Application of external carbon sources for denitrification becomes necessary for wastewater treatment plants that have to meet very stringent effluent nitrogen limits (e.g., 3 to 5 mgTN/L). In this study, we evaluated and compared three carbon sources—MicroCTM (Environmental Operating Solutions, Bourne, Massachusetts), methanol, and acetate—in terms of their denitrification rates and kinetics, effect on overall nitrogen removal performance, and microbial community structure of carbon‐specific denitrifying enrichments. Denitrification rates and kinetics were determined with both acclimated and non‐acclimated biomass, obtained from laboratory‐scale sequencing batch reactor systems or full‐scale plants. The results demonstrate the feasibility of the use of MicroCTM for denitrification processes, with maximum denitrification rates (kdmax) of 6.4 mgN/gVSS·h and an observed yield of 0.36 mgVSS/mgCOD. Comparable maximum nitrate uptake rates were found with methanol, while acetate showed a maximum denitrification rate nearly twice as high as the others. The maximum growth rates measured at 20°C for MicroCTM and methanol were 3.7 and 1.2 day−1, respectively. The implications resulting from the differences in the denitrification rates and kinetics of different carbon sources on the full‐scale nitrogen removal performance, under various configurations and operational conditions, were assessed using Biowin (EnviroSim Associates, Ltd., Flamborough, Ontario, Canada) simulations for both pre‐ and post‐denitrification systems. Examination of microbial population structures using Automated Ribosomal Intergenic Spacer Analysis (ARISA) throughout the study period showed dynamic temporal changes and distinct microbial community structures of different carbon‐specific denitrifying cultures. The ability of a specific carbon‐acclimated denitrifying population to instantly use other carbon source also was investigated, and the chemical‐structure‐associated behavior patterns observed suggested that the complex biochemical pathways/enzymes involved in the denitrification process depended on the carbon sources used.

This study comprehensively investigated the impact of titanium dioxide nanomaterials (nTiO(2)) exposure on cell growth, nitrogen fixation activity, and nitrogen storage dynamics in the primary producer cyanobacteria Anabaena variabilis at various dose concentrations and exposure time lengths. The results indicated that both growth rate (EC(50)-96 h of 0.62 mgTiO(2)/L) and nitrogen fixation activity (EC(50)-96 h of 0.4 mgTiO(2)/L) were inhibited by nTiO(2) exposure. The Hom's law (C(n)T(m)) was used as inactivation model to predict the concentration- and time-dependent inhibition of growth and nitrogen fixation activity. The kinetic parameters determined suggested that the time of exposure has a greater influence than the nTiO(2) concentration in toxicity. We observed, for the first time, that nTiO(2) induced a dose (concentration and time)-dependent increase in both the occurrence and intracellular levels of the nitrogen-rich cyanophycin grana proteins (CGPs). The results implied that CGPs may play an important role in the stress response mechanisms of nTiO(2) exposure and can serve as a toxicity assessment endpoint indicator. This study demonstrated that nitrogen-fixing activity could be hampered by the release of nTiO(2) in aquatic environments; therefore it potentially impacts important biogeochemical processes, such as carbon and nitrogen cycling.

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The present study investigated the impact of nano titanium dioxide (nTiO(2) ) exposure on the cellular structures of the nitrogen-fixing cyanobacteria Anabaena variabilis. Results of the present study showed that nTiO(2) exposure led to observable alteration in various intracellular structures and induced a series of recognized stress responses, including production of reactive oxygen species (ROS), appearance and increase in the abundance of membrane crystalline inclusions, membrane mucilage layer formation, opening of intrathylakoidal spaces, and internal plasma membrane disruption. The production of total ROS in A. variabilis cells increased with increasing nTiO(2) doses and exposure time, and the intracellular ROS contributed to only a small fraction (<10%) of the total ROS measured. The percentage of cells with loss of thylakoids and growth of membrane crystalline inclusions increased as the nTiO(2) dose and exposure time increased compared with controls, suggesting their possible roles in stress response to nTiO(2) , as previously shown for metals. Algal cell surface morphology and mechanical properties were modified by nTiO(2) exposure, as indicated by the increase in cell surface roughness and shifts in cell spring constant determined by atomic force microscopy analysis. The change in cell surface structure and increase in the cellular turgor pressure likely resulted from the structural membrane damage mediated by the ROS production. Transmission electron microscopy (TEM) analysis of nTiO(2) aggregates size distribution seems to suggest possible disaggregation of nTiO(2) aggregates when in close contact with microbial cells, potentially as a result of biomolecules such as DNA excreted by organisms that may serve as a biodispersant. The present study also showed, for the first time, with both TEM and Raman imaging that internalization of nTiO(2) particles through multilayered membranes in algal cells is possible. Environ. Toxicol. Chem. 2011; 30:861-869. © 2010 SETAC.

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