Pool Plant Courses - Blog

For the purposes of pool plant operation, chlorinated isocyanurates (also referred to as stabilised chlorine) can be thought of as a combination of cyanuric acid and chlorine. Why would a pool plant operator want cyanuric acid in addition to just chlorine? The answer is because in outdoor pools that are exposed to sunlight, the chlorine will get diminished by the UV in the sunlight. It needs an additional chemical to be added to prevent this from happening. This chemical is cyanuric acid. It is possible to simply add some cyanuric acid (which comes as a white crystalline power) to the circulation system in conjunction with sodium/calcium hypochlorite. Many outdoor pool operators prefer to add a chemical that contain both the chlorine and the cyanuric acid. There are two chemicals on the market:

1.  Dichloroisocyanuric acid 
55% available chlorine. Comes as a white powder. pH is around 6.5.

2.  Trichloroisocyanuric acid 
90% available chlorine. Comes as either white powder, granules or tablet. pH is around 3.0.

Dichlor and trichlor should never be mixed. When choosing one, the main factors to consider would be the method of dosing, the pH levels and how these are going to affect the rest of the pool plant system, and the amount of available chlorine in each product.

Cyanuric acid is mostly used in outdoor pools. The reason for this is that chlorine is broken down by ultra violet light. Sunlight contains ultra violet light and when the sun shines onto an outdoor pool the chlorine gets depleted very quickly. The pool plant operator needs a way of 'stabilising' the chlorine and that's where cyanuric acid comes in. Cyanuric acid binds with the hypochlorous acid (the disinfectant in chlorine) and makes it more resistant to degradation by UV. The drawback is that it also makes the hypochlorous acid less effective as a disinfectant and for that reason, higher free chlorine residuals must be maintained when using cyanuric acid (2.5 - 5.0 mg/l).

Dosing the correct amount of cyanuric acid can be tricky. Too little and you won't stabilise the chlorine enough, too much and you will over-stabilise and the hypochlorous acid will not be available to act as a disinfectant. The pool plant operator must ensure that the cyanuric acid levels are kept below 200 mg/l, with the ideal range being 50 - 100mg/l. The most effective way of keeping cyanuric acid levels in check is through dilution with fresh water.

Although this video is about the treatment of potable water treatment, many of the processes involved are very similar to the treatment of swimming pool water. In terms of pool plant operation, it's worth a look:

Excess physical pollution floating on the surface of the swimming pool can sometimes cause a problem for pool plant operators. The heavier physical pollution sinks to the bottom, but the lighter stuff remains floating around on the surface and looks extremely off-putting to pool users. This type pollution usually consists of things like; bits of float, plasters, hair etc. There will also be chemical pollution sitting on or around the surface of the pool, stuff like; biofilm, grease, sweat, mucus etc. 

Modern pools are usually deck-level, meaning that the surface of the swimming pool is level with the deck of the pool surround. This system is very good at removing much of this pollution that resides on the surface, and within the top 150mm. because as the water laps over the edge of the pool, the pollution enters the drainage channel that goes around the perimeter of the pool. 

In older pools, the deck-level system is not so common and instead, there may be a skimmer system or an overflow channel. The skimmer system is not very good at removing pollution from the surface as the skimmers do not go around the entire perimeter of the pool like a deck-level drainage channel does, so not nearly enough surface water goes through the skimmer for this to be an effective system. The overflow channel system isn't any better because if the pool level is too high or too low, the swimming pool water will not flow over the channel in the correct way and you'll end up with what is known as a 'scum-line'. This pool plant operator has spent many hours in years gone by kneeling on a float scrubbing away to remove this scum-line and it's not an enviable task.

If you've got either of the older systems (overflow or skimmer), chances are that at some point you've experienced going on to poolside and seeing excess debris floating around on the surface with a public swim session about to commence in a few minutes. There is a quick and easy technique that you could use to quickly and easily bring the swimming pool back to a reasonable standard of appearance in order to get you out of the immediate situation. You'll need 2 people, a rope that is as long as the width of the pool and a few towels. 

1. Drape the towels over the length of the rope while it's lying on the poolside.
2. Get one person on each side of the pool and slowly drag the rope down the length of the pool, from shallow to deep.

What you should see is the towels acting as a filter/barrier. It will catch some of the smaller particles in the material of the towels, while at the same time push some of the larger particles towards the deep end outlets. You will probably also need to get the nets out and spend a few minutes going over the pool surface and collecting any debris that remains.

After you've completed this process, which should only take a matter of minutes, you should see that the appearance of the swimming pool has been greatly improved. 

I should stress that this technique is a 'quick fix' only in order to get you out of trouble when you're under pressure. If any of the pool water test results are outside the correct parameters, then the appropriate action should be taken. But if all test results are OK, then this method should come in handy.

One other thing I will stress is that if you're experiencing this problem regularly, then this is an indication that there is an underlying problem somewhere. The pool plant operator should ask the following questions:
 
1. Are swimmers taking pre-swim showers?
2. Is the bathing load too high?
3. Is the correct circulation rate being achieved?
4. Is there enough fresh water going in (30 litres, per bather, per day)?
5. Are backwashes being carried out frequently enough?
6. Are the skimmer baskets being cleaned out frequently enough?
7. Are you using a coagulant, and are you dosing it correctly?
8. Is the skimmer valve closed, or being throttled back for any reason?
 

Bromine is a disinfectant that works in a similar way to chlorine, in that it oxidises pollution on contact. The active disinfectant produced by the addition of chlorine-based disinfectants is hypochlorous acid, whereas the active disinfectant produced by the addition of bromine-based disinfectants is hypobromous acid.

Bromine-based disinfectants are not widely used in conventional commercial pools, largely because it is known to sometimes cause a condition known as ‘bromine itch’ in some swimmers. Also, in has been found that bromine is not as effective as chlorine at controlling microbiological contamination. Bromine-based disinfectants are sometimes used in large leisure pools and spa pools for reasons that are explained below.

There are two ways of producing the active disinfectant (hypobromous acid):

1. Bromochlorodimethlyhydantion (BCDMH)

This method tends to get used by small pool and spas, mainly because it’s fairly easy and relatively safe to store and dose the product and it has a minimal impact on pH levels, so the use of an additional chemical to correct pH is often not required.

2. Sodium Bromide + Hypochlorite

This method tends to get used by large leisure pools, mainly because the disinfection by-products (bromamines) are not a problem in the same way as the disinfection by-products of chlorine (chloramines) are. In fact, some of the bromamines themselves have disinfectant properties. This makes it easier for large leisure pool operators to avoid the chlorine smell and irritant properties of chloramines.

Ultra violet disinfection is a process whereby the swimming pool water flows through a UV chamber and is exposed to UV light. The UV light is harmful to bacteria and other micro-organisms because it mutates the DNA of the organism, which means that it can no longer reproduce. The UV chamber is installed in the plant room and once the swimming pool water has passed through the chamber, it will have been purified to the extent that any chlorine in the water will have also been removed. UV disinfection is a physical, not a chemical process and nothing is added to the swimming pool water when it passes through the UV chamber. This means that UV treatment is a non-residual form of disinfection and a secondary disinfectant such as chlorine will need to be added before the water recirculates back to the swimming pool. 

Some advantages of UV disinfection systems are that there are no chemicals to handle or store and once installed, the system does not take up too much space or require high levels of expertise to operate (unlike ozone disinfection systems). Also, it does not produce chemical by-products like chlorine does.

For more information and useful fact sheet downloads regarding UV, go here.

Ozone is a disinfection method that uses ozone to oxidise contamination in the pool water. It is a more powerful oxidiser than chlorine and will actually oxidise (and therefore, remove) chlorine from the pool water when it passes through the ozone dosing system. It is a non-residual disinfectant, which means that no ozone remains in the pool water once it has passed through the ozone dosing system in the plant room. This is different to chlorine because under normal circumstances, there will always be some chlorine in all areas of the swimming pool water circulation system, including the swimming pool itself. Because of this, where an ozone disinfection system is installed, there must also be an additional disinfection dosing system (such as chlorine). Without this secondary system, the swimming pool water would be re-polluted as soon as it circulated back into the swimming pool and this pollution would only be removed when the water goes through the pool plant system again. This is not sufficient to control the cross-contamination risk in swimming pools. Bacteria needs to be killed within seconds in order to minimise the risk. Having said that, the use of ozone will allow pool operators to use substantially lower levels of chlorine (as low as 0.5 mg/l). Another advantage is that ozone will kill cryptosporidium, whereas chlorine does not.

Ozone is not delivered to site, it is generated on-site. This means that there are no handling and storage issues. Ozone is a very toxic substance though and the on-site generation method means that extra training is required for pool plant operators. It is generated by passing an electrical discharge through dried air in an ozone generator, then it goes to a mixing vessel where it is mixed with the pool water, once the pool water and ozone have been mixed together it goes into a contact vessel because ozone needs about 2 minutes of contact time with the pool water in order to be effective, then it goes through a filter to remove all of the ozone from the pool water. The purified pool water then carries on its journey through the pool plant system and the air and any undissolved ozone goes through an ozone removal system before being vented externally. As you can see, ozone disinfection requires quite a lot of equipment, which can be an issue, as can the high levels of expertise required to operate and maintain the system safety.

Balanced water testing is something that you should be doing weekly in order to determined whether you pool water is 'balanced'. This refers to whether the water is corrosive or scale-forming.

Corrosive water will eat away at grout, pump seals, pipework etc. (basically, anything it comes into contact with). Scale-forming water does the opposite of corrosive and instead of eating away at things it will form a scale on it (you're likely to see the effect of this on the heating element of your kettle). This scale-forming can be a problem if it is allowed to build up on the pool circulation pipework etc.

The aim of good water quality management is to have water that is neither corrosive, or scale-forming (ie, balanced).

The test works like this:

Step 1. Carry out tests and note results for:

• Calcium hardness
• Total alkalinity
• Pool water temperature  
• Total dissolved solids

Step 2. Convert the results from the above tests to factors as indicated in the chart below (figures in red provide a worked example).

Step 3. Add the factors for temperature, calcium, alkalinity to the pH.

Step 4. Minus the factor for TDS from the figure obtained in step 3.

The ideal result is somewhere between 0.1 - 0.4. The pH level is the thing that has the most impact on water balance test results. A high pH would contribute to scale-forming water, a low pH would contribute to corrosive water. However,it is not advisable to start adjusting pH levels just to try and get good water balance results, as pH is a critical factor in the efficiency of your disinfection and coagulation, both of which are more important than water balance results. 

 A better way to a adjust water balance might be:

• To increase: consider increasing the levels of calcium hardness, or total alkalinity, by adding calcium chloride or sodium bicarbonate respectively, depending on the results of those tests. Another option might be to reduce the TDS levels (if they are particularly high) by diluting with fresh water.

• To decrease: Take a look at your pH result. If it's high, you need to decrease it anyway as your chlorine is not going to be as effective at higher pH levels.   

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Alkalinity is the amount of alkaline salts (carbonates, bicarbonates and hydroxide ions) present in the pool water. It is an important factor in the treatment of swimming pool water because it acts as a pH buffer. If your alkalinity levels are too low, you may find it difficult to control the pH level; when you add and acid, the pH level will drop too fast, then when you try to correct the problem, the pH level will shoot up past the level you want it at. If your alkalinity levels are too high, you will have the opposite problem and may find it difficult to budge the pH level. Recommended alkalinity levels are as follows:

When using sodium hypochlorite: 120 - 150mg/l
When using calcium hypochlorite: 80 - 120mg/l

The usual way of measuring it is to collect a 100ml of pool water and add the total alkalinity tablets one at a time and shake. The colour will start off yellow, when it turns bright red (take a look at the picture), multiply the number of tablets you needed by 20 and then minus 10. Example: 5 tablets to bring about the bright red colour (5 x 20 = 100 - 10 = 80). You could also use a 50ml sample (as in the picture), but you would need to multiply the number of tablets by 40 and then minus 20. 

There is a very handy calculator on the 'Tools' section of the website which will work all of this out for you - all you have to do is enter the number of tablets used for a 100ml sample and the calculator does the rest.

One of the main problems for many swimming pools is condensation. Condensation happens when the water vapour in the air condenses into liquid water and forms a film on surfaces. This will then start to attack the fabric of the building and lead to expensive repair bills further down the road. For the pool operator, the objective is to keep this condensation to a minimum. This can be achieved by being aware of the temperature at which the water vapour in the air will condense into liquid water (otherwise known as the Dew Point). If you have a dew point of, say 24 degrees celsius, this means that if the water vapour in the air stays above that temperature, it will remain as water vapour. As soon as it drops below that temperature (for example, when it penetrates into the porous concrete walls of a building) the water vapour will become liquid water (ie, condensation). Therefore a high dew point is not good.

The two things that affect the dew point are: temperature and relative humidity. The relative humidity refers to the amount of water vapour in the air, expressed as a percentage of the maximum possible amount of water vapour there could possibly be in the air at that time. A relative humidity of 100% means that the air is holding the maximum amount of water vapour that it possibly could and is therefore said to have reached saturation. Warm air is capable of holding more water vapour than cool air.

Click here for an interactive tool that allows you to see how temperature and relative humidity affects the dew point.

Straining is one of the processes taking place during filtration and is perhaps, the one most pool operators will be familiar with. The process involves dirty water passing through the filter media (usually sand) and particles of pollution becoming trapped in the small gaps (pores) between the grains of sand because they are too large to pass through.

Take a look at the picture (click to enlarge). In swimming pool filters, the size of the sand grains is usually 0.5 - 1.0mm. This results in a pore size of approx. 77microns (1mm = 1000microns). Anything to big to pass through the pores will become trapped, anything smaller will pass through. If you consider that the size of bacteria is 1 - 5microns, you will realise that sand filtration on its own will not remove it. This is why it's very important to pay close attention to the process of coagulation, which clumps small particles of pollution together to form what is known as floc. This process of coagulation, combined with the fact that filtration is a progressive process (more and more pollution will be removed each time the water passes through the filter) means that it is possible to remove particles much smaller than 77microns, in fact there is no specific bottom limit to the size of particle that can be removed.

The filtration rate is the rate (in metres per hour) at which the pool water moves down through the filter during normal operation. It is not to be confused with the circulation rate, which is the rate (in cubic metres per hour) at which water is moving through the circulation system.

The filtration rate is calculated by dividing the circulation rate by the surface area of the filter. For example:

Circulation Rate: 100 cubic meres per hour
Filter Surface Area: 5.72 metres squared
Filtration Rate: 17.48 metres per hour (100 divided by 5.72)

You should have a flow-meter fitted onto the circulation pipework in your plant room to tell you what the flow rate is. You can easily calculate what the filter surface area is by multiplying the radius by itself and then multiplying that value by pi (3.14). For example:

Width of filter: 2.7 metres (this is also the diameter)
Radius: 1.35 metres (half of the diameter)
Radius Squared: 1.82 metres squared (1.35m x 1.35m)
Radius Squared x pi: 5.72 metres squared (1.82m x 3.14)

You'll need to know the filtration rate in order to establish whether your filter is a high, medium or low rate filter.

High Rate: 25 to 50 metres per hour (small domestic pools only; to fast to deal with pollution in public pools).
Medium Rate: 10 to 25 metres per hour (recommended for public pools).
Low Rate: up to 10 metres per hour (very good filtration, but requires a vary large surface area). 

Physical pollution is one of the three main categories of pollution that concerns pool plant operators (the other two being chemical and biological). As you might expect, physical pollution is made up of stuff that does not dissolve into the water:

• dirt 
• grit
• sand
• plasters 
• bits of float  

The heavier physical pollution will sink to the bottom of the pool and will need to be removed by sweeping it up to the deep end outlets, or by the use of a vacuum cleaner (either manual or automated).

Free chlorine is measured with the DPD1 test. It indicates how much of the total chlorine in the pool has not yet reacted with any pollution (ie, combined) and is therefore free and available to carry out it's purpose as a disinfectant.

There should always be enough free chlorine in the pool to minimise the risk of cross-contamination. The minimum recommended amount is 0.5mg/l. any less than this and you will need to shut the pool. The maximum recommended amount is 2.5mg/l. There is no need to have any more than this in the pool unless you are superchlorinating the pool for some reason (ie, following faecal contamination etc.)

Every month, all public pools should be tested for bacteriological water quality by a UKAS accredited laboratory. These tests serve as a link of 'health check' and will give the pool operator an indication of the performance of the swimming pool plant system. There are four different tests that are carried out:

Aerobic colony count

Total coliforms

Escherichia coli

Pseudomonas aeruginosa

Aerobic Colony Count
The first test, aerobic colony count, is a test for the number of bacteria in the sample of water which are capable of forming colonies of visible bacteria, when it has been incubated at 37 degrees Celsius (ie, normal human body temperature) for 24 hours. The test doesn't necessarily differentiate between different types of bacteria, but looks at the overall picture and will provide the pool operator with an overview of the efficiency of the pool plant system. The test is expressed as the number of visible colony forming units (cfu's) counted in a sample of one ml of pool water and there should be less than 10 of these present after 24 hours at 37 degrees C in a well managed pool. If you get a result of more than 10 cfu's don't immediately assume that there is a major problem. Remember - the aerobic colony count is a measure of ALL bacteria capable of forming visible colonies. Further tests would need to be carried out in order to get more information about what types of bacteria we are dealing with.

Coliforms
This is where the testing begins to provide more specific information. Coliforms are a type of bacteria that are present in (amongst other places) the faeces of warm-blooded mammals. Coliforms on there own are not normally the cause of serious illness, but since they cannot multiply in pool water, if you get any showing up in your results, it is very likely that they have been introduced into the pool via faecal matter (or possibly environmental contamination as they are also found on plant and soil). This means that there could well be other types of pathogenic (ie, harmful) bacteria present also. You should be getting less than 10 cfu's per 100ml sample of pool water in a well run swimming pool. If your getting results over and above this, along with high results on the aerobic colony count test, it's likely that either the sample was taken shortly after a faecal contamination, or there may be problems with the pool plant system. Another test will need to be carried out to confirm.

Escherichia coli 
E. Coli is are present in the intestines of humans and many strains or non-pathogenic (ie, not harmful). Some strains are pathogenic though and can cause serious illness. If your getting any showing up in your results, then there is little doubt that it got there via faecal contamination. Coloforms and E. Coli are both killed by chlorine, so you should not be getting any more than 10 cfu's in a well-run pool, but similar to above, the sample could have been taken shortly after a faecal contamination. The result needs to be considered alongside all the other results in order to build a picture of what's happening with the water. If repeat tests still show high results, take it as a red flag; something's not right somewhere.

Pseudomonas Aeruginosa
Pseudomonas bacteria are present in in the environment, and on humans, and are capable of growing in water. They can cause ear and skin infections, but are killed by chlorine, so as long as you keep your disinfection levels within the recommended parameters, there should be no issues and you should not see results above 10 cfu's per 100ml in a well-run pool. If your getting any showing up in repeat samples, then it is looking probable that they have colonised the filters, or some other area of the circulation system, probably due to poor disinfection and/or filtration.

The ideal result with all these tests is to have zero across the board. If your getting some results that indicate the presence of any of the above, you need to look further and consider all of the results together, as they are quite closely linked. If your results indicate gross contamination, you will need to close the pool and get to the bottom of the problem. Please see below (click to enlarge) for a simple flow chart, which will help you to interpret the results and determine whether you have gross contamination of your pool water:  
 

Sedimentation is one of the processes that is taking place during filtration, the other two being straining and adsorption. Fine particulate matter settles on the upward-facing surfaces of the sand grains. The process of sedimentation can remove finer particles of pollution than straining. As the amount of sediment increases, the amount of space in between sand grains (pores) decreases. This will cause the velocity of water through the filter to increase. Further sedimentation can then no longer occur and, due to the higher velocity, some sediment could get pushed further down into the filter bed.  

Adsorption is one of the processes taking place during filtration, the other two being straining and sedimentation. It is not to be confused with absorption. With adsorption, very small particles of pollution adhere to the surface of the sand grains. This process is promoted by electrostatic charges within the particles (similar to a balloon 'sticking' to a wall). Once particles begin to adhere to the sand grains, a sticky coating builds up, which promotes further adherence of particles onto the filter media. 

Coagulation is an important process in the swimming pool water treatment process. It helps to remove very small particles from the pool water. In a conventional swimming pool sand filter, the size of the gaps between the sand grains (pores) are around 70 microns. As the filter begins to trap particles, the size of the pores decreases. This is known as 'filter ripening'. When the filter is fully ripened, it will be capable of trapping particles the size of around 5 - 10 microns. The size of cryptosporidium cysts are about 5 microns, bacteria is around 1 - 5 microns, and colloidal matter can be as small as 0.1 micron. 
 
Even with a ripened filter, some of the pollution is too small and will pass through the filter. This is why coagulation is so important. This pollution has to be removed, especially cryptosporidia as it is not going to be killed by the chlorine in the pool. We need to add a coagulant, which works by causing the small particles to bind together to form what are known as flocs. The flocs are typically 20 - 50 microns, so will be large enough to become trapped in the filter. The most common coagulants used in swimming pool water treatment are:

• Polyaluminium Chloride (PAC)
• Polyaluminium Sulpho-silicate (PASS)
• Aluminium Sulphate (Kibbled Alum)
• Sodium Aluminate
• Iron Chlorides
• Iron Sulphates  

Source water refers to where the mains water that supplies your swimming pool comes from. The water companies collect water from a variety of sources. Some of it is collected from ground waters such as wells and boreholes etc., and some of it is collected from surface waters such as lakes, rivers, streams etc. The ground waters have a higher content of calcium carbonate and are known as 'hard' water. The surface waters don't have such a high calcium carbonate content because they haven't permeated through the ground where it comes into contact with various minerals, including calcium carbonate. Hard water leads to scaling as it deposits its calcium onto the surfaces it comes into contact with.
 
Soft water does the opposite and will corrode the surfaces it comes into contact with. This is why water balance testing is carried out weekly and the aim for pool operators is to have the water ever so slightly scale-forming. In practice, it will be very difficult to get the water completely balanced and as long as your pH levels are OK (which means your alkalinity levels are probably also OK) then you are unlikely to have major problems with water balance.

Different parts of the UK have different source waters. Due to the network of pipes water can also transported from one area of the UK to another. This means that source water is not always constant and an area that has been supplied with hard water could find that at certain times (due to low stocks of water for example) it gets supplied with soft water from somewhere else.

Cryptosporidia is an organism that pool operators need to be particularly aware of. The main problem with it is that the chlorine in the pool water will not kill it due to the fact that it is protected within a shell that the chlorine will not penetrate. Cryptosporidia causes an acute gastro-intestinal illness and is introduced into the pool water via faecal contamination. If someone has an accident in the pool and the result is a loose, as opposed to solid stool, you as pool operator are going to have to assume that cryptosporidium is present. Therefore, you will need to get everyone out (because the chlorine won't kill it) and then take steps to remove the contaminant via mechanical means (ie, scoops, nets, hoover at the pool and then coagulation and filtration in the plant room). Two main things to consider are:

1. Cryptosporidia is about 5 microns wide (a micron is 1/1000th of a millimetre) and the spaces in between sand grains in a conventional swimming pool filter are about 70 microns just after an effective backwash and about 5 - 10 microns when ripened. As you can see, cryptosporidia will be small enough to pass through. In order to deal with this problem you will need to add a coagulant to the pool water circulating around the system, before the point when it enters the top of the filter. This is usually an aluminium based compound that will cause fine particles of pollution to bind together to form what are know as 'flocs' which are big enough to become trapped by the filter media.

2. Not all of your pool water will pass through the circulation system during your turnover period. The turnover period refers to the time it takes for an equivalent  volume of water to pass through. Some 'bits' of water will pass through a few times, some other 'bits' might not pass through at all during a single turnover period. This is affected by how well your pool water mixes together due to things like the number and position of inlets and outlets in relation to each other, the pool water velocity around the system, the number of people in the pool and how much they are moving etc. This is the reason why your emergency procedures in the event of faecal contamination should stipulate than the pool remain closed for 6 turnover periods in order to be reasonably sure that all of the cryptosporidia has been removed from all of the pool water.
 
 
 

Oxidation is a process that has a disinfecting effect on swimming pool water pollution. It's basically a reaction between the oxidiser (the chlorine for example) and the pollution. The oxidiser will take electrons from the pollutant substances in the water, which breaks down and kills them. It's a very similar process to combustion (burning) and rusting. You can also observe the process if you peel an apple and see brown areas start to appear after a while (if you don't eat it, that is). What's happening is that the cells of the apple are being exposed to oxygen particles in the air and are being broken down (oxidised). The brownish parts are the result of the oxidation process. In the pool, the chloramines (combined chlorine) are the result of the oxidation process and will need to be removed via a combination of dilution with fresh water, coagulation and filtration.

If you've ever seen a swimming pool being emptied, you will have noticed that as the volume of water decreases, so does the 'blueness' of the water. Why is this? Well, basically, water absorbs light. You probably already know that light at wavelengths that are visible to humans is made up of the seven colours of the spectrum, ie. Red, Orange, Yellow, Green, Blue, Indigo, Violet. Water absorbs light at shorter wavelengths first, so the red gets absorbed and is therefore not visible. The blue is at longer wavelengths and doesn't get absorbed and that's why you can see it. The substances that are suspended in the water also have an effect on the distribution of light in the water by scattering the light at different angles. When you're emptying the pool, you're decreasing the amount of water there is to 'look through', so the effects of the absorption of light become less-visible. The absorption of light is also what causes the oceans to appear blue (rather than the reflection of the sky, which is a popular misconception).

If your total alkalinity levels are too high, or too low; you will have problems controlling the pH levels. High total alkalinity causes 'pH lock', low total alkalinity causes 'pH bounce'. 

As pool plant operators, we all know that the pH of the swimming pool water is a critical factor. If the pH is too high, the disinfectant properties of the chlorine will be minimised. If the pH is too low, the swimming pool water will be too corrosive. Controlling the pH levels is usually a simple process which involves adding an acid (low pH value) in order to reduce the pH levels.

Sometimes, problems controlling pH occur. One of these problems is called 'pH lock' and the result is that, try as you might, you can't seem to bring the pH down, no matter how much acid you add, the pH levels are locked. This is usually caused by high levels of total alkalinity. By total alkalinity, we're referring to the levels of alkaline salts dissolved in the water and NOT the pH level of the swimming pool water.You measure total alkalinity (or should be) every week, along with calcium hardness, TDS etc. 

Another problem is 'pH bounce'. This is the opposite problem to the one described above. Now, rather than not being able to shift the pH level, you find that it shifts too readily and is extremely reactive; you add a little acid to bring the pH down one minute, and the next, you're adding an alkali in order to get the pH back up.

What's happening here is that either the swimming pool is not 'buffered' enough (pH bounce), or it is 'buffered' too much (pH lock). Buffering solutions are used all the time in chemistry in order to keep pH levels stable. In our scenario, the swimming pool water itself is the buffer solution, but it's important to realise that water by itself doesn't have this buffering effect; it has to be given these buffering properties via the addition of certain chemicals. The chemicals that provide this buffering effect are referred to as the 'total alkalinity family' and are comprised of:

• carbonates
• bicarbonates
• carbonic acid  

Sound familiar? They should. You probably have a chemical called sodium bicarbonate on your site. You would add this if you wanted to increase the total alkalinity. Be careful though, if you don't calculate the correct amount to add you'll either not add enough of the chemical and the pH will continue to bounce (because the water is not buffered enough). Or, if you overdo things and add too much, you'll buffer the water too much and the pH level will be over-stabilised (or 'locked). If this happens, you need to dilute with fresh water. If the source water itself has high total alkalinity levels, you may need to look at switching to a different disinfectant.

If you carry out your weekly pool water balance test and find that the total alkalinity levels are low, you're going to need to add some sodium bicarbonate. This chemical will introduce some alkalinity into your pool without having too much of an effect on the pH level. The sodium bicarbonate that you add will go through various chemical reactions and some of it will convert into different chemicals according to what the pH level is at any given time. If the pH is high, some of the sodium bicarbonate will convert to sodium carbonate. If the pH is low, then some of the sodium bicarbonate will convert to carbonic acid.