Hydraulics for Good Pool Water Treatment

PWTAG Technical notes are updates or new material for the standards and guidance given in the PWTG book, Swimming Pool Water and the PWTAG Code of practice and should be read in association with these publications.

  • Subject: Hydraulics for good pool water treatment
  • Date: March 2011

Water treatment systems are an integral part of the architectural, structural and mechanical design of a swimming pool.

The design, selection and operation of swimming pool water treatment plant has to take into account: mains water quality, dilution and drainage, coagulation, filtration and disinfection bathing load, circulation rate, circulation hydraulics and turnover period; and pool operation, water treatment system and plant room.

Pool operators should be aware of the HSE Code of Practice on Legionella.

Mains Water Quality

The water companies’ treatment processes provide a safe water but, especially if from a river or reservoir (surface waters), are likely to contain some or all of: organic materials, including humic acid (a precursor of the undesirable chlorination byproducts, trihalomethanes, which themselves may be present) lime and other alkalis (added to prevent corrosion in the supply network) phosphates (added to prevent lead dissolving from pipework, but which encourage algal growth in the pool) other substances at levels which, if boosted by pool water treatments, may take the levels above recommended safe limits.

So, it is essential that there is careful monitoring of a pool’s disinfection, pH, total alkalinity, calcium hardness, total dissolved solids and filtration. In general, the importance of a balance between public health demands and consumer acceptability applies to both drinking and swimming waters.

Disinfection cannot be compromised, but can be aimed towards minimising both disinfectant levels and the formation of unwanted substances, including disinfection byproducts. Dilution is an important factor in this process.

Source water monitoring

Pool operators should determine, monitor and record the characteristics of the source water they are using, including any likely variation in source. All water suppliers are required by statutory regulations to maintain a public record of the quality of water supplied to each water supply zone. This includes the results of analyses. Users are entitled to inspect the record and have a free copy of it for their water supply. They should, however, also ask water suppliers for details of likely changes in the future.

Bathing load, circulation rate and turnover rate

The bathing load, circulation rate and turnover rate are used to determine the settings used for the running of the treatment plant in order to maintain good water quality. They may be varied according to the prevailing or expected conditions of pool use.

Maximum bathing load

Circulation rate, turnover, treatment plant size and other factors are derived from the maximum bathing load (the greatest number of bathers allowed at any one time), which should have been determined at the design stage for the pool. The maximum bathing load takes into account:

  • the surface area of water in the pool (The HSE book, Managing Health & Safety in Swimming Pools gives a figure for maximum bathing load of 1 bather per 3m2)
  • the water depth
  • the planned bathing activity for the pool.

The maximum bathing load for each pool should be recorded.

Pool operators should be aware of the maximum bathing load and should ensure that this is not exceeded during operation of the pool.

The bathing load may be reduced or increased operationally, to take into account:

  • the design of the pool (eg if it has a moveable floor)
  • the cleanliness of the pool water
  • the performance of the treatment system.

Where the maximum bathing load has not been established at the design stage or is not known, a maximum bathing load can be calculated:

  • shallow water (under 1m) – 1 bather per 2.2m2standing depth water 1 bather per 2.7m2
  • deep water (over 1.5m) – 1 bather per 4m2Operational daily bathing load

The operational daily bathing load should be established using the following equation:

Operational daily bathing load = 25% to 50% of maximum bathing load x 12.

The operational daily bathing load for each pool in operation should be recorded, including details of the basis on which it was calculated.

If the operational daily bathing load is approached or exceeded frequently, then attention may need to be given to:

  • the treatment plant size
  • additional dilution
  • the use of additional treatments, such as ozone or ultraviolet.

Circulation rate

The specific circulation rate should be derived from the formula: Circulation rate (m3/h) = Maximum bathing load (bathers per m2) x 1.7

The circulation rate and turnover period are related and form the basis for sizing new water treatment plants, and for checking the capacity of existing water treatment plants.

Turnover rate

The turnover rate should be calculated from the formula:

  • Turnover period (h) = Water volume (m3) Circulation rate (m3/h)

Types of pool

Different sized pools and pools of different types should have turnover as follows:

  • Competition pools 50 m long – 3 to 4 h
  • Conventional public pools up to 25 m long with a 1 m shallow end – 2.5 to 3 h
  • Diving pools – 4 to 8 h
  • Hydrotherapy pools – 30 to 60 min
  • Leisure water bubble pools – 5 to 20 min
  • Leisure waters up to 0.5 m deep – 10 to 45 min
  • Leisure waters 0.5 to 1 m deep – 30 to 75 min
  • Leisure waters 1 to 1.5 m deep – 1 to 2 h
  • Leisure waters over 1.5 m deep – 2 to 2.5 h
  • Teaching/learner/training pools – 30 to 90 min
  • Waterslide splash pools (separate circulation/filtration) – 30 to 60 min

If the turnover period for an existing pool taken from this Table is less
than that calculated from the formula, then the former should be applied to the pool.

In this case a revised circulation rate should be established using the formula:

  • Circulation rate (m3/h) = Water volume (m3) Turnover period (h)


The turnover of water in shallow areas may be designed to be higher than in other areas. The turnover period of pools with moveable floors should be appropriate to the pool at its shallowest point (ie potentially biggest bathing load).

Pools with restricted access and thus bathing load (eg school, hotel, health club) may operate with a longer turnover period than indicated here, but the turnover rates given above should not be reduced where public use is introduced (e.g. dualuse school pools).

Pool Requirements

Pool water should circulate and be treated 24 hours a day. If the pool has a moveable floor or bulkhead (boom), the circulation system should ensure proper water distribution in all possible positions.

Good circulation hydraulics are necessary to ensure that the whole pool is adequately served in terms of both disinfection and removal of pollutants.

The circulation rate and turnover rate established in accordance with the guideline here should form the basis, along with water velocity and inlet and outlet issues (see later), for the specification of circulation hydraulics, including pumps and pipework.

Surface water removal

Surface water removal by a deck-level system is the most effective method for the removal of surface water, which carries the most pollution. No other method (overflow channels, skimmers) removes over 50% of its water from the surface.

Outlets and inlets

Where possible, outlets, inlets, grilles and covers should be designed in accordance with BS EN 134513 and regularly inspected for obstruction, impact damage or vandalism and to make sure that they are correctly in place. If they are damaged or missing swimming should be suspended. The velocity of water passing through covers, grilles, inlets and outlets can be a hazard to bathers, and depends on the circulation rate and the sizing of such covers, etc.


The velocity of water flowing from the pool through outlets should not exceed 0.5m/s, and there should be at least two outlets to each suction line. Outlets should be sized and located so that they cannot be blocked by the body of one bather.

Grilles in outlets should have gaps no greater than 8mm to prevent entrapment hazards.


In areas less than 800mm in depth and in sensitive areas (steps, teaching points, beside base inlets, etc) the velocity of the water entering the pool should not exceed 0.5m/s. In other areas, the velocity of the water entering the pool should not exceed 2.0m/s.

Some inlets incorporate water features such as geysers, waterfalls, jets and sprays. Where such features are incorporated into inlets, it is acceptable for the water velocity to be increased as necessary.


Separate pools demand separate water treatment systems.

Features in pools such as jets or mushrooms or spas or water slides should receive their water supply straight from the treated flow from the water treatment system, and not from the balance tank or any other supply that would mean short circuiting.

Swimming Pool Water describes a useful additional test for hair entrapment.