With the growing population and industrial development, there is more stress on natural water resources. Additionally, environmental laws make the disposal of waste streams from industries difficult. In this scenario, it is crucial to treat waste streams to recover water and possibly minerals, for reuse in the industry, reducing the dependency on new resources. Reverse osmosis (RO) is a membrane-filtration technology that has been in use for decades. RO systems can waste up to 30% of the feed water through the production of brine, containing the rejected minerals. This rejection is high when dealing with saline waters. Treatment of this brine stream for reclamation of water would increase the overall efficiency of RO systems. In this research, the treatment of RO brine by a closed-circuit configuration of RO – closed-circuit desalination (CCD) - is investigated. In CCD, filtration is done in batches, with the recirculation of the concentrate stream back into the feed stream. The high cross-flow velocity and short filtration batches result in prevention of scaling in these systems.
This research focused on proving the resilience of the CCD system to silica scaling despite supersaturations of silica. Lab-scale experiments were performed on a single-element CCD system, specially built with relevant measurement instruments. Silica scaling was monitored by mass transfer coefficient (MTC) calculations, silica mass balance calculations, and membrane autopsies. Synthetic feed water was used, containing only NaCl (10 mg/L) and varying concentrations of silica (70 – 120 mg/L as SiO2). No anti-scalants were used. Longer experiments were performed, i.e. 20 consecutive cycles of 1 hour each with 120 mg/L of silica in the feed. The cycle duration was increased, 2 cycles of 3-hour each were performed, with 120 mg/L of silica in the feed.
The MTC curves in all the experiments had a gradual decline with the progression of each cycle, but always recovered at the beginning of the next cycle. This decline was probably due to the increasing osmotic pressure in the recirculation loop. There was no permanent decline in the MTC, leading to the observation that there was no scaling in the system. This was supported by the silica in the mass balance calculations, that showed there was no loss of silica from the brine. The reactive silica concentration in the brine was as high as 1800 mg/L in the 3-hour cycle, attaining a recovery of 93%, without signs of scaling. The membrane autopsy showed that the membranes used for the experiments with 120mg/l silica in the feed had higher silicon content compared to the blanks. However, there were too few samples to compare against and make strong conclusions whether there was scaling.
Thus, the designed CCD system was resistant to silica scaling in these conditions, of high pH and in the absence of other components such as iron, aluminium, calcium and magnesium. The results of this study proved that, despite high concentrations of silica in the feed, CCD can improve the total efficiency of RO systems (with regard to the water wastage) by recovering water from the brine produced by RO installations. The extremely high recoveries attained by the system would result in small volumes of very concentrated brine, making the extraction of minerals more cost-effective, because lesser volumes must be treated to obtain the same amount of minerals.