【正文】 inorganic solids from the wastewater. The effluent from primary treatment,therefore, contains mainly colloidal and dissolved organic and inorganic solids. Recent effluent standards and water quality standards require a greater degree of removal of organics from wastewater than can be acplished by primary treatment alone. Additional removal of organics can be acplished by secondary treatment. The secondary treatment process consists of the biological treatment of wastewater by utilizing many different types of microorganisms in a controlled environment. Several aerobic biological processes are used for secondary treatment differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter. 3. Developments in wastewater treatment methods Primary and secondary treatment removes the majority of BOD and suspended solids found in wastewaters. However, in an increasing number of cases this level of treatment has proved to be insufficient to protect the receiving waters or to provide reusable water for industrial and/or domestic , additional treatment steps have been added to wastewater treatment plants to provide for further organic and solids removals or to provide for removal of nutrients and/or toxic materials. Therefore, advanced wastewater treatment is defined as: any process designed to produce an effluent of higher quality than normally achieved by secondary treatment processes or containing unit operations not normally found in secondary treatment. The above definition is intentionally very broad and enpasses almost all unit operations not monly found in wastewater treatment today. . Types of advanced wastewater treatment Advanced wastewater treatment may be divided into three major categories by the type of process flow scheme utilized: ? tertiary treatment ? physicochemical treatment ? bined biologicalphysical treatment Tertiary treatment may be defined as any treatment process in which unit operations are added to the flow scheme following conventional secondary treatment. Additions to conventional secondary treatment could be as simple as the addition of a filter for suspended solids removal or as plex as the addition of many unit processes for organic, suspended solids, nitrogen and phosphorous removal. Physicochemical treatment is defined as a treatment process in which biological and physicalchemical processes are intermixed to achieve the desired biologicalphysicalchemical treatment is differentiated from tertiary treatment in that in tertiary treatment any unit processes are added after conventional biological treatment,while in bined treatment, biological and physicochemical treatments are mixed. Another way to classify advanced wastewater treatment is to differentiate on the basis of desired treatment goals. Advanced wastewater treatment is used for: ? additional organic and suspended solids removal ? removal of nitrogenous oxygen demand (NOD) ? nutrient removal ? removal of toxic materials In many, if not most instances today, conventional secondary treatment gives adequate BOD and suspended solids removals. But advance wastewater treatment is necessary because advanced wastewater treatment plant effluents may be recycled directly or indirectly to increase the available domestic water supply. Advanced wastewater treatment effluents may be used for industrial process or cooling water supplies. Some receiving waters are not capable of withstanding the pollutional loads from the discharge of secondary effluents. Secondary treatment does not remove as much of the organic pollution in wastewater as may be assumed. The performance of secondary treatment plants is almost always measured in terms of BOD and SS removal. A welldesigned and operated secondary plant will remove from 85% to 95% of the influent BOD and SS. However,the BOD test does not measure all of the organic material present in the wastewater. An average secondary effluent may have a BOD of 20 mg/L and a COD of 60 to 100 mg/L. The average secondary plant removes approximately 65% of the influent COD. Thus, when highquality effluents are required, additional organic removal must be acplished. In addition to the organic materials remaining in most secondary effluents, there is an additional oxygen demandresulting from the nitrogen present in the wastewater. . Emergence of membrane treatmenttechnology Biological and chemical treatment methodologies have been developed to handle different treatment scenarios. Yet these applications are often limited by the expensive cost of treatment, continuous additions of toxic chemicals, extensive space required for installation, side effects of secondary pollution, etc. As a result, physical, membranebased separations of liquids from solids have enjoyed increasing popularity over the last 20 years and are being the promising technology for the 21 st century. It is a means of purifying and/or concentrating a wide variety of fluids from water and wastewater to pharma ceutical and chemical products. It is also a pressuredriven process that relies on the pore size of the membrane (typically thin films or sheets of plastic with an accurately sized micropore structure resembling that of a sponge) to separate feed stream ponents according to their pore sizes. The use of membranes is not unmon to human beings。 and tertiary treatment, in which advanced biological methods of nitrogen removal and chemical and physical methods such as granular filtration and activated carbon absorption are employed [2]. The characteristics of industrial wastewaters can differ considerably both within and among industries. The impact of industrial discharges depends not only on their collective characteristics such as biochemical oxygen demand and the a