Operating a Chlorinated Swimming Pool Content from the guide to life, the universe and everything

Operating a Chlorinated Swimming Pool

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A swimming pool.

It's difficult to part the image of summer from swimming. While beaches, rivers and lakes are popular swimming spots, the city-bound, the summer camper and the finicky are more familiar with the swimming pool. Though swimming pools are more versatile than their natural counterparts, their contained structure makes them a challenge to keep sparkling clean and sanitary. Swimming pool1 operators can easily find themselves overwhelmed by the apparently impossible task of balancing the pool water composition. Luckily, a little information can go a long way in relieving their confusion.

Keeping it Clean

Before starting on the many aspects of operating a pool, it pays to clarify some terminology:

Parts of a Pool

Main drain – any in-ground pool will have a large drain at its deepest point. Most of the water drains through here to reach the sanitisation and filtration system.

Gutter drain – some, but not all, pools have a gutter around the edge of the pool to catch displaced water. Most swimmers just think of it as a convenient handhold.

Skimmers – box or cylinder-shaped baskets, skimmers draw in water which they send through the sanitisation system. Any leaves, plasters, hair or other debris get caught in the baskets. Swimmers know these as the funny glugging things in the pool walls.

Water Return Inlet – also interestingly known as outlets, inlets shoot water back into the pool after it's been sanitised. Swimmers frequently view these as convenient water massage jets.

Surge Tank – aka the balance tank or the make-up tank, this tank holds extra water that can be released into the pool to make up the balance of water lost in the course of the day due to splashing, evaporation or maintenance operations.

There are three parts to maintaining a clean pool: Circulation, filtration and sanitisation.


Every pool must be equipped with a filter system. However, the filter system is useless if the water never reaches it. Much of what creates good circulation is built into the pool structure: frequently-spaced skimmers, two main drains in a large pool, and evenly-spaced inlets.

A pool operator can also manipulate the circulation by varying the valves on the drains, keeping skimmers clear, regulating the valves and regulating the pump speed.

Pump speed: the pump draws the water through the sanitisation system. A faster pump ensures quicker circulation. However, the pump speed should not exceed the limit set by the filter, or the filter may fail. A goal rate is the litres- or gallons-per-minute that circulates all water through the system in around six hours.

You can calculate the necessary speed with the following equation:

  1. Calculate the pool volume: width x length x depth
  2. Multiply the cubic volume in metres by 1,000 to get the volume in litres. If your calculations are in cubic feet, multiply by 7.5 to get the volume in gallons.
  3. Divide the resulting number by the amount of minutes in six hours – 360.
  4. The resulting number is the litres per minute or gallons per minute that must pass through the system to complete the entire cycle in six hours.

Inlet valves: The inlets closer to the pump are subject to more pressure and therefore return more water than the inlets further along the piping. You can keep the circulation even by narrowing the valves that lead to the closer inlets. If the inlets don't have valves, you can use inserts to narrow the opening and constrict the water flow. The inlets should also be turned at an angle if possible to ensure thorough recirculation. Slanting all the inlets at approximately 60 degrees to their right or left (just make sure it's consistent) will ensure the water circulates through all the corners.

Drain valves: By varying the openings on drain valves you can control the speed at which the water drains from the pool and enters the filter system. Gutter and skimmer valves should be kept at 100%, while the main drain should be at about 25%. (Opening the main drain further can create a vortex that can virtually glue a swimmer to the drain cover, or worse, eviscerate them. Eviscerated swimmers will not die, but they will be extremely inconvenienced for the rest of their lives. And if they are American, they will doubtless sue. Furthermore, you have to clean the resulting mess out of the filter.)

Skimmers and Gutters: If the skimmers, gutters or filters are clogged with debris, water will have a more difficult time passing through and circulation will necessarily be impaired. In outdoor pools, skimmers usually require daily cleaning.

Filtration system

Filtration removes dirt and particles suspended in the water. There are several types of filters. The most common three are:

Sand filters: Large filters that strain the water through several feet of sand to remove dirt. Every once in a while sand filters need to be backwashed to clean the dirt out of the sand.

Diatomaceous earth filters: Smaller but more efficient, these filters strain the water through a powder made of diatoms2, which is smoothed over a paper sheet. Diatomaceous filters can't be backwashed. The DE has to be replaced.

Cartridge filters: Made out of round cartridges of filter paper. The least efficient type of filter, these are used predominantly for smaller pools. The cartridges usually can be cleaned a few times before they have to be replaced.

Cleaning and backwashing filters: When the pressure gauge for water entering the filter is around 5psi different than the number displayed for water leaving the filter, it is time to clean the filter. Diatomaceous earth is considered a hazardous waste, and needs to be disposed of according to the local regulations. Sand filters can be backwashed into the sewer. Before backwashing, close all the valves leading from the filter to the pool. Open the valves leading from the filter to the sewer. Then open the valves that will run the water through the filter backwards, washing all the dirt out of the filter and into the sewer. Continue backwashing until the water coming out of the filter is clear.


Besides plasters, hair and grime, people who enter the pool also leave behind sweat, fluids and germs from their mouth, nose, skin and, well, you know. To remove this organic matter you need a sanitiser. The most common sanitisers are chlorine and bromine, although ultraviolet light, ionisation, and ozone are also used. Because bromine destabilises in sunlight, it is predominantly used in indoor spas, making chlorine by far the most common chemical for pool chemical treatment.


When chlorine is added to water it immediately disproportionates3 into chloric (i) acid4 and hydrochloric acid. Chloric acid is the compound that sanitises and oxidises.

When people swim in a pool they introduce contaminants, usually in the form of urea and creatine from urine and sweat. (Much of this contamination can be avoided by requiring swimmers to shower before entering the pool.) Sanitising is when the chloric acid combines with the body fluids forming chloramines, also known as combined chlorine, and nitrogen trichloride, which cause the characteristic chlorine smell around pools. Nitrogen trichloride also causes respiratory irritation, and when inhaled in large doses can cause respiratory distress and asthmatic attacks.

Nitrogen trichloride is more of a problem in heated indoor pools because high temperatures and constant water disruption cause more nitrogen trichloride to vaporise. In a poorly ventilated room the vapour will concentrate just above the water level.

The chloric acid that does not initially combine is known as free residual chlorine, and it can either combine with other body fluids or oxidise (remove) the chloramines. If the free chlorine level is high enough to both burn up the chloramines and combine with body fluids, the pool will remain clean and clear of odour. However, when combined chlorine levels get too high, the pool will begin to smell of chlorine.

Test at least daily for free chlorine, and twice a week for total chlorine5, then subtract the free from the total to find the level of the combined chlorine. When the combined chlorine is over 0.5 ppm (parts per million), it is time to take action.

While extremely rare, small pools with large pumps can suffer from over-chlorination. If the chlorine level is above 15 ppm the test kit may not be able to provide an accurate reading. In such a case, the chlorine reading would turn bright pink and then become clear, while the pH reading would turn purplish-blue. If you ever receive such test results try diluting one cup of pool water in nine cups of bottled water and retest it. If you then receive results that indicate high or normal chlorine levels and normal pH levels, it means your pool has dangerously high levels of chlorine. Drain and backwash immediately; retest before opening the pool.

There are two ways to eliminate chloramines; with break-point chlorination and with super-chlorination. In break-point chlorination, additional chlorine is gradually added to the pool every hour until the combined chlorine is completely oxidised. You need to test frequently to see if chlorination should continue.

In super-chlorination, also known as 'shocking', you add enough chlorine to bring the total chlorine level up to around ten times the combined chlorine level. The pool needs to be empty when this is done, and nobody should be allowed in until the chlorine level is back down at around 5ppm.

To raise 10,000 gallons of water 1ppm of free chlorine, use 0.06kg (2oz) of calcium hypochlorite 65%, or 0.05kg (1.7oz) of calcium hypochlorite 75% or 770 ml (26fl oz) of sodium hypoclorite 5%.


pH is the measure of hydrogen atoms dissolved in the water. Water with low pH is acidic, while water with high pH is basic. The centre of the pH scale - or, neutral - is 7.0.

Chlorine is 100% effective when the pH is 5.5. However, a pH that low is too acidic for people to comfortably swim in and the acid will corrode your pipes. At higher pH - up to around 7.0 - algae will grow. A pH of 7.2 is the most comfortable level for swimmers, and it has no adverse effect on the pool's circulation system. However, at 7.2, chlorine is only 50% effective, so free chlorine levels should be at a minimum of 0.6 ppm. The comfort range extends up to a pH of 8.1, where chlorine is 25% effective, and where free chlorine levels should be at least 1.5ppm.

Test for pH every time you test for chlorine – two to three times daily.

To raise 10,000 gallons of water by 0.2 pH units, use 0.17 kg (6oz) of sodium carbonate (also called soda ash). To lower 10,000 gallons by 0.02 use 0.45kg (1lb) of sodium hydrogen sulphate (sodium bisulphate) or 355 millilitres (12fl oz) of muriatic acid (concentrated hydrochloric acid).


pH stability is affected by alkalinity. Low alkalinity can cause unstable pH, which in turn will affect the chlorine. Alkalinity is best when it tests between 80 to 120 ppm.

Test the alkalinity at least every two weeks, more often if your pH is fluctuating.

To raise 10,000 gallons of water by 10 ppm of alkalinity, use 0.68kg (1.5lb) of sodium hydrogen bicarbonate (baking soda). To lower 10,000 gallons by 10 ppm, use 635 millilitres (21.4fl oz) of concentrated hydrochloric acid (muriatic acid) or 0.68kg (1.5lb) of sodium hydrogen sulphate (sodium bisulphate).

Calcium Hardness and Total Dissolved Solids

Calcium hardness and total dissolved solids are measurements of the amount of minerals dissolved in the water. This will vary based on the water source and water location; outdoor pools tend to have higher levels due to greater evaporation. If the water is too hard there will be build up on the tiles and pipes. If the water is too soft, minerals in the grout on the side of the pool will dissolve into the water and the tiles will fall off. Calcium hardness should test somewhere between 175ppm and 275ppm; total dissolved solids should be below 1,500ppm. Higher counts will burn the swimmers' eyes.

Test the calcium hardness and total dissolved around every two weeks, more often if you operate an outdoor pool. The only way to lower the mineral level is to drain off and replace some water.

To raise calcium hardness 10ppm for 10,000 gallons, use 0.45kg (1lb) of calcium chloride 100%, or 0.67kg (1.25lb) of calcium chloride 77%.

Pool Heaters

Heated pools are popular because they extend the season you can use an outdoor pool and control the water temperature, making swimming more comfortable and relaxing.

There are three ways to heat your swimming pool: with a solar, electric or gas heater. Solar heaters require some place for you to put the solar panels that will soak up sunlight to provide the energy to heat your pool. Solar heaters are fairly cheap and have no additional energy costs, but the panels are ugly, won't work during extensively cloudy weather and take up roof-space. It also has the lowest maintainable temperature - so if you want hot, solar is not for you. Electric or heat-pump heaters pump heat out of the air and into the pool. They can maintain extremely high temperatures (32° Celcius or 90° Farenheit) when the outside temperature is as low as 7° C (45° F). They are the most expensive initially, but have relatively low energy costs. Gas heaters are cheaper than electric, heat the pool up faster than any other system and can maintain the temperature regardless of the outside temperature. However, they have higher energy requirements.

Safety and Numbers

Swimming pools are essentially dangerous tubs of water. Stories abound of toddlers who drowned after wandering into an unprotected swimming pool area. Toddlers and infants can drown in six inches of water, so whatever the size, shape and depth of your pool, keep it fenced-in and locked whenever there is no supervision.


For both the safety of the swimmers and the cleanliness of the pool, commercial pools should have a strictly enforced limit on the number of bathers allowed in the swimming pool at any given moment. The New York Department of Health recommends allowing 25 square feet (7.6 square metres) per swimmer; regulation may vary according to the location. If there are no local requirements, set a number that will allow pool users to swim comfortably and will not overload the santisation system. Too many bathers will inhibit the chlorination process and release an unhealthy amount of dangerous chemical by-products that can cause bacterial infections in the ear and skin, breathing difficulties and miscarriage or birth defects.

The square footage is calculated based on the surface area of the swimming pool. In a simple rectangular or square pool the surface area is the width multiplied by the length. Irregularly shaped pools may require recourse to a high school geometry textbook for calculating the area of a circle, ellipse or trapezoid.


Lifeguards are the most public faces of a swimming pool. Their job includes ensuring that swimmers obey safety rules, ensuring that they swim in a depth suited to their swimming level and watching for any signs of struggling or drowning among the swimmers.

The number of lifeguards necessary to guard the swimmers varies according to the pool and its use; a backyard wading pool may require nothing but adult supervision while a wave pool will need many specially trained guards. Pools limited to lap swimming will need far fewer guards than a public free-swim pool. The average age of the swimmers or the experience level of the lifeguards you employ may also affect the number of lifeguards necessary to keep the swimmers safe.

The New York Department of Health requires one lifeguard for every 75 bathers in pools under 3,400 square feet. For pools larger than 3,400 square feet they require one lifeguard per 3,400 square feet or fraction thereof, and an extra lifeguard in the pool is filled over half capacity (capacity calculated based on 25 square feet per swimmer). Local regulations will vary, but it doesn't pay to skimp when the safety of patrons or family is at stake.

1Spas, saunas and hot tubs are even more difficult to keep safe and clean due to their higher temperatures and smaller volume, and are not covered in this entry.2Diatoms are a class of algae which, together with dinoflagellates and a few other algae, are the basis for all life in the sea. They have skeletons made of silica (SiO2) which is very porous and absorbent, and therefore useful for filtering and absorbing materials from various liquids.3That is, splits into two compounds. The long, impressive word for it just means that one chlorine atom is oxidised (gets an additional oxygen atom) while the other is reduced: CL2 (g) + H2O(l) > HOCl(aq) + HCl(aq)4Also know as hypochlorous acid.5A commercial chemical testing kit can be purchased at your local pool supplies store. Ensure before you buy that the kit includes test reagents for all of the necessary chemicals. All kits differ in how they test; spend an afternoon curled up with the manual if necessary.

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