water treatment

hygiene and purification

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Salt Chlorinators - Ionisers - Ozone


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Pool chemicals kill bacteria some of which are pathogenic, and algae which turn the pool green and make it unsightly. Filtration removes the dead bodies of bacteria and algae and so prevents the pool water from turning cloudy. Filtration is insufficient on its own as living bacteria and algae are not killed within the pool, and live organisms are sticky and tend to clog filters particularly cartridge ones. Chlorine is the most widely used chemical. It is inexpensive easy to use and many like its clean fresh smell. The concentration used in domestic pools is lower than that of public pools where most of the unpleasant smell and stinging of eyes is caused by chloramines (chlorine attached to ammonium products largely from urine). There are now alternatives to chlorine for those who do not like the smell or taste of chlorine or for those who are allergic to it. These are excellent but will work out more expensive.


Types of chlorine products

For domestic use chlorine is supplied as a liquid or as granules or tablets. If cyanuric acid is part of the formula then the chorine is said to be ‘stabilized’. This means that it lasts longer because it is attacked less by UV light. Some stabilizer is therefore valuable but problems arise if it becomes too concentrated when a condition called chlorine lock occurs. Then the chlorine remains in a combined form so is not available to kill micro organisms. Excess stabiliser has to be diluted out as it does not get used up like chlorine.

Chlorine without cyanuric acid is ‘unstabilised’ and is referred to as shock chlorine. When shock chlorine is added to a pool it works rapidly killing bacteria and algae and gets used up.

The best solution when sanitising a pool with chlorine is to use a combination of ‘stabilised’ and ‘unstabilised’ chlorine. A low concentration of ‘stabilised’ chlorine of 1-2 ppm should be maintained and periodically, say once a week, the pool should be shock dosed with ‘unstabilised’ chlorine up to 5ppm. This will kill any micro organisms that are resistant to low levels of chlorine. Ideally this should be done at night so by the next day the chlorine level has fallen.

Chlorine concentration is measured using DPD No1 in a test kit. All chlorine treated pools should have a pH between 7.2 and 7.6. A low pH is acidic and corrosive so will damage pool parts and sting eyes. A high pH is alkaline and stops any chlorine present working as well as being scale formin (adjusting pool pH is described later).

Liquid chlorine

(Sodium hypochlorite) This is unstabilised and very effective at sanitising a pool. However it has a short shelf life [less then 3 months] is dangerous to use, bleaches clothing and comes in heavy 20 litre bottles.

Shock granules

(Calcium hypochlorite) This is unstabilised, acts quickly, and has a long shelf life. As some of the calcium does not fully dissolve it can cause some cloudiness unless it is mixed with water and added through the skimmer. The new quick dissolving shock over comes this but with a slight increase in cost.

Non-chlorine shock granules

(Potassium Monopersulphate). SHOCK AND SWIM! NO WAITING! This powerful chlorine-free shock zaps organic pollutants, bacteria, and algae and boosts chlorine’s killing power. Regular use also clarifies and restores sparkle to pool water. Quick dissolving formula requires no pre-mixing and will not bleach your vinyl liner. The perfect shock for weekly maintenance. Compatible with bromine. Active ingredient: Potassium Monopersulfate. Dosage: 0.5kg per 10,000 gallons

Stabilised chlorine granules

(Sodium Dichloroisocyanurate) Long lasting with a long shelf life. Should be added regularly to maintain a concentration of 1-2 ppm. More will be needed during hot weather, warm water, high use, or when the pool cover is off for extended periods. NB With liner pools chemicals should be dissolved first before adding as there is a risk of undissolved chlorine bleaching the liner. Never dissolve two chemicals together

Stabilised Chlorine tablets

(Sodium Trichloroisocyanurate) Long lasting with a long shelf life. These must not be put directly in the pool but held in a feeder where they slowly dissolve. Floating feeders are simple and cheap. Some are ready filled and disposable when empty and others are refillable. Inline feeders in the pump house cost around £100 and provide a semi automatic chlorination system. The amount of chlorine they release is easily adjusted to suit the needs of a particular pool and will deliver chlorine whenever the pump is running. Once set up it requires little attention other than the weekly shock dose of unstabilised chlorine. Some people put chlorine tablets in their skimmer basket. Although this works it will shorten the life of the basket as it makes the plastic brittle and could be dangerous if children put their hands in. NB Never mix pool chemicals together in their undissolved concentrated form as they can react together. Stabilised and unstabilised chlorine, for example, form an explosive mixture if mixed in say a bucket or skimmer basket.


Bromine Tablets

Actually contains both chlorine and bromine combined. Less odour than chlorine and less corrosive in the atmosphere so is useful for indoor pools particularly ones attached to a house. More expensive than chlorine and attacked by light so is not often used in outdoor pools unless bathers are particularly allergic to chlorine. As supplied in tablets it must be added via special feeders. The pH must be kept between 7.2 and 7.6

Blue crystal

This is a chlorine free two part liquid treatment. It has no taste or smell and is harmless to plants around the pool. It also contains no algaecide and is not attacked by light. Treatment involves adding an initial dose of Blue crystal Alpha followed by a weekly dose of Blue crystal Omega. The amount needed is calculated from the pool volume and testing to see how much Omega is used up so it can be added more often if necessary. Shock dosing with unstabilised chlorine every 3-4 weeks is good practice to kill off resistant organisms. Use of Blue crystal is more expensive than chlorine. This product is less pH sensitive


Sanitizes with active oxygen and is a two part treatment: Bayroklar and Bayroplus which activates the Bayroklar. It comes either as a 2 tablet system or as a tablet and a liquid. The amount needed is calculated using pool volume and a repeat dose is needed weekly. Shock dosing with chlorine is possible but not essential. It has no taste or odour and is environmentally friendly but is more expensive than chlorine. The pH of the water must be kept between 7.2 and 7.6


This liquid product is not compatible with chlorine so the pool must be chlorine free before use. A pool left untreated over the winter will be chlorine free or the chlorine can be neutralized with Baqua Start. A start up dose of Baquacil and Baqua Check is added then after 24 hours the level of Baquacil checked with a test kit and topped up if necessary. Then the levels of each are checked weekly with further top ups as required. The pool should be shocked with Baqua Shock monthly. The pH should be kept between 7.2 and7.6


Pool water must have a pH between 7.2 and 7.6.and is tested using Phenol Red in a test kit.

pH minus

A pH above 7.6 (“high”) is lowered by adding pH minus (Sodium Bisulphate or pool acid)

pH plus

A pH below 7.2 (“low”) is raised by adding pH plus (Sodium carbonate or pool alkali)


Chlorine kills algae but using algaesides in addition to chlorine is popular as they help to prevent the pool going green if the chlorine level accidentally falls. Some need to be added in small amounts weekly but the most popular is “long life” with one dose lasting up to 6 months. Those containing copper should be avoided by people with very blond hair who spend a lot of time under water. Algaesides are designed to inhibit the growth of algae but do not kill it. This must be done with shock chlorine.


These can be used with chlorine and bromine treated pools to help remove cloudiness due to fine suspended particles that the filter is having difficulty removing. They are not suitable with cartridge filters.

Granular floc

(aluminium sulphate/alum) This is sprinkled over the surface and the water recirculated [not filtered] for 12 hours. The pump is switched off and the floc is allowed to settle taking with it the particles. It is then vacuumed to waste.

Floc tablets

These are placed in the skimmer basket where they make the fine particles clump together so they are removed by the filter.

Jolly jel

A new “jelly like” cube that when placed in the pump basket removes cloudiness. Highly recommended.

Liquid Clarifiers

Polymer based liquids that cause fine particles to clump together so that they can removed by the filter.


These use copper and silver electrodes which release ions that kill bacteria and algae. They enable pools to be run with lower levels of chlorine but they can not be used on their own.


Ozone is generated when a molecule of oxygen is illuminated by UV light. It destroys algae bacteria and viruses on contact, breaks down harmful chemicals and acts as a flocculent. It also oxidizes combined chlorine and bromine freeing them to be re-used. As micro-organisms are killed only as they pass through the unit a residual of chlorine or bromine must be kept in the pool.


Salt is added to the water at a concentration of human tears and electrolysis used to produce chlorine from it. It is necessary to check that heaters ladders etc are suitable for salt water. Salt chlorinators are not suitable with metal above ground pools.


For Square or Rectangular pools Length x Width x Average Depth x 6.25 = Gallons For Round or Oval pools Length x Width x Average Depth x 4.9 = Gallons. Measurements in feet

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