【文章內(nèi)容簡介】 ed denitrification process The activated sludge tank in the stepfeed denitrification process consists of two to three preanoxic zone units in series: the return sludge is diverted to the first denitrification zone and the wastewater is distributed to each denitrification zone (Fig. ). Although the first experiments on this process were performed in the UK [英國 ] and later in Japan [59], the first results for a fullscale（全尺寸，實比） plant were reported by Schlegel [施勒格爾 ] in Germany. Again, considering plete nitrification in the aeration zones and plete denitrification in the anoxic zones, the effluent nitrate concentration for the last denitrification zone can be calculated by mass balance (Eq. 39): (x Q) SNH4,N = (Q + QRS +QIR,n) 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 Q1 = Q, Q2 = Q, and Q3 = Q. The MLSS would then be, respectively, kg m–3, kg m–3, and kg m–3. Without internal recirculation, the ratio CCOD/SNO3, which determines the degree of nitrate removal, in the three denitrification zones differs considerably（相當?shù)模?. With QIR,1 = – Q, QIR,2 =– Q, and QIR,3 = 0, CCOD/SNO3 takes the same value. In the threestep process with QRS = Q, QIR,3 = 0, and x = , the denitrification efficiency is 86% (Eq. 41). If with moreconcentrated （集中的，濃縮的） wastewater a low nitrate concentration must be maintained, QIR,3 (Eq. 41) has to be selected. QIR,1 and QIR,2 then have to be increased appropriately. The similarity（相似） of the preanoxic zone denitrification process and the stepfeed process is obvious when Eqs. 38 and 41 are pared. To achieve 86% denitrification efficiency in the preanoxic zone process, the recycling ratio must be QR/Q = . The differences between the preanoxic zone process and the stepfeed process are illustrated（圖解） in Figure . The advantages of the preanoxic zone process are that each of the three tanks is operated independently and that the tanks all have the same water level（含量） and depth. In the stepfeed process some head loss occurs as the water flows from one tank to the next。 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 removes 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 SN