Sustainable performance of NF-SWRO pilot plant with low fouling NF membrane

Abstract

NF membrane fouling is a major obstacle in the smooth implementation of this technology. Hence, attempts were made to identify the best NF membranes which can handle fouling and also are capable of maintaining its specific ion rejection properties, which are usually affected by fouling, chemical cleaning, and long-term operation. In this study, low fouling NF membranes were used in a 2:1 configuration, the product of which was used as feed to a SWRO unit as well as make-up to MED unit at different combinations. Moreover, SWRO reject was also used as make-up to MED unit. NF and SWRO operations conditions were mainly directed by the requirement of MED unit. NF unit was most of the time operated at a recovery of 70% and SWRO unit which was fed by the NF product was operated mostly at a recovery of about 48%. NF unit received pretreated seawater which was taken from an open intake located at Gulf Seawater. The pretreatment includes disinfection by chlorine, coagulation using FeCl₃, and antiscalant dosing, where feed water passes through two dual media filters arranged in series. NF membranes were operated at an average flux of 21 LMH at the normal recovery of 70%. A total of four chemical cleanings were only performed during the period of 3 years in operation. A total of only seven NF membranes were only replaced during the test period which amounts to an average of 13% annual membrane replacement rate which is quite acceptable in membrane industries. Some of the chemical cleaning and membrane replacement could have been avoided if problem with isobaric ERD would not have happened. End membrane elements were replaced on two occasions due to heavy CaSO₄ scale, which occurred due to failure of isobaric ERD system. NF unit was most of the time operated at feed pressure of <15 bar and it increased up to 25 bar only during the occurrence of scaling. NF product quality was maintained constant in the narrow range of TDS between 36,600 and 37,600 ppm from a feed TDS of about 45,000 ppm despite large variation in feed temperature. Even the concentrations of scale-forming divalent ions were in the narrow range. These properties of NF membrane were better and much different from that was used at Ummlujj and resulted in steady performance of SWRO unit which received NF product as feed. SWRO membranes which received NF product as feed was mostly operated at recovery of 48% with an average flux of 13.5 LMH. The performance of SWRO membranes was excellent as expected due to very clean feed water as well as moderate flux operation. During the three years period no chemical cleaning and membrane replacement were carried out due to its excellent performance. The product conductivity was mostly below 400 μS/cm despite large seasonal variation in feed water temperature, which ranged between 17 and 36°C. The present study revealed excellent performance of NF membrane, especially its ability to maintain its performance regardless of fouling, cleaning, and long-term operation, which reflected in the outstanding performance of SWRO unit which received feed from NF unit.

Keywords:

• Nanofiltration
• Seawater desalination
• Pretreatment
• Fouling

Notes

Presented at the Conference on Desalination for the Environment: Clean Water and Energy 11–15 May 2014, Limassol, Cyprus