Natural Swimming Pools (NSPs)

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chem geek

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Mar 28, 2007
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As noted in this article, the first Natural Swimming Pool (NSP) has been approved in the U.S. for public use (such pools were already allowed in the residential market as there are no regulations requiring specific disinfection in residential pools -- only restrictions on registered pesticides and their claims).

Bathing Water EC Requirements

I thought I would compare the regulatory water quality requirements in the European Directive 2006/7/EC against that of the field test requirements of EPA DIS/TSS-12. Note that the European Directive does not apply to swimming pools and spas so one cannot call the bathing area that is created a swimming pool. It is generically described as "bathing water".

The bathing water is classified as "poor", "sufficient", "good", or "excellent". By 2015, all bathing waters must be at least "sufficient" in quality and any bathing water that is "poor" for 5 consecutive years will have a permanent bathing prohibition or advice against bathing. The following criteria are for inland waters. Coastal waters and transitional waters have criteria roughly half the levels indicated below.

....................................................... Excellent . Good . Sufficient
Intestinal enterococci (cfu/100ml) ..... 200 ....... 400 ....... 330
Escherichia coli (cfu/100ml) .............. 500 ..... 1000 ....... 900

The criteria for Excellent and Good are at a level of 95%-percentile evaluation while Sufficient is at a level of 90%-percentile evaluation.

Up to 15% of the samples, or one per bathing season, whichever is greater, may be ignored and not counted in the above. These are treated as "short-term pollution" and steps must be taken to warn or prohibit bathing during such times.

By comparison, the EPA DIS/TSS-12 criteria are that the standard plate count (for all heterotrphic bacteria) not exceed 200 cfu/100ml and that coliform bacteria (which would include E.coli) not exceed 2.2 cfu/100ml and that entercoccal organisms not exceed 2.2 cfu/100ml. 85% of the samples must meet this criteria.

Because of the differing criteria for % of samples that must meet a standard, a direct comparison cannot be made, but using rough standard deviation values the EPA criteria is roughly 50-100 times more stringent.

So the discussion would be as to whether the looser European criteria provides sufficient safety. Clearly it is a criteria that prevents uncontrolled bacterial growth. Neither the European nor the EPA criteria are specific to other bacteria (though the EPA does have total plate count) nor to viruses or protozoa (or their oocysts). In the U.S., the primary concern for disease transmission in chlorinated pools is that of the protozoan oocyst Cryptosporidium parvum. It is not clear how the natural pools will handle this pathogen without supplemental coagulation/filtration or supplemental inactivation.

German FLL Requirements

I also compare against the German Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V. (FLL) Recommendations for the Planning, Construction Servicing and Operation of Public Swimming and Bathing Pond Facilities. There are no regulations for E.coli for private ponds, but for public swimming ponds the limit is 100 CFU/100ml while for Enterococci it is 50 CFU/100ml and for Pseudomonas aeruginosa it is 10 CFU/100ml.

Here again, the EPA criteria for the field test for pesticide registration is roughly 20-45 times more stringent. Again, the question is what level of criteria is appropriate for sufficient safety.

Comparing German FLL to EPA DIS/TSS-12

The EPA document Implementation Guidance for Ambient Water Quality Criteria for Bacteria describes the risk level in terms of illness rates over a swimming season. The German FLL requirement for a maximum Escherichia coli level of 100 CFU/100ml corresponds to a risk level of (log10(100/17.742))/1.064 = 0.7% or 7 ill swimmers per 1000 per season while the requirement for a maximum Enterococci level of 50 CFU/100ml corresponds to a risk level of (log10(50/4.656))/1.064 = 1.0% or 10 ill swimmers per 1000 per season. In practice the geometric mean would be kept below these maximum limits, but even if I assume that these limits are 95% confidence levels, then this is equivalent to a geometric mean for E.coli of 100/10^(1.65*0.4) = 21.9 so a risk level of (log10(21.9/17.742))/1.064 = 0.086% or 0.8 ill swimmers per 1000 per season while for Enterococci this is equivalent to a geometric mean of 50/10^(1.65*0.4) = 10.9 so a risk level of (log10(10.9/4.656))/1.064 = 0.35% or 3 ill swimmers per 1000 per season.

The EPA laboratory criteria effectively require a complete kill (6-log reduction) of Escherichia coli in 30 seconds or less and of Enterococcus faecium in 2 minutes or less. The field test requirement requires at least 85% of the samples to have less than 2.2 CFU/100ml. This corresponds to a geometric mean of 2.2/10^(1.03*0.4) = 0.85 CFU/100ml and an illness rate of virtually zero (the essentially zero illness level is at a geometric mean of 17.7 CFU/100ml for Escherichia coli and 4.7 CFU/100ml for Enterococcus faecium). Real data from CDC surveillance summaries shows that this is largely the case where most illness in disinfected water is due to the protozoan oocyst Cryptosporidium parvum because it is so highly chlorine resistant.
 
I read your post and thought...I am not sure this is a good idea...then I went to the link and discovered it is in my town...not sure I will swim in it, but I think I will go take a look when they start the construction.
 
Post up some pictures when things get underway.

Maybe you can talk to the park and get a behind the scenes tour as things develop ... tout the Official TFP Moderator badge ;)

It would not actually surprise me that either the park or the pool company would want to talk it up and show you around.
 
You need to think of these things as artificial ponds. If you read the FLL (unfortunately you must purchase it, as I have), it allows for algae to be in the pool so long as it doesn't reduce visible depth to < 2 meters. The following is an actual paragraph from the standard:

In its totality, zooplankton represents an important functional unit for water purification as it eliminates phytoplankton algae, bacteria and particles from the water by means of biogene filtration and other processes. As daphnia (water fleas) are a very effective means of filtration, their presence should be assured.
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That's interesting because of the paper A colonization factor links Vibrio cholerae environmental survival and human infection. Daphnia primarily feed on planktonic algae.

One thing that is done is to keep the phosphorous level low. Phosphorous is to be kept at <= 0.01 mg/L P which is equivalent to 1000*0.01*94.971/30.974 = 31 ppb phosphate which is very low. Nitrate, on the other hand, is required to be < 30 mg/L and ammonia < 0.3 mg/L. So one of the key ways of controlling algae growth in the utilization area (i.e. where swimming and bathing occur) even if the algae is not circulated is to keep phosphate levels very low. This also helps to slow down bacterial growth. I'm not sure how they keep the plants growing in the treatment area with such low phosphate levels. They refer to phosphate fixation in the soil which helps to remove phosphates, but it seems the presumption is that phosphates are introduced by bathers at a rate that is balanced by their removal in the soil and by plants.

The other way that bather contaminants are eliminated is through water dilution at around 8 gallons per person which is basically the DIN standard of 30 liters per bather.
 
Thanks for the post ChemGeek. Also would be nice to hear from linen on progress if you decide to keep up with it.

A good friend of mine drives truck from either coast to coast at times. He said many years ago, he quit drinking the local water away from home, wherever he has been as it has given him infections & even parasites at one time or another. I know this isn't parallel with swimming water, but it did bring to mind some correlation to the antibodies our systems have & diseases they have built resistance to & are familiar with fighting. Sorry for being off topic.
 
chem geek said:
Nitrate, on the other hand, is required to be < 30 mg/L and ammonia < 0.3 mg/L.

The other way that bather contaminants are eliminated is through water dilution at around 8 gallons per person which is basically the DIN standard of 30 liters per bather.
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4

This is more than an artificial pond... It's going to be a giant fish aquarium.

The nitrogen cycle in a fish tank is essential. Ammonia from fish waste is converted to nitrites, which are then converted to nitrates. Ammonia and Nitrates are highly toxic to fish, nitrates however are less toxic. Once you get this nitrogen cycle established, ammonia is very low to (0 preferably), nitrites are 0, and nitrates on the rise.

The only way to reduce the nitrates is to make water changes (thus the 8L per person water dilution).

Introducing chlorine in any amount will disrupt this cycle. Using tap water to clean the filter media can disrupt it. You can't even introduce a sanitizer into this system. It is all going to rely on some very detailed testing for specific contaminants and if they do find them, they'll have to shut down. Bad business model if you ask me.

In an established aquarium, the fish are healthy, the plants are healthy and contribute to the nitrogen cycle, but it's still a fish tank even when no fish reside in it. They're going to have to introduce bacteria to produce the ammonia to begin the cycle, or perhaps some fish to do some eating and pooping (gravel or filter media from an established tank can speed this process). It can take 6-8weeks for the nitrogen cycle to complete so that ammonia and nitrites are low, with nitrates rising.

What I don't get here is the bacterial safety factor. E-coli is not the only bacteria that can make people sick. When one fish gets fungus in a tank, the others can get it too. If one fish gets sick, it should be moved to a hospital tank so it can be medicated, and doesn't infect the other fish. What if someone with MRSA (unknown to the person presumably) drops some puss into this pool? Pretty sure nothing short of bleach is going to kill that bug.

I'm sure all this is studied, and I am not one to be saying it's crazy because I don't know much if anything about it... but I do know fish tanks. I know how they work, and I know how they smell. Even when ammonia is low. Yuck. I'll swim in a lake or river... but I sure would not want to swim in a fish tank or a fish pond.

I also don't want other fish (people) with unknown diseases or sores, or infections... (think foot fungus) in my tank where I swim infecting me or my tadpoles.

Sorry, but I'm not convinced this will be a good idea at a commercial/public level. It's one thing to do it at home with your family in your back yard... at least the diseases are fairly known at that level. But having this system open to the world to walk into without a health check prior... well no one would put sick fish into an established aquarium, and the ones that do get added at least get a look over prior to addition.
 
The E.coli and the Enterococci are more marker organisms for general fecal discharge. Though these can be pathogenic in higher concentrations, they are mostly used as indicator organisms. The EPA Implementation Guidance... document I linked to goes through the history of how general coliform bacteria counts weren't as correlated as these two indicator bacteria to actual gastrointestinal disease reports. You are right that these aren't the only pathogens that can be in the water. Without disinfection, they are relying on filtration to remove any contaminants. So any bacteria, viruses or protozoa (and their oocysts) that gets shed can be transmitted from person-to-person just as they can in any other untreated water.

They are just figuring that it's no worse, and if properly managed and monitored probably better, than natural waters. They are trading off a greater number of short-term health effects (mostly gastrointestinal illness) against the long-term low cancer risks of chlorinated disinfection by-product exposure (and short-term irritation from nitrogen trichloride, but that's mostly for high bather-load indoor pools with no CYA).
 
chem geek said:
The E.coli and the Enterococci are more marker organisms for general fecal discharge. Though these can be pathogenic in higher concentrations, they are mostly used as indicator organisms. The EPA Implementation Guidance... document I linked to goes through the history of how general coliform bacteria counts weren't as correlated as these two indicator bacteria to actual gastrointestinal disease reports. You are right that these aren't the only pathogens that can be in the water. Without disinfection, they are relying on filtration to remove any contaminants. So any bacteria, viruses or protozoa (and their oocysts) that gets shed can be transmitted from person-to-person just as they can in any other untreated water.
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True, a local popular lake with many summer bathers has been closed a number of times in my lifetime for e-coli bacteria. That's the only time it's ever closed however. I think it's a little shortsighted to only test for one or two bacteria as a marker. Sure, fecal matter is not good but I feel that there's a lot of other infectious diseases out there that should be considered.

They are just figuring that it's no worse, and if properly managed and monitored probably better, than natural waters. They are trading off a greater number of short-term health effects (mostly gastrointestinal illness) against the long-term low cancer risks of chlorinated disinfection by-product exposure (and short-term irritation from nitrogen trichloride, but that's mostly for indoor pools).
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I do see the point, and the logic behind it. I am still wary of the bather load in a public pool, and I am not sure random population based bather load won't overload such a system and introduce more pathogens than just E.coli and the Enterococci.

This pool is likely to become very popular. A public swimming pool is often seen filled with bathers. The load on said pool is going to be extreme at times, far more extreme than any average swimming hole. It will also be a daily load, whereas public waterways often will only be full on weekends. I'm just not convinced they'll be able to mimic a true biological filter successfully in a public pool. I'm afraid this pool will be shut down more often than it's open.

I'm also curious what the water dilution source will be. Tap water is chlorinated or chloramined (is that a real word?) typically, so that rules out the city water (even small amount of chlorine can disrupt a biological filter). Either they treat the water they exchange with chlorine and then neutralize it to return it back to the system, or they dump it and get new. I'm thinking there will be less outsourced chlorine free water than treated and neutralized water used in this system but maybe they have another plan. All in all, this pool is going to be a major undertaking, with countless checks and balances, equipment, scientists, operators, and materials.

If it were in my city, I'd be wondering where the heck the money is coming from to produce such an experiment.
 
I'd still rather swim in natural waters. We have Lake Erie 10 miles away, I feel safe swimming in that, that water gets churned up constantly and it has a very rich ecosystem that includes plenty of seaweed, zebra mussels, and many types of fish.
 

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frogabog said:
If it were in my city, I'd be wondering where the heck the money is coming from to produce such an experiment.
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I am not surprised it is Minneapolis...they find money for all sorts of "unique" projects. The area where this pool is going to located is in area called "North Minneapolis" and is kind of notorious for being a less then desirable region of the city when it comes to crime and poverty (though there are some very nice areas as well there). This particular park got hammered by a tornado that went through North Minneapolis last summer (I happened to be very close to the tornado, as I was traveling through the area coming back from a son's baseball game). Where the tornado hit was particularly devastating...still looks like a war zone over a year later, since the area is generally financially depressed and heavily populated. My guess is some of the money for it must be part of the revitalization effort due to the tornado.
 
Revitalization.... sounds more like using tax money for what is an untested and probably dumb idea.

Oh, wait, Minnesota.... maybe it is really an ice skating pond in disguise, before long it will be closed due to bacterial and parasite issues and only be open for use once winter starts?

I kept "Natural Aquariums" also. Plants, fish, snails, shrimp -- beautiful but a lot of work to keep fertilization and lighting high for lush plants without stressing out the fish. Then I got into discus and planted tanks --- still afraid of eating sushi after seeing so many discus die of parasites.
 
The FLL guidelines give a formula to calculate the bather-load capacity based on the type of regeneration system. The formula is as follows:

N = (1/k)*(VT + VF + A*q)

N = Nominal visitor number in persons/day
k = Person-related thinning factor in m3/person
VT = Regeneration-capable water volume of the utilization area in m3/day
VF = Daily filling water feed in m3/day
A = Treatment area in m2
q = Raw water infeed into treatment area in m3/(m2 x days)

A sample calculation is the following:

k = 10
VT = 3000 m3/day
VF = 20 m3/day
A = 1500 m2
q = 2 m3/(m2 x days)

so N = (1/10)*(3000 + 20 + 3000) = 602 nominal visitors/day

The utilization area is fed a daily volume flow of N*k and is equal to the volume in the utilization area that is able to be cleaned (regeneration-capable) plus the daily fill water (continuous drain/refill) plus the volume fed through the treatment area.

So the bottom line is that the size of the utilization area is irrelevant in their calculation. They instead assume that a volume of water proportional to bather load needs to be treated or replaced. This is where that assumed "k = 10" comes from where basically the steady-state bacterial concentration (independent of utilization area) is the amount shed by one person in 10 cubic meters (2642 gallons) of water. Technically, it should be something like person-hours, but they just figure some average swim time and the bacterial introduction is uneven anyway (i.e. more when one first gets in).
 
I heard on the news about a swimming lake being closed nearby in Washington State. While searching for something linkable online I happened to see a google result of recent beach and natural water area closures.

All due to bacteria, of some kind. It's interesting and relevant that the only way to sanitize such an area to make it safe is to wait. Thus, closures.

Now, insofar as this local closure the only reason they even knew about this particular bacteria is because people got sick. The first case of infection was back on the 17th, and the water was closed today. They traced it back to the lake. Countless people have been exposed since then.
http://www.columbian.com/news/2012/jul/ ... -bacteria/

Various bacteria that cause closures of public natural waterways including ocean beaches are not uncommon, apparently. Most are the result of fecal matter containing the bacteria, it seems. A common thread is that people get sick, and then the water is closed after specific testing. People forgive natural waterways for such thing, and will return after the closure is over.

I'm still not getting exactly why this needs to be done. I see no way to sanitize this pool should something infect it, thus it will have to close and wait any time someone infects the water. Calculations aside, the treatment plan doesn't seem to allow for bacterial infections. While I haven't read all the literature in depth, I also don't see a comprehensive testing program. Did I miss something?

If people have to get sick before this pool is closed, it's popularity will decline at minimum.

This is something like some crack-nut would propose around here. I'm all for natural stuff, really... I like it but I don't trust it.

At least with a chlorinated pool, bacteria is killed. Most pools don't have to close down and wait for bacteria to go away, and they don't have to test for any specific bacteria because as long as the chlorine is high enough, nothing can live. I'm sure relatively few people have been infected with bloody diarrhea, fever, and cramping abdominal pain after swimming in a chlorinated pool.
 
The CDC Surveillance reports are consistent with what you describe. Chlorinated pools pretty much only have Crypto unless they are poorly maintained. You get all kinds of other illnesses and even some deaths from the untreated water venues.

As for testing in these "bathing waters" or "ponds", the FLL specifies that they test for the three bacteria I listed twice a month. If the limits are exceeded, then measures must be taken, mostly seeing what is wrong and trying to fix it. If they are unable to find the source of the problem or to fix it, then "swimming and bathing in the affected facility must be prohibited."

They really are counting on biological filtration/removal of pathogens so if there is some sort of discharge, it basically has to get handled via such filtration. What will be interesting is to see how Crypto is handled. They could add some coagulation/flocculation with filtration to try and reduce it, though it would take time. The CDC is proposing in their Model Aquatic Health Code (MAHC) to use UV or ozone to handle Crypto, at least in high-risk venues (in chlorinated water, of course).
 
They could probably use UV or ozone as a supplement -- they just don't mention that in the FLL "standard". With the ozone, they'd have to have outgassing tanks or make sure the output didn't go directly into the treatment area since any residual ozone probably wouldn't be good for the plants.
 
On a smaller scale

Well I decided to build one in central FL, I begin construction in few weeks.

The recharge area will be a pond adjacent to the pool with water moving between the pool & pond 24/7.

A 25' 4" diameter slotted drain hose (with mesh sock) will be coiled 10" below the pebble bottom of the pond and water will be drawn from that at a prescribed rate discharging thru a water fall or river type return to the pool.

The pool is 10x20 and the pond 8x11, it will be 18" deep with plants no fish what so ever.

The system will have a UV sized to eliminate the bacteria & protozoa ( still working on that unit)

If anyone is interested I can detail the build if the mods will allow it.
 
Nova13,

Feel free to post your progress. It will be interesting, I'm sure

What the forum took exception to was your apparent interest in arguing the merits of a "natural" system vs. a properly sanitized pool.

That subject has been beaten to death here and elsewhere with pretty conclusive results.....we saw no real need to revisit it.
 

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