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CaCO3 deposits in reverse osmosis Part I - Shortcomings of current approaches leading to a new prediction model and monitoring device
S. Hager, A. Bachmann, T. Hofmann, R. Engelbrecht and K. Glas

Reverse osmosis (RO) is a widely used technology for providing brewing and process water in the beverage industry. In order to design the membrane process in a resource-saving and cost-effective way, the prediction of salt deposits in the membrane modules is of great importance. Since calcium and carbonate containing waters are very common, the solubility of CaCO3 is often the limiting factor of the raw water yield. The prediction and thus the plant design is largely based on chemical saturation indices as the LSI and S&DSI. However, the calculation of these parameters is done on the basis of simplifications of the water chemistry, which are no longer necessary due to computer-aided calculation methods. Furthermore, there is no valid theory on CaCO3 crystallisation based on fundamental energetic considerations. The classical crystallisation theory, which had been considered valid for decades, has proven to be insufficient. Improved theories are currently under discussion and a validation for CaCO3 crystallisation is not available today. Nevertheless, the polymorphism of CaCO3 seems to be of great value for the description of different saturation and crystallisation zones. Besides the most stable CaCO3 form, calcite, the monohydrate calcium carbonate (MCC) has been characterised as crucial for the description of metastable solutions. Thus, the solubility product of calcite does not provide a suitable basis for predicting CaCO3 precipitation as applied in the LSI and S&DSI. With this work, we introduce a hydrochemical simulation model of the RO process that can be linked to the chemical equilibria of other crystal forms of CaCO3 such as the MCC. The calculations can be executed in the freely available water chemistry software PHREEQC. Besides the prediction of salt deposits, the literature shows a great need for reliable technical monitoring of fouling and especially inorganic fouling (scaling). In this context, we present a new fibre-optic based fouling detection system. Our proposed minimally invasive measurement technique allows spatially resolved measurements in membrane systems. The detection is based on the interaction of light and deposits such as salt on an optical fibre. The fibre optic sensor has the potential to provide the necessary database for validation and further improvement of the simulative prediction.

Descriptors: brewing water, process water, membrane, reverse osmosis, scaling, salt saturation, LSI, CaCO3, fouling detection

BrewingScience, 74 (September/October 2021), pp. 122-133