Application of the activated sludge model for nitrogen to elevated nitrogen conditions.

The Activated Sludge Model for Nitrogen (ASMN) was evaluated by conducting simulations under both steady-state and dynamic conditions using a wastewater containing high concentrations of chemical oxygen demand (COD) and nitrogen, and an inhibitor of ammonia-oxidizing bacteria. The adopted wastewater characteristics were based on data from several industrial wastewater treatment facilities. The simulations were performed at a variety of temperatures, solids retention times, dissolved oxygen concentrations, pH values, and salt concentrations. The nitrification operating window was defined, and denitrification performance was characterized. The pH and temperature were found to be the most important variables affecting nitrification performance under upset or startup conditions, with lower pH values allowing better performance at higher temperatures for the high-nitrogen wastewater used in the simulations. Emissions of nitric oxide and nitrous oxide were higher than generally thought to occur and were directly linked to depletion of the electron donor in the anoxic reactor. The findings concerning pH, temperature, and gaseous emissions were all consistent with the known growth characteristics of nitrifying and denitrifying bacteria. Parameter and process variable sensitivity studies were performed, and guidelines for improved biological nitrogen removal were developed.

An updated process model for carbon oxidation, nitrification, and denitrification.

The currently available comprehensive activated sludge models, ASM#1 (Grady et al., 1986) and its successor ASM#3 (Gujer et al., 1999), do not adequately describe nitrification and denitrification, with respect to ammonia oxidation inhibition, nitrite accumulation, or emissions of nitric oxide and nitrous oxide. A new comprehensive activated sludge process model, the Activated Sludge Model for Nitrogen (ASMN), is presented. The ASMN incorporates two nitrifying populations-ammonia-oxidizing bacteria and nitrite-oxidizing bacteria-using free ammonia and free nitrous acid, respectively, as their true substrates. The ASMN incorporates four-step denitrification (sequential reduction of nitrate to nitrogen gas via nitrite, nitric oxide, and nitrous oxide) using individual, reaction-specific parameters. Simulation results for ammonia, nitrate, soluble substrate, and biomass concentrations determined by using ASMN for three activated sludge process configurations under steady-state and dynamic municipal-type influent conditions are shown to be comparable with ASM#1 results.

The effect of cyclic aerobic-anoxic conditions on biodegradation of benzoate.

The response of a mixed microbial culture to cyclic aerobic and anoxic (denitrifying) conditions was studied in a chemostat with a 48-hour hydraulic residence time receiving a feed containing benzoate and pyruvate. When the cyclic conditions were 3-hour aerobic and 9-hour anoxic, the bacteria-degraded benzoate aerobically via the catechol 2,3-dioxygenase (C23DO) pathway. The quantity of C23DO remained constant throughout the anoxic period but decreased during the initial portion of the aerobic period before returning to the level present in the anoxic period. Anoxic biodegradation of benzoate was via benzoyl-CoA reductase, which remained constant regardless of the redox condition. The aerobic benzoate uptake capability (AeBUC) of the culture increased during the aerobic period but decreased during the anoxic period. The anoxic benzoate uptake capability (AnBUC) exhibited the opposite response. When the cycle was 6-hour aerobic and 6-hour anoxic, aerobic biodegradation of benzoate proceeded via the protocatechuate 4,5-dioxygenase (P45DO) pathway. The P45DO activity decreased early in the aerobic period, but then increased to the level present during the anoxic period. The level of benzoyl-CoA reductase was constant throughout the cycle. Furthermore, AeBUC and AnBUC responded in much the same way as in the 3/9-hour chemostat. During a 9-hour aerobic and 3-hour anoxic cycle, the culture synthesized both P45DO and C23DO, with the former having significantly higher activity. Unlike the other two cycles, AeBUC changed little during the aerobic period, although AnBUC decreased. The culture was well-adapted to the cyclic conditions as evidenced by the lack of accumulation of either substrate during any cycle tested. This suggests that cyclic aerobic-anoxic processes can be used in industrial wastewater-treatment facilities receiving significant quantities of simple aromatic compounds like benzoate. However, the results showed that the kinetics of benzoate degradation were different under aerobic and anoxic conditions, a situation that must be considered when modeling cyclic bioreactors receiving aromatic compounds.

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 6.2%, 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 <0.050 mg/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.

Relative efficacy of intrinsic and extant parameters for modeling biodegradation of synthetic organic compounds in activated sludge: dynamic systems.

The utility of intrinsic and extant kinetic parameters for simulating the dynamic behavior of a biotreatment system coupled with a distributed, unstructured, balanced microbial growth model were evaluated against the observed response of test reactors to transient loads of synthetic organic compounds (SOCs). Biomass from a completely mixed activated-sludge (CMAS) system was tested in fed-batch reactors, while a sequencing batch reactor (SBR) was tested by measuring SOC concentrations during the fill and react period. Both the CMAS system and the SBR were acclimated to a feed containing biogenic substrates and several SOCs, and the transient loading tests were conducted with biogenic substrates along with one or more SOCs. Extant parameters more closely reflect the steady-state degradative capacity of activated-sludge biomass than intrinsic parameters and, hence, were expected to be better predictors of system performance. However, neither extant nor intrinsic parameters accurately predicted system response and neither parameter set was consistently superior to the other. Factors that may have contributed to the inability of the model to predict system response were identified and discussed. These factors included the role of abiotic processes in SOC removal, disparity in the bases used to evaluate parameter estimates (substrate mineralization) and reactor performance (substrate disappearance), inhibitory substrate interactions under the severe loading conditions of the SBR, changes in the physiological state of the biomass during the transient loading tests, and the presumed correlation between the competent biomass concentration and the influent SOC concentration.

Effects of oxygen on anoxic biodegradation of benzoate during continuous culture.

In this study, various amounts of oxygen were added to denitrifying chemostats receiving benzoate to mimic the input of oxygen to anoxic zones of biological nutrient removal systems. The effect of oxygen on the biodegradative capability of the mixed-microbial culture for benzoate was investigated. The anoxic benzoate biodegradative capability of the culture was not significantly changed as the mass flowrate of oxygen was increased to 40% of the input benzoate chemical oxygen demand (COD) mass flowrate, but was decreased approximately 70% when the mass flowrate of oxygen was increased to 70% of the input benzoate COD mass flowrate. The decrease in the anoxic benzoate biodegradative capability was due primarily to the loss of the denitrifying enzymes (measured by the anoxic pyruvate-degrading ability) and not to the loss of the key anoxic catabolic enzyme (benzoyl-coenzyme A reductase). The proportional increase in the concentration of nitrate as the residual terminal electron acceptor and the lack of synthesis of aerobic ring-cleavage enzymes as the oxygen input to the chemostat was increased suggest that the mixed microbial culture preferred oxygen to nitrate as the terminal electron acceptor, but degraded benzoate using the anoxic metabolic pathway. The concentration of the mixed microbial culture increased as the oxygen input to the chemostat was increased, suggesting that the oxygen was used by cytochrome cbb3 rather than quinol oxidase because the energetic yield of cytochrome cbb3 is higher than that of quinol oxidase or the nitrogen oxide reductases.

Relative efficacy of intrinsic and extant parameters for modeling biodegradation of synthetic organic compounds in activated sludge: steady-state systems.

The performance of intrinsic and extant kinetic parameters as predictors of synthetic organic compound (SOC) concentration in biotreatment systems operated at steady state was evaluated. Two laboratory-scale, completely mixed activated-sludge systems were sampled on a routine basis, and SOC concentrations were quantified using gas chromatography with flame-ionization detection coupled with solid-phase microextraction for analyte concentration. At the same time, intrinsic and extant respirometric tests were performed periodically, and the kinetic parameter estimates obtained were used to predict effluent SOC concentrations for comparison with the measured values. Out of 28 comparisons that could be made between intrinsic and extant predictions, extant parameters were superior in 27 cases and intrinsic parameters were comparable, at best, to extant parameters in the remaining case. Given their superior performance and relative ease of measurement, extant parameters are preferable for use in design and operational decision-making.

Factors influencing deterioration of denitrification by oxygen entering an anoxic reactor through the surface.

The purpose of the paper is to examine the factors that influence the deterioration of denitrification in open anoxic reactors. For this investigation an ASM 1-based simulation model was developed and successfully applied to fit data from batch experiments carried out in lab-scale reactor vessels (uncovered and covered) using both clarified domestic wastewater and synthetic wastewater. Applying the verified model, simulation studies were performed to investigate the effects of available denitrifiable substrate, biomass concentration, oxygen transfer rate, and temperature on deterioration of denitrification in open anoxic reactors. It has been shown that oxygen entering an anoxic reactor through the surface may not just affect denitrification metabolically, but also kinetically, due to increased dissolved oxygen (DO) concentration exerting an inhibitory effect on the denitrification rate. When the exogenous substrate concentration in the reactor vessel is high enough for a high consumption rate, the DO concentration is kept low. The higher the biomass concentration, and thereby the consumption rate of endogenous substrate, the lower the DO concentration during the low-rate denitrification phase. At low substrate removal rates, decreasing temperature will cause the DO concentration in anoxic vessels to increase. The results suggest that assuring removal of available exogenous carbon source at high rate by staging of open anoxic bioreactors may significantly improve denitrification efficiency.

Catabolic enzyme levels in bacteria grown on binary and ternary substrate mixtures in continuous culture.

Bacteria grow on multicomponent substrates in most natural and engineered environments. To advance our ability to model bacterial growth on such substrates, axenic cultures were grown in chemostats at a low specific growth rate and a constant total energy flux on binary and ternary substrate mixtures and were assayed for key catabolic enzymes for each substrate. The substrates were benzoate, salicylate, and glucose, and the enzymes were catechol 1,2-dioxygenase, gentisate 1,2-dioxygenase, and glucose-6-phosphate dehydrogenase, respectively. The binary mixtures were salicylate with benzoate and salicylate with glucose. Measurements were also made of oxygen uptake rate by whole cells in response to each substrate. The effects of the substrate mixture on the oxygen uptake rate paralleled the effects on the measured enzymes. Catechol 1,2-dioxygenase exhibited a threshold response before synthesis occurred. Below the threshold flux of benzoate through the chemostat, either basal enzyme levels or nonspecific enzymes kept reactor concentrations too low for enzyme synthesis. Above the threshold, enzyme levels were linearly related to the fraction of the total energy flux through the chemostat due to benzoate. Gentisate 1,2-dioxygenase exhibited a linear response to the salicylate flux when mixed with benzoate, but a threshold response when mixed with glucose. Glucose-6-phosphate dehydrogenase activity increased in direct proportion to the glucose flux through the chemostat over the entire range studied. The results from two ternary mixtures were consistent with those from the binary mixtures.