Studies on interactions between membranes (RO and NF) and pollutants (SiO2, NO3-, Mn++ and humic acid) in water

In nanofiltration and reverse osmosis membrane systems 75¯90% of the feed water is conversed to product, and as a consequence dissolved compounds are concentrated in the membrane concentrate. Subsequently, sparingly soluble inorganic compounds such as calcium carbonate and barium sulphate become supersaturated and may precipitate in a membrane element (scaling). Scaling leads to an increase in energy consumption and chemical cleaning frequency. This is undesirable and, therefore, an accurate prediction and monitoring of the risk of scaling is important. In this study a model is presented, called Supersaturation Prediction Model, which is able to calculate the supersaturation ratios of sparingly soluble compounds at the membrane surface. The model is a combination of a concentration polarisation model from literature and a model for calculation of the supersaturation ratio of sparingly soluble compounds. The concentration polarisation model from literature was fitted to measured average concentration polarisation factors of magnesium sulphate in a 4ʹʹ×40ʹʹ spiral wound reverse osmosis membrane element operated with different permeate fluxes and linear flow velocities of the membrane concentrate. The fitted concentration polarisation model was also able to predict the average concentration polarisation factors of sodium chloride and magnesium chloride in a 2.5ʹʹ×40ʹʹ spiral wound nanofiltration membrane element. With the aim to detect scaling in an early stage a scaling monitor has been developed and tested. The scaling monitor, named ScaleGuard®, measures the normalised flux of a small membrane element fed by a part of the concentrate of the full-scale plant. This scaling monitor was tested at a pilot plant treating anaerobic groundwater for the production of drinking water. The monitor demonstrated to be able to detect scaling before it took place in the last stage of the pilot plant.