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Key Takeaways:

  • Biofilms are resistant to chlorine and can significantly increase chlorine demand.
  • Biofilms may increase the rate of creation of some disinfection by-products.
  • There is controversy over whether bacteria can be killed by chlorine before forming biofilms though in residential pools and spas this appears to be the case based on a lack of symptoms.
  • There are questions about the frequency at which biofilms form in sand filters in high bather-load pools such as commercial/public pools.


Biofilm is an aggregate of microorganisms in which cells adhere to each other and/or to a surface.  For our purposes in pools and spas, biofilms are on surfaces and excrete chemical compounds (such as alginate slime) that make the microorganisms harder to kill from sanitizers such as chlorine.


Large colonies of bacteria in biofilm can significantly increase chlorine demand and may result in higher disinfection by-products such as nitrogen trichloride (trichloramine).


There are questions (well, at least I have questions) about whether planktonic (free-floating) bacteria are able to be killed quickly enough in properly chlorinated pools and spas before biofilms can form and the effect of different filter types, especially sand filters, on biofilm formation.  The posts in this discussion thread relate to such issues as well as others regarding biofilms.

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I am moving the biofilm part of the discussion from the "Cartridge Filters - Thumbs Up Or Thumbs Down?" discussion into this thread.  I will then (later on) summarize the main points/conclusions/disagreements in the description section for this topic.
The concentration of chlorine used in swimming pools is not sufficient to oxidise bacteria growing on any surface in contact with the water. A slippery tile is not body fat but alginate excreted by bacteria. Bacteria urease enzyme convert urea to ammonium, the ammonium reacts with chlorine on the biofilm surface to form nitrogen trichloride.

Autotrophic bacteria in combination with ammonium and phosphate will use carbonates to produce more bacteria, organic matter and alginates that react with chlorine to form THM`s. 1g of nitrogen will allow the production of 10 g of organic carbon, and it happens quickly, the doubling time of heterotrophic bacteria is only 15 to 30 minutes, autotrophs are slower but they will immediately manufacture alginates when given a nutrient supply.


Just to be clear, you aren't saying that chlorine isn't strong enough to kill planktonic (free-floating) bacteria including that shed from skin, mucous, fecal matter, etc., nor are you saying that biofilms could form underwater or elsewhere where chlorine is circulating because the bacteria would be killed before that could happen.  What you are saying is that bacteria on solid surfaces above the water line (typically on the tile above the waterline) could form biofilms and then what...creep down below the water line creating a larger chlorine demand?  That's analogous to finding algae in the skimmer above the water line and on the lid surface, but not finding it in the pool.  Does physical brushing of the tile using a standard pool brush disrupt the biofilm at all and is that why such weekly maintenance for residential pools is usually recommended?


The slimy tile is sometimes felt if it isn't brushed regularly, but I've never noticed it progressing below the water line.  The closest thing to that is a surface with only a thin layer of water at the top of the ramp in my pool (shown here).  Then again, I'm pretty regular at weekly brushing, but many people reporting on the pool forums aren't so meticulous yet there are virtually no reports of slimy tile problems or slippery steps, etc., at least once chlorine levels are maintained with a high enough FC/CYA ratio to prevent algae growth.  Also, a more realistic example would be spa users since residential spas are high bather load.  There doesn't seem to be any ongoing change in chlorine demand that persists between water changes and I would presume that any biofilms would stick around between such changes.  There is an increase in chlorine demand over time, presumably from the buildup of some slow-to-oxidize organics, but this goes away after a water change.  Also, maintaining the proper FC/CYA ratio by using Dichlor-then-bleach has the water last twice as long (double the standard WRI formula) before the chlorine demand and water quality require a change compared to Dichlor-only that tends to get dull/cloudy much faster (due to CYA buildup that lowers active chlorine levels so slows down oxidation rates).

FYI.  This interesting thread just popped up at Trouble Free Pool (TFP) that gives a real-world example to some of the points you are making about the problems with sand filters and extra organics (biofilms, algae) with chlorine demand.  I'm puzzled by his outdoor pool in the sun with low CYA and low chlorine loss -- never seen that before except, of course, when there was minimal sunlight or an indoor pool or the pool was covered with a mostly opaque cover.

Reply by Howard Dryden on January 19, 2011 at 8:25am

Individual bacteria will be killed in under 30 seconds by free chlorine at 0.2 mg/l at a pH of 6.8.  It is a requirement under DIN specifications that any oxidising agent used in a public pool must comply with this standard.

However above the water surface it is very easy for bacteria to become established. Below the water line, on the inside off pipes, and the surface of sand it takes a bit longer. Through natural selection bacteria that produce copious amounts of alginates such as pseudomonas species will be the first to colonise.  The alginate protects the bacteria from oxidation, and then other bacteria such a Legionella or MRSA can take up residence.  All you need to do is take a swab of any surface and you will find bacteria. There is absolutely no possibility of chlorine preventing the growth of bacteria on a substrate. Indeed we find we have more problems with bacterial coagulation of sand in chlorinated system that we do in non-chlorinated systems because through natural selection the bacteria in chlorinated system produce more alginates that blind the filters.

There are two main groups of bacteria, the heterotrophs use organic matter as a food source, these bacteria double every 15 to 30 minutes and the only parameter limiting their growth is the availability of food, not the level of chlorine.  The second group are the autotrophs that use carbonates as their carbon source. The autotrophs are slower to grow but after a few months they become established in most pools, and this is why the chlorine demand and concentration of chlorine reaction products gradually increase as they start to manufacturing organic matter, it is not due to an accumulation of organic matter.

Howard and Richard,

Great incite on a complicated and controversial topic. It’s like anything else, filter choices have trade offs, and now we are getting into the real nitty-gritty of just what some of those trade offs really are. It’s not just about the water usage and savings, it’s not just about smaller mechanical room footprints that incorporate smaller filters and higher flow rates, it’s not even about what filters you end up using, as they all have their own set of advantages and tradeoffs.


Since chlorine is the standard for use in our pools, it is important to know what effect that really has on us, since we as patrons and swimmers, are ultimately the real cause of the various issues we have with water and air quality. 

To combat this dilemma, there are 2 models that have developed in its treatment, both of which have their own set of tradeoffs to deal with.

  • The US model opted for smaller mechanical room footprints, higher flow rates, less water usage. The cost of building our pools went down due to the smaller footprints, tighter building envelopes, all saving us in overall water usage, heat, chemical, and building costs.
  • Whereas the European model opted for larger mechanical room footprints, slower flow rates, flocculation and coagulation, and more extensive backwashing procedures that included air scouring. It’s the basis of the German Din Standard 16943, that many European countries are mandated to use.

Is one model better than the other…I don’t know?  If we and chlorine are the culprits, then the Din Standard allows for much lower levels than we use here, yet they still have many of the same problems that we face. Both ozone and UVc were championed in Europe, and brought here. Both of these band-aids have their own issues. UV is much more popular here than ozone, although that wasn’t the case 10 years ago. Here’s a link that offers an interesting discussion of filter types.


One of the common threads in both models appears to be the biofilms, and whether or not they are really an issue in well-maintained chlorinated pools. As Richard stated, “slippy tiles” is not something we see that often…but that doesn’t necessarily mean it’s not a problem. “Slippy tiles” is when the pool is getting pretty bad, but to say biofilms can’t form in a well-maintained pool is an overstatement, as there is enough research out there that proves otherwise. The issue shouldn’t get lost in worst-case scenarios... that just have a tendency to turn people off. Many pools are somewhere in-between, and most of us don’t even realize that we have biofilm, only because we don’t t really understand what it is, or what the fuss is all about .

We are never going to get rid of biofilm, and all the new alternatives that are popping up are just a means of managing it maybe a little better...(some real good ones that offer promise, and some that are more kindred to snake oil than anything else). Biofilms are a topic that is being discussed more and more. In fact, it wasn’t that long ago here on PGN, via Rex and Alan Lewis discussed it.

To help clarify what biofilms are all about to those of us who are less knowledgeable, here are some links:

 The Slimy Truth About Biofilm


I have a couple of other links that are applicable to this discussion, and both were presentations at the 2009 WAHC. One is on filtration and cryto, and the other is Chips discussion on urea and chlorine. Because they are attachments, and I don’t know how to put them here, I will send them to both of you by email.

My basic thinking on biofilms and remediation products is very similar to that of phosphates and phosphate removers.  You can't have the problems of biofilms without bacteria and you can't have the problems with algae growth without algae since in both cases there is no spontaneous generation.  If you already have a serious biofilm problem, then sure, you need to deal with it, but using chlorine to kill bacteria BEFORE they form significant biofilms is similar to using chlorine to kill algae FASTER than algae can grow even under ideal nutrient conditions.


The phosphate remover folks sometimes use an analogy of having wood (fuel) feeding a fire (algae growth/bloom), but you can't sustain a fire regardless of how much wood you have if it is constantly being sprayed by water (chlorine) to put out any nascent flame (new algae spores).  Similarly, you can't form biofilms if bacteria are killed before they can be formed.


I agree that most pools are in-between in terms of biofilms and algae growth, but at least for the low bather-load residential pools, chlorine alone has been proven to work when chlorine is understood properly (i.e. FC/CYA ratio).  Of course, if one lapses with sanitizer levels or has poor circulation or a lesser filtration medium in a pool with lots of nutrients, then watch out -- all hell breaks loose since all those organics and nutrients will have bacteria and algae growing very rapidly.  This happened in my own pool (3000+ ppb phosphates) when I misjudged chlorine usage during a spring solar system startup that I describe in this thread (see this post for a chronological summary) where the bacteria/algae converted CYA into ammonia creating a HUGE chlorine demand.  So controlling organics and nutrients are certainly a decent insurance policy, as would use of supplemental growth inhibitors (algaecides, borates, copper/silver ions), but they should not be seen as absolutely required.


Commercial/public and other high bather load pools present far greater challenges.  Nevertheless, the experiences in residential spas, some of which are high bather load, indicates that they too can be controlled by chlorine alone after removal of biofilms and organics as in new spas by treating them with Spa System Flush or equivalent.  I will admit that I am a mere neophyte in my understanding of commercial/public pools though I'm learning more over time.


It's interesting you bring up the U.S. vs. European models because I've been looking at their typical disinfection by-product levels and though the European ones are lower, sometimes by half, they aren't very low considering the rather dramatic efforts being taken.  I'm talking about comparing reasonably maintained pools using each technique.  Obviously, one can go through the roof with DBPs in improperly maintained pools (including air ventilation, not just water quality).


Anyway, we've wandered away from the topic of this thread which is cartridge filters.  To tie things back to that, it seems to me that the problems with channeling and biofilm formation in many sand filters doesn't occur as readily with cartridge filters, though the issues of organic buildup and potential DBP production if not cleaned frequently probably still exist.  Also, coagulation/flocculation could still be used as can adjunct media like Fiber Clear that others have mentioned.

This paper shows that even maintaining 1-3 ppm FC (with no CYA) in a simulated pool, biofilm was able to form on coupons and in a sand filter.  Shocking for 16 hours with 10 ppm FC weekly eliminated any buildup of biofilm.

Though this sounds bad on the surface, the devil is in the details which is, of course, what I always try and look at.  The bacterial inoculum and the bather insult were equivalent to 20 bathers in 7000 gallons per day, but the inoculum and insult were added 3 times per day, not continuously (this is important since it can allow more clumping of bacteria to shield them from chlorine longer).  Nevertheless, compared to a control tank that had no sanitizer, the chlorinated tank had a 5.53-log10 reduction in biofilm on coupons and a 6.70-log10 reduction in biofilm in the filter.  Clearly, chlorine greatly inhibits biofilm formation.


At the conclusion of the 30 day experiment, the accumulation of biofilm was 0.57-log10 (which I take to mean 3.7) colony forming units per square centimeter (cfu/cm^2) while in the filter it was 219 cfu/g.  For comparison, the strict EPA DIS/TSS-12 standard requires general plate counts to be <= 200 cfu/ml while coliform bacteria shall not exceed 2.2/100ml (up to 15% of collected samples can exceed these requirements).  To convert the surface biofilm to volume, I'll use a volume to surface area ratio of 2.5.  If the entire surface of biofilm was distributed into the volume of the pool, this would be around 1.5 cfu/ml or 150 cfu/100ml so would far exceed the EPA standard for coliform bacteria.  However, actual bacterial tests in pools (from the Pinellas County, Florida study, for example), show < 10% of chlorinated pool samples exceeding the standard.  This means that if there is biofilm there, it is usually staying on the surface.


So what does this mean?  First off it shows that the amount of bacteria in biofilms on pool surfaces in chlorinated pools is very low and explains why there isn't a high chlorine demand or other problems seen, at least from pool surfaces.  A colony forming unit is a single viable bacteria.  Yes, ONE bacteria.  The experiment showed that with chlorine there were on average only 3.7 viable bacteria per square centimeter on surfaces.  Sand filters, on the other hand, are worse.  In a 17,000 gallon pool, the experiment's surface biofilm would be a total of 3.2 million cfu's.  If I assume a 200 pound sand filter, then that is 19.9 million cfu's.  Even as large as this number sounds, it's still not enough to create any perceptibly noticeable chlorine demand.  The experiment running for only 30 days simply wasn't long enough for the sand filter to develop the sort of channeling and far more extensive biofilms sometimes seen.


So I should have qualified my original statement by saying that in properly chlorinated pools, minimal biofilm is formed on surfaces.  I should not have implied that no biofilm at all is formed.  In practice, there is no difference since there is no noticeable effect on chlorine demand or other effects (again, when properly chlorinated and using better filtration media than sand or making sure the sand doesn't channel).  Personally, I wouldn't call 4 bacteria per square centimeter any sort of "film".  More like some bacteria that briefly got lucky enough to get to a surface before getting killed by chlorine, but that aren't able to grow uncontrolled.

So .......... Is this insinuating there is little to no build up in cartridge filters and DE filters? It would seem never disrupting the media surface on a cartridge would allow quite a nice build up, the longer between cleanings, the more the buildup. Bumping the DE filter would disrupt buildup any but then again simply allow much to pass thru until it is recoated.


Also coming to light is the fact that it might not be the sand filters, but the fact that they are more often than not undersized for the flow. I see it all the time. To me it still remains the easiest filter to maintain and therefore will most likely be maintained. Cartridges get left too long and the either the flow suffers or the bypass allows much to get by. DE could be nice aside from disposal issues and the fact that proper operational and maintainence must be followed or they dont live up to their potential.

It all comes down to circulatory flow and the ability to get consistent exposure to chlorine.  So any filter medium that gets low to zero chlorine exposure in some area could become a problem.  You are right that cartridge filters that go longer between cleanings build up more organic substances so provide a nice nutrient bed for bacterial growth, but I don't know if or when that becomes a problem.  It sounds like it would be an issue, but you don't hear about this as much with cartridge filters for whatever reason (perhaps there is still fairly even flow even through the nutrient bed).  I do think that the lack of frequent cleaning does have the potential for more disinfection by-product production, but that's just speculation on my part.


I don't know why sand reports more problems.  It could be undersizing of the filter (too fast a flow that makes it uneven) as you say or it could be the nature of the sand particles to more likely clump.  As for DE, I just saw a presentation on this regarding filtration for Crypto that Al sent to me that talks about bumping DE (stopping flow to filter) but that it doesn't filter Crypto as well after that unless you backwash before recoating.


I think that generally there is this rush to the "next big thing" be it removal of phosphates or controlling biofilms rather than taking a look at whether a design solution could reduce the problem, such as understanding FC/CYA in the case of algae control or setting up a stirring/churning mechanism or processing sand to avoid clumping to reduce biofilms.  I'm not disparaging phosphate removers or crushed glass filters since they are real technologies with benefits, but I see these things more as a spectrum of choices with pros/cons.

Howard, Al and I continued the biofilm discussion off-line that I am now adding here, summarizing some portions where we went back and forth a bit to get to a common understanding.  (This will take some time so I'll get back to this later...please be patient).

I now have a chance to add the E-mail discussion Al, Howard and I had on biofilms.  I'm excerpting portions since we went back and forth and clarified some things so I'll try to summarize.


Al wrote:

I think his [Richard's] very last post misses the point of the discussion on bioflms in general. It’s not about technology or jumping to the newest thing. It’s about incorporating a multifaceted approach, using layers of protection.
And yes Richard, circulation patterns do play a role in sand filters, based it on high rate sand, and pattern flow rates through horizontal filters, which have become so popular here in the US for smaller footprints as they are oftened stacked. As for cartridge filters, there aren’t that many on large commercial pools. Starite was loop mainifolding 6-8 System 3’s for a while, and there’s always Harmsco, stacking 600 to 900 filters, but those are all on a decline. Mostly on pools over 100,000 gal, it is horizontal sand, some vacuum DE, and on the rise is the regens.
Did you guys read the article I posted on Mount Olympus Water Park? The part I wanted to bring out was toward the end where they discuss some of the difficulties of the regenerative filters that James Ambergey brings up. That bumping a filter may not be that beneficial, and that captured crypto could get back into the pools.


Richard responded:

I’m all for a multi-faceted approach with layers of protection.  I just want people to go to such approaches with facts, not generalities.  I don’t want people to use phosphate removers because they think they can’t (normally) control algae with chlorine alone and I don’t want people to go to a new filter technology not knowing that perhaps they could use existing technology if done properly (slow-rate, for example).  They can certainly opt for the newer solution, as insurance or an extra layer of protection/prevention or as something that works better or is more practical.  Sorry I didn’t make that clear.


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