The objective of the work was to verify the possibility to reduce the quantity of the sludge produced during the treatment of chrome tannery waste waters. The modified Ludzack-Ettinger process for high nitrogen content waste waters was tested without a chemical-physical pretreatment, normally adopted to eliminate from the tannery waste waters the compounds potentially inhibitory for biological processes. The results obtained in a pilot plant during 4 runs differing in waste waters flow and strength and in MLSS recycling ratio demonstrate the lack of inhibition of denitrification and nitrification by a high content of sulphides and chromium. In the comparison with traditional treatment of tannery waste waters the 60% reduction of the sludge to be wasted and the improvement of a final effluent quality were achieved. The mean overall rate of COD and N removal was 0.38 mgCOD/mgMLVSSxday and 0.125 mgN/mgMLVSSxday, respectively.

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