【正文】 re. Industrial discharges may consist of very strong organic wastewaters with a high oxygen demand or contain undesirable chemicals that can damage sewers and other structures. They may contain pounds, which resist biological degradation,or toxic ponents, which interfere with satisfactory operation of the wastewater treatment plant. A less obvious source, which must be considered an industrial waste, is thermal discharge since it lowers dissolved oxygen values. Many industries use large quantities of cooling water, with the electric power industry being the largest , the primary metal and chemical industries also use substantial quantities of cooling waters. 2. Conventional wastewater treatment processes Conventional wastewater treatment consists of a bination of physical, chemical, and biological processes and operations to remove solids,organic matter and, sometimes, nutrients from wastewater. General terms used to describe dif ferent degrees of treatment,in order of increasing treatment level,are preliminary, primary, secondary, and tertiary and/or advanced wastewatertreatment. 2. I. Preliminary treatment The objective of preliminary treatment is the removal of coarse solids and other large materials often found in raw wastewater. Preliminary treatment helps to remove or to reduce in size the large, entrained, suspended or floating solids. These solids consist of pieces of wood, cloth, paper, plastics, garbage, etc., together with some fecal matter. Removed are heavy inorganic solids such as sand and gravel as well as metal or objects are called grit and excessive amounts of oils or greases. . Primary treatment Primary treatment is designed to remove organic and inorganic solids by the physical processes of sedimentation and 2550% of the ining biochemical oxygen demand (BODs), 5070% of the total suspended solids (SS), and 65% of the oil and grease are removed during primary treatment. Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary sedimentation, but colloidal and dissolved constituents are not effluent from primary sedimentation units is referred to as primary effluent. Table 1 provides information on primary effluent from three sewage treatment plants in California along with data on the raw wastewaters. . Secondary treatment Table 1 Quality of raw wastewater and primary effluent at selected treatment plants in California Quality parameters, mg/l City of Davis San Diego Los Angeles County joint plant Raw wastewater Primary effluent raw wastewater Primary effluent raw wastewater Primary effluent BOD5 112 73 184 134 204 TOC SS 185 72 200 109 219 Total nitrogen NH3N NON 0 0 OrgN TDS 829 821 1404 1406 Alkalinity(CaCO3) 322 332 Hardness(CaCO3) 265 Source: Intemational Desalination Association [3]. The objective of secondary treatment is the further treatment of the effluent from primary treatment to remove the residual organics and suspended solids. In terms of the size of the solids, the distribution is approximately 30% suspended, 6% colloidal and about 65% dissolved solids. The function of primary treatment is to remove as much of the suspended solids as possible. Primary treatment utilizes clarifiers or settling tanks, which remove the settleable organics and settleable 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 wh