Outbreaks associated with recreational water environments have been linked to poor system design. Therefore, the first disease prevention strategy is ensuring the adequate design of recreational water environments given the extent and nature of use. Another common cause of outbreaks is improper operation of controls, such as allowing recreational water environments to be bunkered beyond capacity or engaging in poor operational practices. Design limits should be adhered to and systems should be properly operated at all times.
Treatment systems can reduce contamination levels, but these can become overloaded. Therefore, reliance should not be placed on treatment alone, and multiple barriers should be actively maintained, including:
Pool design needs to be tailored to a realistic understanding of the way in which the pool will be used. For example, the number and type of users, the temperature of use and any special health considerations for particular user groups will all affect the details of how the pool should be designed, constructed and managed. Specific considerations might include:
Swimming-pool and bathing-pool water needs to be safe. These water quality requirements need to be met through optimal matching of the following design factors:
Control of pathogens is typically achieved by a combination of recirculation of pool water through treatment (typically involving some form of filtration plus disinfection) and the application of a residual disinfectant to inactivate microorganisms introduced to the pool by bathers.
A dedicated crew member should be assigned to the operation of the recreational water environment and should be suitably trained.
A. Swimming pools
The pool and its water supply need to be designed, constructed and operated in view of the health and safety protection of bathers. These design, construction and operational issues are summarized in the paragraphs below, and details on specific requirements of various pool and spa types follow.
1. Circulation and hydraulics
The purpose of paying close attention to circulation and hydraulics is to ensure that the whole pool is adequately served. Treated water needs to reach all parts of the pool, and polluted water needs to be removed—especially from areas most used and most polluted by bathers. If not, even good water treatment may not result in good water quality. The design and positioning of inlets, outlets and surface water withdrawal are crucial.
Pools usually use seawater or a potable water supply passing through an air gap or backflow preventer. The fill level of the pool is at the skim gutter level. The pool overflows can either be directed by gravity to the make-up tank for recirculation through the filter system or disposed of as waste. Surface skimmers need to be capable of handling sufficient volume, such as approximately 80%, of the filter flow of the recirculation system. There should be sufficient skimmers, such as at least one skimmer for each 47 m2 of pool surface area.
Circulation rate is related to turnover period, which is the time taken for a volume of water equivalent to the entire pool water volume to pass through the filters and treatment plant and back to the pool. In principle, the shorter the turnover period, the more frequent the pool water treatment. Turnover periods need to suit the particular type of pool. Ideally, turnover must be designed to vary in different parts of the pool: longer periods in deep areas, shorter periods where it is shallow.
Disinfection and treatment will not remove all pollutants. The design of a swimming pool should recognize the need to dilute the pool water with fresh water. Dilution limits the buildup of pollutants from bathers (e.g. constituents of sweat and urine), the by-products of disinfection and various other dissolved chemicals and pollutants.
A drain must be installed at the lowest point in the pool, and drainage facilities need to be sufficient to ensure quick emptying. The drains from the pool should be independent; however, when they are connected to any other drainage system, a backwater valve must be installed in the recreational water environment to prevent cross-connections. Anti-vortex and anti-entanglement type drain covers must be provided, which are constructed of durable, easily visible and readily cleaned material.
Children's pools can have their own independent recirculation, filtration and halogenation system, because children are particularly potent sources of pathogens. The turnover rate of water needs to be sufficient, ideally higher than in adult pools, such as at least once every 30 minutes. Anti-vortex and anti-entanglement type drain covers must be provided that are constructed of durable, easily visible and readily cleaned material.
2. Bather load
A realistic bather load needs to be catered for during both design and operation of pools. The circulation and treatment systems and the hydraulic volume will determine the appropriate safe bather load, but the practicability of maintaining bather loads within design criteria also needs to be considered.
3. Filtration
Controlling clarity involves adequate water treatment, usually involving filtration and coagulation. Filtration is crucial to good water quality, affecting both aesthetic clarity and disinfection. Disinfection will be compromised by reduced clarity, as particles associated with turbidity can surround microorganisms and shield them from the action of disinfectants. In addition, filtration is important for removing Cryptosporidium oocysts and Giardia cysts and some other protozoa that are relatively resistant to chlorine disinfection.
Filters need to be designed to remove particles at a sufficient rate, such as removing all particles greater than 10 µm from the entire volume of the pool in 6 hours or less. Filters can be cartridge or media type (e.g. rapid-pressure sand filters, high-rate sand filters, diatomaceous earth filters or gravity sand filters). All media-type filters need to be capable of being backwashed. Filter accessories, such as pressure gauges, air-relief valves and rate-of-flow indicators, should be provided as required. Sufficient access to sand filters should be maintained so that they can be inspected at a regular frequency, at least on a weekly basis, and the media must be changed periodically.
Some of the factors that are important to consider in the design of a granular media (e.g. sand) filtration system include:
Filtration rate: The higher the filtration rate, the lower the filtration efficiency. Some of the higher rate granular filters do not handle particles and colloids as effectively as medium-rate filters and cannot be used with coagulants.
Bed depth: The correct sand bed depth is important for efficient filtration.
Number of filters: Pools will benefit greatly from the increased flexibility and safeguards of having more than one filter. In particular, pools can remain in use with a reduced turnover on one filter while the other one is being inspected or repaired. Filtered water from one filter can be used to backwash another.
Backwashing: The cleaning of a filter bed clogged with suspended solids is referred to as backwashing. It is accomplished by reversing the flow, fluidizing the sand and passing pool water back through the filters to waste. It should be initiated as recommended by the filter manufacturer, when the allowable turbidity value has been exceeded or when a certain length of time without backwashing has passed. The filter may take some time to settle once the flow is returned to normal, and water should not be returned to the pool until the filter has settled.
A hair strainer is required between the pool outlet and the suction side of the pumps to remove foreign debris such as hair, lint and pins. The removable portion of the strainer should be corrosion resistant and have holes that are smaller than 6 mm in diameter.
Coagulants (and flocculants) enhance the removal of dissolved, colloidal or suspended material by bringing this material out of solution or suspension as solids (coagulation), then clumping the solids together (flocculation), producing a floc, which is more easily trapped in the filter. Coagulants are particularly important in helping to remove the infective cysts of Giardia and oocysts of Cryptosporidium spp., which otherwise would pass through the filter. Coagulant efficiency is dependent on pH, which therefore needs to be controlled. Dosing pumps should be capable of accurately dosing the small quantities of coagulant required and adjusting to the requirements of the bather load. Coagulation is often required as a prerequisite to effective filtration, depending on the filtration process selected.
4. Chemical dosing, including disinfection
Disinfection is a process whereby pathogenic microorganisms are removed or inactivated by chemical (e.g. chlorination) or physical (e.g. filtration, UV radiation) means, such that they represent no significant risk of infection. Recirculating pool water is disinfected using the treatment process, and the entire water body is disinfected by application of a disinfectant residual, which inactivates agents added to the pool by bathers.
For disinfection to occur with any biocidal chemical, the oxidant demand of the water being treated must first be satisfied, and sufficient chemical must remain to effect disinfection.
Issues to be considered in the choice of a disinfectant and application system include:
safety;
comfort (e.g. avoiding skin irritation);
compatibility with the source water (hardness and alkalinity);
type and size of pool (disinfectant may be more readily degraded or lost through evaporation in outdoor pools);
oxidation capacity;
bather load (sweat and urine from bathers will increase disinfectant demand);
operation of the pool (i.e. supervision and management).
The choice of disinfectant used as part of swimming-pool water treatment should ideally comply with the following criteria:
effective, rapid inactivation of pathogenic microorganisms;
capacity for ongoing oxidation to assist control of contaminants during pool use;
a wide margin between effective biocidal concentration and the concentration resulting in adverse effects on human health;
availability of a quick and easy determination of the disinfectant's concentration in pool water (simple analytical and test methods);
potential to measure the disinfectant's concentration electrometrically to permit automatic control of disinfectant dosing and continuous recording of the values measured.
Commonly used disinfectants include the following:
Chlorine: Chlorination is the most widely used pool water disinfectant, usually in the form of chlorine gas, sodium or calcium hypochlorite or chlorinated isocyanurates. Chlorine is inexpensive and relatively convenient to produce, store, transport and use. Chlorinated isocyanurate compounds, which are somewhat complex white crystalline compounds with slight chlorine-type odour that provide free chlorine when dissolved in water, are used in most small outdoor ship pools. They are an indirect source of chlorine, via an organic reserve (cyanuric acid). The relationship between the chlorine residual and the level of cyanuric acid is critical and can be difficult to maintain. Chlorinated isocyanurates are not suited to the variations in bather loads usually found in large pools. However, they are particularly useful in outdoor swimming pools exposed to direct sunlight, where UV radiation rapidly degrades free chlorine.
Ozone: Ozone can be viewed as the most powerful oxidizing and disinfecting agent that is available for pool and spa water treatment. Ozone in combination with chlorine or bromine is a very effective disinfection system, but the use of ozone alone cannot ensure a residual disinfectant capacity throughout the swimming pool. Ozone is most frequently used as a treatment step, followed by deozonation and addition of a residual disinfectant, such as chlorine. Excess ozone must be destroyed by an activated carbon filter, because this toxic gas could settle, to be breathed by pool users and staff. Residual disinfectants should also be removed by the activated carbon filter and are therefore added after this step.
UV radiation: Like ozone, UV radiation is a plant-room treatment that purifies the circulating water, inactivating microorganisms and, to a certain extent, breaking down some pollutants by photo-oxidation. This decreases the chlorine demand of the purified water but does not leave a disinfectant residual in the pool water, so chlorine disinfection is still required. For UV to be most effective, the water must be pretreated to remove turbidity-causing particulate matter that prevents the penetration of the UV radiation or absorbs the UV energy.
Microbial colonization of surfaces can be a problem and is generally controlled through cleaning and disinfection, such as shock dosing.
The method of introducing disinfectants to the pool water influences their effectiveness. Individual disinfectants can have their own specific dosing requirements, but the following principles apply to all:
Automatic dosing is preferable. Electronic sensors continuously monitor pH and residual disinfectant levels and adjust the dosing correspondingly to maintain correct levels. Regular verification of the system (including manual tests on pool water samples) and good management are important.
Hand dosing (i.e. putting chemicals directly into the pool) is rarely justified. Manual systems of dosing must be backed up by good management of operation and monitoring. It is important that the pool remains empty of bathers until the chemical has dispersed.
Trying to compensate for inadequacies in treatment by shock dosing is bad practice, because it can mask deficiencies in design or operation that may produce other problems and can generate unwelcome by-products.
Dosing pumps should be designed to shut themselves off if the circulation system fails (although automatic dosing monitors should remain in operation) to ensure that chemical dispersion is interrupted.
Residual disinfectants are generally dosed at the very end of the treatment process. The treatment methods of flocculation, filtration and ozonation serve to clarify the water, reduce the organic load and greatly reduce the microbial count, so that the post-treatment disinfectant can be more effective and the amount of disinfectant that must be used can be minimized.
It is important that disinfectants and pH-adjusting chemicals be well mixed with the water at the point of dosing.
Dosing systems, like circulation, should continue 24 hours per day.
Production of disinfection by-products can be controlled by minimizing the introduction of their organic precursors (compounds that react with the disinfectant to yield the by-products) through good hygienic practices (pre-swim showering) and maximizing their removal by well-managed pool water treatment. Control of disinfection by-products involves dilution, treatment and disinfection modification or optimization. Because of the presence of bromide ions in salt water, a common by-product formed in the water and air of seawater pools on ships will be bromoform, which can result from either chlorine or ozone treatment.
It is inevitable that some volatile disinfection by-products will be produced in the pool water and escape into the air. This hazard can be managed to some extent through good ventilation.
The use of analysers helps to automate dosing and optimize conditions for pool safety, such as automatic dosing of chemicals for disinfection and pH adjustment. Water sample points must be provided throughout the system for testing halogen levels and routine calibration of the analyser. Analyser-controlled halogen-based disinfection equipment should be provided as required. It may be necessary to ensure that pH is adjusted by using appropriate acids and bases and that a buffering agent is used to stabilize the pH. This can be added to the functionality of the analyser.
5. Legionella control
In recreational water environments, it is impractical to maintain temperatures outside the range 25–50 °C. However, levels of Legionella spp. can be kept under control using appropriate management measures, including filtration and maintenance of a continuous disinfectant residual in recreational water environments and physical cleaning of all spa-pool equipment, including associated pipes and air-conditioning units. Rooms housing recreational water environments must be well ventilated to avoid an accumulation of Legionella spp. in the indoor air. Therefore, it is necessary to design and implement a range of other management strategies, which may include:
adding biocides to the spa water, plumbing and filter. Whirlpool spas shall typically maintain a free chlorine residual between 3 and 10 mg/l or a free bromine residual between 4 and 10 mg/l (
WHO, 2006). To ensure that free halogen is effective for disinfection, there is a need to maintain or regularly adjust the pH, typically in the range 7.2–7.8;
ensuring that staff have appropriate training and skills to operate the recreational facility;
applying a constant circulation of water in the whirlpool and spa pool;
cleaning filter systems (e.g. by backwashing filters);
cleaning pool surrounds;
replacing a portion (e.g. 50%) of the water in each whirlpool and spa pool daily;
completely draining whirlpools, spa pools and natural thermal pools and thoroughly physically cleaning all surfaces and all pipework regularly;
maintaining and physically cleaning heating, ventilation and air-conditioning (HVAC) systems serving the room in which spa pools are located;
installing signs that list standard safety precautions near the recreational water environments, which caution people who are immunocompromised or who are taking immunosuppressant medicines against using the recreational water environments.
Routine cleaning of the whole circulation system, including the spa, sprays, pumps and pipework, is critical and can require quite intensive doses of disinfectant, as Legionella spp. can persist in biofilms (scums on the surfaces of fittings and pipework), making them difficult to inactivate.
Bathers must be encouraged to shower before entering the water. This will remove pollutants such as perspiration, cosmetics and organic debris that can act as a source of nutrients for bacterial growth and as neutralizing agents for the oxidizing biocides. Bather density and duration in whirlpools and spa baths can also be controlled. Spa-pool facilities may require programmed rest periods during the day to allow recovery of disinfectant concentrations.
Testing for Legionella bacteria serves as a form of verification that the controls are working and should be undertaken periodically—for instance, monthly, quarterly or annually, depending on the type of ship environment. This testing should not replace or pre-empt the emphasis on control strategies. Furthermore, the tests are relatively specialized and need to be undertaken by properly equipped laboratories using experienced staff; they are therefore not generally performed by crews or during voyages. Verification sampling should focus on system extremities and high-risk sites.
6. Air quality
It is important to manage air quality as well as water quality in swimming pools, spas and similar recreational water environments. Rooms housing spas should be well ventilated to avoid an accumulation of Legionella spp. in the indoor air. In addition, ventilation will help reduce exposure to disinfection by-products in the air. Adequate ventilation should reduce risks from Legionella spp., but it is important that the system does not create its own risks. All surfaces of HVAC systems serving the room in which the spa or pool is located should be physically cleaned and disinfected to control biofilm.
Other design and construction aspects
The pool mechanical room must be readily accessible and well ventilated, and a potable water tap must be provided in this room. To help with ongoing maintenance, it is valuable to mark all piping with directional-flow arrows and maintain a flow diagram and operational instructions in a readily available location. The pool mechanical room and recirculation system need to be designed for easy and safe storage of chemicals and refilling of chemical feed tanks. Drains need to be installed in the pool mechanical room to allow for rapid draining of the entire pump and filter system, with a sufficiently large drain, at least 8 cm, being installed on the lowest point of the system.
To help reduce drowning risks, the depth of the pool and depth markers must be displayed prominently so that they can be seen from the deck and in the pool. Depth markers must be in either feet or metres, or both, and installed for every significant (1 m) change of depth.
C. Flow-through pools
The flow-through swimming pool is probably the type most practicable for construction, installation and operation on board ships. The number of bathers that can use a swimming pool safely at one time and the total number that can use a pool during one day are governed by the area of the pool and the rate of replacement of its water. Therefore, the pool should be designed with special attention to the probable peak bather load and the maximum space available for the construction of a pool.
The following principles should be applied in the design of flow-through pools:
The design capacity of the pool should be judged on the basis of the area, such as 2.6 m2 per bather. For the maintenance of satisfactorily clean water in the pool, the rate of flow of clean water needs to be sufficient to achieve complete replacement every 6 hours or less. The water flowing through must be delivered to the pool through multiple inlets, located to ensure uniform distribution. These inlets can be served by a branch line taking off from the main supply line, at the pressure side of the filling valve near the pool. Control of the flow must be independent of the filling valve.
The overflow must be discharged into skim gutters or a similar boundary overflow, with multiple outlets spaced not more than 3 m apart, and discharging to the waste system.
The bottom of the pool should slope towards the drain or drains in such a manner as to effect complete drainage of the pool. In the interest of safety, the slope of any part of the pool bottom in which the water is less than a standing depth, 1.8 m deep, should not be more than a 1 in 15 gradient. For safety, there should be no sudden change of slope within the area where the water depth is shallow, less than 1.5 m.
To help reduce drowning risks, the depth of the pool and depth markers must be displayed prominently so that they can be seen from the deck and in the pool. Depth markers must be in either feet or metres, or both, and installed for every significant (1 m) change of depth.
It is preferable to have a separate water supply system, including the pump, for recreational water environments. The water intake must be forward of all sewage and drainage outlets. However, if the pool is to be filled and operated only when the ship is under way, the fire or sanitary water pumps, or a combination of these pumps, may be used, noting that the following can be used to reduce contamination risks:
The delivery line to the pool should be independent of other lines originating at or near the discharge of the pump or the valve manifold or at a point where the maximum or near-maximum flushing of the fire or sanitary water pump is routinely effected.
If seawater is drawn into the pool, water should not be drawn when the ship is in port or, if under way, in contaminated waters. A readily accessible shut-off valve should be located close to the point from which the water is drawn and labelled “CLOSE WHILE IN HARBOURS”.
Flow-through seawater supply systems for pools shall be used only while the ship is under way and at sea beyond 12 nautical miles from land. The pool (when in flow-through seawater mode) should be drained before the ship reaches port and should remain empty while in port. If the pool is not drained before arriving in port, the pool's seawater filling system should be shut off 12 nautical miles before reaching land, and a recirculation system should be used with appropriate filtering and halogenation.
D. Whirlpool spas
Whirlpools are subject to high bather loads relative to the volume of water. With high water temperatures and rapid agitation of water, it may become difficult to maintain satisfactory pH, microbiological quality and disinfectant residuals; therefore, additional care must be taken in the operation of whirlpools.
Potable water supplied to whirlpool systems must be supplied through an air gap or approved backflow preventer. Water filtration equipment needs to be able to remove all particles greater than 10 µm from the entire whirlpool water volume in 30 minutes or less. Filters can be cartridge filters, rapid-pressure sand filters, high-rate sand filters, diatomaceous earth filters or gravity sand filters. A clear sight glass can be added on the backwash side of the filters.
The overflow system must be designed so that the water level is maintained. It is advisable that whirlpool overflows be either directed by gravity to the make-up tank for recirculation through the filter system or disposed of as waste. Self-priming, centrifugal pumps must be used to recirculate whirlpool water.
Sufficient skimmers, one for every 14 m2 or fraction thereof of water surface area, should be provided. The fill level of the whirlpool needs to be at the skim gutter level to enable skimming to take effect.
A temperature-control mechanism is required to prevent the temperature from exceeding 40 °C to avoid scalding and overheating.
A make-up tank may be used to replace water lost by splashing and evaporation. An overflow line at least twice the diameter of the supply line and located below the tank supply line should be used.
The system needs to permit regular (e.g. daily) shock treatment or superhalogenation. Halogenation equipment that is capable of maintaining the appropriate levels of free halogen throughout the use period must be included.
E. Spa pools
Spa pools have different operating conditions and present a special set of problems to operators. The design and operation of these facilities make it difficult to achieve adequate disinfectant residuals. They may require higher disinfectant residuals because of higher bather loads and temperatures, both of which lead to more rapid loss of a disinfectant residual.
A Pseudomonas aeruginosa concentration of less than 1 cfu/100 ml should be readily achievable through good management practices. Risk management measures that can be taken to deal with these non-enteric bacteria include ventilation, cleaning of equipment and verifying the adequacy of disinfection.
Spa pools that do not use disinfection require alternative methods of water treatment to keep the water microbiologically safe. A very high rate of water exchange is necessary—even if not fully effective—if there is no other way of preventing microbial contamination.
In spa pools where the use of disinfectants is undesirable or where it is difficult to maintain an adequate disinfectant residual, superheating spa water to 70 °C on a daily basis during periods of non-use may help control microbial proliferation.
To prevent overloading of spa pools, some countries recommend that clearly identifiable seats be installed for users combined with a minimum pool volume being defined for every seat, a minimum total pool volume and a maximum water depth.
