, not total quantity of organisms.
A spokesman for the Santa Barbara sewage treatment plant once calmly explained that discharge of untreated sewage to the ocean during intense rainfall was not an issue, because "the dilution factor is so great." This is an exceptionally clear case of "concentration blindness."
50,000 kilograms of fecal matter flushed to the ocean in a billion gallons of storm water is no less harmful to swimmers than 50,000 kilograms in a million gallons of sewer water. The amount of water that carries it to the ocean is irrelevant considering the relative size of the oceanwhat matters is how much feces are being added. If anything, feces delivered in a giant slug of fresh water are worse, as the large flow of less dense fresh water might tend to float on the surface where exposure is more likely.
However, according to standards for effluent, which are based on concentration, the latter scenario appears a thousand times worse.
Indicator connection varies
General coliforms, E. Coli, and Enterococcus bacteria are the "indicator" organisms generally measured to assess microbiological quality of water. However, these aren't generally what get people sick. Other bacteria, viruses, and parasites are what we are actually worried about.
Because it is so much more expensive and tedious to do so, actual pathogens are virtually never tested for. Over the course of a professional lifetime pouring over indicator tests, in a context where all standards are based on indicators, water workers tend to forget that the indicators not the thing we actually care about.
What are these indicators?
- General coliforms indicate that the water has come in contact with plant or animal life. General coliforms are universally present, including in pristine spring water. They are of little concern at low levels, except to indicate the effectiveness of disinfection. Chlorinated water and water from perfectly sealed tube wells is the only water I've tested which had zero general coliforms. At very high levels they indicate there is what amounts to a lot of compost in the water, which could easily include pathogens (Ten thosand general coliform bacteria will get you a beach closure, compared to two or four hundred fecal coliforms, or fifty enterococcus).
- Fecal coliforms, particularly E. coli, indicate that there are mammal or bird feces in the water.
- Enterococcus bacteria also indicate that there feces from warm blooded animals
in the water. Enterococcus are a type of fecal streptococci. They are another
valuable indicator for determining the amount of fecal contamination of water.
According to studies conducted by the EPA, enterococci have a greater correlation with swimming-associated gastrointestinal illness in both marine and fresh waters than other bacterial indicator organisms, and are less likely to "die off" in saltwater.
The more closely related the animal, the more likely pathogens excreted with thier feces can infect us.
Human feces are the biggest concern, because anything which infects one human could infect another. There isn't currently a quantitative method for measuring specifically human fecal bacteria (expensive genetic studies can give a presence/absence result).
Ingesting a human stranger's feces via contaminated water supply is a classic means for infections to spread rapidly. The more pathogens an individual carries, the more hazardous their feces. Ingesting feces from someone who is not carrying any pathogens may gross you out, but it can't infect you. Infection rates are around 5% in the US, and approach 100% in areas with poor hygiene and contaminated water supplies.
Keep in the back of your mind that the ratio of indicators to actual pathogens is not fixed. It will always be different, sometimes very different. Whenever you are trying to form a mental map of reality based on water tests, you should include in the application of your water intuition an adjustment factor for your best guess of the ratio between indicators and actual pathogens.
"Best guess?!" I can imagine precision obessessed regulators cringing. Well, it can hardly be better to ignore the fact that the number and virluence of pathogens present in samples with the same number of fecal coliform indicators can be different by a factor of ten to a hundred or more, simply because checking for the pathogens themselves is too cumbersome.
These are the factors to include in your mental indicator to pathogen adjustment factor:
- Feces of non-human origin are of less concern to humans (this is why spreading manure on your vegee garden is not considered insane)
- Feces from human populations with higher infection rates are of greater concern (a currently low rate is not a reason to condone a new fecal to oral disease transmission routewhich will raise the infection rate over time)
- All treatment methods and environmental conditions affect pathogens and
indicators differently. For example, chlorinated sewage effluent may have
zero indicators and zero pathenogenic bacteria, but be laden with nearly all
its original viruses.
Pathogens (and indicators) can "hide" from treatment inside suspended solids. If treated water is turbid, the saftey of the water and the suspended solids can be very different. If the samples don't capture the suspended solids, the reading will be low.
Important public health and engineering decisions are often made with a fuzzy idea of the hazards.
There is a tendency to tighten policy and overbuild facilities until the number of coliforms per 100ml at some point in the process is zero. If the actual sense of the hazard is not in focus, seeking the simple assurance of a zero reading is understandable.
However, this is a poor design guide compared to real understanding. Consider:
- A sewage treatment plant which removes indicator bacteria may not remove viruses; it will test safe but not be safe in reality.
- A beach front community with septics in Santa Barbara is being pressured to hook up to sewers, because a study found about 80 human fecal coliforms per 100 ml in the lagoon water. Sounds awful, whatever it means. But what it means is there was a half-teaspoon of human feces in the 30,000 or so gallons of the lagoon. All the feces for the duration of the study could have come from one disposable diaper, and not from the septics at all. Millions of dollars might be spent on sewer connections for no benefit.
- A study which turned up 84,000 fecal coliform bacteria per 100 ml of kitchen sink water did not consider the possibility that indicators were multiplying and there wasn't really that much feces (or pathogens) in the water. If they realized this equates to about a teaspoon a day of feces down the kitchen sink, they might have paused to consider if this much poop was really being dumped in the kitchen sink. But based on this study, the law did not allow kitchen sink water greywater systems, but it might be OK in reality.
In order to incorporate water quality considerations into my designs in a quantitative way, I first had to convert the measurements to units I could understand. Most other water quality measurements are ratios: parts per million, or billion.
The beauty of this kind of measurement is that by multiplying the concentration by the volume of water it is possible to figure out how much actual stuff you're talking about, as in the kitchen sink and beachfront septics examples above.
It turns out that one fecal coliform bacteria per 100 milliliters closely equates to one part per billion of feces, or one milligram per cubic meter (you can see how I did the conversion below).
One part per billion of fecal matter is an infintesimal amount of contamination; about a grain of sand in five 55 gallon drums, or about what someone drinks in three years. However, this is worth worrying about; it fails the minimum standard for drinking water quality in most of the developed world, which is zero general coliforms per 100mL.
Converting to concentration and absolute quantities enables you to estimate what could account for a given level of contamination, or what level of contamination would result from a given action. For example, a buttwipe (ahem) diluted in a swimming pool of water yields a feces concentration of about 1 part per billion.
Measuring organisms per 100 mL, you can't easily relate a case of contamination either to cause or effect in a quantitative way.
Without further ado, here is a table which shows conventional units and standards, and their conversion to parts per million, parts per billion, and the novice-friendly units of of buttwipes or turds per swimming pool...
To find the conversion factor from any unit to any other, find the bold number 1's, then read across to the other column.
- 1 fecal coliform/ 100ml = 1 ppb = 0.001 ppm = 1 buttwipe per swimming pool = 0.001 buttwipe per bathtub
- 1 turd/ swimming pool = 10,000 mg feces/ m3
Back to the questions we opened with:
If a kid sneaks a swim in an un-chlorinated 50,000 gallon drinking water tank without wiping their butt, what is the likely level of contamination?
Layperson version: 50,000 gallons is twice as big as a swimming pool. One buttwipe per swimming pool is the drinking standard, so half a buttwipe per swimming pool does not exceed the drinking standard.
Scientist version: 50,000 is about 200 m3. A buttwipe is about 100 mg. So there is about 0.5 mg per m3, which does not exceed the drinking standard.
Additional considerations: If the kid is known to have an infectious condition, the cause for alarm is greater. Also, the amount of fecal matter may not be average, it probably won't all come off in the water, and the part of it that does will not be evenly distributed. Most likely, the particles will sink to the bottom or top. If the geometry of the inlets and outlets is designed optimally, almost none of it will make it into the water distribution system.
If there is a bear poop bleeding into the edge of a swiftly flowing river (25,000 gallons per minute), how likely are you to get sick from drinking the water?
Layperson version: That's a swimming pool every minute. If The bear poop takes 1000 minutes, (16 hours) to dissolve, that's a thousandth of a turd per swimming pool - ten times the drinking standard.
Scientist version: The bear bowel movement is 1,000,000 milligrams. The flow is about 100 m3 per minute, times 1000 minutes = 100,000 m3 of water. 1,000,000 divided by 100,000 is 10 mg feces/ m3 - ten times the drinking standard.
Additional considerations: A bear poop is probably bigger than a human poop. However, bear pathogens are less likely to infect humans. The swift flow will probably distribute the fecal matter pretty evenly through the water column before long, without much settling. This illustrates a feature of rivers: while on average they are likely to be clean, infectious level pulses of pathogens are likely to come through.
(Note: you can estimate flow by multiplying the width times the depth of the channel times half the speed of the surface. Ten meters wide, two meters deep on average and a meter per second is about ten cubic meters per second)
If you irrigate your fruit trees with kitchen sink water, how likely is a kid to get sick from eating the dirt under the trees?
A risk assessment analysis of this scenario is viewable in the Arizona Water CASA greywater study. Note that they assume from the high level of indicators that there is a level of pathogens in the water corresponding to nearly a gram a day of fecal matter entering the kitchen sink. This could be accounted for by ten people wiping their butts with thier hands only and washing them off in the kitchen sinkan unlikely scenario, I dare say. (If nothing else, few houses have ten people in them!) Also, note that they assume that 100% of the dirt the child eats will come from the greywater-irrigated area, 365 days in a row.
Considering that even with these wild assumptions, the risk was on the order of 1 in 10,000 of the kid getting sick, the risk is probably not significant.
Much of what I learned about microbiological water quality came from doing hundreds of tests myself (see Water quality testing (download)). I use coliscan plates and an chicken egg incubator. What this system lacks in precision is made up for by the fact that you can afford to test many more samples, so you can begin to understand what is going on. (The total setup to do 100 coliscan "easy gel" tests costs less than $200, enough for only ten lab tests on a good day).
A primary goal in the testing I do is to refine my water intuition and understanding. I'm not so interested in knowing what the exact number of bacteria is at one point in the flow at that moment. What I want to know is what is affecting bacteria levels and how they are changing with time or at different points in the flow.
Instead of grabbing one twenty dollar test from a water tank, I'll test the water the spring as far in as I can reach, at the outlet of the spring box, the bottom of the spring box, the inlet to the tank, the outlet to the tank, the kitchen tap, and in a drinking glass.
If you picked a random rural home system and did this series of tests, you'd could easily find that the bacteria levels varied by a factor of ten. One costly, highly precise test from a certified labthe standard approachlends a false sense of accuracy to the results. If you did two such tests a few hours or a few feet apart they could easily be out of agreement by a wide margin.
Numerous tests provide a rich information stream from which you can learn a lot about what makes bacteria levels rise and fall.
Once you know that, of course you can make a better system.
For potable water, the accuracy of results is more critical, and you need a more sensitive test. I use coliscan membrane filtration tests and Hach presence absence with MUG tests for design of potable systems. Then, if I need certified results, I send one last sample to a certified lab.
The numbers were jiggled so the alternative units came out to be even orders of magnitude from the conventional units and each other.
In some cases the number used was close to the middle of the range, in others it is off the average by 30% or more. Overall, the alternative measures which are represented as equivalent on the table are within a factor of two or three of actual equivalency.
This degree of precision is in line for this area of study.
The conventional measurements use indicator organisms. There is a few orders of magnitude difference in coliforms per gram of feces for different mammals, so the precedent for allowing imprecision of a large order is well established.
Here's the assumptions and math:
- These are typical numbers of fecal coliform bacteria per gram of wet feces: dog=23 million, human 13 million, pig 3.3 million, cow a quarter million)
- Since human feces and easy math are of greatest concern, I assumed 10 million fecal coliform bacteria per gram. This assumption builds into the conversion an overstatement of a factor of 1.3 if the bacteria are of human origin.
- Thus, one coliform bacteria weighs one ten millionth of a gram. Diluted in 100ml=100g of water, that's one part per billion.
- 100 m3 water per swimming pool (a typical swimming pool is more like 75 m3 or 20,000 gallons)
- An average buttwipe is 0.1 gram (based on a few measurements which averaged 0.13 grams)
- Bowel movement is 1000g (one source gave 1113g as the average daily production of feces)
- A bath is 100 L; this is about 8" of water in an average bathtub. Full capacity is about twice this, depending on the displacement of the bather(s).
- 1 turd = 10,000 buttwipes =1,000 grams = 1,000,000 milligrams
- 1 swimming pool = 1000 bathtubs=100,000 bottles = 100 cubic meters = 100,000 liters
If you want to get into these numbers more deeply, see Water quality testing (download), a download packet which includes the editable spreadsheet the calculations were done in.
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