bluepools

Salt water chlorination

Introduction

This series of posts on swimming pool water has been leading up to the subject of salt water chlorination. There is a lot of mis-information being bandied about on various forums and this post is an attempt to look at the subject on a balanced basis. I have set out the post on question and answer basis to get the maximum number of facts recorded in the smallest space.

What is salt water chlorination?

About 150 - 180 kg of water softener type salt is put into the pool when it is first filled resulting in a minimum level of about 3000 parts per million of salt in the pool water. Salt makes the water conductive so that the electricity can pass between the plates in the cell described below. If the salt level goes too low, then the chlorine production simply stops. Salt is just the raw material from which the chlorine is produced.

During filtration this passes around two electrodes in a small (normally translucent) cell unit placed after the filter and heater in the pool pump house - to avoid excessive corrosion of these units. The control unit is a device that sends power to the salt cell. The unit controls how much chlorine is produced by regulating how long the power is applied to the cell. If the control knob is turned down the unit will apply power to the cell for less time, thereby producing less chlorine. The control unit will often sense the level of salt in the pool and indicate the need to add more salt.Self cleaning units have a feature built into the unit that reverses the polarity of the voltage through the cell in order to help clean any scale buildup off of the cell plates (but see below)

What's the difference between a saltwater pool and a pool maintained with packaged chlorine?

Chlorine Level -In Saltwater Pools there is 0.5 to 1.0 ppm chlorine and in traditional Pools there is 3.0 - 10.0 ppm chlorine.

Swimmer comfort - This is subjective - with properly controlled conventional chlorination few people can tell the difference

What is the difference in cost?

Salt water chlorination is substantially more expensive to install (1000 euros or more) and will cost less to run because of the reduced amount of chemicals required.

What are the disadvantages of salt water chlorination?

There are two:

In hard water areas like Charente-Maritime and the Dordogne the very hard water causes scale deposits to form on the electrodes. The cell then needs to be taken apart and the electrodes cleaned off with an appropriate acidic cleaner. This can even happen every month or so if the pH control is not rigorous.

The waste water from the pool needs to be discharged somewhere. There is already legislation in place that is supposed to stop polluted water from reaching streams where salt water can have a highly detrimental effect on vegetation and wild life. It is understood that this legislation may be strengthened in the future with specific reference to private swimming pools. Hence the water will have to be discharged and retained within the property of the pool owner.

Will this have a detrimental affect on the garden ?

It may have but:

• Water Volumes will normally be small
• The salt water concentration is about a tenth or less of sea water salt concentration and the dilution affect of rain will reduce the concentration even further
• The water can be disposed of in a grassy area where salt resistant grass can be sown
• If a pool owner is really concerned a small low cost evaporation pond can be built where the excess salt can be scraped up and put in your bin

Will Witt

Water hardness

Introduction

It is a pleasure to be working inside after the bitterly cold weather we have had in the last few weeks - pool installation in the summer is a very pleasant job, but not in the winter. Few realise that one of the "benefits" of the famous Charente-Maritime micro-climate is that it is always several degree colder here than in England.

Water hardness

After pH and TA - water hardness is another very important aspect of swimming pool water quality.

It is normally measured in parts per million (PPM) but the French measure it in degrees f - but just multiply by 10 to convert degrees f to PPM

How hard is the water at your French property? - Click on the link below to get some idea.

http://www.salines.com/index.php?cat=4&mot=16

But this does not give the full picture because the hardness can vary from commune to commune - your marie should have details from the local water company and be able to give you the range of likely hardnesses for your property.

The water in Charente-Maritime and the Dordogne is amongst the hardest in the world - the French classifications of soft to hard relate to roughly double the calcium content of the comparable US classes.

By now  you are probably wondering why you should care how hard your swimming pool water is.

If you have a vinyl lined pool there is no need for the calcium hardness to be above 100 PPM. The higher it is and the hotter the water the more likely you are to get cloudy water and scale deposits that can occur throughout the pump and filtration system and even on the vinyl lined pool walls. This is particularly likely at calcium hardness levels are above 350 PPM and where the pH and TA are above their ideal levels - these are 7.2 and 120. As you will note from my previous post this pH level is slightly lower than the ideal 7.4 to 7.6 range for optimum chlorination. High pH and a high TA lower the soluability of the calcium and other minerals, thus encouraging their deposition.

Please note that if you have deposits that are not pure white they may be the deposition of other minerals such as iron or copper.

If you have a gunnite or a tiled concrete pool the Calcium hardness should be maintained at about 200 PPM - otherwise the water will etch the walls until it reaches equilibrium with the calcium in the pool walls.

How do you get rid of scale deposits? - drop the pH value to below the optimum for a period - possibly as low as 6 until it has disappeared. You could also add some mineral treatment that will reduce the likehood of deposition of Calcium and other deposits.

How do you avoid their re-appearance? - rigorously control pH and TA and AVOID THE USE OF CALCIUM HYPOCHLORITE by using sodium hypochlorite.

Can you use a water softener? - not really - these cost an arm and a leg to run just for household water - the cost of softening pool water would be difficult to control and very expensive.

Will Witt

 

Swimming pool water chemistry

Introduction

As it is a holiday and I have some spare time from pool installation I thought I would add some more detail on water quality following our posts on this topic in October. This is the first on a series of posts that will end with a commentary that is a very hot topic in the pool industry - the move towards salt water chlorination.

pH

pH is the single most important element in swimming pool water chemistry. It affects every other chemical balance in pool water.

pH is a measure of hydrogen ion (H+) concentration in water. It indicates the relative acidity or basicity of pool water. pH is measured on a scale of 0 (strong acid) to 14 (strong base) with 7 being the neutral pH.

In pools a slightly alkaline pH of 7.4 to 7.6 is apparently the most desirable because this range is most comfortable to the human eye and provides for optimum use of free chlorine while maintaining water that is not corrosive or scale forming - but this is not always true as we will see in later posts.

If pH is too low (below 7)

• Water becomes acidic
• Chlorine residuals dissipate rapidly
• Eye irritation occurs
• Metal fittings, pump impeller, heater core may corrode
• Dissolved metals may leave stains on walls
• Rapid Loss of alkalinity

If pH is too high (above 8)

• Chlorine activity is slowed and inefficient
• Scale formation and discoloration of pool walls
• Water becomes cloudy
• Filter is overworked
• Eye irritation may occur

pH Adjustment

To avoid the problems listed above, pH must be maintained between 7.2 and 7.8.  If the pH is too low raise it by adding pH Plus (sodium carbonate) at the recommended dosage for the pool volume.  Be sure the pump is running when chemicals are added. Allow to recirculate then retest to determine if further treatment is necessary. If problems with low pH persist, it may be necessary to raise total alkalinity to stabilize the pH.

Total alkalinity (TA)

Total alkalinity is closely associated with pH but it is a measure of the ability of a solution to neutralize hydrogen ions. Expressed in parts per million (ppm), total alkalinity is the result of alkaline materials including carbonates, bicarbonates and hydroxides - mostly bicarbonates. This acid neutralizing (buffering) capacity of water is desirable because it helps prevent wide variations in pH whenever small amounts of acid or alkali are added to the pool.

Total alkalinity is a measure of water's resistance to change in pH.

Total alkalinity in a swimming should be maintained in the range of 80 to 150 ppm.

If total alkalinity is too low:

pH changes rapidly when chemicals or impurities enter the water. pH may drop rapidly, causing corrosion etc.

If total alkalinity is too high:

pH becomes difficult to adjust. High pH often occurs causing other problems such as; cloudy water, decreased disinfectant effectiveness, scale formation and filter problems - more on this in later posts.

Total alkalinity can be raised by the addition of soda ash (sodium bicarbonate, baking soda)  - there are other proprietry products on the market that are even more effective - just follow the instructions on dosage.

Total alkalinity can be lowered by adding muriatic acid (hydrochloric acid) or sodium bisulfate or other proprietry products.

Just remember your pool water will only be OK if the pH is right - and that will depend on total alkalinity.

Will Witt

Swimming pool. Charente Maritime, France

The swimming pool at Cherac, near Cognac, Charente Maritime is nearing  completion. The pool has a sand coloured liner with Carthage Vert tile band and sand coloured roman end. The paving is Ardoise Jaune by CaraTerra and is installed with the margelles flush and all joints lining through to give a finish that is very pleasing to the eye. The pool has been fitted with a winter safety cover and will also be protected with a pool safety fence by Encloture. The pool filtration, sanitisation and heating system comprises a 0.75CV pump, 600 dia sand filter, in-line chlorine doser and 12kW heat pump. The photos show the pool nearly filled and ready for the skimmers to be installed. Notice how the sand coloured liner makes the water appear turquoise which can help marry the pool into the natural surroundings.

Swimming pool complete. Dordogne, France

The pool at Serres et Monguyard, near Eymet, Dordogne is now complete and is very similar to the pool we recently completed at Monteton for the same client. The pool has a sand coloured liner with matching roman end and Sahara margelles. The pool has been fitted with a winter security cover which will be augmented with a pool safety fence in the spring once the paving has been installed.

The pool house is also very similar and is fitted with a 1CV pump, 600 dia sand  filter, saltwater chlorinator, pH regulator, pump/light control box and 12kW heat pump (installed out of sight behind pool house).