Performance and internal environment of partial nitrification-entrapped cells under changes in cell-to-matrix ratio and substrate concentrations

Abstract

In this study, partial nitrification performance was investigated in entrapped cell-based reactors prepared using phosphorylated polyvinyl alcohol gel (PPVA) and operated using oxygen limiting strategy. The study was divided into three main parts: 1) to investigate the community of nitrite-oxidizing bacteria (NOB) in the reactors operated at different bulk dissolved oxygen (DO) concentrations, 2) to investigate the effect of cell-to-matrix ratio (1% and 4%) on partial nitrification performance of the entrapped-cell-based reactors and to observe internal environment, microbial community, and microbial localization within the gel matrix during long-term operation of the reactors, and 3) to study effect of ammonia and organic loading rates on performance and pathways of nitrogen removal as well as nitrous oxide (N2O) production in the entrapped-cell-based reactors. In the first part, a comparable level of partial nitrification (> 60%) was achieved in entrapped-cell-based reactors operated at bulk DO concentration of 2 and 3 mg l-1. NOB having distinct oxygen affinity, including Nitrobacter and Nitrospira lineage I and Nitrospira lineage II, coexisted in both reactors. The results indicated that the entrapped-cell system allow a variety of NOB to exist and perform some activity, although the majority of nitrite-oxidizing activity was inhibited in the system. In the second part, two entrapped-cell-based partial nitrification reactors were operated using gel beads containing different cell-to-matrix ratios of 1% and 4%. Results showed that oxygen concentration gradient determined the periods of which partial nitrification could be achieved. However, after partial nitrification was achieved, both reactors showed similar degree of nitrite accumulation during long-term operation. The oxygen-limiting zone (DO = 0.5-1.5 mg l-1), where nitrite-oxidizing activity was suggested to be suppressed, occurred at 10-230 µm from the surface of gel matrix. In the last part, varying ammonia and organic loading rates showed that partial nitrification could only be maintained in the reactors without organic feeding and with ammonia loading rate of ≥0.3 kg m-3 d-1. While, complete nitrification occurred in the reactors without organic feeding and with ammonia loading rate of ≤0.2 kg m-3 d-1. Simultaneous nitrification and denitrification was found in most reactors fed with organic. Moreover, N2O was produced between 0.1 and 0.24% of the ammonia loading rate in the studied reactors.