【正文】 r circular and have mixers in the center or propellers （螺旋槳） (Fig. ). Rectangular tanks can be visualized as a series of square tanks. In closedloop, tanks either propellers or vertical shaft（軸） impellers（推動(dòng)器） maintain the circulating flow. Finebubble diffusedair aeration(曝氣 ) systems can be installed in almost any type of tank. Tanks for vertical shaft surface aerators are either square or rectangular in which the length is a multiple of the width (Fig. ). Tanks for cyclic（循環(huán)） aeration are preferably（更好的，更可取的） circular or of closed loop type. Aeration equipment as well as appropriate mixers must be installed. Mixing can also be performed by rotating bridges on which diffusers are mounted. (Fig. ). Since the aeration is switched off cyclically（循環(huán)的） , only nonclogging aeration systems, ., membrane diffusers for fine bubble aeration are appropriate. Simultaneous nitrification and denitrification in practice is performed mainly in closedloop tanks equipped with horizontalaxis（水平的，軸） surface aerators, ., mammoth（巨大的） rotors（旋翼） or vertical shaft surface aerators, as in the carousel（轉(zhuǎn)盤） process. Simultaneous denitrification can also be performed with air diffusers arranged in ‘ fields’ in closedloop tanks . Carbon removal processes In industrialized countries today, removal of both carbon and nitrogen is increasingly required。 ? preanoxic zone denitrification 。環(huán)境生物技術(shù) 沈陽(yáng)大學(xué) 07環(huán)境工程 20239gying 3 Activated Sludge Process ? Singlestage process ? Fig. Flow diagram of anactivated sludge plant. 3 Activated Sludge Process ? Singlestage process ? An activated sludge plant is characterized by four elements: ? An aeration tank equipped with appropriate aeration equipment, in which the biomass is mixed with wastewater and supplied with oxygen. ? A final clarifier, in which the biomass is removed from the treated wastewater by settling or other means. ? Continuous collection of return sludge and pumping it back into the aeration tank. ? Withdrawal of excess sludge to maintain the appropriate concentration of mixed liquor. Twostage process ? The twostage process has several advantages. Harmful substances can be removed in the first stage, which is important for the treatment of industrial wastewater。 and in the lowload second stage, due to the high sludge age microanisms can be maintained that are able to remove slowly biodegradable anics or to oxidize ammonia. Furthermore, bulking sludge is only rarely observed in either stage. ? The disadvantages are that about twice as many clarifiers are needed as in the onestage process and that nitrogen removal, as well as enhanced biological phosphate removal, may be inhibited owing to missing anics, which are removed in the first stage. ? Single sludge carbon, nitrogen, and phosphorous removal ? In the early 1960s three different methods for nitrogen removal were demonstrated: ? postdenitrification 。 ? simultaneous denitrification 。 however, in developing countries and for pretreatment of industrial and trade wastewater, removal of anic carbon only is important. In designing, provisions should be made for ? a sludge that settles well ? a sufficiently high volumetric（體積的，容積的） oxygen transfer rate ? a robust（強(qiáng)壯的） aeration system, especially in developing countries and in industrial plants Sludge settling（沉淀） is improved by plugflow（阻塞流） type aeration tanks, aeration tanks constructed as a cascade（串聯(lián)） , and/or use of a selector（選擇器） . Twostage plants may be a choice if the wastewater is highly concentrated. In Germany, numerous plants that receive wastewater with a high fraction of readily（容易的） biodegradable anics have been operated with a plastic media trickling filter（滴濾池） as the first stage followed by an activated sludge plant. Often, intermediate（中間的） settling to remove trickling filter sludge was not implemented. Although the aeration efficiency (expressed as kg oxygen transferred per kWh at zero DO) of surface aeration systems measured in clean water may be lower than of fine bubble（微泡） diffused air systems, surface aeration is in some respects preferable. First, no problems with diffuser clogging or destruction have to be considered。 Q) SNO3,e (39) The flow (x ? Q) is the fraction of the total wastewater flow entering the last denitrification zone, and x must not be equal to the inverse number（倒數(shù)） of units. To increase nitrate removal in the stepfeed process, it is necessary to decrease the fraction of wastewater diverted to the last denitrification zone (decrease x) or to increase the return sludge flow and/or the internal recirculation of the last unit (QIR,n) (Eqs. 40 and 41). In existing plants, however, internal recirculation is generally not used with the stepfeed process. Since increasing the return sludge flow (QRS) can hinder（阻礙） the final clarification（凈化，清潔） , it seems more appropriate to decrease x. This is possible only if the anic carbon contained in the wastewater flow (x ? Q) is sufficient to denitrify the ining nitrate load. Due to the stepwise（逐漸的） ‘ dilution’ of the return sludge with wastewater, the MLSS drops from unit to unit. The result is a higher average concentration of MLSS than in the effluent of the last aeration tank. This is considered an advantage, since the MLSS of the effluent determines the size of the final clarifiers. To maintain the same loading rates in the three partments（隔室） it is possible to chose appropriate tank volumes or, which is easier, to distribute the wastewater flow appropriately. This would occur when Q