The detailed evaluation of the data from the daily logbooks of municipal clarification plants led to the finding that the nitrification capability of the plants is in essence dependent on the BOD5- and nitrogen sludge load. The evaluation of the data showed that, above all, aeration units with surface aerators did not operate well in respect to nitrification. Comprehensive investigations of different clarification plants led to the identification of shortages in the oxygen content of the aeration basins as the main reason for these deficiencies.

The detailed examination of the results of these investigations shows clearly that in order to maintain low levels of ammonia nitrogen in the effluent, more attention must be paid to the factors influencing the oxygen concentration and distribution.

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