Pools for collective use

Type 1

Public swimming pools, recreational pools and water parks (in the 2016 edition: categories A1 and A3).

Type 2

Swimming pools of hotels, campsites, agritourism and non-hotel accommodation facilities, clubs and associations, gyms and beauty centres without medical supervision, schools and boarding schools, residential buildings, residential care homes and therapeutic pools with medical supervision (in 2016: A2, B and C).

Type 3

Swimming pools used exclusively for diving, military training, lifesaving training and underwater activities.

Surface draw-off

At least 75% of the circulation flow rate passes through the surface draw-off. The overflow channel, where provided, covers at least 50% of the perimeter (30% in type 2 pools up to 100 m²); skimmers are only permitted in type 2 pools up to 100 m², at least one every 20 m².

Submerged outlets and dye test

Main drains, wall outlets and pool cleaner outlets must prevent the risk of entrapment according to the UNI EN 13451 series. Circulation must pass the dye test of UNI EN 15288-1, with uniform colouring of the pool within 15 minutes.

Recirculation times

The systems operate continuously 24 hours a day, with maximum recirculation times that depend on the depth and use of each zone of the pool: for example, 3 hours above 1.35 m, 2.5 hours between 0.6 and 1.35 m, 1 hour between 0.4 and 0.6 m, half an hour up to 0.4 m.

Filters and pumps

At least two equivalent filters and two pre-filters in parallel (one in type 2 pools up to 100 m²); as many circulation pumps as there are filters, plus one standby pump in rotation.

Water renewal

Automatic make-up and renewal, overflow and water meter on the supply water. The daily renewal, recorded, is at least 30 litres per bather or, if the number of bathers cannot be determined, 1.5% of the pool volume plus 60% of the balance tank volume.

Flocculation

The flocculant, when used, is dosed continuously upstream of the filters with dedicated dosing pumps; each tank must guarantee at least 24 hours of autonomy.

Hardness

It indicates the concentration of calcium and magnesium ions. Continuous variations in pH and temperature, together with the natural tendency of hardness to precipitate, form limescale on the floor, walls, fittings, pipes and filter, eventually blocking the filtration circuit. Products that prevent precipitation or softening systems that remove calcium and magnesium are used.

Alkalinity

It indicates the concentration of carbonates, which must be between 80 and 130 ppm (mg/l): lower values make the water aggressive for bathers and fittings, higher values make pH control difficult.

Turbidity

With well water, pre-filtration may be necessary to prevent soil and small debris from entering the pool.

Metals

Iron, copper and manganese in high concentrations, besides being harmful on human contact, cloud and colour the water on contact with hypochlorite: a metal sequestrant or an iron removal system is used.

Filtration

The water passes through a bed of sand or zeolites, which have adsorbent properties and are therefore more effective than sand, retaining colloids and coarse pollutants (skin, saliva, cosmetics, soap, fats), among the main causes of turbidity.

Flocculation

The effectiveness of filtration increases by dosing a flocculant (aluminium sulphate, ferric chloride, ferric chlorosulphate, polyaluminium hydroxychloride and polyaluminium hydroxychlorosulphate, sodium aluminate). Iron and aluminium in the pool must not exceed the values for drinking water: an excess indicates a leak from the filter and the return to the pool of the retained impurities.

UV rays

UV lamps help to destroy chloramines and reduce the microbial load, but they do not replace the disinfectant: they act only as the water passes under the lamp, with no residual protection, and even a small dust particle can shield microorganisms.

pH below 7.2

Attack on metal materials and irritation of bathers’ eyes and mucous membranes.

pH above 7.4

Chlorine is converted into hypochlorite ion, which has little disinfecting power: for the same level of pollution, more chlorine is needed. A high pH also promotes the precipitation of hardness and limescale deposits.

Continuous control

Bathers and alkaline disinfectants (sodium hypochlorite) tend to raise the pH, dichlor and trichloroisocyanurate to lower it. To reduce the pH, sulphuric acid is normally used, which, unlike hydrochloric acid, does not produce chlorides: in excess, chlorides make water discharge difficult.

More filtration hours

Increase the filtration running time.

More effective filter

Replace the filter bed or dose a flocculant.

Adjust the pH

Bring the pH back into the correct range.

More disinfectant

Increase the free chlorine value.

Bacterial count at 22 °C and 37 °C

It estimates the quantity of bacteria present and provides information on the hygiene level of the pool and on potential regrowth.

Escherichia coli and enterococci

They indicate faecal contamination; enterococci are more resistant to disinfection. Their absence does not rule out the presence of more resistant protozoan cysts.

Staphylococcus aureus

It indicates non-faecal organic material released by bathers; it causes skin rashes and infections of wounds, urinary tract, eyes and ears.

Pseudomonas aeruginosa

It indicates the possibility of microbial regrowth; it causes respiratory and urinary tract infections, otitis, and infections of the cornea and hair follicles.

Legionella

It proliferates where water stagnates, at temperatures between 25 and 55 °C; it is transmitted by inhaling droplets of contaminated water (showers, whirlpools, fountains). It is controlled with an oxidant such as chlorine or chlorine dioxide.

Chloramines

Chlorine-based products (hypochlorite, salt electrochlorination, trichloroisocyanurate) react with the nitrogenous substances introduced by bathers, such as urea, forming monochloramines, dichloramines and trichloramines: together they make up combined chlorine, with negligible oxidising power. Trichloramines, volatile and irritating, cause the typical odour of poorly chlorinated pools.

Shock chlorination

Monochloramines and dichloramines remain in the water for days: they are eliminated with activated carbon, UV lamps or shock chlorination (break-point), which oxidises the chloramines, converting them into nitrogen gas. It must be limited to what is strictly necessary, because it forms trihalomethanes, volatile compounds that can be absorbed through the skin and by inhalation.

Total chlorine

The sum of free chlorine and combined chlorine is total chlorine: by measuring it and subtracting free chlorine, combined chlorine is obtained. Combined chlorine and stabiliser alter the Redox measurement; the closed amperometric cell measures hypochlorous acid only.

Sodium hypochlorite

Liquid product dosed by automatic systems that continuously measure free chlorine and control the dosing pump to keep it at 1.5 mg/l, also adjusting the pH. Hypochlorite contains sodium hydroxide (caustic soda), which tends to raise the pH. It is the most widely used form of chlorine and the one with the fewest side effects.

Salt electrochlorination

The electrochlorinator produces chlorine by electrolysis of the sodium chloride dissolved in the pool, at a concentration of 4 – 6 g/l, much lower than that of seawater: the disinfectant is generated on site, with no storage. Critical aspects: oxidation of metal parts not made of AISI 316L stainless steel or not connected to an earth electrode, discharge of chloride-rich water, precipitation of hardness in hard water.

Trichloroisocyanurate

Granular or in tablets placed in the skimmers: a strongly acidic, slow-dissolving compound, it is converted into hypochlorous acid and cyanuric acid, which stabilises chlorine against UV rays. Cyanuric acid accumulates at the bottom: at around 75 ppm it drastically reduces the effectiveness of chlorine and makes pH control difficult. It can only be reduced by replacing water through the main drain.

Ozone

A powerful but very unstable disinfectant, generated on site by generators that pass an electrical discharge through the oxygen in the air. It turns back into oxygen almost immediately and leaves no safety residual; indeed, it must be completely removed before bathing. Chlorine must still be added to protect the pool.

Bromine

Little used because of its odour; in water it forms hypobromous acid and hydrobromic acid and is less dependent on pH than chlorine. In swimming pools, bromochlorodimethylhydantoin (BCDMH) is used: bromamines, less stable and less irritating than chloramines, recombine into hypobromous acid. A weaker oxidant than chlorine, it requires 2 – 6 mg/l of free bromine; high concentrations of dimethylhydantoin reduce its effectiveness and the Redox potential.

Hypochlorous acid

It is the form of chlorine with the highest and fastest disinfecting power: it prevails the closer the pH is to 7.

Hypochlorite ion

It is the form with a considerably lower disinfecting power: it prevails the more the pH tends towards 8.

Combined chlorine and total chlorine

Chlorine that reacts with the nitrogenous substances introduced by bathers forms chloramines, which cause irritation of the eyes and mucous membranes and the characteristic odour: together they constitute combined chlorine, which must not exceed 0.4 mg/l. The sum of free and combined chlorine is total chlorine.

pH 7.2

Approximately 69% of the free chlorine is hypochlorous acid.

pH 7.6

Hypochlorous acid falls to approximately 47%: the two forms are roughly equal.

pH 8.0

Approximately 26% hypochlorous acid remains: three quarters of the chlorine dosed have a reduced disinfecting power.

Higher chlorine consumption

Cyanuric acid is organic material: chlorine treats it as a pollutant to be oxidised and an increasing share of each tablet is consumed in the attempt. More and more tablets are needed, which in turn add stabiliser.

Higher pH corrector consumption

Excess cyanuric acid keeps the pH very low and makes it difficult to raise, even when a pH increaser is dosed.

Measurement errors

Redox systems and open-cell amperometric systems measure the current that chlorine releases in the water: cyanuric acid releases additional current, which the measurement attributes to chlorine; the DPD method also reads the chlorine bound to the stabiliser as free chlorine. Only the closed amperometric cell is unaffected.

pH first

Given the close dependence between chlorine and pH, the pH must also be controlled automatically: systems that first adjust the pH and only then dose the hypochlorite are preferable.

Two probes

Two probes, mounted in a flow-through probe holder supplied with a water sample taken after filtration, measure pH and free chlorine and tell the pumps how much corrector and how much hypochlorite to dose. There is only one measuring principle for pH; for chlorine the choice is between Redox and amperometry.

Dosing in solution

While the pool is open to the public, chlorine and pH correctors are not added to the pool or at points accessible to bathers (skimmers, overflow channel), but only in solution into the circulation system, with the pH corrector downstream of the filter. Direct dosing into the pool is only permitted for extraordinary treatments when the pool is closed.

Dosing safety

The dosing pumps are interlocked with circulation, so as not to dose in the absence of flow. The tanks guarantee at least 24 hours of autonomy, have containment systems and, in type 1 pools, are housed in a separate, ventilated room (also recommended for types 2 and 3).

Ozone, UV and electrolysis

Ozone and UV rays are complementary to chlorination: medium-pressure UV reduces combined chlorine, while ozone must be removed before the water enters the pool. Chlorine generators using electrolysis of sodium chloride are covered by UNI EN 17818.

pH

Phenol red comparators or photometers, or potentiometric meters, with a resolution of at least 0.1 units; control instruments have outputs to control dosing, preferably proportional, and automatic recording of values is recommended.

Free chlorine

Comparators or photometers using the DPD method, or amperometric meters, with a resolution of at least 0.1 mg/l. For automatic control the same indications as for pH apply: control outputs, proportional dosing, recording of values.

Redox

Permitted only as a supplementary indication, to monitor the trend of organic pollution and to support the chlorine probe in controlling disinfection, with a resolution of at least 10 mV. Deriving the value of chlorine or other disinfectants from the Redox reading is not permitted.

Open cell

The two electrodes are in direct contact with the water and read all the current generated, including that of combined chlorine. Compared with Redox, it is less affected by pH variations, compensates for temperature and requires periodic calibration, but less frequently. Range 0 – 10 mg/l.

Closed cell

The electrodes are immersed in an electrolyte and separated from the water by a selective membrane cap: hypochlorous acid passes through the membrane, reaches the electrolyte and generates at the electrodes a current proportional to its concentration. The membrane excludes hypochlorite ion, combined chlorine, cyanuric acid and the other components that would alter the measurement; with pH and temperature compensation, the reading shows the mg/l of free chlorine actually present, with a resolution of 0.01 mg/l. Range 0 – 20 mg/l. A dedicated closed cell measures total chlorine: combined chlorine is obtained by difference.

Constant flow rate

With a low flow rate less hypochlorous acid reaches the membrane and the cell reads less; with an excessive flow rate it reads more. The cell is installed in a flow-through probe holder in by-pass, with adjustment and monitoring of the inlet flow rate and without exceeding the working pressure.

Clean membrane

Organic deposits and biofilm on the membrane reduce over time the quantity of hypochlorous acid passing through it and lower the reading: the membrane must be checked and the membrane cap replaced at the specified intervals.

Polarisation

The controller must always remain powered, to maintain the polarisation of the electrodes; when the filtration pump stops it goes into stand-by, continuing to measure without dosing.

The reaction

The DPD1 reagent, in tablet or liquid form, contains a buffer solution that brings the sample to pH 6.2 – 6.5 and reacts with all the free chlorine, hypochlorous acid and hypochlorite ion, developing a pink colour proportional to the concentration.

The reading

The photometer measures the absorbance of the sample at the wavelength of the pink colour and converts it into mg/l of free chlorine; in the manual comparator the colour is compared by eye with a scale.

Total and combined chlorine

Adding the DPD3 reagent to the same sample measures total chlorine; the difference from free chlorine is combined chlorine.

A snapshot, not a film

Each measurement describes the water at the moment of sampling: it is suited to checking and calibration, not to following the process or controlling dosing.

The pH of the measurement

The DPD method brings the sample to pH 6.2 – 6.5 and measures the free chlorine as if it were all hypochlorous acid. The closed cell measures the water at its actual pH: at pH 7.6 – 7.8 hypochlorous acid is less than half of the free chlorine and the cell reads considerably less than the photometer.

Not a calibration error

In these cases the cell must not be recalibrated to the DPD value: it indicates the share of chlorine that actually disinfects, whereas the DPD method indicates the overall free chlorine, including the part that has a reduced disinfecting power at the pH of the pool. To bring the two readings closer, the pH is brought to the optimum value.

The stabiliser

In the presence of cyanuric acid, the DPD method also reads as free chlorine the chlorine bound to the stabiliser, which the membrane of the closed cell excludes.

Continuous or spot

The cell follows every variation in real time; the DPD method captures a single moment, with the margin of error of sampling and of the operator.

pH not representative

The colour develops at the pH of the buffer solution, not at that of the pool: the measurement does not represent the actual disinfecting power of the chlorine.

Temperature

The reaction rate depends on temperature: with very cold water or reagents the reaction slows down and the times specified by the method are no longer correct.

Reaction times

A reading taken too early finds incomplete colour development and underestimates the chlorine.

Other oxidants

If other oxidants are present in the sample, the reaction is slow or progressive and the value is overestimated.

Sampling

Air bubbles in the cuvette, turbidity of the sample, cuvettes dirty inside or outside and expired reagents alter the reading.

Combined chlorine

Chloramines produced by bathers add an unwanted contribution to the free chlorine reading: with monochloramine above 1 mg/l the photometer reads up to 20% more than the closed cell.

Closed (membrane) amperometric cell

Open amperometric cell

DPD method

Passage

The photocell on the stainless steel column detects a person passing and opens the solenoid valve on the delivery line.

Filling

The water flows through the ball valve, the check valve, the solenoid valve and the water meter and reaches the basin through the filling line.

Dosing

The water meter measures the volume supplied and the pump doses the antifungal agent proportionally, or in a preset quantity at each activation.

Drainage

The GRP basin is fitted with a trapped drain and an overflow drain line.

Dosing pump

Injects the antifungal agent into the filling line of the basin; 230 VAC power supply.

Pulse-output water meter

Measures the water supplied to the basin and controls the pump in proportion to the volume.

Photocell and solenoid valve

The photocell, on a 0.80 m high stainless steel column, controls the solenoid valve on the delivery line; 24 VAC power supply.

Tank

In PE with safety bund, suction lance and level probe.

Surface water

Table 3: free active chlorine not exceeding 0.2 mg/l.

Sewer

Table 3: free active chlorine not exceeding 0.3 mg/l, in the absence of different limits set by the competent authority or by the operator of the final treatment plant.

Soil

Table 4: active chlorine not exceeding 0.2 mg/l, in addition to the other parameters specified for discharge on soil.

With water presence sensor

Constant dosing, activated automatically when water flows in the discharge pipe.

With pulse-output water meter

Dosing proportional to the volume of water discharged, measured by the water meter.

Water presence sensor

Capacitive, with no contact with the water: it is installed on the outside of the pipe with a clamp saddle and a 3/4" fitting. Sensitivity adjustable by trimmer, signal LED, IP 67 protection rating.

Dosing pump

Activated by the sensor, it doses a constant quantity of dechlorinating agent set with the knob, from 0 to 800 or from 0 to 8,000 g/h; product level probe.

Tank with safety bund

In PE, with safety bund against spills and suction lance with level probe.

Pulse-output water meter

Turbine or Woltmann type, 1” 1/2 or 2”, on the discharge pipe: it sends the pump a pulse for every set volume of water.

VMS MF dosing pump

Multifunction pump in volumetric proportional control: it doses the dechlorinating agent in ppm according to the pulses from the water meter. Versions of 3 bar / 10 l/h or 2 bar / 15 l/h; product level probe included.

Tank with safety bund

In PE, with safety bund against spills and suction lance with level probe.