Cleaning of filtration membranes

Whatever the membrane process, membrane material or fluid being filtered, it is clear that clogging of varying degrees occurs systematically during filtration (both tangential and frontal). In addition to limiting flow and altering the selectivity of transfers across the membrane, clogging, when of organic origin, provides nutrients for micro-organisms which, if not eradicated, are then free to develop in the equipment, potentially causing production safety problems following the quasi-irreversible installation of a biofilm that can be defined as micro-organisms embedded in a "cement" of exopolysaccharides [1] . This general observation can, however, be qualified depending on the application.

In water filtration, the fluid of interest is the permeate. Having passed the physical barrier of the membrane, the permeate is effective in retaining bacteria (microfiltration, MF) and possibly viruses (ultrafiltration of the order of 30 kg · mol ^–1 , UF), and presents little risk of contamination, especially as legislation requires additional disinfection, as membranes are not approved as a disinfection process. Production issues (UF, MF) are therefore much more focused on flow management, resulting in an original implementation involving regular retrofiltration (reversal of flow from permeate to retentate) with variable frequency depending on the origin of the water to be treated. This regular physical unclogging, combined with low chlorine levels, enables acceptable production flows to be maintained and delays the need for chemical cleaning, which may only be required once a week [2] .

Spiral nanofiltration (NF) and reverse osmosis (RO) membranes are a special case in this landscape, as membrane materials, generally made of polyamide, are rapidly degraded by disinfection in an oxidizing environment: disinfectants such as chlorine irreversibly hydrolyze the covalent bonds in the polymer matrix, weakening the active skin. As a result, NF membranes are rarely disinfected, even though they are regularly cleaned [3] and sold with a maximum cumulative chlorine exposure content.

The spiral-shaped reverse osmosis membranes used to desalinate brackish water or seawater may only need to be cleaned once or twice a year, but are the site of extensive biofilm development, visible to the naked eye [4] .

Applications in the food and beverage industry (BFSI)...