In my experience, when a pool is in good shape, keeping it in good shape is reasonably easy. However, if something goes wrong, it can be quite hard to figure out what went wrong, and hard to get things back under control.
Beware! If you have been using tablets of stabilized chlorine for more than a couple of months, you almost certainly have accumulated too much CYA (cyanuric acid, aka stabilizer). You need some stabilizer in any outdoor pool, to protect the chlorine from sunlight – but if you have too much it defeats the purpose of the chlorine, and you wind up with algae and pathogens in the water. See reference 1.
This is an extremely common problem.
The “trichlor” product (in tablet or powder form) is about half chlorine and half stabilizer. “Dichlor” has an even larger proportion of stabilizer. The chlorine naturally goes away after a few days, but the stabilizer lasts essentially forever, and accumulates. If you’ve been using stabilized chlorine for a year or more, the CYA level is probably astronomical.
Short-term recommendation: Grab a clean plastic jug, rinse it once in pool water, then fill it with pool water and take it to the local pool store. They probably have a machine that performs a water analysis and prints a report. They will probably do this for you for free once or twice a year.
They’re happy to give you a free report, because they use it as a tool to get you to buy stuff. However, you should usually ignore everything they say. Pool-store advice is not reliable. In particular, if the CYA is high, you have to replace the water, and nothing you buy is going to change that.
In the medium and longer term, you should get a test kit that allows you to check the CYA level, using the melamine turbidity test. I would not trust any other kind of test.
If you have excess CYA, one common symptom is an outbreak of mustard algae and/or black spot algae attached to the surfaces of the pool. Before draining the pool, brush it with a steel-bristle brush to knock loose as much of this as you can. Then vacuum it up, discarding the water as you go (rather than running it through the filter). This means that the water you are removing serves some useful purpose.
As you are refilling the pool, when it is about 10% full, add enough stabilized chlorine to chlorinate the entirely full pool, so that the concentration is about 50 ppm, i.e. ten times higher than you would want for swimming. Then do some more brushing, to kill whatever algae is still clinging to the surface. You want to really clobber it with concentrated chlorine. The 10x chlorine won’t do much harm to intact skin, but you wouldn’t want to get it in your eyes or mouth, so be careful. Soak all the pool toys in the 10x chlorinated water, to kill whatever algae is clinging to them. Machine-wash all bathing suits, using a good detergent and hot water.
After you are sure you have clobbered the algae, fill the pool the rest of the way.
Chlorinate the new water using stabilized chlorine at first ... until the level of stabilizer gets up to 25 or so. Thereafter chlorinate the pool using unstablized chlorine.
Lowes sells a carton containing two gallons of “chlorinating liquid” for less than $7.00. It’s like ordinary chlorine bleach, only more concentrated. Beware that this stuff has a very limited shelf life, so buy the freshest stuff you can. Buy from some place has a high turnover, not from the grocery store, where the stuff has probably been sitting there so long that it has lost most of its potency. Don’t buy more than a one-week supply, especially during the summer. Remember, it is unstabilized. Store it in a cool, dark place.
Assuming you have some stabilized chlorine tablets lying around, just keep them. They have a nice long shelf life. Whatever you have is probably a lifetime supply, since you will use them only on special occasions. The CYA level will very gradually drift down, due to water splashing out of the pool if nothing else. If it starts getting too low, use one of the stabilized tablets.
If you want to add a whole lot of stabilized chlorine quickly, e.g. when filling a pool with fresh water, you can smash up the tablets, to increase the surface area. Put a brick under water, put a tablet on the brick, and whack it with another brick, or a hammer, or whatever. Do this under water, because you really don’t want to breathe the dust.
The point of the so-called “chlorinating” agent is that it serves as an oxidizer. The fact that it contains chlorine is almost irrelevant. To a first approximation, any oxidizing agent works as well as any other. This includes
In theory, at low enough pH, plain old molecular oxygen (O2) is a sufficiently strong oxidizer. In practice, you don’t want to operate your pool at such a low pH, but it’s not off by much.
CYA stabilizer protects the HOCl from being destroyed by sunlight, but this protection comes at a price: The CYA makes the HOCl much less aeffective as an oxidizer. See the chart in reference 1.
There exist instruments that will measure the oxidation-reduction potential (ORP). If the ORP is less than 700 millivolts, the pool does not have sufficient protection against pathogens. It doesn’t matter how much of this-or-that chemical you have in the pool; if the ORP is less than 700 millivolts you have a problem. For a public pool, state regulators will close it instantly if they find the ORP is below 700 millivolts.
A professional pool service technician should have an ORP meter and should know how to use it. For the generic homeowner I don’t know what to recommend, because ORP meters are somewhat expensive and require quite a bit of maintenance.
You also have to keep after the pH. If the pH is too high, it doesn’t matter how much chlorine is in the pool; it won’t be effective. If the pH is too low, the chlorine is super-effective at first, but it won’t last very long.
Alkalis have high pH. Acids have low pH. Around here the fill water is alkaline and the dust is alkaline, so you can plan on adding acid every so often. Use muriatic acid, i.e. hydrochloric acid. The most economical form is concentrated to about 32%. The local pool store sells a carton containing two gallons for $11.00. The local discount stores and hardware stores are not an economical source, because they charge the same price for something that is much less concentrated.
Unlike liquid chlorine, muriatic acid has a more-or-less unlimited shelf life, so don’t worry if the amount you buy exceeds the immediate need. Store the acid somewhere away from the chlorinating liquid, because the combination of acid + bleach liberates chlorine gas, which is very toxic. You don’t want any scenario, even a remote scenario such as an earthquake and/or a collapsing shelf, to break an acid bottle and a chlorine bottle at the same time.
Concentrated acid will eat the plaster off the walls of your pool. Therefore you should dilute the acid, and then add the less-concentrated acid to the pool very slowly, with vigorous mixing. Start by adding a pint of acid to a five-gallon bucket of fresh water, and then feed that into the pool, in small batches at different places, as far from the edge as you can manage, with the filter running. Keep the filter running until you are sure the acid is well mixed into the pool.
This is probably more than you want to bother with, and is not a requirement in the short run, but I found it to be well worth it in the long run: I rigged up a venturi system that sucks the acid out of the bucket and mixes it with a huge amount of water before it enters the pool. It empties the bucket over the course of a couple hours.
One useful lesson from high school chemistry is that when dealing with concentrated acid, always add acid to water, not the other way around. The reason is density: The acid will sink into the water, whereas the water would just sit on top of the acid, react, boil, and splatter acid all over the place. (With dilute acid you don’t have to worry about this.)
There is a bunch of complicated chemistry related to CO2. Gaseous CO2 can dissolve in water to a rather large extent, remaining as molecular CO2. Then some of the dissolved CO2 reacts with water to form carbonic acid, H2CO3. Since you have lots of alkali ions in the pool, such as sodium and calcium, they will react to from carbonates, such as sodium carbonate (which is soluble) and calcium carbonate (which tends to precipitate out, depositing scale on various surfaces).
If you have a lot of carbonates in the pool, pH control will have some mild surprises. Here’s the usual scenario: You add enough acid to lower the pH to a good level, and then the next day you find that the pH has climbed back up. Don’t worry; you did the right thing. What has happened is that some of the carbonates have turned back into carbonic acid, which has turned into dissolved CO2, which has gradually turned into gaseous CO2 and bubbled away into the atmosphere. Compared to most chemical reactions, this is a rather slow process.
The correct procedure is to just add some more acid. You might have to add acid every day for a week. Eventually the amount of carbonate and CO2 equilibrates at the desired level and the pH stabilizes. Don’t try to solve the problem all at once. You’re dealing with a process that is very slow compared to most chemical reactions, so don’t let that surprise you. Don’t add more than a pint of concentrated acid at once. Wait to see what happens before adding more.
You can get to a stable pH more quickly by aerating the water. This encourages the dissolved CO2 to leave the system more quickly.
A pool should not have any smell.
If a pool “smells like chlorine” you are almost certainly not smelling chlorine per se. That smell comes from molecules in the chloramine family, i.e. compounds containing nitrogen plus chlorine. If somebody urinates in the pool it adds a bunch of nitrogen, leading to chloramines. Also if you have an algae bloom and kill it with chlorine, the dead algae will release nitrogen into the water, leading to chloramines.
The pool store will tell you to destroy the chloramines using a super-high level of chlorine. The problem is, that leaves you with a chlorine level that is too high for swimming. It will last for many days.
My preferred way is to run the aerator night and day for a while. The reason you can smell the chloramines is because they are bubbling out of the water on their own, and the aerator speeds up this process. The aerator makes an especially big difference when the pool is covered.
Chloramines do not make the water unsanitary; indeed they tend to kill pathogens and algae more effectively than plain old chlorine does.
The problem with chloramines is that they sting the eyes and lungs of the swimmers. This can be a huge problem with indoor pools. With outdoor pools the problem is somewhat less, because the wind tends to blow the chloramines away.
Remove leaves and dead bugs from the pool promptly, because they can be a source of nitrogen, leading to chloramines. They can also be a source of nitrates and phosphates, which act as fertilizer, allowing algae to grow extra-quickly if the chlorine level ever gets too low.
If the pool chemistry is correct but the pool is still appears somewhat cloudy, it means there are micron-sized particles suspended in the water. These could be the carcasses of dead algae, or plain old dirt and dust that got blown in, or possibly globules of oil. A sand filter is not very effective at catching such particles, especially when the filter is clean.
In such a situation, you can clear up the water by adding a couple ounces of diatomaceous earth to the sand filter. Just dump it into the skimmer, while the pump is running. If the added DE causes the backpressure to go up, you added too much; next time use half that much. This is a highly nonstandard trick, but I find that it works for catching the small stuff. The sand catches the DE, and then the DE catches the tiny particles. If you add too much DE it decreases the capacity of the filter, so you will have to backwash more often, but if the water was clear of everything except the tiny particles this should be no problem.
The so-called “test strips” or “dip strips” are very cheap and convenient. You can use them to obtain a rough measurement of pH, free chlorine, and combined chlorine (i.e. bleach plus chloramines).
I have seen test strips that purport to measure other things, such as CYA level, but they are so inaccurate as to be worse than useless.
The dip strips tend to work OK under ideal conditions, but if something goes wrong they can be very misleading. Among other things, if the chlorine level is very high, it can cause the strip to report a paradoxically low reading, which can be dangerous.
For example, if the pH is high, you don’t really care how high it is; what you care about is the acid demand. The wet chemistry will tell you that, but the dip strip will not.
Algae grows much more quickly in warm water. Therefore, if you make a mistake with the chlorination in the middle of winter, not much is going to happen ... but if you make the same mistake in the middle of summer, there could be a tremendous algae bloom.
Changing the temperature is not really an option. The only action item here is to be careful always, but extra-careful when the water is warm.
The term “algae” does not have a well-defined scientific meaning. The word means different things to different people. Here is how I prefer to think about it:
My definition of algae excludes:
Algae is “usually” not considered directly dangerous, but it is a warning sign. It’s like the canary in the coal mine, in reverse. If you have enough chlorine in the pool to kill pathogens, algae cannot grow.
Actually it’s slightly worse than that, because there is a vicious circle: If you have algae, it will consume chlorine, making it difficult to maintain a sufficient chlorine level. So algae not only indicates a problem, it can indirectly make the problem worse.
The organisms that cause Black Spot Syndrome are cyanobacteria. According to my definition, they are not algae. A chlorine level that is low enough to allow swimming may be insufficient to kill cyanobacteria, even if it is more than enough to kill algae.
Here is my recipe for combatting cyanobacteria:
This will not kill the cyanobacteria overnight. However, you will know that you are making progress, because every day the infestation will be slightly smaller.
As always, the primary defense against algae and pathogens is to maintain a sufficient chlorine level.
As a second-order correction, if you ever make a mistake with the chlorine, the rate at which algae grows depends on the availability of fertilizer in the water. That’s because algae is a microscopic plant, and requires the same sort of nutrients as any other plant.
A bag of fertilizer typically has a label of the form 15–10–10, representing N–P–K, where:
That’s because protein consists of amino acids, which are nitrogen compounds. Any time you hear the stem -amine- you know there is nitrogen involved.
Dead bugs, dead algae, and leaves that get into the pool will leach nitrogen into the water. To stop this, it does not suffice to vacuum stuff off the bottom and collect it in the strainer; you have to empty the strainer, to remove the stuff from contact with the water.
The cyanobacterium that causes “black spot” infestations is one of the very few organisms that can use N2 directly.
Nitrogen compounds do not accumulate over the long term, because they eventually react to form oxides of nitrogen, chloramines, or N2. These slowly leave the system in gaseous form.
The best way to get chloramines out of the water is by aeration.
There are ways of destroying chloramines chemically, by using huge amounts of chlorination, but this is expensive and time-consuming and prevents you from using the pool (because of the excessively high chlorine levels). Also, if the chloramine level is high, it is essentially impossible to get the chlorine level high enough to destroy it. Aeration is cheaper and easier and better.
Phosphorus in water is principally in the form of phosphoric acid (H3PO4) or perhaps some version of polyphosphoric acid. Near pH 7, i.e. under normal conditions in the pool, the singly- and doubly-dissociated forms predominate: H2PO4− and HPO4−−.
There is a long-running pointless controversy about whether it is worth worrying about phosphate in the pool water, as discussed in section 13.
The main good reason, possibly the only good reason, for worrying about phosphate has to do with the cyanobacteria that cause Black Spot Syndrome. They can be rather resistant to chlorine. You can kill them with chemicals, but the chemicals are so harsh that you can’t swim in the pool, which sorta defeats the purpose.
If you have copper ions in the pool, don’t add phosphate. By the same token, if you have phosphate in the pool, don’t add copper. Otherwise you will get copper phosphate, which forms strongly colored blue scale, probably in places where you didn’t want it.
One reasonable way to reduce the phosphate level is to add aluminum ions. See section 11.4. Aluminum ions react with phosphate to form aluminum phosphate, which is exceedingly insoluble. It’s isolectronic with silica (i.e. beach sand, SiO2).
Testing for phosphate requires special effort. There exist specialized test strips, and specialized wet chemistry kits. The local pool store might do it for you for free, once or twice a year. Another approach is just to add a bunch of aluminum hydroxide. That will zero out the phosphate, and any remaining aluminum hydroxide won’t do any harm.
To a first approximation, almost any aluminum salt that you add will convert to aluminum hydroxide, if the amount is not too large. That’s because there is an endless supply of of OH− ions in the water. For example, adding alum (i.e. aluminum sulfate) is the same as adding aluminum hydroxide plus sulfuric acid. Aluminum hydroxide is only sparingly soluble. That is, you can have at most 1 ppm of aluminum hydroxide in solution at any given time. The rest initially forms a sticky gel, which is an effective way to remove tiny particles from the water. Over time the gel turns to crystals and settles out.
Aluminum ions react with phosphate, as discussed in section 11.3.
Aluminum ions in the water are not a problem. Antacids such as Maalox contain huge amounts of aluminum hydroxide, and many antiperspirants contain aluminum chloride. The amount of aluminum in pool water is vastly less.
The pool store sells alum (i.e. aluminum sulfate), but I refuse to use it because I don’t want sulfate ions in the pool. See section 11.8.
The best source of aluminum ions I have found is aluminum hydroxide, in the form of fine white crystals, which you can obtain from the local pottery / ceramics supply store. (They use it as an ingredient in some glazes.) They call it “alumina hydrate” or “hydrated alumina”. It sells for only a couple bucks per pound, which makes it vastly cheaper than the “phosphate reducer” or “flocculent / clarifier” chemicals they sell at the pool store.
You can use the Al(OH)3 crystals as-is to remove phosphate from pool water. At normal pH, the crystals are only sparingly soluble, but soluble enough to do the job, via the following process:
You can also use aluminum ions as a filter aid, as follows: dissolve aluminum hydroxide crystals in dilute hydrochloric acid, and add the resulting aluminum chloride to the pool. As soon as it hits the water it will convert to aluminum hydroxide plus hydrochloric acid. Initially the aluminum hydroxide will take the form of a sticky gel. Over the course of a few days it will convert to crystals.
Some algacides contain copper ions.
If there is any appreciable amount of phosphate in the pool, don’t add copper. Otherwise you will get insoluble blue copper phosphate, which will get deposited some place where you don’t want it. Also it nullifies the intended algacidal effect of the copper.
Copper reacts with oxygen to form dark black CuO. If this gets into the pores of a plaster pool surface, it causes hard-to-remove stains.
In my experience, macroscopic pieces of copper metal perform well in the pool environment. They do not appear to corrode. I use copper to make weights, to counter the buoyancy of aerator hose, cleaner suction hose, et cetera.
There are lots of ways potassium can get into the pool.
There is no way to remove potassium chemically. All potassium compounds are soluble. The only way for potassium to leave the system is when water splashes out of the pool, or when the filter is backwashed.
Potassium is not a problem, except insofar as it is a nutrient for algae. Since it’s not a big problem, and there’s nothing you can do about it anyway, the best policy is to just ignore it.
In a plaster pool, you need a certain amount of calcium in the water. Otherwise the water will leach calcium out of the plaster, eating away at the plaster.
Where I live, there is so much calcium in the municipal water supply that when the pool is first filled the water has all the calcium it will ever need. Thereafter, when water evaporates and is replaced by more municipal water, the calcium level goes up and up.
If you have too much calcium, it will deposit scale, probably in places where you don’t want it. In particular, both calcium carbonate and calcium sulfate have the unusual property of becoming less soluble as the temperature increases ... so they will tend to deposit scale wherever the water is warmest. If you have a pool heater (solar or otherwise) it could easily get fouled with scale.
Calcium carbonate scale is easy to remove using any mild acid. Calcium sulfate scale is much harder to remove.
If any soap gets into the pool, e.g. from people who have soap on their skin, it will react to form calcium stearate, calcium palmitate, calcium oleate, et cetera. These are hard greasy or waxy substances that make up the “soap scum” that is an all-too-familiar presence in bathtubs and showers.
I don’t want sulfate ions in the pool, because they combine with the calcium to form calcium sulfate, which forms hard-to-remove scale. (Calcium carbonate scale is annoying, but calcium sulfate scale is much worse.) Like calcium carbonate, calcium sulfate becomes less soluble as the temperature increases. This means that if you have a pool heater, you probably don’t need to worry about scale forming in the pool; it will form in the heater instead.
The pool store sells alum (i.e. aluminum sulfate) as a flocculant and/or filter aid. I refuse to use it, because it introduces sulfate into the pool. There are better sources of aluminum ions, as discussed in section 11.4.
You need a cover. Otherwise there will be excessive evaporation, which means (a) you waste water, and (b) the pool will be cold.
This doesn’t have to be fancy. Low to the ground is generally preferable to tall. Depending on details, you might want the ability to wheel the cover around after it has been rolled up, but even so, you need the ability to lock the reel in place while you are rolling it up.
The cover is designed to trap heat. If it is rolled up and left in the sun, it will cook itself to death. The cover will last much much longer if you shade it when it is rolled up.
I have several old political campaign signs that I use for this purpose. They are made of corrugated plastic, 4 foot by 4 foot. They are sturdy, lightweight, and opaque. This is not an elegant solution, but it works.
The cleverest solution I’ve seen is a bench with a built-in reel. This requires some clever engineering, so that the bench is cantilevered out over the rolled-up cover, without interfering with its operation.
Some institutional pools have a trench around the pool. The reel for the cover is hidden in the trench.
A sand filter has a lower initial cost, lower operating cost, and more convenience than a cartidge filter.
A sand filter does not necessarily catch the very finest particles, but there are various tricks for helping the filter, if you want super-clear water.
Pumping flow rate should not exceed the design flow rate of the filter. That is to say, a bigger pump is not better if you are overdriving the filter.
If you have an old one-speed pump, you might as well live with it for a while longer. However, modern pumps are more efficient. For starters, it is advantageous to have (at least) a two-speed pump, or (preferably) a variable-speed pump. The point is, the filter works best when the flow rate is low, so it uses less electricity and produces a better result if you run the pump on low for 24 hours, rather than running it on high for two hours.
A two-speed pump on the low setting will not generate enough pressure or enough flow to operate a Kreepy.
A two-speed pump on the low setting will not perform well if the filter is a little bit dirty. This is where the variable-speed pump shines. You can increase the pump speed a little bit and then continue using the filter. This increases the interval between backwashes, and therefore saves water.
For an above-ground pool you have to buy a skimmer mechanism. Get one that attaches in a nice way.
Most inground pools have one or more weirs, i.e. openings in the side at the surface level. Each weir should have a flap (also called a blade or gate), so that it draws water from the thinnest possible surface layer, not from the whole depth of the weir.
The model shown in figure 4 has a spring-loaded hinge pin, so it can be used in a wide variety of settings, even if the weir was not originally designed to have a flap.
I am perfectly happy with the classic Kreepy Krauly. It’s cheaper than most, reasonably effective, and easy to use.
The Kreepy sometimes doesn’t pick up leaves as quickly and thoroughly as you might wish. Also, there are sometimes twigs and other things that no automatic vacuum will ever catch. Some places, such a steps, will always have to be cleaned manually.
This goes in series with the vacuum hose, downstream of the Kreepy or the manual vacuum, and upstream of the pump.
I find it useful to have two of these things. Sometimes when you get in the pool there is a lot of leaves and krud on the surface, and you want a team effort to get rid of it before you swim.
A 12-inch width is about right. Bigger is not better.
Beware! If there are power lines anywhere near the pool, make sure you cannot possibly come near the wires with a metal pole. This is a good way to get killed.
You can use the same pole for the vacuum head, wire brush, and skimmer. On the other hand, I find it worthwhile to have multiple poles: One sturdy non-telescoping pole, as well as one telescoping pole (adjustable from 5 to 15 feet).
It really helps to aerate the pool.
Beware that when the back pressure goes up, the flow rate goes down. For example, a pump that is nominally rated 1300 GPH “and” 5.22 psi should really be rated 1300 GPH or 5.22 psi. With this pump, if you try to pump air to a depth of 10 feet, very little air will make it. On the other hand, if you only want to go down 6 feet, the air flow will be rather substantial. ‘
You can get a 12” long sintered air stone from aquarium supply shops for a couple of bucks.
There is a long-running pointless controversy as to whether phosphorus is worth worrying about. The usual not-very-smart not-very-helpful argument goes like this:
|Minimize Phosphate||Don’t Bother|
|It is easy to reduce the amount of phosphate to very low levels. This stands in contrast to the other macronutrients (nitrogen and potassium) which cannot be eliminated.||If you maintain a proper chlorination level, it will kill algae and pathogens, and you don’t care about the phosphate or lack thereof.|
|The advantage of low phosphate is that if you ever make a mistake with the chorination, algae cannot grow very fast in the absence of phosphate.||You still need to maintain proper chlorination, to kill pathogens. Algae is not your only concern. So the best advice is, don’t make mistakes with the chlorination.|
|If you go on vacation for 9 days, and on the last day the chlorine level gets a little bit low, there might be pathogens in the pool, but you don’t care, because you’re not swimming in it. When you return, restore the chlorine to a proper level and hold it there for a couple of hours before swimming.|
One huge problem with that argument is that you need to worry about cyanobacteria, not just pathogens and algae. See section 11.3.