Performance, restoration and autopsy of NF membranes used in seawater pretreatment, Desalination 178

To evaluate the newly developed configuration of NF-SWRO system on a commercial scale, the single Umm Lujj SWRO desalination plant was converted to a dual NF-SWRO desalination process by introducing NF membrane pretreatment ahead of SWRO desalination train. The permeate flow from the dual NF-SWRO system was increased to 130 m3/h compared to 91.8 m3/h permeate flow from single SWRO desalination process. After about 6000 hours of continuous operation of NF unit, one pressure vessel was isolated from NF unit for chemical cleaning at low pH followed by high pH as the plant permeate flow was reduced from 234 m3/h to 200 m3/h and the feed pressure increased from 30 bar to 38 bar as a result of decrease in feed temperature from 32 to 23oC as well as suspected fouling. The chemical cleaning test resulted in improvement of the NF permeate flow, i.e., 100 % recovery of the permeate flow, but with slight reduction in hardness ions rejection. Thereafter, low and high pH cleaning was successfully carried out on entire NF unit to restore its performance. After about 2000 hours from the first cleaning of NF unit, the permeate flow was again gradually reduced to 190 m3/h from 234 m3/h. Only a high pH cleaning in presence of Sodium Dodecyl Sulfate (SDS) surfactant, in addition to other components was carried out to the NF unit to restore its performance in the second occasion. The permeate flow of NF unit was recovered back to 234 m3/h whereas the rejection of hardness ions of NF unit was reduced as in the first cleaning. Hence, autopsy of two NF membranes was carried out to find out the reason for decline in permeate flow. Before doing autopsy, both lead element and end element membranes were subjected to performance evaluation and it was found that the performance of the end element was much better than the performance of the lead element. Autopsy and analyses results showed that th