How Swimming Pool Salt Water Chlorination Works

Salt Water Chlorination of pools is now in increasingly common use, especially in the United States. However it is not making the same inroads in Europe and Australia. This article describes the water chemistry of how it works and how it compares with liquid chlorine injection that is becoming the dominantly used system outside the US. One of the questions that we are asked most frequently is this; "Can I have automatic salt water chlorination in my pool?" Our answer is always - Yes, But it may not be the best solution. But first of all - How does it work?

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. For the chemists among you this is the equation; NaCl + 2H2O = HOCl + NaOH + 2H2 What does all that mean? - on the left side we have salt + water - and some electric current in a cell that does the nadgery bit!

On the right hand side of the chemical equation we have the HOCl that is the hypochlorous acid that kills all the germs and bacteria. However Sodium Hydroxide and Hydrogen are also produced alongside the HOCl. The Sodium Hydroxide (or Caustic Soda as it is more commonly known) increases the Ph value of the pool water. This is why salt water chlorination must be installed with Ph measurement and adjustment system that will cater for the need to constantly measure PH and add acid to the pool water to keep it in the correct range. An overly acidic pool will eat the tiles and tile grout as well as your heat pump and be thoroughly unpleasant to swim in.

Hydrogen that is also produced is normally just lost into the atmosphere. But oxygen can also be a side product of the cell when water molecules are split into hydrogen and oxygen there have been a few explosions in the US when the cell has been faulty and produced freak conditions.

There are a lot of other disadvantages in using salt water chlorination and it is very harmful to the environment - but please read my other article on Salt Water Chlorination to find out more about this.

The primary disadvantage of salt water chlorination is that in hard water areas the cells are very prone to the build-up of calcium carbonate deposits. The electronic boxes that come with the system switch the polarity of the probes in the cells to reduce this, but periodically the cell will have to be taken apart and cleaned - a thoroughly nasty little job. This should not be such a problem in soft water areas - but we don't have many of these in France! In Europe a fully automatic liquid chlorine injection and Ph control system can be installed for about the same price. Both systems are very reliable - but slightly more complex to set up. However they control the chlorine and Ph levels very accurately and will last as long as the pool with the replacement of a few parts. The process is quite similar to salt water chlorination in that there are a couple of small in-line cells through which all the pool water is passed in about 6 to 8 hours. As the water passes through the first cell the Ph and chlorine levels are measured, first of all the Ph is adjusted by the addition of caustic soda or acid and then the liquid chlorine is injected a drip at a time in the second cell. All the these parts and the whole installation are normally very reliable and can last for 10 years or more without any attention.

So in Europe and Australia most professional pool installers steer their Customers towards the automatic chlorine injection system simply because it is more reliable and cost effective. Salt Water Chlorination is also very unfriendly to the Environment but read my other article to find out more about this. For more information follow this link Swimming Pools in France.

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Swimming Pool Surrounds Part 2 - Decking

Part 1 of this article describes how reinforced concrete slabs should be used to support paving flags around a swimming pool. These slabs carry the paving flags across the backfilled gap between the back of the pool walls and the ground around the original pool excavation so that they do not subside. This article is designed to help choose between paving flags and timber decking and describes how the decking around a pool should be supported in the same way as the paving flags. 

The first question that needs to be answered is when decking should be used in place of paving flags. Decking is softer aesthetically and gives the appearance of not being a form of hard landscaping - which of course it is! 

If the pool is to be installed in the middle of a landscaped garden, decking may be a better answer then paving flags. However if it is intended to have meals on the area around the pool then fully grouted paving flags are best because food that is dropped can be properly cleaned up so that wildlife is not attracted to the pool area.

Another alternative is to have paving flags just at the Roman End where the sitting / eating area is and to surround the other 3 sides of the pool with decking. 

When decking is being considered, the type of decking should be taken into account because of the cost involved. When decking is being used around a swimming pool where people will be walking with bare feet DECKING THAT CAN SPLINTER MUST NOT BE USED. This more or less rules out cheap softwood decking and the more expensive exotic hardwoods have to be used in most circumstances. So do not make the choice between paving and decking until you have confirmed the cost and availability of appropriate decking material. 

Decking should be supported by the same type of reinforced concrete slabs that are recommended in Part 1 of this article. Many so-called experts will recommend that decking can be installed on lots of small pad footings and softwood timber framing. Well it can be - but it is very likely to be more costly and the likelihood of settlement is high. There is just no point in buying very costly decking and then supporting it in a way that will allow it to settle. 

You may also think that this is a very costly way of supporting timber decking. In practice it is not because you will find that if individual support pads are used for the decking, they are very time consuming to install at exactly the right level. The timber framework will also need to be made in treated softwood, and even when treated, this will eventually need to be replaced before the costly hardwood decking starts to deteriorate. 

There is one fundamental difference between installing decking for subsequently covering with either paving slabs or decking - the top level of the concrete slabs.

Hardwood decking is normally 30 or 35 mm thick and should be screwed down onto hardwood bearers 75 x 50 mm in size. These are leveled up on mortar pads and then fixed down by plugging and screwing into the underlying concrete slabs. 

This requires the supporting concrete slabs to be kept 120 below the top level of the adjacent coping stones if a step is to be avoided. It is also very important to make sure that these slabs have a surface slope away from the pool of at least 1 in 50 in order to shed rainwater. The hardwood bearers also need to be supported by packer plates so that rainwater can run beneath them. 

In most circumstances properly installed decking will be more expensive than paving flags and so an over-riding aesthetic consideration might be the only reason why it would be installed.   

Thanks To : Civil Engineer Civil Engineering Forum Civil Engineering Webboard Construction Control

Infinity Edge Swimming Pools and Their Cost

 Infinity pools are also known as vanishing edge or negative edge pools. They have one or more sides where the pool edge terminates in a weir that is 15 to 25 mm lower than the median pool water level. The water cascades over the weir into a collection trough that runs along beneath the whole length of the weir. The effect when viewed by a bather in the pool is very dramatic when the edge appears to merge with sea in the background or even the sky if the pool is located high up on a hillside. 

A properly designed infinity edge pool has two circulation systems. The infinity edge system is only designed to run when the pool is being used and consists of a circulation system that takes water from the collection trough, filters it and returns it to the main pool. The other system works in exactly the same way as a conventional pool, filtration and heating circulation system except that the ugly conventional skimmers are not required and the water circulates back through the pump via the drain in the pool floor. 

The additional cost of an infinity edge pool arises mainly from the provision of:- 

The collection trough that acts as a buffer tank
The water-proofing of the weir and the collection trough
The large capacity infinity edge circulation pump and filter
 The electronic autofill sensor in the collection trough

The automatic level sensor / filling device is needed to ensure there is enough water in the system to prime the edge pump and to compensate for any waves sent over the weir by bathers and rainwater. For a 12 x 6 metre pool with one infinity edge the additional cost should be in the 8 to 10,000 Euros range excluding any taxes. The trough and autofill, edge filtration system and the additional water proofing costs will each amount to about one third of this or about 3,000 Euros each. The water-proofing may seem to be costly but a conventional factory made PVC liner is not suitable for an infinity edge pool and needs to be replaced by a PVC "liner armee" system that involves cutting and welding the PVC liner on site. 

There is a very interesting alternative to infinity edge pools that is beginning to be sold into the French pool market where Bluepools operates. These are called mirror pools and they are an exciting option when the views from a garden are less than spectacular. In a mirror pool the infinity edge is taken around the entire perimeter of the swimming pool and the weir is drained by a perimeter trough that is much smaller than the collection trough of an infinity pool. The perimeter trough is drained by a network of gravity fed downpipes that take the water to a central collection tank that provides the buffer water storage normally provided by the collection trough in a conventional infinity pool. The cost of a mirror pool will be a little more than the cost of an infinity pool of the same size with the infinity edge along one long side. 

The additional operating costs of either an infinity or mirror pool will not be that high because the big pump needed to run the infinity edge system will only operate when the pool is in use. The extra costs will be incurred from:- 

The power the pump uses
 The cost of the water that is lost due to evaporation from the flow over the weir
 The cost of replacing the heat loss caused by evaporation
The heat required for the water replacing the evaporated water 

The total additional operating cost is difficult to assess accurately because it does depend on so many variables but it will certainly be less than the cost of heating a conventional pool of the same size using a heat pump. 

The development of infinity pools has coincided with the increase in a new trend towards designer pools that are now often seen at exclusive properties and on magazine covers. Many people have property in an ideal location for the installation of an infinity edge pool even more have gardens that would benefit from a mirror pool and I hope this article encourages the development of both. 

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www.bluepools.com

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