Induction time measurements in two brackish water reverse osmosis plants for calcium carbonate precipitation

Abstract

In this research work, the induction time of two brackish water reverse osmosis (BWRO) plants was measured with and without antiscalant (AS) operation with focus on calcium carbonate precipitation. This study focused on two BWRO plants in the Netherlands. The scaling potential of RO concentrate in the plant at recovery of 80% was calculated using the PHREEQC program. Induction times in RO concentrates were measured by collecting RO concentrates directly from the plants, with and without an AS dose in an air-tight glass reactor. The solution was continuously stirred for homogenization and maintained at room temperature using a thermostat. The change in pH of the solution over time was monitored. The time required to change the nucleation phase to just the start of the crystal growth was noted from the pH versus time graph, which was defined as induction time. At the end of the experiments, precipitate with RO concentrate with and without AS dose (plant B) were collected and an X-ray powder diffraction (XRD) and scanning electron microscope (SEM) analysis were performed. The results showed that the measured induction times for RO concentrate with AS dose in the two plants were longer than 100 and 280 h, respectively. The plants were operated at the same recovery (80%) but with different types of feed water and different types of AS. In a plant where the induction time could be measured without AS, the induction time turned out to be short namely about 4 h. The XRD results of this plant with and without an AS dose revealed calcite as the exclusive precipitate. The shapes of the crystals obtained with AS, shown by SEM images, were smaller, less clustered, and had more rounded edges than crystals without AS. The results suggested that the measured induction times of 100–280 h in both plants are much higher since the detention time in RO systems is just 1–1.5 min and even seconds in the last membrane. Therefore, for safe operation, induction time lower than what we measured in two BWRO plants might be needed.