【文章內(nèi)容簡介】 selectively separate a fluid from its ponents. There are four established membrane separation processes, including microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO) and nanofiltration (NF). RO separates dissolved and ionic ponents, MF separates suspended particles, UF separates macromolecules and NF is selective for multivalent ions. MF and UF can be used as a standalone technology for treating industrial wastewater, but RO and NF are usually employed in water desalination. Membrane technology operates two types of filtration processes, crossflow filtration or deadend filtration, that can be a pressure (or vacuum)driven system. Microfiltration/ultrafiltration MF has the largest pore size (–3 181。m) and is typically used for the removal of suspended solids and turbidity reduction. It can operate in either crossflow or deadend filtration. UF pore sizes are between and 181。m. They are employed in the removal of colour, odour, viruses and colloidal anic matter . UF is the most effective method for oil removal from produced water in parison with traditional separation methods , and it is more efficient than MF for the removal of hydrocarbons, suspended solids and dissolved constituents from oilfield produced water . Both MF and UF operate at low transmembrane pressure (1–30 psi) and can serve as a pretreatment to desalination but cannot remove salt from water . Polymeric/ceramic membranesPolymeric and ceramic membranes are used for UF/MF treatment of water. Polymeric MF/UF membranes are made from polyacrylonitrile and polyvinylidene and ceramic membranes from clays of nitrides, carbides and oxides of metals . Ceramic UF/MF membranes have been used in a fullscale facility for the treatment of produced water . Product water from this treatment was reported to be free of suspended solids and nearly all nondissolved anic carbon . Ceramic UF/MF membranes can operate in both crossflow filtration and deadend filtration modes and have a lifespan of 10 years. Chemicals are not required for this process except during periodic cleaning of membranes and precoagulation (used to enhance contaminants removal). Reverse osmosis and nanofiltration RO and NF are pressuredriven membrane processes. Osmotic pressure of the feed solution is suppressed by applying hydraulic pressure which forces permeate (clean water) to diffuse through a dense, nonporous membrane . Seawater RO can remove contaminants as small as 181。m, but its major disadvantage is membrane fouling and scaling . Early studies on using RO to treat produced water failed due to insufficient process integration and poor treatment . Nicolaisen and Lien however reported a successful RO treatment of oilfield produced water in Bakersfield, California. The pilot system which was operated for over 1700 h in 6 months produced 20 gpm of clean water. Benchscale studies have shown the potential of brackish water RO membranes to successfully treat oil and gas produced water. Experiments indicated that RO membrane technology would be excellent for oilfield produced water treatment with appropriate pretreatment technology . Capital costs of RO membrane systems vary depending on the size of rejection required, materials of construction and site location. Operating costs depend on energy price and total dissolved solid (TDS) level in the feed water. RO membrane systems generally have a life expectancy of 3–7 years. NF is a robust technology for water softening and metals removal and is designed to remove contaminants as small as 181。m. It is applicable for treating water containing TDS in the range of 500–25 000 ppm. This technology is similar to RO. NF membranes were employed for produced water treatment on both bench and pilot scales. Mondal and Wickramasinghe studied the effectiveness of NF membranes for the treatment of oilfield produced water. Results showed a minimal improvement when pared with the effectiveness of brackish water RO treatment of the same feed water. Thermal technologies Thermal treatment technologies of water are employed in regions where the cost of energy is relatively cheap. Thermal separation process was the technology of choice for water desalination before the development of membrane technology. Multistage flash (MSF) distillation, vapour pression distillation (VCD) and multieffect distillation (MED) are the major thermal desalination technologies . Hybrid thermal desalination plants, such as MED–VCD, have been used to achieve higher efficiency . Although membrane technologies are typically preferred to thermal technologies, recent innovations in thermal process engineering make thermal process more attractive and petitive in treating highly contaminated water. Multistage flash MSF distillation process is a mature and robust technology for brackish and sea water desalination. Its operation is based on evaporation of water by reducing the pressure instead of raising the temperature. Feed water is preheated and flows into a chamber with lower pressure where it immediately flashes into steam. Water recovery from MSF treatment is ～20% and often requires posttreatment because it typically contains 2–10 mg/l of TDS. A major setback in operating MSF is scale formation on heat transfer surfaces which often makes this process require the use of scale inhibitors and acids. Overall costs vary depending on the size, site location and materials of construction . Its energy requirement is between and kWh. Globally, MSF market share has significantly decreased due to petition of membrane technologies, but it is a relatively costeffective treatment method with plant life expectancy of more than 20 years, and can be employed for produced water treatment . Multieffect distillation MED process involves application of sufficient energy that converts saline