Membrane processes used for potable water quality improvement

The tap water is often used as water source for different industries, however, the water repurification is required to meet the specification of technology. The effect of application of ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO) and membrane distillation (MD) processes on the quality of treated water was investigated. The removal of suspended solids and colloids obtained by UF process allowed the reduction of SDI15 from 8 to 2. The NF process permitted to achieve the complete removal of dissolved organic carbon and the rejection of hardness between 60% and 87%. The rejection of TDS obtained in the RO system was at a level of 99.7%. The highest quality water (0.8 µS/cm, 0.6 ppm TDS) was produced in the MD installation.

Potable water used in the study contained significant amounts of solutes and suspended solids, which prevent it from a direct use in the technological processes. The UF process was efficient only in the removal of suspended solid and colloids, which allow achieving an appropriate SDI15 index at a level of 2 from potable water (SDI15 index 7–10); thereby this water can be directly used in RO process. The quality of produced water from UF–RO integrated membrane system corresponds to quality of distilled water. The increase of conversion does not result in the decrease of ion rejections. NF process used for polishing of potable water allowed the rejection of divalent ions exceeding 90%, whereas, the rejection of monovalent ions was below 60%. The NF process was found to be very efficient in the rejection of organic matter from water (0.8–2.1 TOC/dm3). The integration of UF with NF process increased the effectiveness of removal of organic compounds. High purity water was produced in the MD process. However, a direct purification of potable water by MD results in the deposition of significant amounts of CaCO3 on the membrane surface. Preliminary softening of water in NF process results in the decrease of fouling. Addition of small amounts of HCl to the feed (pH = 5) permitted to carry out the MD process without a decline of the efficiency. The best results were obtained in the RO–MD integrated membrane processes.