【正文】 and in a second stage a fixedfilm reactor converted nitrite to nitrogen gas (N2) by autotrophic bacteria using ammonia as electron donor. However, this is not regarded as an activated sludge process. Postdenitrification with external anic carbon Few plants around the world use external carbon as the sole carbon source for denitrification in singlestage activated sludge plants. One such plant was constructed to treat the whole wastewater of the Salzgitter Steel Works in Germany. The wastewater originates from treated blast furnace gas (high ammonia loads), treated coke oven gas (phenols, cyanides, ammonia, etc.), and several other discharges. The total flow of about 50 000 m3 d–1 contains about 35 mg L–1 ammonia nitrogen. Because soft water is used for cooling, the alkalinity is low. Therefore, experiments were first carried out using the preanoxic zone process to gain as much alkalinity as possible. Since the anic carbon content of the wastewater was too low, methanol had to be added to ensure sufficient nitrate removal, but the process was very unstable due to the toxic ponents of the wastewater. The process was therefore changed to postdenitrification, with satisfying experimental results [72]. The fullscale plant is shown in Figure . It is possible to operate the first two aeration tanks (1 and 2) in parallel, to prevent a strong concentration gradient of the toxic substances, but this has not been necessary. The center zone of tank 4 (1500 m3), to which methanol is added in response to the measured nitrate concentration in the tank, serves as a denitrification zone. In the final aerated zone (1500 m3) accidentally overdosed methanol is oxidized. Tank 3, for maintenance purposes, is constructed like tank 4. During normal operation both parts of this tank are aerated. To remove 1250 kg of nitrate nitrogen per day, 3500 kg of methanol per day are consumed, which is in the practical range of to kg methanol per kg nitrate nitrogen to be denitrified [72]. Design procedure Detailed design information may be taken from handbooks [., 75, 76]. In addition, for final clarifier design, Ref. 73 is remended. The main design steps for singlestage activated sludge plants prise: 1. Determination of the design loads (., BOD5, COD, suspended solids, nitrogen, phosphorous), the average alkalinity, the wastewater flow (daily and peak), and the peak storm water flow (m3 h–1). Existing data should be checked for annual fluctuations in the wastewater temperature, the loads, and the flows. If nitrification is required, the design load should be selected in bination with the wastewater temperat。lfrath, Germany) that developed this process are available. The J252。nster, Gera, Paderborn, TelAviv, and in numerous smaller munities in which oxidation ditches were converted to the simultaneous mode by means of automatic control. Whether and how much of the readily biodegradable anics are oxidized by dissolved oxygen (and therefore lost for denitrification) depend on the placement of the wastewater inlet and its distance to the next aerator. It is advisable to introduce the flow near the tank bottom, since anoxic conditions are predominant there. The closedloop circulating flow tanks for simultaneous nitrification–denitrificationcan be regarded as pletely mixed. In addition, due to the high dilution of the ining wastewater by the circulating flow, conditions favorable for filamentous bacterial growth are established. If the wastewater contains a higher fraction of readily biodegradable anics, a selector or an anaerobic mixing tank for enhanced biological phosphate removal is remended. Intermittent nitrification–Denitrification process Aeration tanks for intermittent nitrification–denitrification must be equipped with an aeration system and mixing devices. The design of the aeration system has to take into consideration the aerationoff periods. To improve the aeration efficiency of plants with diffused air aeration, it may be advisable to also operate the propellers during the aeration periods. In intermittent nitrification–denitrification plants the fraction of readily biodegradable anics, which enters the tank while aeration is operating, may be oxidized by dissolved oxygen and therefore lost for denitrification. This must be considered when calculating the nitrate to be denitrified. Precautions against the growth of filamentous anisms should be taken, because the tanks are pletely mixed. The duration of the aeration periods and the aerationoff periods can be set with a timer. Any other control system described in Section can be used to achieve a more stable effluent quality. In Germany, especially in the large number of extended aeration plants, an ORP controller is frequently used. NitraReg is another lowcost control system, which requires only continuous measurement of the dissolved oxygen concentration [66]. By knowing the oxygen transfer capacity, the oxygen uptake rate is automatically calculated and used as the control parameter. Intermittent nitrification–denitrification processes with intermittent wastewater feeding Intermittent wastewater feeding is used in three processes: ? alternating nitrification–denitrification process (BioDenitro) ? TriCycle process ? J252。 therefore, either the water depth differs from tank to tank (same bottomlevel) or the water depth is kept constant (different bottom levels). For maintenance, it is necessary to have a bypass（支路） for each tank in the stepfeed process. In both processes, dissolved oxygen that enters the denitrification zone rem