A large citric acid factory is situated close to a rather small river, so the discharge of its wastewater caused severe water quality problems. A highly concentrated stream holds more than 90% of the total COD-load, the lowly concentrated stream about 80% of the flow. Anaerobic pretreatment of the highly polluted stream and additional aerobic treatment of all wastewater reduces the total costs to less than one third in comparison to the aerobic treatment alone. The anaerobic treatment is linked with several problems like high sulphate, high ammonia and high calcium concentration leading to H2S and NH3 toxicity as well as calcium carbonate precipitation, about 3 times more than biomass growth. A special anaerobic reactor (EKJ-reactor) was designed to realize an equal distribution of the wastewater in the sludge bed and to prevent sludge and lime deposition. In 1986 the first stage of the full scale treatment plant for the high concentrated wastewater consisting of an equalization tank, an anaerobic reactor (10 000 m3) and a highly loaded activated sludge plant (2 000 m3) went into operation. In 1987 a second anaerobic reactor was built. In 1988 an additional activated sludge plant with simultaneous nitrification-denitrification (aeration tank volume 15 000 m3) for treatment of all the wastewater was designed in order to keep the effluent concentrations within the strict effluent limits. This plant went in operation in 1990. The anaerobic reactors have shown very good results throughout all the five years of operation. At a COD loading rate of 3 to 7kg/m3·d) a COD removal of almost 90% could be achieved. Because of the high COD removal rate of the anaerobic stage, nitrification occurred in the first post-treatment plant but only to nitrite due to high ammonia concentration and high pH. In the second activated sludge plant stable nitrification was obtained. The content of NH4-N as well as NO2-N in the effluent is generally less than 2 mg/l. Some highly concentrated wastewater is fed directly into the second stage in order to obtain rather complete denitrification. So the NO3-N-concentration in the effluent can be kept below 20 mg/l (85% nitrogen removal) mostly.

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