Tech Talk: Superchlorination – The Basics of Breakpoint Chlorination

The original intention was to have this PrP issue cover only the weird and the wonderful about problem superchlorination. A hasty poll of some of our readers suggests, however, that we might aughta' start at the beginning so we're all tuned to the same channel. Let's do:

When we talk of breakpoint chlorination, we're talking about superchlorination. Breakpoint is the less customary yet more correctly descriptive name for the subject at hand. Although some consider the two to be different, we'll use them pretty much interchangeably.

Let's define superchlorination, if we can. There are many descriptions of "superchlorination," most designed to remain mysterious and to keep the process thought of as magical – a necessary routine your pool can't live without. Superchlorination has been compared to fire, burn-out, overkill, boiling, sterilization, even electrical shock, in attempts to give pool owners a sense of what's going on while keeping the mystique intact. And the term "shock" is even harder to grasp, as some "professionals" consider it to be a super super-chlorination to lofty residual heights in order to do everything from condition the water to confuse the algae. Still others say it's a milder version of over-chlorination – more of a maintenance thing. Some use it interchangeably. This author thinks it's simply a term designed to create a "different" process in order to sell more products; he avoids the use of the word "shock" altogether.

Superchlorination has been used to cure stubborn algae, unseen causes of eyeburn, stains, turbid water, odors, bad tastes... to sterilize filters, fix water hardness, prevent AIDS, cure paint, and soften water. It has been blamed for green hair, plaster failure, disintegrating bathing suits, rashes and earaches, even the loss of suntans. And it has also simply been "what we do every Saturday morning", with no better reason at all.

`Truth is, superchlorination was designed for one thing alone, the eliminating of the offensive and unwanted ammonia compounds of chlorine. Secondarily it serves to do damage to any establishing algae colonies, but this is a side-benefit that a professional pool operator shouldn't have to deal with anyway.

Of course the process is necessary for all pools, right? Nope, not necessarily. A well, managed, well outfitted pool (under admittedly better-than-average, fully predictable conditions) may almost never have to superchlorinate. OK, it seems like wishful thinking, but it has been done.

And if a pool utilizes non-chlorine sanitizers, then the whole discussion is academic because superchlorination is not needed, right? Nope again; it becomes very important to consider superchlorination when bromine, ozone, peroxide, ionization and most other alternatives are used, as you'll see elsewhere in this or in an upcoming PrP issue. In these cases, what amounts to the side benefits in chlorinated pools – handling algae, organics, even some pathogens – are no longer simply bonuses but may become the primary reasons for superchlorinating non-chlorinated pools.

Superchlorination, then, is narrow in purpose and, if necessary at all, critical in importance. Let's look at what it is we're getting rid of, why we need to superchlorinate, what happens and how to do it.

Look at ammonia compounds of chlorine. In earlier issues (and in your certified courses) we examined chlorine and its detractors, one being created by the combining of the good-guy chlorine compound (HOCl) with ammonia. These ammonia compounds of chlorine are also called chloramines, combined chlorine, the bad guys and even some non-printable names you're free to recall or imagine. Chloramines are lousy sanitizers and oxidizers, have an offensive chlorine-ish odor, irritate eyes and mucous membranes, don't do much to preclude algae, waste otherwise effective chlorine, and simply cost money.

You know where ammonia comes from, right? Common sources are urine, sweat and decomposing organic matter. The first, urine, can be minimized with good education and rules, handy bathrooms, time-out breaks, and maybe removal of the coffee machine. Ridicule sometimes helps, but is not recommended. (Have you ever heard of that chemical additive for the pool water that shows a tell-tale red cloud around the guilty party...?) The next two, perspiration and decomposition of everything from sluffed dead skin to leaves, dust and small dead animals, are even less preventable. Of course the coach can give easy workouts and outdoor pools can be fenced or moved indoors but, the truth is, "nitrogenous substances" are pretty unavoidable; with the pool used by those nasty beings called humans, ammonia is here to stay.

Technically, there's both organic and inorganic ammonia – and the former is tougher to deal with than the latter. Let's keep it simple and lump them together. We'll almost always get the results we want; that's safe, comfortable water.

Ammonia is represented chemically by the term NH3. It is the balanced combination of N, nitrogen (the most common, in-every-breath gas on earth), and H, hydrogen, (the lightest gas, that highly flammable one). As a liquid, it is sold for use as a household cleaner and for other purposes. As a chemical produced or released into pool water, however, it becomes a pain in the pumproom. Ammonia produces at least three separately named compounds when in the presence of small amounts of chlorine: monochloramine, di-chloramine and tri-chloramine.

It's fun to dazzle folks with these three names, but we're perfectly safe and probably more correct to generalize and call them all just chloramine. Tri-chloramine, said to form, ironically, during the attempt to rid our pools of this very family of problem compounds, might require special treatment – a focus of the article below. White's Handbook on Chlorination says that di-chloramine and especially tri-chloramine require low to very low pH values for stable formation so, if we're lucky, it's really only monochloramine we're dealing with anyway.  In any case, this combined chlorine is tough on eyes and mucous membranes, so we'd like it to just go away.

Would you say that eyeburn is the biggest water-related concern of your swimmers? (OK, second to temperature...) While dozens of things are blamed for common swimmers' eyeburn, chloramine is so much the dominant contributor to this classic problem that hardly anything else is worth addressing. There's really no better indicator of the need to superchlorinate, nor better reason to do so, than the almost inevitable eyeburn complaint. In a brief hour or two, successful superchlorination can render innocuous – that is, non-irritating – pool water which has been producing roadmap eyes in two laps of freestyle. It can tame a staggering chloramine odor in an equally short time. (For a very large pool or one indoors, you'd better set aside a day – reasons discussed later.) A half-part-per-million combined chlorine could easily be as offensive as this description; that water needs the "cure" of superchlorination.

In order to clean up the problem, the value of chloramine remaining following this super-chlorination process should be zero. And zero is the desired value to maintain. While you've all been through this at your pools and your schools, let's review how it works and when it works – for those who want to brush up.

Techniques of Superchlorination: You may have heard of, or even practiced, the routine, calendar-scheduled superchlorination with a given amount or chlorine product or to a standard target value – say each Saturday, to 10 ppm. You should already be using the more "scientific" – or at least more reasonable – method, using only what you need when you need it. Let's decide upon a typical sample set of conditions which will indicate the need to superchlorinate, and calculate the level of chlorine to shoot for.  And let's decide right now to skip superchlorination altogether if the pool doesn't need it!

First we must decide when it's time to start heaping the chlorine into our pool, and then plan the work carefully to fit the schedule of programs on the facility's calendar. Leaving the psychology of scheduling, down-times and priorities up to you, let's at least stress that the pool must be closed during most of the process.

When taking those routine test-kit readings, a skilled operator frequently notes the total chlorine readings. Ideally they are the same as the "free" readings, indicating an absence of combined chlorine. If, however, a little insidious chloramine has begun to accumulate, he or she will carefully observe any rise in the combined chlorine value over a period of days (maybe even weeks) until it gets to a point of intolerance for you or your swimmers. (If you are a little slow in noticing, they will let you know...)

More judgment is required here, as everyone's got a different idea about when, in terms of a no-longer-tolerable quantity of chloramine, to superchlorinate. There's a widely agreed-upon .4 ppm threshold, where eyeburn complaints seem to rise rapidly, but the number is quite objective. The following example uses that value.

1.4 ppm   Total   -1.0 ppm   "Free"     0.4 ppm   Combined    X 10 = 4 ppm target for breakpoint

Simplified Superchlorination Curve, Breakpoint Example

In the above figure, the example shows the .4 ppm difference between 1.4 total and 1.0 "free". The simple rule that seems universally accepted is to multiply the chloramine value times ten. You'll arrive at the target level to which you should elevate your water's chlorine residual.

So ten times .4 is 4 ppm, the goal.

The chlorine required to reach that level is easy to calculate by proportionate dosage. Remember the 120,000 gallon pool which contains 1,000,000 pounds of water? Divide that 120,000 into your pool size to get the "pool size factor"; you'll then know how much pure chlorine to add to the pool. We say "pure", because only gas chlorine can be figured pound for pound. Calcium hypochlorite (granular) needs to be used at a rate of 1.6 pounds equating to one pound of gas, while sodium hypochlorite (liquid) requires one full gallon (over eight pounds) for the same result.

In a 230,000-gallon example pool, the pool size factor is just under 2. (Don't, for heaven's sake, use 1.917; none of our pool measurements supports such false accuracy.) The chloramine level times 10 was 4 ppm, so 2 (the size factor) x 4 (the target) = 8 pounds gas chlorine or equivalent.

The operator could use eight pounds of gas if available, however it takes many hours to get it in there during which time it's dissipating – not a very good technique. (At a 50 pounds-per-day feed rate, one-fourth of that 50 – maybe 12 pounds – is fed in one-fourth of a day. That might work to leave the eight pounds you need if done at night, but even the 12 pounds fed in six hours will never reach the desired 8 ppm during a sunny, hot day.)

If the operator elects to use liquid chlorine, often a wise choice, it works gallon-for-pound (if it's fresh, 12-ish percent) so eight gallons could be added directly and quickly. Breakpoint will be achieved as soon as the chlorine is distributed evenly throughout the entire volume of the pool. That, in itself, may take a couple of hours or more if your circulation is poor or if you've added it all in one spot off the diving board.

Granular calcium hypo is a third choice, with the required 4 ppm multiplied by both the 2 for the pool size and the 1.6 because it's not all chlorine (60 to 65% strength, divided into 1 pound). For our purposes, that result is 13 pounds to add to the pool. (Again, please, not 12.8.)

Look at the curve in the figure above, showing the rise in total residual in the pool. As that value climbs towards the ten-to-one level (called the break-point), note that the combined chlorine climbs some as well, scavenging all remaining ammonia in the water. When that point is reached where the chlorine finally overwhelms the ammonia products, the combined value drops abruptly to zero. From then on, whether the chlorine continues to rise (over-shooting the actual breakpoint) or declines (as chlorine additions are stopped and it dissipates), all the chlorine is "free". The odor's gone, the irritant is gone, and, even at residuals as high as 10 ppm – if the local health inspector and the swim-team mom with the test kit will allow – you can safely swim the swimmers.

Seems simple enough, but before you have trouble with the process or ask a lot more questions, we'll look at a little "theory" and a number of important considerations:

Just where does the ten-times rule come from, and can it be used to prevent the formation of chloramine as well as to effect its elimination? Look at one small parcel of water, say a water-glass full, with some chlorine in it. Now let's flip a drop of sweat in that glass. There's a magical molecular-weight ratio – a little under ten-to-one – that, if matched or exceeded by the chlorine over any ammonia present, precludes the formation of ammonia compounds or chloramines. Differently said, if there's at least ten times the chlorine in the water as there is ammonia in the sweat, it just doesn't happen. But if there's less chlorine, say five to one or one to one, the ammonia combines with the chlorine to produce the bad guy, chloramine. Superchlorination, with the times-ten rule, will soon be necessary. It's pretty amazing; the two materials produce entirely different products, according to their ratio when they meet.

Like the tough guy, Ammonia-breath, who enters the corner bar... if there's just a few mild-mannered brothers from the Chlorine family there, it might behoove the brothers to invite the bully in. But if there's ten or more of the little Chlorine fellas on hand, they can throw the big guy through the swinging doors out onto the street!

If the maintenance of a "times ten" level precludes the formation of chloramine, it makes sense that superchlorination with the same multiplier will get rid of it. Forcing the ten-to-one conditions on a particular pool containing considerable accumulated chloramine should burn it out, leaving the water containing "free" chlorine only. (We love to use that over-generalized and somewhat inaccurate term "burned out"; we could even say blasted, smashed or blown out of the water. That last one may be the most accurate of all...) In reality, the ammonia, NH3, loses its nitrogen to atmosphere and all sorts of other chemical activities happen, from salt, alcohol and nitrate production to the gassing off of chloroform.

Now what if the chlorine's at zero? Can chloramine form? No, no more than chloramine can form in a natural creek or pond. But any ammonia in the pool waits around till you feed it a little chlorine, then there's plenty of the offensive stuff. If you have heavy chloramine and you let the chlorine dissipate to zero, is the chloramine gone? Yes. Is the ammonia gone? Not a chance; as soon as you restore the chlorine, the amines are back, and back worse than ever if any organics were added during the time the chlorine was low.

When calculating our superchlor dosage, shouldn't we figure in the chlorine already in the pool? No, you'll notice that every time we round off, estimate or guess we do so on the generous side, in order not to miss the required amount for breakpoint. The one-ppm level you had an hour ago when you made the tests may be all gone by now, or, if not, you still might need that cushion to make up for the loss during the calculating, measuring and dosing phase. Forget it; the bonus can only help.

How do you know when you reach breakpoint? What if your test kit doesn't read high levels of chlorine? You have a number of choices. Buy and use a hi-range kit. Or dilute a sample of your pool water with a known ratio of grocery-store bottled water then multiply your standard kit reading of that diluted sample by that ratio. You could even count on your "calibrated nose"; you know you've hit breakpoint if you detect the absence of odor. Then, of course, you could simply be confident in your calculations. The final way is to open the pool for swimming and see if anyone hollers. (Just kidding.)

Is there a time when the times-ten rule doesn't work? Yes. How about an example: Say you have only a couple tenths of total chlorine. And let's say only one tenth is "free". That's a case of 50% chloramine, yet – because the total value happens to be very low – the calculated value of combined chlorine is only .1 ppm. Ten times that apparent chloramine level is only 1 ppm, suspiciously low for a superchlorination target. There's very little likelihood that you'll hit breakpoint when bringing the pool to 1 ppm, as yet-uncombined ammonia will almost surely cause a proportional elevation of the chloramine as the total chlorine rises. By the time you arrive at 1 ppm the chloramine may well show up on the kit as .4 or .5 ppm rather than zero, holding that 50% ratio and never even approaching breakpoint! Boiled down to a simple statement, the times-ten rule only applies when the pool's free residual is in the "normal" range near 1 ppm, so the observer can be sure that virtually all the ammonia is engaged with chlorine when performing the calculation.

What about pH when superchlorinating? The answer may be surprising. We think that everything about chlorine works better at a low pH; here, however, the more offensive forms of chloramine are more prone to develop at pH values near 7 (and certainly so for pH values lower yet). Breakpoint reactions work best in the sevens so, for superchlorination day only, choose the high end of the decade, say 7.7 to 7.9 or even 8.0. Makes high-pH chlorine products look pretty good for this purpose, doesn't it?

Can I superchlorinate under a pool blanket? No, no, no. Complete break-out of amines requires that interface with atmosphere. Where else will the gaseous products of the process go? (It's tough on the cover if left in place, too!)

Think about this one: Chlorine is introduced in a pipe full of water heading back from the filters to the pool. The concentration of chlorine therein is somewhere from 50 to 200 ppm. Why then doesn't this near-constant high chlorination on water cycling through the system up to four times per day maintain superchlorination conditions and preclude any possibility of chloramine build up? The only reasonable answer is that the pipe is entirely enclosed. It's trying; the process just doesn't work. If it would work in the plumbing, we could have skipped this article – there wouldn't be anything to talk about.

Now indoor pools are difficult to superchlorinate, cover or not. There, doors and windows need to be opened, floor fans used – anything to get air circulating over the surface of the water to aid in carrying off what's gassing off. (Much more on this subject follows in the feature on superchlorination difficulties...)

So whether we've shocked the pool, blasted or superduperchlorinated it, our purpose has been accomplished – we have rid the pool of unwanted byproducts of chlorination and the chlorine residual shown by the test kit has been re-established as "free" chlorine only. If only we could keep it that way...

~kw

©1997 Professional Pool Operators of America