Biological treatment of gaseous nitrogen oxides using modular denitrifiers

Abstract

Nitrogen oxide emissions caused by human activities are accelerating the imbalance in nitrogen cycle. The importance of reducing emissions from engineered systems is emphasized to resolve the serious environmental problems arising from alterations in nitrogen cycle. This thesis was a study on the biological treatment system of nitrogen oxides emitted from indestrial off-gas. In the first research topic of the thesis, the feasibility analysis for various concentration of $N_2O$ removal was performed by operate the pilot-scale biotrickling filter integrated into the anoxic-anaerobic-oxic ($A_2O$) and partial nitritation-anammox (PN-A)-type wastewater treatment plants (WWTPs) during 1 year and 6 months, respectively. Cost-efficiency operation was achieved by using dissolved organic carbon (DOC) in primary sedimentation sewage as an electron donor and self-conveying $N_2O$ as covering the aeration section. The stability of the $N_2O$ removal performance was confirmed through the combination of the bench-scale system operated in parallel at the full-scale $A_2O$ WWTP. The robustness and resilence of microbial compositions in the biofilm to seasonal variations were demonstrated by microbial community analyses based on 16s rRNA sequencing. Taxonomic classification of 157 diverse nosZ genes was identified using metagenome analysis, and the distribution of nosZ genes maintained similar within both systems, regardless of variations in the $N_2O$ concentration and operating conditions. Therefore, the long-term stability of $N_2O$ abatement in this system was identified in highly variable WWTP conditions.

In the second research topic was development of biological treatment system for gaseous nitrogen oxide ($NO_x$) removal generated from the combustion of industrial fuels. Considering that oxygen permeation impact on anoxical reation of denitrification and iron-reduction, the reactor system was designed as packed bed plug flow system, and Fe(II)EDTA-NO$^{2-}$ reduction which formed by NO absorption into aqueous Fe(II)EDTA$^{2-}$ solutions was caused by denitrification as inoculating the activated sludge. Moerever, the regeneration of aqueous solution of Fe(II)EDTA$^{2-}$ was performed by iron-reducing bacteria, suggesting the cost efficiency could be increased. The long-term monitoring was performed to assess the stable removal efficiency of $NO_x$ and the conversion rate of Fe(II)EDTA$^{2-}$. As a results, the 16s rRNA sequencing analysis revealed the predominance of Clostridium sensu stricto species, which are known for fermentative anaerobes. This finding suggests that these species play a crucial role in the recirculated PFR system.