Nitrous oxide (N(2)O) emission from biological nitrogen removal (BNR) processes has recently received more research attention. In this study, two lab-scale BNR systems were used to investigate the effects of various operating parameters including the carbon to nitrogen (C/N) ratio, ammonia loading, and the hydraulic retention time on N(2)O production. The first system was operated in a conventional BNR mode known as the Ludzack-Ettinger (LE) process, consisting of complete denitrification and nitrification reactors, while the second one was operated in a shortcut BNR (SBNR) mode employing partial nitrification and shortcut denitrification, which requires less oxygen and carbon sources. As the C/N ratio was decreased, a significant increase in N(2)O production was observed only in the anoxic reactor of the LE process, indicating that N(2)O was released as an intermediate of the denitrification reaction under the carbon-limited condition. However, the SBNR process did not produce significant N(2)O even at the lowest C/N ratio of 0.5. When the SBNR process was subjected to increasing concentrations of ammonia, N(2)O production from the aerobic reactor was rapidly increased. Furthermore, the increasing production of N(2)O was observed mostly in the aerobic reactor of the SBNR process with a decline in hydraulic retention time. These experimental findings indicated that the increase in N(2)O production was closely related to the accumulation of free ammonia, which was caused by an abrupt increase of the ammonium loading. Consequently, the partial nitrification was more susceptible to shock loading conditions, resulting in a high production of N(2)O, although the SBNR process was more efficient with respect to nitrogen removals as well as carbon and oxygen requirements.

This work was designed to investigate the removal efficiency as well as the ratios of toluene and xylene transported from air to root zone via the stem and by direct diffusion from the air into the medium. Indoor plants (Schefflera actinophylla and Ficus benghalensis) were placed in a sealed test chamber. Shoot or root zone were sealed with a Teflon bag, and gaseous toluene and xylene were exposed. Removal efficiency of toluene and total xylene (m, p, o) was 13.3 and 7.0 μg·m(-3)·m(-2) leaf area over a 24-h period in S. actinophylla, and was 13.0 and 7.3 μg·m(-3)·m(-2) leaf area in F. benghalensis. Gaseous toluene and xylene in a chamber were absorbed through leaf and transported via the stem, and finally reached to root zone, and also transported by direct diffusion from the air into the medium. Toluene and xylene transported via the stem was decreased with time after exposure. Xylene transported via the stem was higher than that by direct diffusion from the air into the medium over a 24-h period. The ratios of toluene transported via the stem versus direct diffusion from the air into the medium were 46.3 and 53.7% in S. actinophylla, and 46.9 and 53.1% in F. benghalensis, for an average of 47 and 53% for both species. The ratios of m,p-xylene transported over 3 to 9 h via the stem versus direct diffusion from the air into the medium was 58.5 and 41.5% in S. actinophylla, and 60.7 and 39.3% in F. benghalensis, for an average of 60 and 40% for both species, whereas the ratios of o-xylene transported via the stem versus direct diffusion from the air into the medium were 61 and 39%. Both S. actinophylla and F. benghalensis removed toluene and xylene from the air. The ratios of toluene and xylene transported from air to root zone via the stem were 47 and 60 %, respectively. This result suggests that root zone is a significant contributor to gaseous toluene and xylene removal, and transported via the stem plays an important role in this process.

Air streams commonly emitted from industrial sources generally contain various mixtures of volatile organic compounds (VOCs), and these complex mixtures can present challenges with respect to bioreactor design and applications. In this study, therefore, a modified Monod-type model using interaction parameters was employed to describe the biodegradation kinetics of mixtures of aromatic compounds by a Pseudomonas isolate. In addition, the model and estimated parameters were utilized to predict the performance of a bubble-column bioreactor for the treatment of mixtures of benzene, toluene, p-xylene, and styrene (BTXS). Benzene, toluene and styrene, as individual substrates, were actively degraded by the bacterial culture, whereas p-xylene was not degraded as a single substrate. Relative to the single substrate experiments, the degradation of benzene and toluene was inhibited by the other compounds, while the degradation of styrene was significantly stimulated in the presence of the other BTXS compounds. The cometabolic degradation of p-xylene was observed in the presence of benzene and toluene. The estimated interaction parameters indicated that the degradation of benzene was substantially inhibited in the presence of styrene, whereas the degradation of styrene was strongly stimulated by toluene. The kinetic coefficients and interaction parameters were used to successfully predict the biodegradation kinetics and performance of a bioreactor subjected to the quaternary mixture. Overall, the model was able to provide reasonable predictions when substrate interactions, including inhibition, stimulation, and cometabolism, play significant roles in biodegradation processes.

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