Autotrophic denitrification for treatment of wastewater with high concentration of sulphur and nitrogen compounds

Abstract

Anthropogenic activities have contributed to the imbalance of nitrogen and sulphur natural cycles which causes many negative effects in nature due to the emissions of sulphur and nitrogen compounds and their transformations, e.g. rain acid, eutrophication, bad odours etc. To avoid such negative effects on environment, effluents containing high concentrations of both nitrogen and sulphur compounds must be treated previously to their discharge. Nitrification/denitrification is the conventional process to remove nitrogen compounds from wastewater. However, when effluents are characterized by low C/N ratios, the nitrogen removal efficiency is limited by the low amount of organic matter available to carry out the denitrification process. In this aspect, autotrophic denitrification supposes a suitable alternative to heterotrophic denitrification. This process is carried out by Thiobacillus-type bacteria which combine nitrate and reduced sulphur compounds to generate dinitrogen gas and sulphate or elemental sulphur. Therefore, the autotrophic denitrification process is able to simultaneously remove both nitrogen and sulphur compounds without presence of organic matter. Compared to other processes, this treatment would also permit to minimize the environment impact in several aspects: the protection of ozone layer by decreasing the nitrogen oxides emission, the decrease of the greenhouse effect by reducing the energy needed for the process, the reduction in the raw materials (as organic matter) consumption and reduction of the amount of sludge generated due to its low sludge production. The present thesis is focused on studying the autotrophic denitrification process. The main objective of this project was the operation of a Sequencing Batch Reactor (SBR) under autotrophic denitrifying conditions for simultaneous removal of nitrate and sulphide, being the general objectives: • Optimize the enrichment of autotrophic denitrifying biomass to be used as seed for the SBR system. • Test the operation of an autotrophic denitrifying sequencing batch reactor for simultaneous removal of nitrate and sulphide. • Test the operation of a nitrifying/autotrophic denitrifying system for the post-treatment of an effluent coming from an anaerobic digester treating fish canning industry wastewater. • Study the influence of different parameters on autotrophic denitrification when sulphide is used as electron donor. • Characterize, by means of molecular techniques, the evolution of the microbial community during its enrichment and along the operational period of the SBR system.