Swimming Pool Water Treatment

Guide to Swimming Pool Water Treatment

In this expert guide the pool specialists at Water Treatment Services take a brief look at swimming pool water treatment techniques including how to maintain pool water balance, the importance of biocides, controlling algae and the role of filtration.

Getting your pool water treatment plan under way

The first step in treating swimming pools is to determine the volume of water by doing some simple calculations. Dosage rates on most swimming pool products you will use are usually expressed in weights per 1000 litres of water. Your maintenance manual should also give the capacity of your pool, spa or hot tub and guidance on water treatment techniques to keep the water clear, clean and safe for use.

Maintaining pool water balance

If the balance or pH of the swimming pool water is too low, the usual treatment is to add sodium carbonate or soda ash at the rate of 10g per 1000 litres every day until the water reaches the correct levels.

If your water is too alkaline, use dry acid (a pH reducer) at the same concentration, again repeating testing to make sure you’re keeping the water in the right pH range at all times.

If you are in any doubt as to the correct chemical dosage rates to use, always check the manufacturers instructions first.

Treating your swimming pool using biocides

As well as controlling the pH, you’ll also need to disinfect your pool water on an on-going basis. This means using a pool biocide to kill off and control any bacteria or algae. Bacteria gets into the water from the air, bathers skin or bodily fluids, and from environmental matter such as leaves dropping into it. If you’re not taking steps to maintain suitable levels of disinfectant in the swimming pool water, bacteria could start to grow rapidly. There are several options for disinfecting the water in your pool, and all have their advantages and disadvantages. Here we consider two of the most common approaches.


Most of us have heard of chlorine and it’s the chemical which instantly springs to mind when thinking of keeping your pool or hot tub clean and free from dangerous bacteria. In some forms, chlorine can be extremely toxic, but the commercially available chlorine based swimming pool products are safe when used according to the manufacturer’s instructions.

When you first set up your new pool or spa you’ll need to perform a starter or shock treatment, and after that, keep the levels topped up to the recommended levels. You might also need to give the water an extra treatment known as a “super chlorination” if you notice algae growing in the pool or after a weekend where it’s been used heavily.

Salt-water chlorination

Another popular way of keeping your swimming pool water clean is by using a salt-water chlorination process. This involves using standard salt and an electrolytic cell which breaks the salt down to produce chlorine. This method is better for those who find their skin and eyes irritated by standard chlorine, and who don’t like the bleaching effect which it can have on your clothing and hair. The electrolytic system also adjusts the levels of salt automatically to ensure the chlorine is maintained at optimum levels.

Controlling algae in pool water

One of the other big issues affecting water quality in swimming pools, spas and hot tubs is the growth of algae. Algae is a type of single-celled plant which lives in water and can grow very quickly when there’s not enough disinfectant in the water to keep it under control.

Algae will discolour the water leading to unsightly pool conditions and poor water clarity. The type of algae present will determine the most appropriate treatment.

Green algae

Green algae is the most common type, and can be dealt with using a shock treatment of chlorine combined with an algaecide to prevent it reoccurring.

Mustard algae

Yellow or mustard algae is trickier to deal with, and along with a suitable shock treatment, the walls and other surfaces of the pool or spa will need to be manually scrubbed.

Black algae

Black algae is one of the toughest types of algae affecting pools and to remove effectively may require several shock treatments. A strong algaecide will also be required to stop it returning, and suitable chlorine levels will need to be maintained. In severe cases of black algae, the only solution might be to drain the pool or hot tub, clean and scrub thoroughly and then refill.

Using ultraviolet light to treat algae in pools

Another, easier way of treating algae is by using ultraviolet light. This technique completely destroys algae and other bacteria in the water. UV treatments are fitted directly into the pool management system and as the water passes over the UV light, bacteria and algae are killed. UV treatment systems tend to be cost-effective to run and can be retrofitted to most swimming pools. UV treatment won’t however deal with algae that is already present and growing on the walls of your hot tub or pool.

Filtering your swimming pool

Over many weeks of use, dissolved solids might start to build up in the water of your swimming pool or spa – all that sun cream, pollen, skin and dirt has to end up somewhere. Dissolved solids are often too small to get trapped in the filter of your pool or hot tub, which is designed to trap larger particles and debris.

If you use an ultraviolet (UV) system in your pool or hot tub, then this will reduce the amount of dissolved solids in the water. However, if you don’t operate one of these systems, then filter aids, including specialist pool flocculants and coagulants can be used to encourage the small particles including Giardia & Cryptosporidium oocysts to “clump” together until they are big enough to become trapped in the filter.

Safety note – Swimming pool chemicals can be dangerous and should always be transported, stored and used strictly in accordance with the manufacturers instructions. Always read the label and follow all safety information provided.

Expert swimming pool water testing solutions

Contact Water Treatment Services today to find out more about our laboratory water analysis services including microbiological testing of pool, spa and hot tub waters.

With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained technicians and consultants we can offer professional, cost effective microbiological water analysis solutions to support your swimming pool water treatment programme.

Contact us today for more information or for your FREE, no obligation quote.

Further reading…

More information about our swimming pool water testing and microbiological analysis … here →

Guide to Swimming Pool Water Treatment
Article Name
Guide to Swimming Pool Water Treatment
In this expert guide the pool specialists at Water Treatment Services take a brief look at swimming pool water treatment techniques including how to maintain pool water balance, the importance of biocides, controlling algae and the role of filtration.
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Water Treatment Services
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