Inhibitors for CaCo3-scaling in reverse osmosis-plants - influence of suspended matter on membrane clogging

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Courtesy of Kurita

Abstract
Calcium carbonate (CaCO3) is the most common source of mineral scaling in reverse osmosis plants, and it is a standard approach to add so-called anti-scalants to avoid membrane blocking. Although anti-scalants are used for many years, it remains a challenge to determine the most effective anti-scalant and its necessary dosage for a given water.

The reason why, is the complex process of the crystallization phenomenon, which can be described as a sequence of three processes: supersaturation, nucleation and growth of crystals. These processes can take place both in the bulk solution as well as on the membrane surface. The nucleation and the crystal growth steps are controlled in particular by the degree of supersaturation.

To measure the effectiveness of anti-scalants in a reliable way a membrane-based method has to be applied, because the interaction with the membrane and the real operating conditions of an RO plant are neglected in standard jar tests. Furthermore, the supersaturation in the standard tests is by far higher than in a real plant, so that nucleation preferably takes place compared to crystal growth.

In order to achieve results that match with the real operating conditions, a new membrane-based method (Fig 1 and Fig 2) combined with a laser diffraction particle-size analyzer (Beckman Coulter LS 13320) was applied, investigating the performance of three phosphorus-free anti-scalants of different chemical structures to prevent calcium carbonate scale. The focus of this study was the influence of suspended matter on the membrane clogging.

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