Fluctuation of microbial activities after influent load variations in a full-scale SBR: recovery of the biomass after starvation.

Due to variations in the production levels, a full-scale sequencing batch reactor (SBR) for post-treatment of tannery wastewater was exposed to low and high ammonia load periods. In order to study how these changes affected the N-removal capacity, the microbiology of the reactor was studied by a diverse set of techniques including molecular tools, activity tests, and microbial counts in samples taken along 3 years. The recover capacity of the biomass was also studied in a lab-scale reactor operated with intermittent aeration without feeding for 36 days. The results showed that changes in the feeding negatively affected the nitrifying community, but the nitrogen removal efficiencies could be restored after the concentration stress. Species substitution was observed within the nitrifying bacteria, Nitrosomonas europaea and Nitrobacter predominated initially, and after an ammonia overload period, Nitrosomonas nitrosa and Nitrospira became dominant. Some denitrifiers, with nirS related to Alicycliphilus, Azospirillum, and Marinobacter nirS, persisted during long-term reactor operation, but the community fluctuated both in composition and in abundance. This fluctuating community may better resist the continuous changes in the feeding regime. Our results showed that a nitrifying-denitrifying SBR could be operated with low loads or even without feeding during production shut down periods.

Distinctive microbial ecology and biokinetics of autotrophic ammonia and nitrite oxidation in a partial nitrification bioreactor.

Biological nitrogen removal (BNR) based on partial nitrification and denitrification via nitrite is a cost-effective alternate to conventional nitrification and denitrification (via nitrate). The goal of this study was to investigate the microbial ecology, biokinetics, and stability of partial nitrification. Stable long-term partial nitrification resulting in 82.1 +/- 17.2% ammonia oxidation, primarily to nitrite (77.3 +/- 19.5% of the ammonia oxidized) was achieved in a lab-scale bioreactor by operation at a pH, dissolved oxygen and solids retention time of 7.5 +/- 0.1, 1.54 +/- 0.87 mg O(2)/L, and 3.0 days, respectively. Bioreactor ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) populations were most closely related to Nitrosomonas europaea and Nitrobacter spp., respectively. The AOB population fraction varied in the range 61 +/- 45% and was much higher than the NOB fraction, 0.71 +/- 1.1%. Using direct measures of bacterial concentrations in conjunction with independent activity measures and mass balances, the maximum specific growth rate (micro(max)), specific decay (b) and observed biomass yield coefficients (Y(obs)) for AOB were 1.08 +/- 1.03 day(-1), 0.32 +/- 0.34 day(-1), and 0.15 +/- 0.06 mg biomass COD/mg N oxidized, respectively. Corresponding micro(max), b, and Y(obs) values for NOB were 2.6 +/- 2.05 day(-1), 1.7 +/- 1.9 day(-1), and 0.04 +/- 0.02 mg biomass COD/mg N oxidized, respectively. The results of this study demonstrate that the highly selective partial nitrification operating conditions enriched for a narrow diversity of rapidly growing AOB and NOB populations unlike conventional BNR reactors, which host a broader diversity of nitrifying bacteria. Further, direct measures of microbial abundance enabled not only elucidation of mixed community microbial ecology but also estimation of key engineering parameters describing bioreactor systems supporting these communities.

Competition between Nitrospira spp. and Nitrobacter spp. in nitrite-oxidizing bioreactors.

In this work the question was addressed if in nitrite-oxidizing activated sludge systems the environmental competition between Nitrobacter spp. and Nitrospira spp., which only recently has been discovered to play a role in these systems, is affected by the nitrite concentrations. Two parallel chemostats were inoculated with nitrifying-activated sludge containing Nitrospira and operated under identical conditions. After addition of Nitrobacter to both chemostats, the nitrite concentration in the influent of one of the chemostats was increased such that nitrite peaks in the bulk liquid of this reactor were detected. The other chemostat served as control reactor, which always had a constant nitrite influent concentration. The relative cellular area (RCA) of Nitrospira and Nitrobacter was determined by quantitative fluorescence in situ hybridization (FISH). The nitrite perturbation stimulated the growth of Nitrobacter while in the undisturbed control chemostat Nitrospira dominated. Overall, the results of this experimental study support the hypothesis that Nitrobacter is a superior competitor when resources are abundant, while Nitrospira thrive under conditions of resource scarcity. Interestingly, the dominance of Nitrobacter over Nitrospira, caused by the elevated nitrite concentrations, could not be reverted by lowering the available nitrite concentration to the original level. One possible explanation for this result is that when Nitrobacter is present at a certain cell density it is able to inhibit the growth of Nitrospira. An alternative explanation would be that the length of the experimental period was not long enough to observe an increase of the Nitrospira population.

Ultrastructure of Nitrobacter agilis grown under autotrophic and heterotrophic conditions.

Nitrobacter agilis, grown through seven transfers heterotrophically in the absence of nitrite, was examined in the electron microscope. The ultrastructure of such cells closely resembled that of autotrophically grown N. agilis. It was thus futher established that the organisms growing heterotrophically were indeed N. agilis and, therefore, that N. agilis is a facultative autotroph. Acetate incorporation into poly-beta-hydroxybutyrate was confirmed cytologically.

Lack of distinction between Nitrobacter agilis and Nitrobacter winografskyi.

No adequate criteria were established to distinguish between Nitrobacter agilis and N. winogradskyi. However, very gentle preparative techniques permitted demonstration of flagella in N. agilis.