Sviluppo di tecnologie alternative per il trattamento biologico di reflui ad elevato contenuto di azoto

The research project concerned the study of the feasibility of the double stage SHARON (Single reactor for High activity Ammonium Removal Over Nitrite) -ANAMMOX (Anaerobic AMMonium Oxidation) process for the biological treatment of nitrogen-rich refinery wastewater (sour water) produced at Saras SpA facilities located near Sarroch, on the southwestern coast of Sardinia (Italy). The Sarroch refinery's operating capacity is approximately 15 million tons per year (or 300,000 barrels per day), accounting for approximately 15% of Italy's total refining capacity. The wastewater of concern was characterized by a high content of ammonium (up to 4,000 mgN/L), as well as by the simultaneous presence of toxic compounds like cyanides, phenols and sulfides. The wastewater is currently treated at full scale by steam stripping up to a final NH4-N concentration of 25 mg/L: if NH4-N level in the effluent from the stripping unit can be set at 2,000 mg/L and treated by the SHARON-ANAMMOX process, it will imply a considerable reduction in energy requirements and operating costs. The SHARON process was applied successfully and proved to be reliable enough to tolerate the inhibitory substances contained in the wastewater. Depending on influent inorganic carbon to nitrogen molar ratio (Ci/N), it was able to provide an effluent suitable for the subsequent treatment by ANAMMOX or heterotrophic denitrification via nitrite. The ANAMMOX process was shown to be more sensitive to the inhibitory compounds contained in the real wastewater. In particular, the research activity has shown that the extreme variability of industrial refinery wastewaters, combined with and the high sensitivity of ANAMMOX biomass may actually hinder the full-scale application of the SHARON-ANAMMOX process for the treatment of such wastewater: nevertheless, the alternative represented by the SHARON-heterotrophic denitrification via nitrite combined system is worthy of being investigated. During the research abroad at the Technical University of Delft (The Netherlands), the influence of temperature on the ANAMMOX process was investigated: batch tests were carried out in order to determine the maximum specific activities and energies of activation of ANAMMOX bacteria at different temperatures. Moreover, a reactor was started up for the study of the single stage CANON (Completely Autotrophic Nitrogen removal Over Nitrite) process at 15 °C, in order to evaluate its feasibilty for the treatment of municipal wastewater, even in Countries with a cold climate, or during the winter time. The results have shown the considerable dependence of the ANAMMOX process on temperature: in particular, the specific activities measured at low temperature (10÷15 °C) were much lower than those observed at high temperatures (30 °C). The difficult CANON reactor start-up at 15 °C, due to lower growth rates and specific activities of the microorganisms involved in the process, demonstrated the need for a start-up at elevated temperatures (30 °C), before the gradual decrease of the operating temperature to the target value, in order to acclimate the bacteria involved at lower temperatures.