Autopsies of membranes fouled on Mery-sur-Oise pilot units: Many lessons for the behaviour of the water treatment plant Original Research Article

In membrane processes, the fouling of membranes keeps being the major issue: it is necessary to reduce it at most. Indeed, a good control of the deposits on the membrane will limit operating costs, mainly due to in the energy spending, and will enhance the lifetime of membranes. The fouling of membranes will cause an increase in the feed pressure to maintain a constant output and in the cleaning frequency. The membrane fouling phenomenon, because of its large variety, is very complex. Four main categories of fouling can be distinguished: biological, organic, colloidal and mineral. The autopsy of membranes is a destructive analytical method but remains an excellent tool to characterize very precisely the nature of deposits. The Mery-sur-Oise Drinking Water Treatment Plant (DWTP), which treats surface water, has now been operating for three years. Membrane studies, carried out on pilot units, started five years before the starting of the DWTP, and are continuing even at present. They aim at selecting the treatment processes, and then the operating conditions which minimize at most the risk of membrane fouling. Autopsies have been performed for several years on nanofiltration membranes, fed by process waters of Méry-sur-Oise DWTP, and which have been filtering for some weeks or for several years. This article presents all the results of the autopsies performed on nanoftltration membranes stemming from pilot units of Méry-sur-Oise. All these autopsies showed that the fouling of membranes fed by the treated Oise river water is a mixture of organic and colloidal deposits. Very few salts deposits were and appeared only on the last modules of the unit. This nature of deposit can be modified during failures or during particular events; it became mainly mineral (calcium carbonate and sulfate) in absence of antiscalant add ing. Algae blooms, which arise especially in spring, are periods of critical treatment for the nanofiltration membranes; whole algae or their skeletons were shown on almost all membranes. Autopsies performed on cleaned modules allowed to assess the efficiency of the used cleaning procedures. For all the tested procedures, some deposit, mainly composed of organic matter, could not be removed. In the future, it will be, interesting to compare the nature of the deposits analyzed on the membranes of the pilots of Méry-sur-Osie DWTP with those analyzed on the membranes of the industrial unit.