A new approach to membrane and thermal seawater desalination processes using nanofiltration membranes (Part 1)

Abstract

In this new approach to membrane and thermal seawater desalination processes developed by the Saline Water Conversion Corporation (SWCC), R&D Center, a nanofiltration (NF) membrane unit, which received non-coagulated filtered seawater feed, was placed ahead of the SWRO and the MSF pilot plant units to form, for the first time ever, fully integrated desalination systems of an NF-SWRO, NF-MSF, and NF-SWRO reject-MSF. Preliminary results were presented at the IDA World Congress, Madrid '97. Further results obtained in this investigation at a pressure of 22 bars showed that the NF unit reduced turbidity and microorganisms, removed hardness ions of Ca ++, Mg ++, SO 4 =, HCO 3 −, and total hardness by 89.6%, 94.0%, 97.8%, 76.6% and 93.3%, respectively. The system also resulted in the reduction of the monovalent ions of Cl −, Na +, K + each by 40.3% and the overall seawater TDS by 57.7%. The seawater (NF permeate water) produced by this process is considerably different in composition from (Gulf) seawater, and quality-wise is far superior to it as a feed to seawater desalination plants, and moreover without the problems normally associated with high concentration in seawater of scale forming ions, high TDS, high turbidity and microorganisms. This made it possible to operate both the SWRO and MSF pilot plants at high water recovery: 70% and 80%, respectively. It also allowed for the successful operation of the MSF unit at top brine temperature of 120°C without the addition to the make-up of antiscalant or acid or antifoam. The said desalination arrangements lead to significant improvement in the seawater desalination processes by lowering their energy consumption, by about 25–30%, and reducing chemical consumption thereby making the process more friendly to the marine environment. The observed increases in their product water output and recovery ratio by more than 70% resulted in the ultimate benefit of lowering the estimated cost of fresh water production by more than 27%. The SWRO permeate produced from the NF-SWRO arrangement has very low TDS, ≤200 ppm, making the requirement for a second-stage RO treatment of the SWRO permeate unnecessary. Moreover, the use of the desalination arrangement NF-SWRO reject-MSF should allow for the conversion of up to 90% of the NF product into fresh water, where about 65% of the NF product is converted to fresh water by the SWRO unit and 25% of SWRO reject is converted by the MSF unit. Definitely, the achievement of the above results represents a milestone in seawater desalination technology. The paper describes the results obtained, the experimental approach used in this investigation along with description of the NF-SWRO, NF-MSF and NF-SWRO reject-MSF pilot plants employed in this study. A simulated desalination model which compares the operation of existing SWRO plants with and without the NF modification is also described along with the technoeconomics of process analysis.