8 Water Circulation and Water Treatment

Chapter 8

Water circulation and water treatment

WATER CIRCULATION For swimming pools, an efficient system of water circulation is essential for the health and safety of the users, to ensure relative freedom from pathogenic bacteria and maintenance of a high standard of clarity in the pool water. There are two basic systems of water circulation:

Figure 8.1 Diagram of flow-through pool.

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Figure 8.2 Diagram of typical layout of water treatment plant for small pool. 1, outlet main from pool; 2, strainer; 3, circulating pump; 4, coagulant dosing; 5, pH regulator; 6, filter(s); 7, heater; 8, aerator; 9, water disinfecting equipment; 9A, alternative position for disinfecting equipment; 10, treated water main to pool.

1. The simplest method is where a pool draws its water from a stream, lake or the sea, so that the water in the pool is continuously changed. These are often referred to as flow-through pools (Figure 8.1). 2. The normal method for the vast majority of pools is the provision of a system of pumped water circulation. This requires inlets to the pool and outlets from the pool connected to a pump so that the pool water is kept in continuous circulation, and fresh water is only added to make up ‘losses’. At predetermined intervals which can vary from a year or longer, the pool is emptied for general cleaning, detailed inspection and repairs. See Figure 8.2 for general layout of treatment plant for small pools.

8.1 Flow-through pools Even with this simple type of water circulation, certain principles should be adhered to. The inlets and outlets should be located so that as far as practical, the whole of the water in the pool is changed at a calculated rate, and there are no ‘dead’ pockets of uncirculated water. Screens should be provided at appropriate locations. The screens require regular inspection, with special attention after heavy rain (for pools fed from a stream) and after storms for sea water pools. It is essential that a reasonable standard of clarity of the water in the pool be maintained for the safety of the bathers. See Section 8.5.1. Figure 8.3 shows a stream used as a swimming pool in Switzerland.

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Figure 8.3 View of a natural flow-through pool in Switzerland. Courtesy, Edward Schwartz.

Figure 8.4 View of one of a series of three main circulating pumps for pools in a private leisure centre. Courtesy, Pool Water Treatment Advisory Group.

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8.2 Pools where the pool water is in continuous circulation It is a basic principle that the inlets and outlets should be designed and located so that the circulation is as complete as possible and that there are no pockets of ‘dead’ water. The maximum contamination is in the surface water and in the area of the pool where the bathing load is heaviest. The methods adopted to achieve adequate circulation will depend on the size and shape of the pool and the use to which it is put. That is, whether the pool is used by members of one family and their friends, when the maximum bathing load is likely to be very light, or whether it is a hotel, club, school or public pool when the loading can vary from very light to very heavy. Success depends largely on the experience of the designer; the system must be in reasonable balance, i.e. the inflow of water must keep pace with the withdrawal of water. With heated pools, particularly open-air ones, the even distribution of the heated water throughout the whole pool is important for the comfort of the bathers (Figure 8.4). The principal factors relating to the efficiency of water circulation in swimming pools are: 1. 2. 3. 4. 5.

the turn-over period. See Section 8.2.1; the pool loading. See Section 8.2.4; the amount of make-up of fresh water used. See Section 8.2.5; the hydraulic design of the system (size of pumps and size and layout of pipework); the type and location of outlets and location of inlets. See Section 8.2.6 and 8.2.7.

8.2.1 The turn-over period Typical turn-over periods are set out below:

Private house pools Hotel and club pools School pools Public pools Teaching/learner pools Diving pools

6–8 hours 2.5–4 hours 2.0–3 hours 2.0–3 hours 1.0–1.5 hours 4–6 hours

The circulation rate is the volume of water in the pool divided by the turn-over period. The effect of the turn-over period is largely governed by the mixing efficiency which depends on the location and number of the inlets and the method of drawoff from the pool to the filters.

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Figure 8.5 Sketch of skimmer outlet.

Figure 8.6 Sketch of standard scum channel. Courtesy, Pilkington’s Tiles Ltd.

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8.2.2 Circulation systems for small pools for private houses The bulk of the contamination in the pool water is in the surface layers and this is why it is most important to remove effectively this surface water. Usually the shallow end of the pool is the more heavily loaded and consequently the water is more contaminated. The system in general use consists of skimmer-weir outlets and one outlet in the floor at the deep end; the incoming water from the treatment plant is distributed through spreader inlets. Provided there is an adequate number of inlets and outlets properly located, this constitutes an effective method of circulation. Figures 8.5 and 8.6 show a skimmer-weir outlet and a scum channel. The following are suggestions for location of outlets and inlets for relatively small private pools using skimmer outlets. However, the circulation system must be in balance and the number of outlets and inlets should be calculated by the designer of the circulation system and treatment plant. 1.

Rectangular Pools (a) Water area 40 m2 say 10 m×4 m (i) Outlets: Two skimmer-weirs in each long wall towards the shallow end of the pool (should be a reasonable distance from the inlets) and one skimmer outlet in the centre of the short wall at the deeper end of the pool. One outlet in the floor at the deeper end of the pool. Total: five surface outlets and one floor outlet. (ii) Inlets: Two inlets in each long wall towards the shallow end and one in the short wall at the shallow end. The inlets should not be close to the outlets as this can result in short-circuiting. Total: five inlets. (b) Water area 133 m2, say 16.67 m×8.00 m (i) Outlets: Three skimmer-weirs in each long wall towards the shallow end, one skimmer outlet in the short wall at the deeper end of the pool, and one outlet in the floor at the deeper end of the pool. Total: seven surface outlets and one floor outlet. (ii) Inlets: Two inlets in the short wall at the shallow end, and two in each long wall towards the deeper end of the pool. Total: six inlets. 2. Free-Formed Pools The outlets and inlets should be located in accordance with the principle that the heaviest contamination is in the area of shallow water, that short circuiting should not occur, and that the turn-over period is generally as given in Section 8.2.1.

8.2.3 Circulation systems for larger pools for hotels, clubs and schools and public pools For hotels, clubs and schools, it is recommended that either scum channels are used for the outlet of the pool water or the pool is designed as a deck-level pool

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with a properly designed perimeter channel and balancing tank. The inlets for the treated water would discharge the water through spreaders. Scum channels, and the provision of a perimeter channel in deck-level pools are appreciably more effective in removing surface contamination than skimmer weirs. Their use is recommended for pools where a heavy bathing load is anticipated. Scum channels consist of heavy glazed ceramic units (Figure 8.6). Unfortunately, the perimeter channel of deck-level pools is sometimes left as unlined concrete which is certainly not satisfactory. The best finish is obtained by the use of special glazed ceramic units; a rather less satisfactory method is to form the channel in insitu concrete, finished with a smooth surface which is then finished with two coats of chlorinated rubber paint or an epoxy-based coating. It has been mentioned previously in this chapter that it is of the utmost importance that the system for the withdrawal of contaminated water and the distribution of the purified water should ensure that the whole of the pool water is circulated during the turn-over period. The water treatment system (filters and water disinfection plant) should maintain the whole of the pool water at the required standard of purity (and temperature if the water is heated). Some information on water treatment is given elsewhere in this chapter. Notes on heating swimming pools and energy conservation are given in Chapter 9. It is recommended that the design of the water circulation system and the water treatment plant should be the responsibility of one firm, either as a ‘package deal’ or by a firm of independent consultants experienced in this field. The water circulation system for a large free-formed pool incorporating wavemaking equipment requires careful design by an experienced firm.

8.2.4 Pool loading It is obvious that the number of persons using the pool at any one time is directly related to the contamination entering the pool water, and the removal of this contamination is related to the turn-over period/circulation rate, filters and treatment plant. The amount of this contamination affects the quality and clarity of the pool water. Bathing loads should be controlled under two main headings, physical safety of those using the pool, and the maintenance of water quality. For an acceptable standard of physical safety: 1.

2.

The maximum number of persons in the pool at any one time should be limited. The HSE booklet Managing Health and Safety in Swimming Pools recommends 3 m2 per person. A high standard of clarity is maintained in the pool water. The clarity of water from a public supply is not necessarily adequate for use in a swimming pool. The term clarity includes turbidity and colour. It is essential that a bather who

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is in trouble on the floor of the pool should be clearly seen from the pool sides.

8.2.5 Make-up water An important factor in the water purification sytem is the amount of ‘make-up’ water used; this is the amount of fresh water introduced into the pool at intervals. This is referred to by the Institute of Baths and Recreation Management as ‘progressive dilution’. In Europe, the amount of fresh water is appreciably greater than that used on the average in the UK. Depending on the design of the system, the admission of large quantities of fresh water can increase the heating cost.

8.2.6 Inlets and outlets While with smaller pools (referred to in Section 8.2.2), inlets for fresh water are invariably located in the pool walls. In larger pools for local authorities, inlets are sometimes located along the centre line of the floor, but this can result in the incoming water which is under pressure from the circulating pumps finding its way under the floor screed resulting in lifting and damage to the tiling. Outlets should be in the form of either scum channels or a deck-level pool with a perimeter channel. These systems are much more efficient in removing the heavily contaminated surface water than individual skimmer outlets used for smaller pools. There must also be an outlet or outlets in the deeper end of the pool which are also used for emptying the pool for general maintenance and cleaning. The scum channel/deck-level overflow should take at least 60% of the circulating water, the remaining 40% (maximum) being removed through the outlet in the floor at the deep end of the pool. Sometimes more than one floor outlet is provided to help ensure that dangerous suction does not develop. Gratings must have small openings to prevent injury to bathers’ toes.

8.2.7 Deck-level pools This type of pool has become very popular, particularly in leisure centres. The circulating water flows over the side of the pool into a continuous perimeter channel. This channel discharges into a balancing tank. There is no generally recognised method for calculating the size of the balancing tank; treatment plant manufacturers develop their own design and are reluctant to divulge the details. The amount of water discharging to the perimeter channel can vary considerably due to wave action which depends on the activities of the bathers; a large group doing exercises would create more wave action than persons swimming. For the actual dimensions of the tank, an adequate allowance should be made for ‘free-board’, say, 300 mm.

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Figure 8.7 Sketch of perimeter/circulating channel for deck-level pool. Courtesy, Pilkington’s Tiles Ltd.

The dimensions and gradient of the perimeter channel has also to be calculated as it forms an essential part of the circulation system. The perimeter channel is closed at the top with a removable grating which is usually made of extruded PVC, but stainless steel (austenitic) is sometimes used in high-class installations. The openings in the grating must be toe and finger ‘proof’ (Figure 8.7). Both perimeter channel and balancing tank should be finished with a smooth, durable coating or glazed ceramic units.

8.3 Ducts for pipework These days, pipework for the circulating water system is almost always unplasticised PVC; reference should be made to BS 3505 Unplasticized PVC Pressure Pipes for Cold Water and BS 3506 Unplasticized PVC Pipe for Industrial Uses, and CP 312 Code of Practice for Plastics Pipework (Thermoplastics Material). It is recommended that all pipework should be in accessible ducts unless otherwise laid/fixed so as to be reasonably accessible for inspection and repair.

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Although this may add a significant amount to the first cost of the installation, this increase is far less than that incurred for repairs/replacement of leaking pipes necessitating breaking up of floors etc. This cost has to include the loss suffered by the closure of the pool for a fairly long period. A further point is that it is extremely difficult to repair an opening made in the floor or wall of a water-retaining structure so that it is watertight.

WATER TREATMENT It was stated in Chapter 1 that in the UK legislation directly relating to the purity of water in swimming pools only applies to pools which are open to the public, mainly local authority pools. There are no detailed unambiguous standards laid down by law which apply to all classes of swimming pools. However, there can be no doubt about the moral obligation of every one responsible for the operation of a swimming pool to ensure that the water in the pool is clear and is good quality. Also that the combination of chemicals used in the treatment of the water does not result in distress to the pool users, and in the concentrations used, is not aggressive to the materials of which the pool is constructed and finished, including the pipework and fittings.

Figure 8.8 Diagram of complete treatment control for swimming pool water. Courtesy, USF Stranco.

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Figure 8.9 View of three air blowers forming part of the complete water treatment plant for pools in a private leisure centre. Courtesy, Pool Water Treatment Advisory Group.

Figure 8.10 View of plant room with fully automatic water treatment equipment. Courtesy, Buckingham Swimming Pools Ltd.

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The size and type of plant required depends on the size and use to which the pool is put. At one extreme, there is the private house pool used only by the owner and his family and friends. At the other extreme, there is the large public pool with heavy pool loading (see Section 8.2.4). However, all methods of satisfactory treatment have much in common (Figures 8.8–8.10). It will be appreciated that the proper control of swimming pool water is complicated even for small pools with a low bathing load. For the large public pools, considerable practical experience supported by sound theoretical knowledge of the chemistry of water treatment is necessary. Reference should be made to the publication Swimming Pool Watertreatment and Quality Standards prepared by the Pool Water Treatment Advisory Group, and to the publications of The Institute of Baths and Recreation Management. Many of the chemicals used in water treatment are potentially hazardous to health and special care is needed in their use and storage. Reference should be made to the requirements of the Health and Safety Executive and to the brief comments in Appendix 5.

8.4 Layout of treatment plant The equipment recommended for a small pool is shown in the diagram at Figure 8.2. The plant would consist of: 1. 2. 3. 4. 5. 6.

strainer; circulating pump and electric motor; coagulant dosing equipment; pressure filter; disinfecting equipment; heater.

The coagulant dosing equipment may be omitted for small private house pools, and the heater may not be included in the owners brief.

8.4.1 Plant rooms The equipment is expensive and needs to be properly maintained. The recommendations which follow are intended to refer to plant rooms generally, irrespective of size. All the above should be installed in a properly constructed plant house/ room, which should also provide space for the storage of chemicals used in the coagulant dosing equipment and the disinfecting equipment and as well as tools and spares. The plant room should have a concrete floor, clay brick or concrete block walls,

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adequate windows and permanent ventilation which in small plant rooms can consist of two louvered openings each about 250 mm×76 mm covered with wire mesh. It must be designed with easy access and adequate floor space so that the plant can be installed, serviced and removed without difficulty. The roof should be of durable materials and completely weathertight. While many plant rooms have plain concrete floors, it is better if the concrete floor is finished with a high-quality durable paint, such as chlorinated rubber, polyurethane or epoxy which is resistant to the chemicals used as these are certain to be spilt on floor. The floor should be laid to a gradient of about 1 in 60 discharging either to the drainage system via a floor gulley or to outside the building, depending on the circumstances of each case. Electrical wiring and equipment should be of the best quality and there should be an accessible control panel with fuses/circuit-breakers. The recommendations of the Institution of Electrical Engineers should be followed. For large installations, it may be necessary to install a gantry for the moving of heavy items of plant. Bunds should be provided around tanks containing chemicals in liquid form. In large municipal pools, the plant room is often below the pool walkways and the changing accommodation. The walkway slabs and floors of the wet changing areas must be completely watertight; see Sections 4.12 and 7.7.

8.4.2 Notes on circulating pumps Centrifugal pumps are used for water circulation with directly coupled electric motors, operating on AC 3-phase, usually 440 V, but very small capacity pumps may operate on 220 V. The pumps should be self-priming. For large installations, the pumps are in sets of two, three or four operating in parallel. In this way, pumps and motors can be taken out of operation for maintenance without an undue effect on the water circulation. With the larger pumps, it is an advantage if they are of the split casing type as this enables the top half of the casing to be removed for inspection of the bearings and impeller. The ‘characteristics’ of the pumps should be such that delivery does not fall off significantly with increase in delivery head caused by build-up of deposits in the filters. For solution feed of chemicals, a different type of pump is used, usually a piston/ displacement type.

8.5 Filtration and filters The basic requirements for a satisfactory swimming pool water are closely connected and have been discussed in the Introduction. The filters have two functions. They must ensure that the water leaving the filters has a high degree of clarity by reducing the matter in suspension and, as

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these particles are mainly organic and some may contain micro-organisms, filtration assists the disinfection of the water. The filters are assisted by the strainer, shown at 2 in Figure 8.2 as this holds back the coarser material in suspension.

8.5.1 Clarity of pool water Clarity is reduced by suspended and colloidal matter in the water, and by colour. There are two principal reasons for requiring that water in a swimming pool should possess a high standard of clarity: user safety and public health. In fact, the user safety aspect can be more important as the cases of water-borne disease which are established as originating from a swimming pool hardly ever occur in the UK and other developed countries with a similar climate. The same cannot be claimed for fatalities due to bathing in a swimming pool containing water of sub-standard clarity. If a bather gets into difficulties and sinks below the surface of the water, it can be very difficult for other users to notice what has occurred and to locate the body unless the water has a high standard of clarity. In practice, the water should be sufficiently clear that the bottom of the pool can be easily seen at the deepest part by persons on the walkway around the pool. In the UK, the type of filter in general use is the pressure sand filter and these are described briefly in Section 8.5.3. There is also the precoat type of filter which is also commented on in Section 8.5.4.

8.5.2 Aids to efficient filtration There are differences of opinion on the type of floculent/coagulant which should be introduced into the circulating water before it enters the filters. The purpose of these chemicals is to form a ‘floc’ (a gelatinous precipitate) which is retained in the upper layers of the filter and assists the filtration process. The material in general use is aluminium sulphate which when dissolved in water forms an acidic solution. As acidic solutions are aggressive to ferrous metals and to cement-based materials (see Sections 3.5, 3.6 and 3.7), pH correction is usually needed. This pH control can be manual or automatic. For public pools, automatic pH control should be adopted. The pH should be maintained in the range 7.2 to 7.8. The pH can be measured approximately by indicator papers, or more accurately by a pH meter.

8.5.3 Pressure sand filters The principal type of filter in use for the treatment of swimming pool water in the UK is the pressure sand filter. Pressure sand filters use graded sand as the filter medium in circular steel

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or glass-reinforced plastic (grp) tanks. These vary in size from small single units for private house pools to a battery of large units for public pools. There are two types, vertical downward flow, and horizontal. The former are considered more efficient. The steel shell requires a high-quality protective lining, and should comply with the relevant clauses in BS 5500 Steel Pressure Vessels, Unfired, Fusion Welded. In recent years, steel shells have been replaced by glass fibre shells which are cheaper in first cost but appear to have a shorter life. For all except small pools for private houses, there should be at least two filters. See Section 8.4.1 for design of plant rooms with particular reference to provision of adequate access for installation and removal. Filters are normally rated on the basis of m3/m2/hour, and the rate is classified as low, medium and high. For club, hotel and private pools, high-rate filters are usually installed, while for public pools and school pools medium-rate filters are usually selected. High-rate filters operate in the range 30–50 m3/m2/hour and medium-rate filters in the range 20–30 m3/m2/hour. Pressure sand filters have to be ‘back-washed’; the frequency depending mainly on the efficiency of the filter in removing suspended and coloidal matter and the bathing load. As the deposit on the filter increases, there is a loss of head through the filter which is measured by two pressure gauges on the two main connections to the filter, one near the top of the filter and the other near the bottom. In many installations, the back-washing is assisted by the agitation of the filter media (sand), either by mechanical rakes or by compressed air. The amount of water used in back-washing filters can be considerable; an average flow rate is about 25 m3/m2/hour. For a 2.5 m diameter filter, filter area 4.9 m2, medium flow rate, the back-washing would take about 8 minutes, would use about 16 m3 (3590 gal) of water. The discharge to the drainage system would be about 2000 litres/minute (440 gal/minute) per filter. This figure is determined by the filter manufacturer and should be followed. Filters have a viewing window on the outlet and the clarity of the wash water should be checked before back-washing is stopped.

8.5.4 Precoot filters In a precoat filter, the filter medium is a very fine powder mixed with water and deposited on ‘carriers’ known as candles. The only advantage with this type of filter is a considerable saving of space, which may be attractive for small installations for private houses, hotels etc. They have not found favour for use in public pools in the UK nor in Europe. Precoat filters also require cleaning from time to time as the coat on the candles becomes blocked. This cleaning is done by compressed air as directed by the manufacturers of the filter.

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8.6 Chemical dosing of the pool water The addition of chemicals to the pool water (in addition to those needed to form a floc prior to filtration) is required for the following reasons: 1. 2. 3.

to control the pH so that it is maintained in the range 7.2 to 7.8 (slightly alkaline); to maintain the water in proper ‘balance’; to disinfect the water to ensure a reasonable standard of bacterial purity.

8.6.1 Control of the pH, alkalinity and a balanced water control of the pH The chemical characteristics of the incoming fresh water, usually from a public supply, may also influence the pH, see Sections 3.7 and 3.8. The control of the pH is essential for efficient water treatment. The pH is the hydrogen ion concentration; the neutral point is 7.0; values less than 7.0 indicate an acidic solution and values above 7.0 indicate that the solution is alkaline. The pH scale is logarithmic, so that a water with a pH of 5.0 has 100 times the hydrogen ion concentration of a water with a pH of 7.0. The main disinfecting agent used in swimming pools is chlorine. This may be

Figure 8.11 View of three small and one large dosing pumps for sodium hypochlorite solution for pools in a private leisure centre. Courtesy, Pool Water Treatment Advisory Group.

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in the form of chlorine gas, or derived from a salt containing chlorine such as calcium or sodium hypochlorite or from an organic compound containing chlorine (Figure 8.11). When chlorine gas is dissolved in water, hypochlorous acid and hydrochloric acid are formed. Hypochlorous acid is very effective in destroying bacteria and the formation of this acid lowers the pH. Therefore, the higher the concentration of hypochlorous acid, the lower the pH and the more effective is the solution in killing bacteria. The formation of hydrochloric acid (which is a strong acid) is undesirable as it lowers the pH still further, and it is not a very effective bactericide. Acidic solutions attack ferrous metals and cement-based materials and therefore the pH must be controlled and kept within the range previously mentioned, 7.2–7.8. Both sodium and calcium hypochlorite are strongly alkaline, and if the pH is raised too high and the water is hard, calcium compounds may be deposited. Chlorine reacts with ammonia to form compounds known as chloramines. These are unstable and in the presence of chlorine break down to produce hydrochloric acid, which as stated above is undesirable as it is aggressive and is not effective in destroying bacteria. If aluminium sulphate (alum) is used as a coagulant (to form a floc) before the water enters the filters, the pH is lowered as alum in solution is acidic. To raise the pH to the required level alkali is added, usually in the form of sodium carbonate (soda ash). The amount of soda ash added has to be determined by the pH. To counteract the effect of the high alkalinity of sodium and calcium hypochlorite, it is often necessary to add an acid salt such as sodium hydrogen sulphate (known as ‘dry acid’). It can be seen from the brief comments above that there are many factors involved in effective treatment of pool water.

8.6.1.1 Alkalinity Alkalinity is expressed as mg/litre (ppm) of equivalent calcium carbonate (CaCO ), and indicates the amount of alkaline compounds in solution in the 3 pool water. The Pool Water Treatment Advisory Group recommend a general minimum level of alkalinity at 75 mg/litre which is value required for effective coagulation. High values can cause difficulty in maintaining the pH in the range of 7.2 to 7.8.

8.6.1.2 Maintaining balance in the pool water There is need to maintain the pool water in ‘balance’ and the main factors which determine whether or not a water is in ‘balance’ are the total hardness expressed as calcium carbonate (CaCO ), the total alkalinity, and the pH. All these are related, 3 but in a complex way. A balanced water is not corrosive to cement-based materials but will corrode unprotected ferrous metals. There are two principal tests which can be used to

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determine whether a water is in balance. These are the Langelier Index, and the Palin test. The Langelier Index has been briefly discussed in Section 3.8. The following table indicates a classification of water based on the Langelier Index by the International Standards Organisation (ISO). The Palin test is the simpler to apply and is favoured by pool operators. In this test, three variables are considered: the pH; the total hardness (as calcium carbonate, CaCO ); the total alkalinity. 3 The adverse effect of a soft moorland water on the cement-based grouted joints in a public swimming pool is shown in Figure 7.6. The water was not in balance and the joints were seriously attacked and large-scale remedial work was required.

8.7 The disinfection of pool water The words purification, sterilisation and disinfection are used for the process which is aimed at the destruction of bacteria in the pool water. In this book, the word disinfection is used. Purification really refers to the work of the whole treatment, while sterilisation suggests the complete elimination of all bacteria which is certainly not practical nor necessary. The DoE publication The Treatment and Quality of Swimming Pool Water states ‘that when coliforms are absent and a satisfactory level of free residual chlorine is maintained throughout the pool, the risk of infection to bathers from the small number of organisms remaining in the pool water is minimal.’ The PWTAG in their treatise Swimming Pool Water and Quality Standards recommend that a reasonable bacterial standard for pool water is that the number of bacterial colonies in 1 ml should not exceed ten and there should be no E. coli in 1 millilitre. It is generally agreed that the disinfecting agent used should remain active in the pool water after passing through the treatment plant. This stipulation is necessary because as soon as the treated water enters the pool fresh contamination occurs and this will remain (and increase) as the water is circulated in the pool until it again passes through the treatment plant which may take several hours. See Turnover Period in Section 8.2.1. There are a number of methods for disinfecting swimming pool water and these are briefly described in the following sections.

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8.8 Chlorination The most effective disinfectant for pool water is chlorine as it is not only very effective in the destruction of bacteria, but it is also a powerful oxidising agent and can deal effectively with organic matter in solution and in suspension. With proper dosing, a ‘residual’ remains in the pool water. This residual is not elemental chlorine but consists of compounds containing available chlorine. This is usually expressed as ‘free residual’ chlorine. The smell of chlorine in pool halls is not due to a very low concentration of chlorine gas but arises from complex chlorine compounds. One such compound, dichloramine, can cause irritation to the eyes and throat of bathers. Chlorine reacts with ammonia to form chloramines which are to a limited extent bactericidal but are slow reacting and are therefore more stable than free chlorine which reacts very rapidly. Ammonia is present in pool water as it is introduced by the bathers by the decomposition of nitrogenous compounds. In the UK, chlorine gas compressed in steel cylinders is no longer used for the disinfection of pool water but is still used in Europe and in the USA, as it is very efficient and effective. This virtual elimination of the use of gaseous chlorine lead to the extensive use of solution feed using sodium hypochlorite or calcium hypochlorite. Sodium hypochlorite is normally supplied as a solution, while calcium hypochlorite is supplied as a powder. Both compounds are strongly alkaline, and acidic solutions have to be added to correct the pH and maintain it in the range 7.2 to 7.8. This correction of the pH is achieved by the use of either hydochloric acid, sodium hydrogen sulphate, or carbon dioxide. Hydrochloric acid is highly corrosive and if it comes into contact with the ‘raw’ sodium or calcium hypochlorite chlorine gas is liberated which can be very dangerous and special precautions must be taken to ensure that this contact does not occur, particularly as this can happen accidentally in a storage area. A concentrated solution of sodium hypochlorite will attack Portland cement concrete and it is advisable, if this compound is used, that the concrete floor of the storage area be protected by a high-quality epoxy-based coating. Modern dosing equipment makes control easy and safe. This equipment automatically controls the chlorine residual in the pool water at a predetermined level, and regulates the pH of the water within acceptable limits.

8.8.1 Break-point chlorination Chlorine dissolves in water forming hypochlorous acid and hydrochloric acid: Cl +H O=HOCl+HCl 2

2

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The hypochlorous acid reacts with ammonia in a complex reaction and in the presence of excess chlorine, breaks down to form hydrochloric acid and nitrogen. The point at which the chloramines start to be broken down is called the ‘break-point’ and the technique of achieving this is called ‘break-point chlorination’. A free chlorine residual of 1.0 ppm should be adequate to maintain satisfactory bactericidal conditions in pool water. This may be increased to a maximum of 1.5 ppm when the pool loading is very high.

8.8.2 Chlorinated isocyanurates Another source of chlorine is compounds in which the chlorine is combined as in chlorinated isocyanurates. These are not used much in public pools but are popular in the private sector. The compounds can be in the form of tablets or as a solution, which are often fed by hand directly into the pool, a procedure which is unsatisfactory and is not recommended. With both types, cyanuric acid is formed and this lowers the pH and adjustment is required to obtain the necessary balance.

8.9 Ozone Ozone (O ) is a very effective bactericide and a powerful oxidising agent. When 3 correctly used it produces a water with no unpleasant taste nor smell. It effects a very rapid ‘kill’ of bacteria and oxidises organic matter in the water as it passes through the plant but there is virtually no residual ozone left in the water when it is returned to the pool. When ozonised water enters the pool from the treatment plant, it starts to become contaminated by the bathers and as there is no residual, the newly introduced bacteria and organic matter are not ‘dealt with’ until the water again passes through the treatment plant. This disadvantage of ozone can be overcome by the injection of a comparatively low dose of chlorine; the chlorine is derived from sodium or calcium hypochlorite and is injected immediately before the treated water enters the pool. An activated carbon filter is sometimes provided to remove excess ozone before the treated water enters the pool. With disinfection by ozone, the free residual chlorine can be maintained at a low level of about 0.5 ppm. In the UK, the number of public pools using ozone as the main disinfectant has increased considerably in recent years, but reliable information on the number is not available. The disinfection of swimming pool water with ozone is very popular in Europe, e.g. Germany, Switzerland and France, and the USA. In Germany, it is mandatory to provide for the injection of a small amount of chlorine to ensure a free residual chlorine in the water in the pool. In Switzerland, this is not generally considered

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entirely necessary provided the pool is properly operated, including generous use of make-up water. See also Section 8.2.5. Ozone is a poisonous gas and therefore safety warning signals should be installed to operate when the concentration of ozone in the plant room exceeds a predetermined level. This safety system should include an automatic plant shutdown device.

8.10 Bromine Bromine (Br ) is in the same group of elements as chlorine (the halogens) which it 2 closely resembles in chemical properties. While chlorine is a gas at normal temperature and pressure, bromine is a liquid which freezes at -7.3 °C and its boiling point is 58.8°C. It is a red liquid with a pungent smell and is very soluble in water. Bromine is claimed to be popular for use in small swimming pools for private houses, clubs and hotels, but is not used in public pools in the UK. It is a strong oxyidising agent and powerful germicide. In solution in water, it reacts with ammonia to form bromamines (in a similar way to chlorine-forming chloramines). The concentration of bromine residual is recommended by the PWTAG to be in the range 4.0 to 6.0 mg/litre using DPD tests. It is claimed that the use of bromine does not cause any irritation to the eyes, nose or throat and does not give rise to objectionable odours. However, there is some reason to suspect that it can cause irritation to the skin of some bathers. It can be dispensed into swimming pools by means of tablets introduced into the pool water by a brominator.

8.11 Chlorine dioxide Chlorine dioxide (ClO ) is a heavy yellow gas which in its pure form is unstable 2 and explodes violently on heating. It can now be prepared in patented stable solutions. Two stable forms of chlorine dioxide are Ultrazon and Dichlor. It is a strong oxidising agent and is claimed to have powerful germicidal properties. When used on its own, there is a tendency for the pool water to become rather cloudy and develop a yellowish-green colour. To overcome this, it is usual practice to dose with chlorine as often as necessary to maintain the necessary clarity and good appearance of the water. It is used to a limited extent in hotel and private pools in Europe.

8.12 Metallic ions (silver and copper) Probably the first reference to the use of metallic ions for the sterilisation/disinfection of water was work by Dr. Krause of Munich in 1929. This method became known as the ‘Catadyn Silver Process’. An account of experimental work on this process

Copyright 2000 Philip H Perkins

is contained in a paper by E.V.Suckling in the Proceedings of the British Water Works Association in 1932. It was used to a very limited extent in Europe for purifying small quantities of water. Ions are derived from atoms, but unlike atoms they possess electrical charges. Ions derived from hydrogen and metals have a positive charge, while ions from non metals and acid radicals have a negative charge. In the early 1960s, it was used for treating water in the swimming pool of a large hotel in Flims Waldhaus, Switzerland. At this time it was known as the ‘Vellos Casanovas’ process and a brief description of the installation is given below. Water is drawn from the pool and passes through a strainer and then through a series of copper plates. A pulsating electric current passes through the plates and due to the difference in potential between the plates metallic ions are liberated into the circulating water. The suspended and colloidal matter in the water are attracted to the liberated ions and form what the patentees term a micro-floc which is much finer than the floc formed by coagulants (see Section 8.5.2). The micro-floc penetrates into the filter medium and this is claimed to increase its efficiency so that the rate of flow is about double that through a high pressure sand filter. After filtration, the water is passed through a battery of silver plates similar to the copper plates used for the micro-floc formation. The electric current liberates silver ions and these have a strong sterilising/disinfecting effect. It is also claimed that the silver ions remain in the water as it is returned to the pool and thus have an effect similar to that of free residual chlorine. There is no simple test for detecting copper and silver ions in pool water. It is important that the pH of the water is controlled within fairly narrow limits and for small pools for private houses, hotels and clubs this can present practical difficulties in overall control. There is very little information on the use of this system of swimming pool water treatment in the UK. A considerable amount of work has been carried out in Switzerland on the bactericidal effect of silver ions in water. Bulletins issued by the Federal Institute for Water Supplies in Zurich showed that silver ions do have a significant destructive effect on E. coli in water.

8.13 Ultra-violet radiation Ultra-violet (UV) radiation for the sterilisation of small quantities of water has been known since the early part of the 20th century. An essential feature for the disinfection of water is that the UV radiation must secure maximum penetration of the water being treated. In addition, there is an optimum wave-length band for effecting maximum kill of the bacteria and viruses. This optimum wave-length band is claimed by the suppliers of the UV equipment to be 2500–2800 angstroms (250–280 nm).

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The need for maximum penetration of the water means that suspended and colloidal matter must be at a minimum and the total dissolved solids (tds) must also be low, particularly iron salts and nitrates. This requires an effective check on the chemical characteristics of the water supply to the treatment plant and the filters must operate at maximum efficiency. The pH of the water should be in the range 7.2 to 7.8. The advantages claimed for this method are: Over-dosage is impossible; No chemicals are added to the water; When correctly designed the plant obtains a very high percentage of ‘kill’ (over 90%). A serious disadvantage for its use in the treatment of pool water is that there is no residual in the water after the water has passed the treatment point. Also, compliance with the tight control procedures can, in practice, prove difficult. Nevertheless, it can be an attractive method of water disinfection for small pools for private houses, clubs and hotels. The UV radiation is produced by low, medium and high pressure mercury vapour discharge lamps; up to about 50 m3/hour (11 000 gal/hour) can be treated.

8.14 The base-exchange process for softening pool water The purpose of softening water is to reduce the hardness and this has many advantages for swimming pool water, especially if it is heated. Hardness is due mainly to the presence in solution of bicarbonates and sulphates of calcium and magnesium. Boiling will reduce the bicarbonate hardness but not that due to sulphates. The bicarbonate hardness is known as temporary hardness and the sulphate hardness as permanent hardness. Most domestic and small industrial water softeners operate on the base-exchange (or ion-exchange) process which removes both bicarbonate and sulphate hardness. In this process, the active material is a natural or artificial zeolite, a sulphonated carbonaceous material, or a synthetic resin which has ion-exchange properties. Water flows through a bed of the active material and the calcium and magnesium ions combine with the zeolite as shown: Calcium bicarbonate+sodium zeolite=calcium zeolite +sodium bicarbonate Magnesium sulphate+sodium zeolite=magnesium zeolite +sodium sulphate After a time all the sodium zeolite is used and the softener needs to be regenerated by the addition of common salt (sodium chloride) as follows:

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Sodium chloride+calcium zeolite=sodium zeolite+calcium chloride Sodium chloride+magnesium zeolite=sodium zeolite+magnesium chloride The calcium and magnesium chlorides are in solution in the wash water which goes to waste. There is no change in the total dissolved solids (tds) in the water. Water of almost zero hardness can be obtained which is not desirable for most purposes including water for swimming pools. To prevent this happening, the softened water should be blended with a percentage of the ‘raw’ water to give the required degree of hardness. The process is expensive and it is not used when large volumes of softened water are required. For use in swimming pools the Langelier Index should be positive and the pH in the range 7.2 to 7.8.

8.15 Sulphates in swimming pool water A further matter to be considered is the possible build-up of sulphates in the pool water arising from the use of aluminium sulphate and sodium hydrogen sulphate, for reasons previously given. Sulphates in solution are aggressive to Portland cement and therefore tile joints, rendering and screeds are vulnerable to attack. British Standard BS 5385 Part 4 Code of Practice for Ceramic Tiling and Mosaics in Specific Conditions, clause 13.1 states: ‘Ideally, the sulphate concentration (expressed as SO ) in the water of 3 swimming pools should not exceed 300 ppm. Where this level cannot be achieved, consideration should be given to the use of impermeable adhesives and grouting materials that are not affected by sulphates.’ It is recommended that when compounds containing a sulphate radical are used in the treatment process, regular testing for the concentration of the sulphate ions should be part of the control tests. Recommendations for mitigating or preventing sulphate attack by the use of appropriate materials in the finishes of the pool shell are given in Chapter 7.

Further reading Amateur Swimming Association. Acceptability of Swimming Pool Disinfection by Different Methods, 1984. Department of the Environment. Treatment and Quality of Swimming Pool Water, HMSO, London, 1984. Elphick, A. Treatment of Swimming Pool Water with Sodium Hypochlorite, Wallace & Tiernan, Tonbridge, 1978.

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Institute Of Baths and Recreation Management. Practical Leisure Centre Management, Vol. 2. Langelier, W.H. The analytical control of anti-corrosion water treatment, Journal AWWA, 28(1), October 1936, pp. 1500–21. Pool Water Treatment Advisory Group. Swimming Pool Water Treatment and Quality Standards, 1999. Wuhrman, K. and Zobrist, F. Investigations into the bactericidal action of silver in water, Information Bulletin No. 142, Federal Institute of Water Supplies, Zurich, 1958 (translation).

Copyright 2000 Philip H Perkins