【正文】 oves anic carbon and hence（因此） decreases denitrification. Therefore, nonaerated outlet zones are shown in the preanoxic zone tanks. In the stepfeed process only the first and second aeration tanks may be equipped with a nonaerated zone. Simultaneous nitrification and denitrification The key to nitrogen removal by the simultaneous nitrification–denitrification process is to appropriately set the aerators so as to establish sufficiently large aerobic and anoxic zones simultaneously (Fig. ). Since the load of any wastewater treatment plant fluctuates diurnally, the concentrations of nitrate and ammonia vary inversely when the aerator setting is constant. To achieve the desired nitrogen removal, a process control is therefore required. Pasveer in 1964 [16] was the first to report on simultaneous denitrification in an oxidation ditch. He achieved this by setting the optimal immersion depth of the surface aerator so as to create a sufficiently large anoxic zone. At the Vienna Blumenta plant (Section ), the oxygen uptake rate was continuously measured with a special (homemade) respirometer. The respirometer output was used to switch the appropriate number of aerators to achieve the desired nitrogen removal [61]. The first real process control for simultaneous denitrification was developed by Ermel [62]. A continuous sample flow was separated from the mixed liquor by ultrafiltration and diverted to a nitrate monitor. At the Salzgitter Bad plant the two closedloop aeration tanks are operated in parallel. Each tank is equipped with three mammoth rotor surface aerators. One rotor in each tank is operated continuously to create sufficient circulating flow. The two other rotors of each tank are automatically switched on if the nitrate concentration in the sample flow drops to, ., SNO3 = 3 mg L–1. The aerators were stopped if, ., SNO3 reached 6 mg L–1 and were switched on again after the set point of SNO3 = 3 mg L–1 was reached. Figure shows a daily time course of ammonia and nitrate levels measured in one of the aeration tanks. Due to the stilllow denitrification rate during the period of high ammonia load (8 to 12 AM) the concentration of ammonia rises to about 4 mg L–1 N. As the denitrification rate increases after noon, the offperiods of the two aerators bee shorter and the ammonia concentration decreases. A more sophisticated control system based on monitoring both nitrate and ammonia was used in the wastewater treatment plant in Hildesheim [63]. In Salzgitter Bad an ORP controller was tested as a very simple, lowcost control method. Since the oxidation–reduction potential (ORP) drops sharply, if at zero DO nitrate reaches zero, the controller switches the additional aerators on at a certain slope of ORP. The additional aerators are then operated for a preset period of time. Figure shows the time course of ORP. The effluent during the time when the ORP controller was used was as good as when the nitrate controller was used [64]. At about the same time, similar experiments with ORP were conducted in Canada [65]. Some newer plants for simultaneous nitrification and denitrification are equipped with propellers (in addition to surface aerators) to maintain a sufficient flow velocity independent of the number of aerators in operation. Such plants can also be operated in intermittent nitrification–denitrification mode. In addition to the cities already mentioned, plants with simultaneous nitrification–denitrification are also operated in, ., Osnabr252。 SNH4,N = (Q + QRS +QIR,n) And second, under process conditions the aeration efficiency of fine bubble systems is lower than in clean water, which is not true of surface aeration systems. The weak points of surface aerators are the bearings（軸承） and the gears（齒輪） . If these are properly designed, maintenance is limited to lubrication（潤(rùn)滑） and changing gear oil. Nitrogen removal processes Introduction The singlestage activated sludge process for nitrogen removal, when the anic matter of the wastewater is used for denitrification, incorporates （合并，一體化，混合） the dilution of ammonia nitrogen to a concentration equivalent to the desired effluent concentration of nitrate nitrogen. Consequently, the anics are diluted by the same ratio. Whether conditions favorable(適宜的 ) to the development of filamentous（絲狀的，細(xì)絲質(zhì)的） bacterial growth are created depends on the wastewater characteristics, the dilution ratio, and the process configuration. Often anaerobic contact tanks （厭氧接觸池） for enhanced biological phosphate （磷酸鹽） removal are considered for the purpose of suppressing（抑制） filamentous growth. The processes for nitrogen removal can be divided into three groups: ? Subdivided（細(xì)分） tanks with distinct partments（隔室） for denitrification and nitrification, ., preanoxic zone denitrification, stepfeed process, or postdenitrification process. ? Completely mixed or closedloop tanks in which conditions for nitrification or denitrification are established periodically（周期性的） , ., intermittent（間歇的） nitrification–denitrification, alternating（交替的） nitrification–denitrification (BioDenitro process), or intermittent nitrification–denitrification with intermittent wastewater feeding (JARV process). ? Closedloop tanks in which anoxic zones for denitrification and aerobic zones for nitrification are established at the same time (simultaneous nitrification–denitrification). Activated sludge plants for nitrification only can suffer from a toolow residual alkalinity（堿度，堿性） . Especially if the aeration tank is pletely mixe