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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Few people understand the commonly used measurements for microbiological water quality...

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.

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).

Concentration blindness

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 ocean—what 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?

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:

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Policy is being made and facilities built with incomplete understanding of hazards

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:

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Alternative measurements that broaden understanding

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.

For example:

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Examples of using these units to understand reality better

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 sink—an 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.

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Do your own tests

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 lab—the standard approach—lends 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.

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Unit derivation notes for the scientifically inclined

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:

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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