Calcium Hypochlorite

• dry product (pellets, tablets or granules)
• 65 – 70% chlorine
• pH of approx 10 – 11 (strong alkali)
• recommended for soft water areas
• long storage time

This is a dry product and typically gets supplied as pellets or granules. It often goes by the name of HTH, but this is a common brand name, not the name of the chemical. It usually gets mixed with water to form a solution and then gets injected into the circulation pipework either before or after the filters (systems that include non-residual disinfection, such as ultra violet or ozone will always have their chlorine dosed after the non-residual disinfection point of contact, which will always be after the filters). Sometimes it gets dosed via a circulation feeder (discussed in a later section).

As the name suggests, calcium hypochlorite contains quite a lot of calcium compared to sodium hypochlorite. Therefore, it is usually not recommended to use this type of product in hard water areas, as there will already be a lot of calcium in the source water. Because of the high calcium content, calcium hypochlorite creates a rather ‘gritty’ solution when mixed with water. This grit settles out of solution on the bottom of the tank and can also clog up the feed lines and injector points etc.

Calcium hypochlorite is a dry and relatively stable compound of chlorine, calcium and oxygen. It must be kept dry and free from contact with all organic materials including paper products, oil and oil products, detergents, cleaning fluids and acids. Contact with organic materials, including isocyanurates and other chemicals, causes a heat reaction, and can lead to explosion, fire and the emission of toxic fumes. Contact with acids liberates toxic chlorine gas.

Spillage should be avoided, as mixture with other chemicals already on the floor or other surfaces could also cause these problems. It should be stored in sealed containers, off wet floors and away from pipes and hot water heaters.

There must be ‘no smoking’ signs in the storage area where this chemical is kept. Suitable personal protection should be used when handling and the provision of an emergency shower considered in large installations.

Sodium Hypochlorite

• wet product (liquid)
• 10 – 15% chlorine
• pH of approx 13 (strong alkali)
• recommended for hard water areas
• short storage time

This is a liquid product and is very similar in appearance and odour as ordinary household bleach. It usually gets delivered in plastic carboys and is then transferred to the day tank(s) via a hand pump. In larger facilities it gets delivered via a tanker that fills up a bulk tank at a filler point and then the product is transferred to the day tank.

Sodium hypochlorite is the recommended disinfectant for areas that have hard source water. If calcium hypochlorite was used in a hard water area, the result would be calcium hardness levels that are too high, leading to problems with scaling.

As sodium hypochlorite is a liquid; if a liquid acid is used with it, there should be safeguards to prevent any confusion between them. The inadvertent direct mixing of an acid with sodium hypochlorite will liberate toxic chlorine gas and the system should be designed to prevent this taking place.

Sodium hypochlorite can also react vigorously with oxidising materials such as chlorinated isocyanurates. Suitable personal protective equipment should be used when handling and there should be ready access to an emergency shower where bulk tanks are used.

Electrolytic Generation of Sodium Hypochlorite

Some sites have a system whereby they create their own sodium hypochlorite. This is achieved by making a brine solution (water + salt) and then passing this solution through an electrolysis unit that converts a proportion of the brine solution into sodium hypochlorite. The sodium hypochlorite gets transferred to a day tanks and gets progressively stronger with each pass of brine solution through the electrolysis unit. The big advantage to this system is that staff are not required to handle the sodium hypochlorite. All they need to handle is the salt when they top up the brine tank. Some of the disadvantages of the system are that it produces hydrogen gas, which is explosive and the equipment required can be expensive.

Hydrogen gas released during the electrolytic process should be vented safely into the open air. Selection and siting of any electrical equipment associated with the electrolytic generator requires careful consideration.

Maintenance of electrical equipment is likely to be a job for specialist staff, but staff should be aware of the general hazards of using electrical equipment near these processes.

Chlorinated Isocyanurates (Stabilised Chlorine)

For the purposes of pool plant operations, 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? Because in outdoor pools, 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 to the circulation system in conjunction with sodium/calcium hypochlorite (but dosed separately – never allow them to mix). Many outdoor pool operators prefer to add a chemical that contains both the chlorine and the cyanuric acid. There are two chemicals on the market:

• Dichloroisocyanuric acid (dichlor). 55% available chlorine. Comes as a white powder. pH is around 6.5 (slightly acidic).
• Trichloroisocyanuric acid (trichlor). 90% available chlorine. Comes as white powder, granules or tablet. pH is around 3.0 (fairly strong acid).

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.

There is a range of products in this category, with many brand names. They are white or off-white granules or tablets with a chlorine odour. Confusion with other similar-looking chemicals must be guarded against. The granules are stable when dry but will slowly liberate chlorine when in contact with water. They can explode in contact with calcium hypochlorite, ammonium salts and other nitrogenous materials and will react vigorously with strong acids, alkalis and reducing agents. Chlorinated isocyanurates should be kept well-sealed in a cool, well-ventilated place, away from combustible materials. Feeders must be designed for the particular chemical, and not used for any other.

We do not recommend the use of chlorinated isocyanurates in commercial pools, unless there is the need to stabilise the chlorine against UV degradation (such as in outdoor pools). A better choice would be a hypochlorite disinfectant, which is not going to push the levels of cyanuric acid up and potentially cause chlorine lock. Otherwise, the pool operator will probably always be battling to dilute the excess cyanuric acid out of the pool, which will cause unnecessary work and also has cost implications related to the heating of potentially large volumes of fresh water.

Bromochlorodimethylhydantion (Bromine)

• dry product (pellets, tablets or granules)
• 61% bromine and 27% chlorine
• pH of approx 3-5 (strong acid)

This product, in stick or tablet form, is stable when dry but will emit bromine/chlorine gas in contact with water. When applied, it is important not to mix the product with other chemicals and to keep it well away from all alkaline substances, eg sodium carbonate, calcium hypochlorite, etc.

A circulation feeder device is normally used for the application of this chemical, and it is important that no other chemicals are placed in this device and that, when refilling, splashing should be avoided by lowering the water level. Strong concentrations of this chemical can cause severe burns to the skin and eyes.

BCDMH should be stored in safe containers in secure premises which are cool, dry and away from oxidisable materials such as paper, solvents, wood, oil, etc.

A potential negative aspect on the use of BCDMH is that a small proportion of bathers develop an itch followed by a visible rash within 12 hours of exposure to water treated with this chemical. The problem is unusual in children and more common in bathers of say 50 years or more.

It’s not a good choice for outdoor pools as it can’t be ‘stabilised’ against ultra violet degradation like chlorine can.