Onsite Treatment of Raw Produced Water through the Use of Forward and Reverse Osmosis

The U.S. Department of Energy (DOE) projects the production of oil and gas (O&G) to increase 48 and 56 percent by the year 2040, respectively, compared to 2012 production levels. Once a well has been developed, every barrel of oil produced will also generate roughly 9.2 barrels of produced water. Produced water is known to be saline, high in chemical oxygen demand (COD), and contain large concentrations of minerals, factors that pose a danger to human and environmental health if not disposed of properly. For these reasons the handling of produced water is a costly logistical challenge to transport, treat, or dispose of. The most common disposal practice is deep well injection of produced water; however, tighter regulations in the eastern U.S restricting deep well injection and unpredictable water resources create a need for a new onsite treatment process.

 Forward osmosis (FO) coupled with reverse osmosis (RO) could provide high quality permeate capable for industrial and agricultural reuse, alleviating water tensions in arid regions and disposal logistic of raw produced water. Compared to current onsite treatment trains, the FO/RO system has exceptional product water quality due to a dual semipermeable membrane system, which has a 97 percent removal of a broad range of contaminants. This study focused on the long term performance of a pilot scale FO/RO system treating raw produced water from the O&G industry. The system consists of a 200 gallon FO Feed tank, 4.93  of FO membrane and 8.4  of RO membrane. The FO/RO system was operated at water recoveries of 0, 25, and 50 percent. The system performance was evaluated through the following parameters: specific flux, specific reverse salt flux, trace organic removal, and metal removal. The FO/RO system was operated continuously for periods of two weeks, during which the FO water flux decreased from 3.1 LMH to 1.8 LMH and produced 4,200 L of high quality permeate.

There are three potential factors that drive this decrease in water flux over time 1) The reverse salt flux increases the salinity of the feed water, therefore decreasing the osmotic driving force across the membrane 2) over time a fouling layer is developed on the FO membrane by the raw produced water, reducing water flux across the membrane, and 3) a decreasing water permeability coefficient due to chemical changes of the polymeric FO membrane. This value corresponds to the membranes physical composition, which should remain constant after the system stabilizes. Because this value never remains constant, it could be hypothesized that the produced water is interacting with the membrane surface, possibly altering the surface chemistry that facilitates water diffusion through the membrane. From these findings a standard operating procedures was developed, which optimizes backwashing intervals, minimizes decrease in water flux, and maintains a constant water permeability coefficient. These improvements will increase the viability of the FO/RO technology, possibly resulting in a commercial implementation.