Design and Development of Hybrid Nanofiltration-Reverse Osmosis (NF/RO) System for Improved Water Recovery of Brackish Water RO Desalination Plant

Due to certain global mega trends (increasing population, urbanization, economic growth, global warming and rapid industrialization) there is severe imbalance between drinking water supply and demand worldwide. In the past few decades, clean water supply has become very critical due to excessive water demand, stringent regulations on drinking water and contamination of the existing clean water sources. The situation is even worst in water deficient and arid areas such as Africa, Middle East, GCC countries including Kingdom of Saudi Arabia and Mediterranean region. However, desalination of seawater or brackish water has made it possible to cope up with global stress between water supply and demand. There are two widely employed methods of desalination, namely thermal based and membranes based. Nevertheless, membranes based desalination processes are gaining popularity owing to their simplicity, strong separation capabilities, cost-effectiveness and ease of integration with other processes. Reverse osmosis (RO) has developed and matured industrially as the major desalination process both for sea and brackish feed water. However, fouling is the major bottle neck for the utilization of ROs to their full potential. In addition, another serious problem faced by inland desalination (desalination of ground or surface water) facilities is concentrate management of enormous volumes of concentrate and its disposal cost. Concentrate or brine is the rejected part of the feed water and needs to be disposed of into a suitable environment. One of the simple and environment-friendly ways of minimizing the volume of concentrate is to increase the product water recovery of an RO plant. However, the product water recovery is limited by several factors, of which, presence of scale forming ions (which are mainly bivalent cations and anions such as carbonates, bicarbonates and sulphates) in feed water is the most severe and critical. Scale forming ions therefore need to be removed/minimized from the feed water before they enter into RO section. Pre-treatment of feed water is an important and integral step in RO desalination process. Pre-treatment precedes the RO step and works in perfect synergy with the final RO step, and thus determines the success and efficiency of an RO process. More recent trend is membranes-based pre-treatment of seawater that utilizes nanofiltration (NF) membranes. NF membranes have pore size ~1 nm and have been reported to reject multivalent ions. These membranes offer key distinguished properties such as low rejection of monovalent ions, higher rejection of divalent ions, higher flux compared to RO membranes, and capability of removing hardness, turbidity, pesticides and some micropollutants from the feed water. This combination of properties have allowed NF to be used in widespread applications especially for seawater desalination, water and wastewater treatment, biotechnology, pharmaceutical and food engineering. Although, few studies have been performed to remove the scalant from brackish feed water employing NF prior to RO step. However, these studies are limited to the utilization of NF for lower salinity and lower water recoveries. Furthermore, through investigation of the performance of NF membranes at higher recovery (>85%) and associated fouling (scaling) propensity caused by decreased solubility limit of mineral scalant (due to increased salinity) at such a higher recovery is missing. Therefore, there is an urgent need for the design and development of NF/RO hybrid system for the BWRO plants in view of enhancing product water recovery and minimizing the scaling of feedwaters with higher salinity. Such a design will be paving the path for an efficient and environment-friendly way of scalant removal from the brackish feed water. Thus the proposed project will mainly undertake experimental approach for the generic design and development of an indigenous hybrid NF/RO desalination system with the objective of enhancing product water recovery along with efficient scalant removal from the brackish feed water. In a follow up project, the optimized hybrid NF/RO system can then be adapted to a pilot-scale to develop an efficient and eco-friendly NF-BWRO plant suitable to Saudi Arabian conditions.