In France, many small sewage treatment plants are of the activated sludge/extended aeration type and, generally, only receive a part of their nominal organic load. They are mostly equipped with surface aerators whose Standard wire Aeration Efficiency is in the range 1.3-1.9 kg 02 kwh−1. Consequently the sludge age and the supply of oxygen are sufficient to obtain a large elimination of nitrogen from domestic waste water even when the sludge temperature is low.

However the concern is with the optimizing of the nitrogen treatment by the nitrification of ammonia in the aeration basin whilst avoiding any parasitic denitrification in the clarifier.

Amongst other investigations, 4 small domestic wastewater treatment plants were studied over several months. A number of adjustment modifications were imposed on them in order to optimize the elimination of nitrogen : The final concentrations to be reached were : N-NH4 lower than 5 mg.l−1 and N-NO3 lower than 3 mg.l−1.

The daily operating time of the aerators depends on the received load, the sludge concentration and the oxygenation capacity. When the works are under-loaded, the non-operational periods of the aerators should be as long as 1 1/2 and 2 hours in order to achieve the nitrates reduction. When the load is higher, the time required to attain anoxia after the shutdown of the aerators is shorter and the length of the non-operational periods can be reduced. These field experiments have allowed an evaluation of the tolerances around an optimum adjustment of the aeration operation : a reduction of 5 to 10 % of the daily aeration time may bring about an increase in the residual concentration of Kjeldahl nitrogen which reaches 10 rag.1−1. Conversely, sludge losses may take place in the clarifiers if the daily aeration time is increased by 5 to 10 % in the case of completely mixed basins equipped with slow vertical shaft aerators, and more than 15 % in the case of oxidation ditches (better denitrification probably due to the continuation of the current after stopping the rotors). The sludge concentration should be kept within fairly tight limits (± 0.5 g.1−1). It is however essential to rectify the aeration adjustments during the seasons : a rise in the sludge temperature of 10°C generates an increase in the total oxygen demand by 3 to 5 %.

The particular case of plants with anoxic zone ahead is finally discussed.

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