A rational approach for the design of the activated sludge process based on wastewater and biomass characterization techniques as applied to the upgrading of the treatment plant of Volos, a city in the central part of Greece, is presented. The study investigates possible nitrification inhibition and carbon inadequacy, due to high salinity, industrial inflows and pre-precipitation by iron salts. The experimentation was carried out by means of batch Ammonium Uptake Rate (AUR) and Nitrates Uptake Rate (NUR) tests. The results show that within the studied range, 900-4000 mg l−1, chlorides did not inhibit nitrification. Contrary to this, the industrial wastewater entering the plant was found to be toxic. With the existing 1:15 ratio of industrial to total wastewater flow a 50% inhibition to the nitrification process was observed, which is higher than the 25 - 30% inhibition caused by typical domestic sewage. Industrial contributions exceeding 20% resulted in complete inhibition of nitrification. With respect to denitrification it was found that the industrial wastewater provided a suitable source of carbon, without any adverse effects on denitrification. Pre-precipitation removed about 25% of the filtered COD, thus reducing the amount of nitrates which could be rapidly denitrified. Design of the biological reactors on the basis of the findings indicate that a significant under-design may result if typical nitrification and denitrification rates obtained from the literature and practice concerning typical domestic sewage are adopted.

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