Nitrifying and heterotrophic population dynamics in biofilm reactors: effects of hydraulic retention time and the presence of organic carbon

Two biofilmreactors operated with hydraulic retention times of 0.8 and 5.0 h were used to study the links between population dynamics and reactor operation performance during a shift in process operation from pure nitrification to combined nitrification and organic carbon removal. The ammonium and the organic carbon loads were identical for both reactors. The composition and dynamics of the microbial consortia were quantified by fluorescence in situ hybridization (FISH) with rRNA-targeted oligonucleotide probes combined with confocal laser scanning microscopy, and digital image analysis. In contrast to past research, after addition of acetate as organic carbon nitrification performance decreased more drastically in the reactor with longer hydraulic retention time. FISH analysis showed that this effect was caused by the unexpected formation of a heterotrophic microorganism layer on top of the nitrifying biofilm that limited nitrifiers oxygen supply. Our results demonstrate that extension of the hydraulic retention time might be insufficient to improve combined nitrification and organic carbon removal in biofilm reactors.Ministério da Ciência, Tecnologia e Ensino Superior. Fundação para a Ciência e a Tecnologia (FCT) - PRAXIS XXI BD/15943/98). Deutscher Akademischer Austauschdienst (A/99/06961). European Comission - T.M.R. BioToBio project. Deutsche Forschungsgemeinschaft

Effect of Self-Association on the Phase Stability of Triphenylamine Derivatives

The self-association equilibrium, i.e. formation of noncovalent dimers, in two triphenylamine derivatives, TPD (<i>N,N</i>′-bis­(3-methylphenyl)-<i>N,N</i>′-diphenylbenzidine) and mMTDAB (1,3,5-tris­[(3-methylphenyl)­phenylamino]­benzene), in solution was evaluated by ¹H NMR spectroscopy. The gas-phase energetics of the respective dimerization processes was explored by computational quantum chemistry. The results indicate that self-association is significantly more extensive in TPB than in TDAB. It is proposed that this fact helps to explain why TPB presents a stability higher than expected in the liquid phase, which is reflected in a lower melting temperature, a less volatile liquid, and possibly a higher tendency to form a glass. These results highlight the influence of self-association on the phase equilibria and thermodynamic properties of pure organic substances