Application of Nanofiltration and Reverse Osmosis Membranes for Produced Water Treatment
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Membrane Applications 2010
Application of Nanofiltration and Reverse Osmosis Membranes for Produced Water Treatment Arun Subramani, Ron Schlicher, Jim Long, Jack Yu, Geno Lehman, Joseph Jacangelo MWH Americas Inc. 618 Michillinda Avenue, Suite 200 Arcadia, California 91007 ABSTRACT 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.
KEYWORDS Oily wastewater, silica polymerization, organic fouling, recovery optimization.
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Membrane Applications 2010
INTRODUCTION Produced water, water that is co-produced during oil and gas extraction, represents the largest source of oily wastewaters (Mueller et al., 1997). The volume of produced water can be as much as ten times the volume of oil extracted (Mondal and Wickramasinghe, 2008). Produced water consists of a combination of organic and inorganic compounds and production chemicals. Typical organic compounds present are aliphatic, aromatic, and polar compounds. Inorganic components include sodium, potassium, calcium, magnesium, chloride, sulfate, carbonate, silicates, and borates. Production chemicals can include emulsion breakers to improve separation of oil and water and corrosion inhibitors (Mondal and Wickramasinghe, 2008). The concentration of these contaminants can vary significantly due to natural variation in the geological formation and the type of oil-based product being produced (Franks et al., 2009). The typical method of dealing with produced water is deep well injection. A portion of the produced water is reinjected into oil producing zones to improve oil recovery through water or steam flooding (Visvanathan et al., 2000). The other portion of produced water is disposed off through deep well injection. Deep well injection is limited by the capacity of the injection wells and discharge limits set by local governing agencies. Due to this limitation in discharging the produced water, oil production companies are looking into options for treating the produced water for surface discharge. Example of discharge limits based on the class and purpose is listed in Table 1. Certain discharge limits (Class 2AB) require a chloride concentration of less than 230 mg/L in the treated water. Treatment of such wastewater streams to meet low chloride and boron discharge limits requires a technology, such as NF and RO, which can serve as an absolute barrier for various contaminants. The use of membrane technology also offers other advantages such as a smaller footprint, high level of automation, and applicable for both onshore and offshore oil exploration (Mondal and Wickramasinghe, 2008). Table 1. Example of surface water discharge limits for produced water. Class 3D 3B
Purpose Beneficial use for livestock watering Aquatic life, ephemeral streams
Potable water, preserving fish life
Discharge limit TDS < 5000 mg/L, Oil and grease < 10 mg/L, 6.5 < pH < 9, Turbidity < 10 NTU, Chloride < 2000 mg/L, Boron < 5 mg/L TDS < 5000 mg/L, Oil and Grease < 10 mg/L, 6.5 < pH < 9, Turbidity < 10 NTU, BOD5 < 30 mg/L, TSS < 30 mg/L, Cadmium < 0.54 ug/L, Selenium < 5 ug/L, Sulfur < 2 ug/L, Chloride < 2000 mg/L, Boron < 5 mg/L TDS < 5000 mg/L, Oil and Grease < 10 mg/L, 6.5 < pH < 9, Turbidity < 10 NTU, BOD5 < 30 mg/L, TSS < 30 mg/L, Cadmium < 0.54 ug/L, Selenium < 5 ug/L, Sulfur < 2 ug/L, Chloride < 230 mg/L, Boron < 5 mg/L, Benzene < 2.2 ug/L, Toluene < 1 mg/L, Ethylbenzene