The chlorines: their advantages and limitations

Macquatics Training
By John McKenny, Macquatics Training
Thursday, 02 June, 2016

The chlorines: their advantages and limitations

In this article — the first in a series — John McKenny from Macquatics Training outlines the basics of chlorine: what’s what, and what to do with it. This is the first of several non-technical articles containing helpful information and hints on how to properly operate any swimming pool. Keep your costs down and your pool clean and safe for swimming by following ‘Good water matters’ in each publication.

All commercial pools — the ones that let the public in, including all learn-to-swim pools — must use chlorine for adequate and necessary disinfection. Necessary, because various governments stipulate its use, and rightly so, although there are a host of chlorine alternatives out there now and some work well.

The amount of chlorine needed to kill off the pool water’s contaminants varies depending on a host of factors, but we’re talking about tiny amounts of chemical. For example, an average indoor 25 m pool needs only 1 L or kg of pure chlorine for general sanitation. That’s about 8–10 L of normal liquid chlorine, because it’s usually available at about 12% strength in some states and 10% in others.

The golden rule here is to initially test the water and add recommended quantities of the chlorine you’re using, depending on the test, pool volume and maybe the weather if it’s an outdoor pool.

Chlorines are oxidisers. This means that they’ll destroy pathogens by the chemical attachment of oxygen (or the removal of hydrogen) to the unwanted substance. (Getting more technical than this simple explanation is unnecessary for our purposes.)

As long as chlorine is used at correct levels and other factors such as pH, water balance, low total dissolved solids (TDS) and adequate vacuuming are attended to, most people can swim in chlorinated water for extended times and not be aware of the presence of any chlorine.

Chlorine: the basics

When any chlorine is added to water, there is a simple chemical reaction:

Chlorine + water => hypochlorous acid

Hypochlorous acid is also known as ‘free chlorine’. We say it’s free because it is free and available to attack any contaminants.

Regardless of which chlorine is used, there are other unwanted compounds produced. Then the fun part starts: adding what I call ‘corrective chemicals’ to keep the water balanced and keep it clean, safe and comfortable for swimming.

Let’s look briefly at the common chlorines available:

1. Sodium hypochlorite (AKA hypo)

Form: liquid

Concentration: 10–14% chlorine


  • Price and availability.
  • Easily automated and added in small amounts via metering pumps.
  • Commercially, delivered in large amounts with no manual handling required.
  • Kills all pool water contaminants including Cryptosporidium. (Note: Sodium hypo will take 7–10 days to kill Crypto at a normal pool concentration. By increasing the chlorine concentration and lowering the pH, Crypto can be destroyed overnight. The use of chlorine dioxide is recommended also and is mentioned further on in this article.)


  • Hazardous. If added or used manually, must be handled correctly.
  • Reactive. Can react dangerously with other chemicals (acids).
  • Doesn’t store well. (My own trials show that at 26° and stored out of sunlight, hypo will weaken at 30% per month. In six months, it will be a very weak sanitiser, down from 13% to 1.53% chlorine. What a waste!)
  • When added, the pH will increase, so an acid is needed to bring the pH back to the ideal level of pH 7.4.

2. Calcium hypochlorite (AKA cal hypo)

Form: granular, powder or briquettes

Concentration: 65–70% chlorine


  • Easily automated.
  • Less residual chemical in the water as less is needed for adequate sanitation.
  • Provides the vital chemical calcium to the water along with low TDS.
  • Very little breakdown over extended periods if stored correctly.
  • Will also kill all contaminants and can quickly increase pool chlorine levels.
  • Is ideal for algae control; dry cal hypo can be sprinkled over the pool surface in small amounts. Remember always: it must never be mixed with anything!


  • Very dangerous if not used exactly according to instructions. It must be stored away from all forms of acid and never mixed with any other chemical.
  • Considered by some as expensive when used in large public pools.
  • High calcium levels often result if it is continuously added.
  • All spills must be treated as if the chemical is now contaminated, and must be correctly disposed of in a non-sealed container to avoid reactions and/or chemical fires.

3. Chlorine gas

Form: gas

Concentration: 100% chlorine

Used for pools and town water treatment. This chlorine produces acids — hypochlorous acid (free chlorine) and hydrochloric acid — when added to pool water, whereas the first two chlorines produce an alkali. With gas chlorine, the addition of an alkali is needed to keep the pH up to the desired pH 7.4.


  • No handling required. Supplied in pressurised vessels and fed directly into the water.
  • Large one-tonne cylinders can supply enough chlorine for a full six-month season, so handling and freight costs can be comparatively low.


  • Gas leaks, although unusual, can be very dangerous.
  • Special equipment — gantry, hoist, leak detector, chlorinator, own room, expensive automated equipment, breathing apparatus and other personal protective equipment (PPE) — is needed for safe handling and use.
  • A good supply of sodium carbonate (soda ash) is also needed for pH correction.

4. Lithium chloride/hypochlorite

Concentration: 35% chlorine


  • Excellent shelf life.
  • Easily handled.
  • Good solubility.


  • Cost.
  • Hazardous.
  • Can cause health risks at high concentrations.

5. Sodium dichloroisocyanuric acid (AKA dichlor) and trichloroisocyanuric acid (AKA trichlor)

Concentration: 50% chlorine, 50% cyanurate (dichlor); 90% chlorine, <10% cyanurate (trichlor)

pH: approximately 7.0 (dichlor); approximately 3.0 (trichlor)


  • Both will chlorinate and provide stabiliser (cyanurate) to the water.


  • Both chemicals are often misunderstood and ultimately misused. The chlorine in these compounds is used up during sanitation, but the cyanurate accumulates, eventually causing unacceptable and often unsafe water conditions. In properly managed water, cyanurate should always be maintained between 15–20 mg of cyanurate, but I have found pools with cloudy water, algae growing and no-one swimming, with cyanurate levels in the hundreds — only because these two chemicals have been continuously added.

6. Chlorine dioxide (AKA the Crypto killer)

From: tablets and gel

pH: 6.0–7.0


  • Attacks and destroys Cryptosporidium approximately 100 times faster than chlorine at normal levels. (So we’re told.)
  • Using chlorine dioxide following a lose stool faecal accident in pool water results in minimal pool downtime, along with proven and complete disinfection when a recommended operating procedure (SWMS) for the particular swim centre/backyard pool is followed.


  • Cost.
  • Having yet another hazardous chemical for a faecal emergency may prove unnecessary.
  • The active life — particularly of the chlorine dioxide gel — is limited.

7. Sodium chloride (AKA salt)


  • Cheap form of chlorine for smaller and backyard pools.
  • Safe.
  • Stores well.


  • Quite often, pool owners are advised to add enough salt to their pools for a salt concentration of 4500 mg/L or more, whereas a level of 2800–3200 mg/L is sufficient and should avoid the salt taste when swimming. Excessive salt means using extra amounts of other chemicals to keep the pH and other tests correct for comfortable swimming.

What’s next?

Stay tuned for the next instalment of ‘Good water matters’, which will focus on pH.

John McKenny has managed and leased swim centres for more than 30 years. He commenced TAFE teaching in swim centre operations and management 25 years ago and continues today. John is the author of The Complete Swimming Pool Handbook and the Leisure Pool and Spa Handbook. Obligation-free advice given any time.

Image credit: ©Shawn Hempel/Dollar Photo Club

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