【正文】 /3 of the cycle period). Valves in the connecting pipes and at the inlet and outlet of the tanks guide the mixed liquor through the three tanks. Mixed liquor is discharged to the final clarifier from the aeration tank that is aerated for the longest period. This process is used in at least six plants in Germany. Unfortunately only brochures of the pany (GVA, W252。lfrath, Germany) that developed this process are available. The J252。lich Wastewater Treatment Process (JARV) was developed to overe problems with bulking sludge in pletely mixed aeration tanks. The key of the process is a rather short feeding period pared to the cycle period [69]. This requires a balancing tank upstream of the aeration tank and an effluent gate to equalize the flow to the final clarifiers. Balancing is avoided if, ., four parallel aeration tanks are consecutively fed with wastewater. The effluent from the aeration tanks may contain considerable ammonia peaks. It is, therefore, advisable to build a small postaeration basin between the aeration tanks and the final clarifiers. Only two plants in Germany are operated by this process. The differences of the three processes relative to the intermittent nitrification–denitrification process are that almost all readily biodegradable anics are available for denitrification, that there is a higher concentration gradient by which the sludge volume index can be improved, and that during the discharge period the mixed liquor is aerobic (except in the JARV process if a post aeration tank is omitted). The anoxic tank fraction (VD/V) can be estimated as for the preanoxic zone denitrification process (Eq. 26). Special processes for low COD/TKN ratio Wastewater from the food industry may be characterized by high concentrations of anics as well as considerable nitrogen concentrations. Anaerobic pretreatment of such sewages is favorable because of the negligible sludge production and energy requirement. If nitrogen removal is required, the COD/TKN ratio can however decrease too much for sufficient denitrification. Since denitrification of nitrite requires about 35% less anics than denitrification of nitrate, a process in which Nitrosomonas were inhibited was developed by Abeling [70]. The preanoxic zone process was operated with a control system to maintain the pH at about pH = 8 by dosing the aerobic tank with NaOH and to keep ammonia at SNH4 = 10 mg L–1 by appropriate aeration control. According to the findings of Anthonisen et al. [45] and Nyhuis [46] (Fig. ), these are conditions under which Nitrobacter are inhibited. Because there is always some ammonia and some nitrite in the effluent, a second stage using a fixedfilm reactor was implemented. A different method of removing high nitrogen concentrations without any anic substrate was published by Jetten et al. [71]. In a continuousflow stirred reactor as a first stage, about 50% of ammonia was converted to nitrite。 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 temp