Nice post, JFN,.....as Homer would say, "stale crackers....mmmmmmm".
I have never quite been able to understand why this discussion on phosphate remover is so controversial. The results seem negligible.
Let's say the average pool has 50 CYA so 7.5% FC (3.75). Reducing the FC/CYA ratio to 5% brings the FC to 2.5 ppm, right?
Taking an average daily loss of 30% of the available FC (maybe a bit high) means this lower 5% ratio saves .3 ppm in FC daily. Let me repeat that for emphasis...... .3 ppm FC savings. To me, it has no practical merit.
There seems a passion for convincing others it not only works but has real use in the real world. I don't see it....at least, not yet.
Well, I don't think it's controversial at all (is just basic science and some math) and I don't think the results are negligible for everyone. So let me see if I can make a convincing argument...in a
dispassionate way.
The first point to make is that phosphate levels have little to do with daily FC loss. Daily FC loss is, for residential outdoor pools, entirely dominated by UV photolysis of the active chlorine compounds in pool water, specifically the hypochlorite anion (OCl-). But let's just take a step back and map out all of the things that contribute to FC loss as it is important. Here's a "fancy" diagram (a little fuzzy from shrinking the original) -
So UV light, chemical oxidation and biological disinfection are the main sources of FC loss in a pool. In a clean pool, UV loss dominates and this is why outdoor pools MUST use cyanuric acid (CYA) to stabilize the chlorine or else it would dissipate so rapidly that chlorine disinfection would not be economical. The next most important factor is bather waste oxidation and that is typically easy to handle as there is more than enough FC available to oxidize bather waste. The least likely contributor to FC loss is actually biological contamination. However, biological contamination is the thing that is most worrisome because, while the other two factors are a constant and known load on the FC loss rate, biological contamination has the ability to multiply rapidly and can quickly consume all of the chlorine in a pool. Thus, the FC level we use in pools is chosen based on a ratio of FC to CYA in which there is enough active chlorine (hypochlorous acid) to properly disinfect the water. In my simple diagram above, I added a dashed line between the boxes for "UV Light" and "Disinfection" to denote that the two are linked because of CYA. So we add enough CYA to the water to protect the chlorine from UV photolysis while, at the same time, increasing the FC concentration so that the available active chlorine level (hypochlorous acid) is high enough to disinfect. TFP typically uses a Target FC level to produce an FC/CYA ratio of ~7.5%.
But here's the thing to remember, the FC/CYA ratio is not some rigorously derived, absolute value. It's a statistical quantity that was derived from experience and some data on pathogen kill rates (Ben Powell's "Best Guess Chart" and Richard Falk's chemical analysis of chlorine/CYA buffer chemistry). That is to say, TFP
recommends a value of 7.5% but that is not absolute. There are lots of pools that can operate cleanly and safely with FC/CYA ratios below 7.5% and there are some "problem-child" pools that seem to need a ratio that is a little higher (maybe 8% or 9%). So the 7.5% rule in itself has some wiggle room that is associated with the particulars of the pool's local environment and setup. Another clear example of this are pools that use salt water chlorine generators (SWGs). Anyone is free to look at the TFP website for the
Chlorine/CYA Chart and what will immediately standout to them is the fact that there are two different recommendations on that page. For manually dosed bleach pools, the Target FC value is based on a 7.5% FC/CYA ratio. For SWG pools, the FC value is based on an FC/CYA ratio of 5%. Right there is a 33.3% difference in the amount of chlorine needed to operate a bleach pool versus an SWG pool and it's based on nothing more than the equipment configuration of the pool.
Still another example of
potential chlorine savings comes form the use of borates. Now people will say that borates are algaecides but that is not quite right. At the concentrations TFP typically recommends using borates at (50ppm), the boron in pool water acts more like an inhibitor of algae growth as opposed to actively killing algae. But, borate users will often find that their FC seems to hang around longer or, in my case, the FC seems to rise a bit when the SWG is left at the same output and the only difference is borates. Now some of that may be a boost in SWG cell efficiency but it could also be caused, in part, to a lower biological load. In my own experience with borates, I noticed last season a few times when my pool water FC was below the minimum value recommended and I did not suffer any harm (cloudy water or increased FC demand).
So will phosphate removers do something similar. Well, that's hard to say. It's a fact that phosphates are absolutely necessary for biological life (both plants and animals need a source of phosphates to live). Algae is essentially plant life and, as any gardner knows, there are three absolutely essential nutrients to fertile soil - nitrogen (nitrates & urea), potassium (as potash) and
phosphorous. All three are required or else nothing grows. The same is true in aquatic plant life - without phosphorous, water is essentially sterile to life.
So all of that blabbering is to say this - phosphate removers are not important to saving the pool owner on daily loss rate (that's controlled by CYA). The real savings for phosphate removal comes in with the overall FC/CYA ratio. Phosphate removers can possibly allow the pool owner to go from having to maintain an FC/CYA ratio of 7.5% to some lower value, maybe half that, maybe not. There's no easy way to say before hand what the savings would be except through direct experimentation. But if, after experimenting, you find that your FC/CYA ratio can be lowered by 25%, then that's 25% less bleach needed over a entire season. Is that worth it? I dunno, depends on how much your bleach costs.
Now one argument against phosphate removers would be, "
well, you're just trading the money you saved on bleach and using it to buy expensive phosphate removers." Well, true, there is some cost shifting based on the phosphate remover chemicals. However, as I posted previously, most people with a reasonable phosphate load and no constant input source of phosphates (municipal water treatment, HEDP sequestrant use, etc) would not have to spend much more than ~$40 on a 1 quart bottle of commercial grade phosphate remover (HaloSource SeaKlear or Orenda PR-10000). A typical treatment for a standard sized pool runs about 8oz or less. So the 1 quart bottle has roughly four treatments in it and, without an incoming phosphate load to the pool, one treatment is usually enough for a full year. So, in rough numbers, you'll spend about $10 on phosphate remover per year. Again, does that save someone money, I can't say you'd have to experiment.
To that end, I plan on experimenting this season. I'll keep more detailed records than last year and we'll see how it goes.
Regards,
Matt
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