Dehydration model for RO-membrane fouling: a preliminary approach

The proposed model is based on the hydration theory of dissolved chemical species in water. The molecular dehydration occurs when the solution system is subjected to critical water loss associated with re-displacement of ionic charges from attached water molecules towards the dissolved molecule. This mechanism allows the formation of solid phase, i.e. fouling. In order to estimate the true fouling load, it is necessary to define the type and molar concentration of the hard dissolved compound individually, then estimate the fraction available for deposition in a given chloride content. The mathematical model is illustrated and tested for six RO plants, and the obtained results are in agreement with field observation.

Dehydration model provides a reasonable explanation of the fouling mechanism occurring during RO membrane dewatering. The model showed that not all hardness ions are participating in fouling and scaling, as it wa thought, but follow selective preferential rules based on the associated chloride concentration in the RO brine solution. The present model indicates that the available hard species ready for fouling and scaling are lower than expected. The mathematical correlation between the theoretical and experimental fouling results proved that the proposed model is an accurate and valuable tool for predicting the inorganic fouling load. As a guideline for the RO design, it is recommended to keep the number of hard atoms in 1 mm×1 cm2 brine layer to be lower than 2×1018 by using the proper pretreatment or/and lowering the permeate recovery ratio. The possible source of error could arise from the interference with added chemicals, membrane type and wide variation in temperature.