Selective nitrate removal from groundwater using a hybrid nanofiltration–reverse osmosis filtration scheme

• A hybrid NF/RO filtration scheme for nitrate removal is proposed and tested.
• Production of low salinity brines allowed to be discharged to sewerage systems.
• The new scheme can be applied in single or double NF stage modes prior to RO step.
• Appropriate NF membranes for the process should reject Cl− better than NO3−.
• The results show the process to be both technically feasible and energy efficient.

A novel and potentially cost effective filtration scheme for removal of nitrate from groundwater, characterized by production of low salinity waste brine that can be easily discharged to sewerage systems and high product-water recovery, is proposed. The inherent preference of particular NF membranes for rejecting chloride and sodium over nitrate ions is utilized in a preliminary NF stage to remove Na+, Cl+, Ca2+ and Mg2+ to a side stream. In a second stage, RO is applied to remove NO3− and the RO permeate is mixed with the side stream of the NF stage to create product water low in nitrate, yet with a balanced composition consisting all the required species and minerals. The number of NF stages depends mainly on the rejection efficiency of the NF membrane. Based on Israeli regulations for both drinking water and required composition of brines discharged to the sewage, a treatment scheme composed of a single and double NF stages followed by RO is shown to reach water recoveries of 91.6% and 94.3%, respectively. Each NF stage raises the energy cost by approximately 0.5 cent/m3 product water. However, this cost is easily paid back by the inherent additional advantages of the combined scheme, i.e., less water treated by the RO, significant increase in total recovery ratio, no need in re-mineralization of the product water and minimization of calcium carbonate precipitation potential on the RO membrane. The principles for process design are described, making the specific treatment scheme proposed here easily adjustable to other regulatory requirements and other water characteristics. A provisional patent has been filed.