Rotating biological contactors : a review on main factors affecting performance

Rotating biological contactors (RBCs) constitute a very unique and superior alternative for biodegradable matter and nitrogen removal on account of their feasibility, simplicity of design and operation, short start-up, low land area requirement, low energy consumption, low operating and maintenance cost and treatment efficiency. The present review of RBCs focus on parameters that affect performance like rotational speed, organic and hydraulic loading rates, retention time, biofilm support media, staging, temperature, influent wastewater characteristics, biofilm characteristics, dissolved oxygen levels, effluent and solids recirculation, stepfeeding and medium submergence. Some RBCs scale-up and design considerations, operational problems and comparison with other wastewater treatment systems are also reported.Fundação para a Ciência e a Tecnologia (FCT

Calibration of a complex activated sludge model for the full-scale wastewater treatment plant

In this study, the results of the calibration of the complex activated sludge model implemented in BioWin software for the full-scale wastewater treatment plant are presented. Within the calibration of the model, sensitivity analysis of its parameters and the fractions of carbonaceous substrate were performed. In the steady-state and dynamic calibrations, a successful agreement between the measured and simulated values of the output variables was achieved. Sensitivity analysis revealed that upon the calculations of normalized sensitivity coefficient (Si,j) 17 (steady-state) or 19 (dynamic conditions) kinetic and stoichiometric parameters are sensitive. Most of them are associated with growth and decay of ordinary heterotrophic organisms and phosphorus accumulating organisms. The rankings of ten most sensitive parameters established on the basis of the calculations of the mean square sensitivity measure (δjmsqr) indicate that irrespective of the fact, whether the steady-state or dynamic calibration was performed, there is an agreement in the sensitivity of parameters

Aerobic and anoxic biodegradation of benzoate: Stability of biodegradative capability under endogenous conditions

Aromatic organic compounds are degraded by different enzyme systems under aerobic and anoxic conditions. This raises the question of how bacteria in biological nitrogen removal processes, which cycle bacteria between aerobic and anoxic environments, regulate their enzyme systems for degrading aromatic compounds. As a first step in answering that question, mixed microbial communities were grown on benzoate as sole carbon source in chemostats under fully aerobic and fully anoxic (nitrate as the electron acceptor) conditions and tested for their ability to degrade benzoate in batch reactors after exposure to aerobic or anoxic conditions in the absence of substrate. Aerobically grown biomass retained its ability to degrade benzoate without loss of activity after endogenous exposure to aerobic conditions for up to 8 h. However, when exposed to anoxic conditions, the biomass rapidly lost its aerobic benzoate degrading activity, retaining less than 20% of the initial activity after 8 h. Similarly, anoxically grown biomass retained its ability to degrade benzoate without loss of activity after endogenous exposure to anoxic conditions for up to 8h. However, when anoxically grown biomass was exposed to aerobic conditions. only 20% of its initial activity was lost in the first 2 h: after which the remaining activity was retained for up to 8 h. Similar experiments with pyruvate showed that the 20% loss of activity was not due to loss of denitrifying enzymes, suggesting that it was due to loss of catabolic enzymes. (C) 2001 Elsevier Science Ltd. All rights reserved

Effects of oxygen on biodegradation of benzoate and 3-chlorobenzoate in a denitrifying chemostat

A mixed microbial culture degraded a mixture of benzoate (863 mg/L), 3-chlorobenzoate (3-CB) (69.7 mg/L), and pyruvate (244 mg/L) under denitrifying conditions in a chemostat. Biodegradation under denitrifying conditions was stable, complete (effluent concentrations below detection limits), and proceeded without the production of toxic intermediates like chlorocatechols. The addition of oxygen at mass input rates of 6.21/,,, 15.5%, and 43.9% of the mass input rate of chemical oxygen demand (COD) (337 mg COD/h) did not induce the synthesis of aerobic biodegradation pathways and thus did not disrupt biodegradation. Rather, the oxygen was used as a terminal electron acceptor, displacing a stoichiometric amount of nitrate, leading to microaerobic conditions (dissolved oxygen concentration <0.050mg/L) in which oxygen utilization and denitrification occurred simultaneously. The reduction of nitrate occurred fully to N-2 gas with no accumulation of nitrite, nitrous oxide, or nitric oxide, although the ability of the culture to transfer electrons to the nitrogen oxides decreased as the oxygen input was increased. The anoxic benzoate uptake capability was unaffected by the increase in oxygen addition, but the anoxic 3-CB uptake capability increased, as did the level of benzoyl-CoA reductase in the cells. (C) 2004 Elsevier Ltd. All rights reserved

Differences in benzoate-degrading denitrifying cultures associated with changes in the residual terminal electron acceptor in a chemostat

A change in the nature of the residual terminal electron acceptor in an anoxic (denitrifying), carbon-limited, benzoate-degrading chemostat from nitrate to nitrite was taken as a sign of a change in the structure and/or function of the microbial community. The results from denaturing gradient gel electrophoresis showed that although the structures of the microbial communities in the chemostat were very similar under the two conditions, there were also differences. Biomass samples were collected from the chemostat during the nitrate residual and nitrite residual periods to see how they differed in terms of the stability and inducibility of their benzoate biodegradative capability (BBC). Biomass taken from the chemostat during the nitrate residual period almost completely lost its anoxic BBC within I h when exposed to aerobic conditions in a fed-batch reactor (FBR). In addition, following 16-h exposure to aerobic conditions, it did not recover that capability within 9 h after being returned to anoxic conditions. In contrast, biomass taken from the chemostat during the nitrite residual period retained at least 20% of its anoxic BBC after 9 h in the aerobic FBR and rapidly recovered that capability within 3 h upon being returned to anoxic conditions. Biomass taken from the chemostat during the nitrate residual period did not develop aerobic BBC within 9 h in an aerobic FBR; however, biomass taken from the chemostat during the nitrite residual period developed that ability within 6 h and then slowly lost it over a 6-h period in an anoxic FBR. Nitrite itself was not responsible for these different observations. Rather, they appeared to depend upon the nature of the bacteria in the communities

Effect of oxygen on the stability and inducibility of the biodegradative capability of benzoate

Anoxic zones in biological nitrogen removal systems are typically open to the atmosphere and receive oxygen from the atmosphere and the recirculation flow from the aerobic zone. This raises the question of how such oxygen input might influence the stability and inducibility of the enzyme systems involved in biodegradation of aromatic compounds. To investigate this, various amounts of oxygen were added to mixed culture denitrifying chemostats receiving benzoate at 667 mg/h as chemical oxygen demand (COD), and the stability and inducibility of the culture's benzoate biodegradative capability (BBC) were tested in aerobic and anoxic fed-batch reactors (FBRs). Cultures from chemostats receiving oxygen at 0, 33, 133, 266, and 466 mg O-2/h lost almost all of their anoxic BBC within one hour after being transferred to an aerobic FBR and the first three cultures did not recover it upon being returned to anoxic conditions, The last two cultures recovered their anoxic BBC between 9 and 16 h during the 16 h aerobic exposure period that preceded their return to anoxic conditions and continued to increase their anoxic BBC as they were retained under anoxic conditions. In contrast, the culture from a chemostat receiving oxygen at 67 mg O-2/h retained its anoxic BBC longer, recovered it within 3 h after its return to anoxic conditions, and increased it linearly thereafter. None of the cultures developed any aerobic BBC during the 16 h aerobic exposure period in FBRs. The results suggest that higher oxygen inputs into anoxic reactors helped the mixed microbial cultures recover and/or induced anoxic BBC more easily when they were exposed to alternating aerobic/anoxic environments. The exceptional behavior of the culture from the chemostat receiving oxygen at a rate of 67 mg O-2/h may have been caused by the presence of a protective mechanism against the toxic forms of oxygen