Stage 1 A of the Valenton purification plant, located to the South West of the Paris conurbation, was designed to treat 15 0,00 0 m3/day, which represents pollution of 500,000 population-equivalents. Due to its situation upstream of Paris, the plant was designed primarily with nitrogen removal in mind. The nitrification-denitrification process with an anoxic zone at the plant inlet was selected.

This paper looks at results obtained over a three-year period. The results achieve expected levels of elimination of both carbonaceous and nitrogenous pollution.

The results achieved in terms of age of the aerated sludge and nitrification kinetics measurements clearly indicate that a nitrification over-capacity exists. However, the denitrification process varies as a function of the concentration levels of organic carbon in the settled water. During periods of rainfall with low BOD5 levels, denitrification is moderately efficient. When the nominal water concentration levels are observed, all the nitrates are reduced in the anoxic zone, and efficiency of the overall nitrogen elimination process reaches 60%.

In any case, and particularly in winter, high sludge concentration levels are required to maintain satisfactory kinetics in the nitrification and denitrification stages.

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