Water filtration in public swimming pools: ANSES publishes its recommendations on membrane processes

Water in public swimming pools can be contaminated by:

• particulate matter (such as hair or flakes of dead skin, which can carry pathogenic micro-organisms);
• dissolved organic matter (urine, sweat, etc.), mostly brought in by bathers.

To safeguard bather health, therefore, the water must be filtered and disinfected, and maintain its disinfecting properties at all times. The water treatment system set up for this purpose must also minimise disinfection by-products (DBPs), whose number, nature and concentration vary greatly according to the conditions of operation and use, and even according to the origin of the water supplying the pools. The presence of DBPs can pose a health risk for both users and operators.

ANSES stresses that the most economical and effective way to improve water quality in pools is to minimise contamination, which is achieved first and foremost through the application of personal hygiene measures by bathers. The Agency reiterates that it is crucial for facilities to raise awareness of hygiene rules and for users to comply with them. This would help minimise the amounts of disinfection products used and the costs of installing and operating the treatment systems.

Comparison of the different membrane filtration processes for public swimming pool water

There are several techniques for filtering pool water, including diatomite filters, glass bead filters, sand filters or membrane processes.

Sand filtration is currently the most widely used water filtration process in French public swimming pools. However, when used alone, this technique cannot always meet the regulatory limits for water quality. To achieve this, therefore, operators increase the rate of water renewal in the pools.

Membrane filtration is a physical separation process using a membrane that is both permeable and selective. Depending on its intrinsic characteristics and the way in which it is used, the membrane constitutes a barrier allowing the transfer or retention of certain compounds. There are four types of membranes according to the average size of their pores and their cut-off points (i.e. the size of the molecules that can pass through their pores):

• reverse osmosis (RO);
• nanofiltration (NF);
• ultrafiltration (UF);
• microfiltration (MF).

Regardless of the technology used, membrane processes are particularly effective in producing clarified water of excellent quality. On the other hand, micro-organism retention will depend on the type of filtration membrane used. Nevertheless, this process is still more efficient than retention using sand filters. Lastly, the removal of chemical contaminants and the DBPs themselves is also highly dependent on the type of membrane.

Thanks to their physico-chemical characteristics (resistance to temperature, pH, etc.), NF, UF and MF membranes can replace the various filters (sand, diatomite, glass bead) currently used to "clarify" pool water. Nevertheless, to ensure that chemical contaminants and DBPs are also eliminated, the Agency recommends using a hybrid treatment process, which could incorporate activated carbon, for instance. These additional treatments also help preserve the membrane's characteristics.

If the membrane filtration process is installed in a pool whose water is disinfected by ozone paired with chlorine, the experts recommend installing the membrane filtration process after the mandatory ozone-removal step before the water reaches the pool, in order to preserve the membrane's characteristics.

The Agency's recommendations on filtering the water used for backwashing filters

Microfiltration (MF) and ultrafiltration (UF) membranes have the best technical characteristics for filtering backwash water. In order to optimise the filtration of this type of water, ANSES recommends that the backwash water be discharged into the wastewater system for 1 to 2 minutes at the start of the wash cycle, to ensure that the water with the highest organic matter content is properly removed. The water must then be pre-filtered using a filter with a 200 micrometre cut-off before being filtered through the membrane system. It can then be:

• either kept in a "clean water tank" with a view to reusing it for backwashing filters;
• or chlorinated and then sent to the holding tank as make-up water.

The Agency points out that not all membrane processes can be regarded as disinfection processes. For this reason, it is not feasible to reduce disinfectant concentrations in the pools below the minimum value set by the regulations (free active chlorine between 0.4 and 1.4 milligrams per litre in the absence of stabiliser, free available chlorine greater than 2 milligrams per litre if stabilised chlorine is used).

Membrane filtration processes for swimming pool water: the Agency proposes introducing a health compliance certificate

Membrane modules used for filtering drinking water must have a health compliance certificate (ACS in French[1]) before they can be placed on the market. This ACS confirms that the products and materials are not likely to affect the water’s characteristics by releasing the constituents of the membrane modules. For the same reasons, the Agency proposes that the membrane processes used to filter swimming pool water should also have an ACS for hot and chlorinated pool water.

[1] Amended Ministerial Order of 22 June 2012 on conditions for placing on the market and implementation of membrane filtration modules used for the treatment of drinking water, issued in application of Article R-1321-50 (I and II) of the French Public Health Code.