【正文】 the ions through the membranes and concentrates them between each alternate pair of membranes. Partially desalted water is left between each adjacent set of membrane pairs. Scaling or fouling of the membranes is prevented in most ED units by operationally reversing the direction of the electrical current around the stacks at 1530min intervals. This reverses the flow of ions through the membranes so that the spaces collecting salty concentrate begin collecting less salty product water. Alternating valves in the water collection system automatically direct the flow in the appropriate direction. Typical freshwater recovery rates for ED (reversal) range from 8090% of the feed water volume. . Ion exchange (IX) In this process undesirable ions in the feed water are exchanged for desirable ions as the water passes through granular chemicals, called ionexchange resins. For example, cationexchange resins are typically used in homes and municipal water 。s largest plant in Saudi Arabia produces 128 mgd of desalted water. In contrast, 12% of the world39。 or wastewater can be treated pletely at the plant and either reused or discharged directly into receiving waters. Industrial wastewaters are the discharge of industrial plants and manufacturing wastewaters can represent, collectively, an important part of munity wastewaters and must be considered for successful wastewater treatment plant operation. In some locations industrial wastewater discharge is collected together with other munity wastewaters and the mixed wastes are treated together. In other instances, industries may provide some pre treatment or partial treatment of their wastewaters prior to discharge to the municipal sewers. In still other situations, the volume and character of the industrial waste are such that separate collection and disposal are necessary. Industrial wastewaters vary widely in position, strength, flow and volume, depending on the specific industry or manufacturing establishment in the munity. The specific position and volume of the industrial waste will, of course,depend on the use to which the water has been put. Typical industries which produce significant volumes of wastewaters include paper and fiber plants, steel mills, refining and petrochemical operations, chemical and fertilizer plants, meat packers and poultry processors, vegetable and fruit packing operations and many more. 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