CaCO₃ deposits in reverse osmosis: Part III – Incipient Scaling detection via polymer optical fibre sensors. Comparison to hydrochemical prediction and image analytical methods

Reverse Osmosis (RO) is a widely used technology for water treatment in the beverage industry to produce brewing water, process water or tap water. The weak points of RO-membrane filtration are membrane degradation due to oxidative water constituents or membrane fouling in the difficult-to-clean, spacer-filled feed channel. The fouling can be distinguished into two main categories. The first one is caused by feed waters with a high organic and microbial load. The second type of fouling is triggered by supersaturated salt solutions, which leads to the precipitation of salts on the membrane surface. Both fouling types are destroying spiral wound RO-modules if they are not detected in an early stage. This work presents and validates a new optical online detection method via polymer optical fibres (POF) for inorganic fouling as CaCO₃, CaSO₄, BaSO₄, SrSO₄. The new detection method is tested by three experimental set-ups and compared with common prediction and detection methods such as saturation calculation, measurement of salt rejection, permeability, and the pressure drop in the feed channel. In addition to conventional online analysis methods for detecting deposits in reverse osmosis systems, this study presents an image analysis method that provides reliable evidence of POF sensor operation. The POF sensor is able to detect incipient crystal formation during the RO process. This gives this study the opportunity to discuss current crystallisation theories such as the induction time theory and the role of monohydrated calcium carbonate (MCC) as a precursor in the formation of CaCO₃ deposits.