Pool chlorine and pool acid, and why they are kept apart

What the alert describes

WorkSafe Victoria issued a safety alert about the hazards and risks of automatic chemical dosing systems at swimming pools, and it was written about one particular plant: a public aquatic centre. It is set out here as exactly that, an account of what an automatic system in a public pool did, because it is the plainest public description of what happens when pool chlorine and pool acid meet.

The sequence in the alert has four steps and no gaps. An electrical fault shut the water filtering pump down, so no water was flowing in the pipeline. The automatic dosing system that administers chemicals to the pool water then inadvertently mixed sodium hypochlorite, which is pool chlorine, and hydrochloric acid, which is pool acid. Both chemicals went into the pipe that normally carries filtered water back to the pool, and with nothing flowing, nothing carried them away. The result was a toxic chlorine gas. Employees and swimming pool patrons were injured when it was released into the pool area. The alert states the condition plainly: adequate water flow in that pipeline is what prevents chlorine gas.

What chlorine is

The same alert describes the chemical that was released. Chlorine inhibits bacterial growth in water, and it is an active ingredient in several cleaning products and in disinfecting pool water. It is poisonous to touch, to swallow and to breathe in, and chlorine poisoning can be fatal: it causes symptoms through the body, among them coughing, trouble breathing and fluid in the lungs.

From WorkSafe Victoria: swimming pool chemical mixing causes toxic gas.

What the alert asks of the plant it was written about

The alert's own rules are addressed to whoever operates a pool with an automatic dosing system, and a home pool with no dosing plant has none of this machinery. They are here because they are the rules that were written after the failure, and a pool serviced by hand meets the same two chemicals.

• No dosing without flow. Dosing is to be interlinked with what the filtered water line back to the pool is reading, the dosing system is to be interlocked with the filtered water pump, and no dosing should occur while that pump is not in operation.
• Fail-safe pumps. On a power failure, all the pumps on the filter water and the dosing system are fail-safe and have to be restarted by hand when power comes back.
• Distance between the containers. The alert asks that sodium hypochlorite and hydrochloric acid be kept at least 3 metres apart, each in its own separate bund or spill tray.
• A barrier between them. A physical barrier has to stand between the two containers, so that a dosing line failing, splitting or coming disconnected cannot let them mix.
• Paper and clothing. Pool chemical containers must always be clearly labelled, workers must be given clear instructions for storing and handling the chemicals safely, and safe work procedures are needed for restarting the dosing and filtration system after a shutdown. Protective equipment is to follow the manufacturer's safety data sheet when the containers are handled and the dosing lines charged, spills are to be cleaned up immediately following the data sheet, and the alert asks for adequate supervision and re-assessment of competency in using the system.
• The law behind it. In Victoria the Occupational Health and Safety Act 2004 puts a duty on employers to keep a workplace safe and without risks to health, as far as is reasonably practicable; where a risk cannot be removed altogether, it has to be brought down as low as is reasonably practicable; and people other than employees must not be exposed to risks arising from the employer's undertaking. Employees must be given the information, instruction, training and supervision they need, and must take reasonable care for their own and others' safety. The Dangerous Goods (Storage and Handling) Regulations 2012 set duties for whoever occupies premises where dangerous goods are stored or handled, and a code of practice gives the practical how.

What WorkSafe Queensland asks a pool shop

The other workplace safety source here answers the questions a pool chemical retailer asks, and its answers apply to retail situations: pool shops, hardware shops and other general retail shops where the chemicals are sold in retail pack sizes and limited aggregate quantities. The bigger stores it names, such as stock rooms, warehouses and dedicated storage locations, may need stricter practice. A home pool's shed is neither; the section after this one sets out what WorkSafe Queensland says about storage at home.

Keeping them apart, in the regulator's numbers

• The regulation requires the identification of any risk of a chemical or physical reaction, and a distance between incompatible chemicals can be reduced with physical barriers, such as rigid impervious splash barriers, that ensure the products cannot interact. Separate spill compounds may be required to keep highly reactive combinations apart, acids and hypochlorite solution among them, and a dedicated compound contains leaks and spills in the immediate area so they cannot reach incompatible products elsewhere.
• Calcium hypochlorite is a Class 5.1 oxidiser in Packing Group II, and it reacts with many things, including water and moisture, dust, dirt and grime. A Packing Group II product has to be kept 5 metres from any incompatible product it could react dangerously with, acids among them, and a barrier that prevents the interaction can reduce that distance.
• In a retail display, packs of calcium hypochlorite should sit at least 1 metre from other incompatible solid products. The liquids it names, from liquid pool chlorine to clarifiers, acids, paints and algaecides, should not sit on shelves above the dry pool chlorine.
• Residues must never be mixed while cleaning up, because some combinations are highly reactive: there have been instances where residues of different white powders and other material were swept together into one waste container and caught fire from the reaction.

The store itself

• A bund has to be impervious and resistant to chemical attack, and a masonry bund, such as a block wall, needs an impervious membrane so spilled product cannot soak into the brickwork. A bund is there to contain the contents of a tank if it leaks, so anything else kept inside it takes up the volume being relied on for the spill and gets in the way of inspecting it. The regulation requires leaks and spills to be contained and managed safely. Where a bulk tank is used, AS 3780-2008 recommends a bund holding at least 110 per cent of the tank's capacity, so a 2000 litre tank needs at least 2200 litres of it, with the bund wall standing at least half the tank's height away from the tank.
• Ventilation is part of the design: the standards recommend enough natural or mechanical ventilation to disperse dusts, mists or vapours, and a closed cupboard needs airflow, which also keeps down the chlorine odours these products give off.
• A chlorine drum should carry a vented cap if it does not already have one, so gas can escape as the product ages; without that release a container can over-pressurise and rupture. A container should also be checked for ageing, with a look out for UV degradation, embrittlement, crazing or discolouration of the plastic, and a container that is not UV stabilised ages faster if it is stored outside.

The amounts, and the triggers

This is the part of the regulator's answer most often misread. The regulation it works under, the Work Health and Safety Regulation 2011, sets no limit as such: the maximum quantity at a workplace is determined by its design and layout, with spill containment among the influences, and certain duties apply depending on the quantity. The prescribed quantities are not limits but triggers for further action: past 250 litres of hydrochloric acid or 1000 litres of hypochlorite solution a hazardous chemical placard is required, and past 2500 litres of acid or 10,000 litres of hypochlorite solution the workplace has to keep a manifest and notify Workplace Health and Safety Queensland. AS/NZS 3833:2007 gives tailored requirements for maximum storage quantities and pack sizes in retail situations, such as display areas.

From WorkSafe Queensland: pool chemical retailers FAQs.

Where a home's own store sits

Queensland's Work Health and Safety Regulation 2011, the one the pool shop above works under, has a different edge for a place that is not a workplace. It puts a ceiling on how much of these materials a non-workplace may store and handle before the hazardous chemical requirements begin, and those requirements are the labelling of containers, a register, safety signs, placarding, spill containment, protection against damage, fire protection, and the installation and operation of tanks. A non-workplace, a domestic residence for instance, may hold up to the quantities in the regulation's Table 328 and be exempt from those particular requirements, and once those thresholds are passed the whole of that part of the regulation applies alongside the general provisions.

For corrosive substances there is a published number to work against. AS 3780-2008 sets minor storage limits for corrosives of 12.5 kilograms or litres of Packing Group II and 50 kilograms or litres of Packing Group III inside the dwelling, and 25 kilograms or litres of Packing Group II and 100 kilograms or litres of Packing Group III at the location. Past those quantities the standard recommends a dedicated storage installation with its own design features, and the regulator's answer adds one more consideration: storing hazardous chemicals beyond normal household needs may have implications under an insurance policy.

Safe Work Australia's own information sheet is written for a workplace that stores, displays or sells swimming pool chemicals, and its opening lines give the reason this guide exists: swimming pool chemicals can be very dangerous when stored incorrectly. The sheet identifies which chemicals should not be stored together, sets out other general storage requirements, and covers retail displays.

From Safe Work Australia: storage of swimming pool chemicals.

The words on the container, and the paper behind it

Where a pool shop fills a customer's own container, the regulator is clear that the shop has to make sure the container is fit for purpose and properly labelled before it is filled, and the label must carry the product name, hypochlorite solution for instance. Decanting pool chemicals into other containers is allowed, provided the container is fit for purpose, labelled, and clearly identifies what is inside it. Package sizes are capped for these products under the Listable Chemical Product (Home Swimming Pool and Spa Products) Standard 2007: a liquid must not exceed 25 litres and a solid must not exceed 25 kilograms.

Behind the label sits the safety data sheet. Workers have to be able to get the data sheet whenever they handle or use a hazardous chemical, and where protective equipment is required to keep the risk down, the regulation requires that it suit the work, be maintained, and actually be worn. WorkSafe Queensland's answers to pool shops' questions describe what handling looks like when it is done properly, and they are worth reading for the reasons behind each item: splashes have caused serious injuries, and in one case a delay in tending to a contaminated sock and boot led to serious skin damage and hospital treatment. The regulator's answer to a spill is equally specific: the data sheet gives the method, dry sand or a proprietary inert absorbent such as kitty litter will collect the liquid, an acidic material such as sodium hydrogen sulphate must never be applied to a hypochlorite spill because it reacts dangerously and releases toxic chlorine gas quickly, and the supplier can name the absorbents and neutralisers that suit the product.

Why this sits on the circuit

On the drawing this guide is the dotted branch to the chemical store and the dosing point beside it, because that is where the two chemicals live in a working pool: stored apart, and added to the water one at a time. WorkSafe Victoria's alert says the two are added separately to the filtered water as it returns to the pool.