Nanofiltration, used in the Méry-sur-Oise water treatment plant (90 MGD) provides a softened water with a low organic matter content from which micropollutants and microorganisms have been removed. The experience acquired on the site since 1992 has revealed that the use of a crystal formation inhibitor is essential to slow down the clogging of the nanofiltration membranes, essentially on the third stage. Given the complexity of the action of the antiscalants (threshold effect, metal ion sequestration capacity, particle dispersion capacity), it is difficult to theoretically define the type of product to be used and its optimal dosing level. In addition to laboratories or modelling studies, pilot experiments are needed to assess antiscalants and optimise their doses. Nanofiltration cycles being long (1-2 months), it is nevertheless difficult to compare results of cycles, and the objective of the project was to develop a methodology for sequestering agent testing.

To carry out strictly comparative studies of commercially available products as well as studies using the same product but at several different dosage levels, a pilot unit comprising three parallel nanofiltration trains was built. The pilot unit is fed with 2nd stage concentrate, produced without sequestering agent, and each trains has its own separate antiscalant injection point. The first cycle aimed to provide answers to the following: (i) choice of the antiscalant best adapted to the treatment of water from the river Oise, (ii) identification of the clogging agents, (iii) the efficiency of the chemical regeneration procedure.

A production cycle using the same antiscalant at three different doses (0, 1 and 3 g.m-3) demonstrated that this product acted through a threshold effect. Analysing the cleaning waters provided further information on the type of clogging agents deposited on the membrane. Although for all the cycles the overall nature of the clogging is identical, essentially composed of organic matter and mineral elements (calcium, phosphorus, aluminium, etc.). The similarity of the water permeability and salt rejection at the beginning of two consecutive cycles tends to demonstrate that the chemical regeneration procedure (caustic soda/citric acid) is efficient.

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