Is Reverse Osmosis Effective for Produced Water Purification: Viability and Economic Analysis

Abstract

Abstract The ultimate goal of produced water management is to remove dissolved components and use the desalinated water for beneficial uses that can effectively alleviate environmental impact and water shortage. Presently, many of the efforts have been focused on membrane technologies including reverse osmosis and electrodialysis. Unfortunately, no large scale implication of produced water desalination by membranes has been reported. The main obstacle against the deployment of desalination technologies for produced water purification has always been the complicated chemical composition and associated high operating cost. Membrane technologies are generally believed to be energy efficient due to single-phase operation comparing to thermal-based desalinations. However, the presence of dissolved organics and scale deposition on membrane surfaces require sophisticated pretreatment and frequent membrane replacement, adding to the water treatment costs. Reverse osmosis membranes including polymeric membranes and molecular sieve zeolite membranes were investigated for ion removal from produced water by a cross-flow RO process. Considerable flux decline with elapsed operation time was observed from 11.5 to 6.8 L/m2.h at a transmembrane pressure of 3.5 Mpa. Pretreatments including nanofiltration and adsorption by active carbon were studied for their influence on the RO performance and impact on the overall desalination cost. Both polymeric membranes and molecular sieve zeolite membranes have been tested for actual produced water from oilfield and coalbed methane site. The study has revealed that (1) most of permeation tests lasted less than 3 months due to serious fouling and drastic flux decline (>30%), (2) scale precipitation and organic sorption are the major fouling mechanisms of membranes, (2) multistage pretreatment is crucial to extend membrane lifetime, and (3) nanofiltration is the only effective process tested that can extend the life of a RO membrane to over 6 months. But periodic chemical cleaning, typically twice a week, is necessary to maintain the desired water flux. The economical efficiency of these processes was discussed from the aspects of produced water chemistry, energy consumption, and water treatment capacity. Considering small to mid-sized water treatment capacity (50 m3/day), the cost of produced water desalination by RO membranes is around $3.7/m3 including nanofiltration pretreatment. Pretreatment and membrane replacement are the major factors that increases the operation cost and limits the economic efficiency of membrane technology for produced water desalination.