Application of Nanofiltration and Reverse Osmosis Membranes for Produced Water Treatment

Produced water is a term used to describe water that is obtained along with oil and gas production. Produced water constitutes the single largest waste stream from oil and gas exploration and production activities and contains high levels of oil and grease, total dissolved solids (usually sodium chloride), hydrocarbons, and refractory organics. If treated appropriately, produced water can be employed as a true water resource to augment existing surface water streams and creeks. Due to stringent surface discharge limits being imposed in the United States, produced water needs to be managed and treated before being discharged to surface water streams and creeks. Certain discharge limits require a chloride concentration of less than 230 mg/L in the treated water. Treatment of such wastewater streams to meet low chloride, selenium, and boron discharge limits requires a technology, such as nanofiltration (NF) and reverse osmosis (RO), which can serve as an absolute barrier for various contaminants. In this study, different types and configurations of NF and RO processes were pilot tested to determine their applicability in treating produced water obtained from natural gas wells at a location in the western United States. In order to reduce the fouling potential on NF and RO membranes, dissolved air floatation (DAF), ceramic ultrafiltration (UF), MYCELX cartridges, and organoclay filters were tested as pretreatment alternatives. It was determined that the fouling potential of NF and RO membrane was not substantially different for the various pretreatment processes utilized. In order to handle high silica concentrations in the feed water and increase the overall feed water recovery, a two pass NF-RO system was tested. The first pass NF system was used to remove hardness and alkalinity from the feed water. The pH of permeate from the first pass NF system was increased to 10.0 to increase silica solubility and used as feed to a second pass seawater RO system. A combination of spiral wound and disc tube configuration was effective in achieving more than 90 percent recovery for the first pass NF membranes while an overall feed water recovery of more than 70 percent was achieved for the entire NF-RO membrane system and also resulted in meeting the discharge limits.