Results of a three year experience on a combined anaerobic-anoxic-oxic municipal waste water treatment process - named ANANOX® - are presented.

This process demonstrated to be highly efficient, with 89.6% CODt, 89.2% TSS and 81.2% N removal, and a sludge production of only 0.2 kg TSS.kg COD removed−1, a value which is roughly 50% less if compared with traditional nitrification/denitrification processes.

Sulphates play a very significant role in the process because, after being reduced in the anaerobic step, where they give a contribution to the organic matter degradation, they are reoxidized in the anoxic step by nitrates, reducing the organic matter need for denitrification.

Due to the high dependence of efficiency on temperature, the system proposed has advantageous uses for sewage treatment, particularly in warm climates and in tourist and recreational areas where the population increases during the warm season.

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