The water in public swimming pools can be contaminated by:
- particulate matter (such as hair or scales – small pieces of skin that detach from the epidermis – which can constitute a support for pathogenic microorganisms);
- dissolved organic matter (urine, sweat, etc.), brought mainly by swimmers.
Thus, to ensure the health safety of bathers, the water must be filtered, disinfected and disinfected at all times. The water treatment system set up for this purpose must also limit disinfection by-products (SPD) as much as possible, the number, nature and concentration of which vary greatly depending on the operating and usage conditions, or even depending on the origin of the water supplying the basins. Indeed, their presence can represent a health risk for users and operators.
ANSES stresses that the most economical and effective way to improve the quality of pool water is to limit its contamination as much as possible, which requires first and foremost the application of personal hygiene measures by bathers. It recalls the importance, for establishments, of making these rules known and, for users, of respecting them. The quantities of disinfection products used and the costs of installing and operating the treatment channels will thus be minimized.
Comparison of the different membrane processes for the filtration of basin water in public swimming pools
There are several pond water filtration techniques: diatomite filter, glass bead filter, sand filter or membrane processes.
Sand filtration is the most commonly used public swimming pool water filtration process in France today. Used alone, this technique does not always make it possible to comply with regulatory limits relating to water quality. To achieve them, the operators increase the rate of renewal of the water in the basins.
Membrane filtration is a physical separation process using a membrane that is both permeable and selective. This membrane, according to its intrinsic characteristics and its mode of use, constitutes a barrier allowing the transfer or the retention of certain compounds. Four types of membranes exist depending on the average size of their pores and their cut-off thresholds (i.e. the size of the molecules that can pass through their pores):
- l’osmose inverse (OI) ;
- la nanofiltration (NF) ;
- l’ultrafiltration (UF) ;
- la microfiltration (MF).
Whatever the technology used, membrane processes are particularly effective in producing clarified water of excellent quality. On the other hand, the retention of microorganisms will depend on the type of filtration membrane used. This process nevertheless remains more effective than the retentions carried out using sand filters. Finally, the elimination of chemical contaminants and of the DBPs themselves is very 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 ball) currently used for “clarification » basin waters. Nevertheless, with the aim of also eliminating chemical contaminants and DBPs, the Agency recommends implementing a hybrid treatment process, which can include activated carbon in particular. These additional treatments also make it possible to preserve the characteristics of the membranes.
If the membrane filtration process is installed in a swimming pool whose water is disinfected by an ozone/chlorine coupling, experts recommend installing the membrane filtration process following the mandatory “deozonation” step before the water reaches the pelvis, in order to preserve the characteristics of the membrane.
Agency recommendations for the filtration of filter wash water
MF or UF membranes have the most suitable technical characteristics for the filtration of filter washing water. In order to optimize the filtration of this type of water, ANSES recommends that the filter washing water be discharged into the wastewater network for 1 to 2 minutes at the start of the washing cycle, so as to ensure that the water most laden with organic matter is correctly evacuated. The water must then be pre-filtered using a 200 micrometer cut-off filter before being filtered through the membrane system. The water is then:
- either kept in a so-called “clean water tarp” with a view to its reuse for washing the filters;
- is chlorinated and then sent to the buffer tank as make-up water.
The Agency recalls that not all membrane processes can be considered as disinfection processes. As such, it is not possible to reduce the concentrations of disinfectants in the pools below the minimum value set in the regulations (active free chlorine between 0.4 and 1.4 milligrams per liter in the absence of stabilizer, free available chlorine greater than 2 milligrams per liter if stabilized chlorine is used).
Swimming pool water filtration membrane processes: the Agency proposes to set up a health compliance certificate
Membrane modules used for the filtration of water intended for human consumption (EDCH) must have, before being placed on the market, a health compliance certificate (ACS)[1]. The ACS makes it possible to check that the products and materials are not likely to alter the characteristics of the water by releasing the constituents of the membrane modules. For these same reasons, the Agency proposes that the membrane processes used to filter swimming pool water also have an ACS suitable for hot and chlorinated swimming pool water.
[1] Order of 22 June 2012 as amended relating to the conditions for placing on the market and implementing membrane filtration modules used for the treatment of EDCH taken pursuant to article R-1321-50 (I and II) of the CSP .
