Comparison of three antiscalants, as applied to the treatment of water from the River Oise

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. In order 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 build. 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 cycles 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. On completion of comparative tests on three commercially available products, only one gave redhibitory water permeability and pressure drop results. The two other products, applied at the dose levels recommended by the manufacturers, gave results that *Corresponding author. Presented at the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France, July 7–12, 2002. 0011-9164/02/$– See front matter © 2002 Elsevier Science B.V. All rights reserved

The aim of this study was to develop an experimental report to evaluate the efficiency of sequestering agents. Looking at the experiences already acquired from the bibliography, the use of a pilot plant with only a single membrane set, revealed interesting results but did not allow a comparison to be made, in the strict sense of the word, between the performance of the different sequestering agents. A pilot plant, comprising three parallel lines, was constructed to operate under operating conditions which were representative of the third stage of nanofiltration. This unit was supplied with a 2nd stage concentrate, produced by a second pilot plant running without the addition of a sequestering agent. Each of the three lines had its own sequestering agent injection station. It was therefore possible to compare the three different product types or three different doses of the same product. Three commercially available products of different composition were tested at the dose recommended by the manufacturer. The results showed an equivalent and satisfactory performance for two products, whilst the third did not appear suitable for treating the water of the River Oise. Combining the pilot studies and membrane autopsies to include a global analysis of the fouling and observation of the cross sections of the membranes under the scanning electron microscope, provided us with very precise information on the genesis and deposit stratification in relation to the nature of sequestering agent used. It transpired that these products have very limited action on the organic matter deposit and that the composition of the mineral deposits depends on the nature of the sequestering agent being studied. Autopsies were performed on the modules following cleaning. These showed that the sequence of chemical regeneration is not completely efficient, especially with regard to organic deposits. Monitoring membrane performance after cleaning (water permeability and salt retention) would not seem adequate to assess the efficiency of cleaning. To conclude, this study allowed us to develop a new method of evaluating the performance of sequestering agents under deposit-forming conditions very similar to those encountered in an industrial plant. Obviously the response time following testing is long (several weeks) but combining the pilot study with membrane autopsies produced very precise results on the onset of scaling along the membrane surface and on its nature, in relation to the product and/or dose tested. Transposing the results to industrial level should be feasible without great problem.