This is a report on the design and operating results of two waste water treatment plants which make use of biological nitrogen and phosphate elimination. Both plants are characterized by load situations that are unfavourable for biological P elimination.

The influent of the HILDESHEIM WASTE WATER TREATMENT PLANT contains nitrates and little BOD5. Use of the ISAH process ensures the optimum exploitation of the easily degradable substrate for the redissolution of phosphates. Over 70 % phosphate elimination and effluent concentrations of 1.3 mg PO4-P/I have been achieved.

Due to severe seasonal fluctuations in loading the activated sludge plant of the HUSUM WASTE WATER TREATMENT PLANT has to be operated in the stabilization range (F/M ≤ 0.05 kg/(kg·d)) in order not to infringe the required effluent values of 3.9 mg NH4-N/l (2-h-average). The production of surplus sludge is at times too small to allow biological phosphate elimination to be effected in the main stream process. The CISAH (Combined ISAH) process is a combination of the fullstream with the side stream process. It is used in order to achieve the optimum exploitation of biological phosphate elimination by the precipitation of a stripped side stream with a high phosphate content when necessary.

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