Reaction Kinetics of Immobilized-Cell Denitrification. I: Background and Model Development

A simplified, quasi-steady-state model has been formulated for groundwater denitrification using immobilized cells. The model takes into account the diffusion-limited penetration of nitrate and nitrite into the immobilized-cell biocatalyst particle, uniform intraparticle cell density, equivalent slab geometry for the spherical particle, zero-order intrinsic reaction kinetics of the immobilized cells, sequential reduction of nitrate to nitrite and nitrite to dinitrogen within the particle, no inhibition of the sequential reactions by either nitrate or nitrite, and ideal plug flow conditions in the bioreactor. As a result, the reaction in the bulk fluid of the plug-flow bioreactor can be described by a half-order kinetics, and the process is characterized by the half-order reaction rate constants for nitrate and nitrite reduction. These constants incorporate the intrinsic reaction kinetics of the immobilized cells, the size and packing density of biocatalyst particles in the reactor, and the effective diffusion coefficients of nitrate and nitrite in the particle matrix.     

Modeling Enhanced In Situ Denitrification in Groundwater

A two-dimensional numerical solute transport model was developed for simulating an enhanced in situ denitrification experiment performed in a nitrate-contaminated aquifer on Cape Cod, Massachusetts. In this experiment, formate (HCOO?) was injected for a period of 26 days into the carbon-limited aquifer to stimulate denitrification. Calibration of the vertical-profile site model was demonstrated through error analysis and comparison with formate, nitrate, and nitrite concentration data monitored along a transect of three multilevel groundwater sampling wells for 75 days after initial injection. Formate utilization rates were approximately 142 and 38 μM/day for nitrate and nitrite reduction, respectively. Nitrate and nitrite utilization rates were approximately 29 and 8 μM/day, respectively. Nitrate utilization rates under enhanced conditions were 1 order of magnitude greater than previously reported naturally occurring rates. The nitrite production rate was approximately 29 μM/day. Persistence of nitrite was attributed to a combination of factors, including electron donor (formate) limitation late in the experiment, preferential utilization of nitrate as an electron acceptor, and greater nitrite production relative to nitrite utilization.     

Initial Alkalinity Requirement and Effect of Alkalinity Sources in Sulfur-Based Autotrophic Denitrification Barrier System

The present study describes the effects of initial alkalinity and various solid alkalinity sources such as calcite, dolomite, and oyster shell on nitrate removal in a sulfur-oxidizing autotrophic denitrification process. The results showed that denitrification rate increased as the initial alkalinity present in the system increased. Denitrification rates determined by a half-order kinetic model were 0.269, 0.976, 2.631, and 3.110?mg?NO3–N1/2/L1/2?day corresponding to the initial alkalinity of 300, 600, 1,200, and 1,800?mg?CaCO3/L, respectively. This amount of consumed alkalinity closely matched the theoretical alkalinity requirement. However, when 300?mg?CaCO3/L of alkalinity was initially present the sulfur-based denitrification was greatly inhibited. The data indicate that approximately two times initial alkalinity of theoretically required alkalinity is needed for a desirable sulfur-based denitrification reaction. The initial alkalinity dissolution rates were 88, 38, and 14?mg?CaCO3/L?day from 5?g of oyster shell, calcite, and dolomite, respectively. Accordingly, only 1.6 and 5% of initial nitrate remained in 7?days for oyster shell and calcite, respectively, but about 15% was still detected when dolomite was used.     

Acetate Limitation and Nitrite Accumulation during Denitrification

Nitrite accumulated in denitrifying activated sludge mixed liquor when the carbon and electron source, acetate, was limited. If acetate was added to obtain a carbon-to-nitrogen (C:N) ratio in the range of 2:1 to 3:1, nitrate was completely consumed at the same rate with no nitrite accumulation, indicating that nitrate concentration controlled the respiration rate as long as sufficient substrate was present. However, when acetate was reduced to a C:N ratio of 1:1, while nitrate continued to be consumed, >50% of the initial nitrate-nitrogen accumulated as nitrite and 29% persisted as nitrite throughout an endogenous denitrification period of 8–9 h. While nitrite accumulated during acetate-limited denitrification, the specific nitrate reduction rate increased significantly compared with the rate when excess acetate was provided as follows: 0.034 mg-NO3-N∕mg-mixed liquid volatile suspended solids∕h versus 0.023 mg-NO3-N∕mg-mixed liquid volatile suspended solids∕h, respectively. This may be explained by nitrate respiration out-competing nitrite respiration for limited acetate electrons. Complete restoration of balanced denitrification and elimination of nitrite accumulation during denitrification required several weeks after the C:N ratio was increased back to 2:1.     

Development of a Response Surface for Prediction of Nitrate Removal in Sulfur–Limestone Autotrophic Denitrification Fixed-Bed Reactors

Sulfur–limestone autotrophic denitrification (SLAD) processes are very efficient for treatment of ground or surface water contaminated with nitrate. However, detailed information is not available on the interaction among some major variables on the design and performance of the SLAD process. In this study, the response surface method was used by designing a rotatable central composite test scheme with 12 SLAD column tests. A polynomial linear regression model was set up to quantitatively describe the relationship of the effluent and influent nitrate–nitrogen concentration and hydraulic retention time (HRT) in the SLAD column reactors. This model may be used for estimating the effluent nitrate–nitrogen concentration when the influent nitrate–nitrogen concentration ranges between 20 and 110?mg/L and the HRT ranges between 2 and 9?h. Based on our model and the requirement for nitrite control, we recommend that the HRT of the SLAD column reactor be kept ≥ 6?h and the nitrate loading rate less than 200 g NO3?–N/day?m3 media to achieve high nitrate removal efficiency (>99%) and prevent nitrite accumulation from being >1?mg/L NO2?–N.     

Nitrite formation and nitrosation potential of heterotrophic biological denitrification of drinking water in laboratory conditions

In a laboratory construction for heterotrophic biological denitrification of drinking water treatment, the formation of nitrite, the potency of nitrosation and the genotoxic activity were tested. Parameter as nitrate concentration, the water flow rate in the system, nitrite and morpholine addition and the pH-value were checked. For testing the potency of nitrosation and formation of nitrite in the reactor we took morpholine, a fast nitrosing amine. The results show for the break down rate of nitrate, there is no influence of the initial nitrate concentration (80-195 mg/L), the nitrite addition (5-20 mg/L) and the water flow rate (45-100 min) in the system. pH-values below 5.5 showed a little break down rate of nitrate. There was no correlation between the starting point of nitrate concentration and the formation of nitrite, although there was a positive correlation between the length of stay and the formation of nitrite. Nitrite concentrations of 5 mg/L with morpholine concentrations of 5 and 10 mg/L didn't show detectable formation of nitrosomorpholine. The analyses of different watertests in the construction didn't show significant results for DNA damage by the sister-chromatid exchange (SCE). The results of the Salmonella microsome assay (tester strain TA1535) didn't show any mutagenic effects relating to the potency of nitrosation. According to our experiments the potency of generalising nitrosamides or nitrosamines by drinking water denitrification seems to be low. There is no final assessment of detriment to health by denitrifying drinking water.     

Kinetic Modeling of Inhibition of Ammonia Oxidation by Nitrite under Low Dissolved Oxygen Conditions

In recent years the application of partial nitrification techniques has been denoted as very promising. These methods are based on the oxidation of ammonia to nitrite and the inhibition of the nitratation using different strategies. In most cases, this inhibition causes an increase in the concentration of nitrite. However, the effect of high nitrite concentrations under low dissolved oxygen (DO) conditions on the nitrification process is not well understood. In this paper, the effect of ammonia, nitrite, and nitrate concentrations on the nitrification process under low dissolved oxygen concentrations were studied using respirometric techniques. Results showed that the specific oxygen uptake rate (SOUR) followed a Monod-type equation with respect to the DO concentration. The coefficient SOURm was constant with respect to the ammonia, nitrite, and nitrate within the tested concentrations; in addition, KO was constant with respect to ammonia and nitrate but it increased linearly with the nitrite concentration, suggesting that nitrite was a competitive inhibitor of the SOUR. The inhibitory effect of nitrite was reverted by washing, in accordance with a competition model. From the data obtained using the open respirometer, the ratio between the oxygen consumption (OC) corresponding to each pulse of ammonia at different nitrite concentrations and the OC in the absence of nitrite (OCO) was calculated. The experimental ratio OC/OCO was almost constant with respect to the nitrite concentration and it was close to the literature value. Finally, simulation results agree with the experimental data confirming that the proposed competition model represented adequately the inhibitory effect of nitrite on the respiration rate of ammonia-oxidizing bacteria.     

钢铁企业的焦化废水短程生物脱氮的研究

铁碳电池预处理－厌氧－好氧－缺氧多级SBR法处理焦化废水取得了良好的COD、氨氮脱除效果。在SBR反应器中创造条件，既保证了亚硝化反应的顺利进行，同时又保证了生成的亚硝酸盐与NH4^＋及有机物发生反应实现共同降解，亚硝酸盐与NH4^ 的反应弥补了反亚硝化过程中碳源的不足。系统运行稳定，氨氮浓度大于600mg/L时，却除率大于95％。该方法成本低，处理效果较好。     

Denitrification by actinomycetes and purification of dissimilatory nitrite reductase and azurin from Streptomyces thioluteus

Many actinomycete strains are able to convert nitrate or nitrite to nitrous oxide (N2O). As a representative of actinomycete denitrification systems, the system of Streptomyces thioluteus was investigated in detail. S. thioluteus attained distinct cell growth upon anaerobic incubation with nitrate or nitrite with concomitant and stoichiometric conversion of nitrate or nitrite to N2O, suggesting that the denitrification acts as anaerobic respiration. Furthermore, a copper-containing, dissimilatory nitrite reductase (CuNir) and its physiological electron donor, azurin, were isolated. This is the first report to show that denitrification generally occurs among actinomycetes.     

Use of Sequencing Batch Reactor for Biological Denitrification of High Nitrate-Containing Water

The efficiency of a sequencing batch reactor in denitrification of drinking water with relatively high nitrate concentrations (40–250 mg∕L as N) was evaluated. Ethanol at a COD∕N of 2.00 was found sufficient to reduce nitrate concentrations to acceptable levels (<10 mg∕L as N). Within the first 6 min of reaction, nitrite accumulation in the range of 0.03–3.5 mg∕L as N was observed increasing with the increase of initial nitrate concentrations. In the first hour, nitrate removal was significantly high in the range of 85.7–91.5%. Anoxic reaction times of 3, 5, and 7 h were required for nitrate concentrations of 40–160, 200, and 250 mg∕L (as N) to achieve acceptable levels of nitrate and nitrite. Alkalinity of the denitrified water increased on average by 3.53 mg as CaCO3 for each milligram of nitrate reduced and pH increased from 7.3 to the range of 8 to 9. Idle times between the operation cycles, in the range of 1–14 h, had an insignificant effect on denitrification. Residual COD concentrations in the range of 5–15 mg∕L and sulfide concentrations (at initial nitrate concentrations ≥120 mg∕L as N) in the range of 0.2–0.4 mg∕L were recorded in the finished water. Elevated concentrations of COD in general are not advisable in drinking water, and specifically in this case, it could result in toxic sulfide formation in the treated water. There is a need to further study the optimization of the use of ethanol and polishing of the treated water. A sequencing batch reactor has the potential of being used as an alternative configuration for biological denitrification of drinking water.     

Zero-Valent Iron-Assisted Autotrophic Denitrification

Porous reactive?barriers containing metallic iron and hydrogenotrophic denitrifying microorganisms may potentially be suitable for in-situ remediation of nitrate-contaminated groundwater resources. The main objective of the research described here was to determine the type and concentration of metallic iron to be used in such reactive?barriers so that ammonia formation through metallic iron-assisted abiotic nitrate reduction was minimized, while a reasonable rate of biological denitrification, sustained by hydrogen produced through metallic iron corrosion, was maintained. Initial experiments included the demonstration of autotrophic denitrification supported by externally supplied hydrogen, either from a gas cylinder or generated through anaerobic corrosion of metallic iron. Next, the effect of iron type on abiotic nitrate reduction was studied, and among those types of iron tested, steel wool, with its relatively low surface-area-to-weight ratio, was identified as the material that exhibited the least propensity to abiotically reduce nitrate. Further, long-term experiments were carried out in batch reactors to determine the effect of steel?wool surface area on the extent of denitrification and ammonia production. Finally, experiments carried out in up-flow column reactors containing sand and varying quantities of steel wool demonstrated biological denitrification occurring in such systems. Based on the results of the final set of experiments, it appeared that to minimize ammonia production, the steel-wool concentration up-flow columns must be even below the lowest value—0.5 g steel wool added to 125?cm3 of sand—used during this study. To counter any detrimental effect of lowered steel wool concentration on the extent of hydrogenotrophic denitrification, increase of the retention time in the columns to values higher than 13 days (the maximum value investigated in this study) may be necessary.     

Construction and calibration of a low-cost bandage pressure monitor

Pregnant sheep with a microdialysis probe implanted in the fetal cerebral cortex were used to determine if nitrate and nitrite anions (nitrate/nitrite) could be quantitated in the microdialysate as an indirect index of in vivo nitric oxide formation. Pregnant ewes (term, about 147 days) were surgically instrumented at gestational day (GD) 90 (n = 3; preterm) and GD 121 (n = 3; nearterm). Three days later, following an overnight probe equilibration period, five dialysate samples were collected continuously on ice at 1-h intervals (infusion rate of 1 (microl/min). The nitrate/nitrite concentration was determined by reducing a 10-microl aliquot of each dialysate fraction with hot acidic vanadium followed by chemiluminescence quantitation of the nitric oxide product. The lower limit of quantitative sensitivity of the method is 25 picomoles. Nitrate/nitrite concentration was 16.6+/-7.3 microM for the preterm fetus and 19.7+/-1.9 microM for the nearterm fetus. The data demonstrate that nitrate/nitrite, as an index of in vivo nitric oxide formation, can be quantitated in microdialysate samples collected from the intact fetal sheep cerebral cortex.     

The kinetics of leaching galena with ferric nitrate

The kinetics of leaching galena with ferric nitrate as oxidant has been studied. Experimental results indicate that the rate of galena dissolution is controlled by surface chemical reaction. Rate is proportional to the square root of the concentration of ferric ion. The addition of more than one mole/liter sodium nitrate decreases reaction rate. With nitrate additions below this concentration, rate either remains constant or is slightly enhanced. An activation energy of 47 kJ/mol was measured, and rate is proportional to the inverse of the initial size of galena particles. These results are explained in terms of mixed electrochemical control. The anodic reaction involves the oxidation of galena to lead ion and elemental sulfur, and the cathodic reaction involves the reduction of ferric ion to ferrous ion.     

Aerobic Methane Oxidation Coupled to Denitrification: Kinetics and Effect of Oxygen Supply

Aerobic methane oxidation coupled to denitrification (AME-D) is a process in which aerobic methanotrophs oxidize methane and release organic compounds that are used by coexisting denitrifiers as electron donors for denitrification. This process is potentially promising for denitrification of wastewater or landfill leachate poor in organic carbon using methane produced onsite as external electron donor. We studied the kinetics of an aerobic methane-oxidizing denitrifying culture and investigated the effect of dissolved oxygen (DO) concentration and air supply rate on AME-D using a batch reactor and a semicontinuous reactor setup. At methane concentrations of 18–33% in air and air flow rates of 15–35?mL?air?L?1?liquid?min?1, the DO concentration was less than 0.01?mg?L?1 and the nitrate removal reached a maximum value of 56.7?mg?NO3–N?g?1?VSS?d?1 with 79% being attributed to denitrification. When the air supply rate was increased to 70?mL?air?L?1?liquid?min?1 resulting in a drop in methane content to 10%, the DO concentration in the bioreactor rose to about 0.8–1.0?mg?L?1 and the total nitrate removal dropped to about 10?mg?NO3–N?g?1?VSS?d?1 with none of it being attributed to denitrification.     

Anaerobic benzene biodegradation linked to nitrate reduction

Benzene oxidation to carbon dioxide linked to nitrate reduction was observed in enrichment cultures developed from soil and groundwater microcosms. Benzene biodegradation occurred concurrently with nitrate reduction at a constant ratio of 10 mol of nitrate consumed per mol of benzene degraded. Benzene biodegradation linked to nitrate reduction was associated with cell growth; however, the yield, 8.8 g (dry weight) of cells per mol of benzene, was less than 15% of the predicted yield for benzene biodegradation linked to nitrate reduction. In experiments performed with [14C]benzene, approximately 92 to 95% of the label was recovered in 14CO2, while the remaining 5 to 8% was incorporated into the nonvolatile fraction (presumably biomass), which is consistent with the low measured yield. In benzene-degrading cultures, nitrite accumulated stoichiometrically as nitrate was reduced and then was slowly reduced to nitrogen gas. When nitrate was depleted and only nitrite remained, the rate of benzene degradation decreased to almost zero. Based on electron balances, benzene biodegradation appears to be coupled more tightly to nitrate reduction to nitrite than to further reduction of nitrite to nitrogen gas.     

Treatment of High Nitrate-Containing Wastewaters by Sequential Heterotrophic and Autotrophic Denitrification

The treatment performance of sequential heterotrophic and autotrophic denitrification process was evaluated using synthetic wastewaters containing high nitrate concentrations. The effluents from two sequentially connected reactors, for heterotrophic denitrification and sulfur-based autotrophic denitrification, were analyzed for more than 200 days. Experimental results indicated that higher than 95% of the nitrate removal could be achieved with volumetric nitrate loading rates of 2.16, 3.24, and 4.32 kg/m3?d. The maximum denitrification rates, with 1,000 mgN/l influent nitrate concentrations, for the heterotrophic and sulfur-packed autotrophic reactors, were found to be 2.47 and 3.61 kg/m3?d, respectively. A sequential heterotrophic and autotrophic denitrification process is considered a good alternative for the sole autotrophic denitrification process, providing excellent nitrate removal, especially for nitrate-rich wastewaters with very low organic contents.     

Effect of Metallic Iron Concentration on End-Product Distribution during Metallic Iron-Assisted Autotrophic Denitrification

The main objective of this study was to determine the optimum composition of a reactive porous medium containing sand and metallic iron, to be used for Fe(0)-assisted hydrogenotrophic denitrification. This determination is important to ensure that the end-product distribution after such treatment is acceptable, i.e., ammonia formation due to abiotic nitrate reduction by metallic iron in such media is minimized, while a reasonable rate of biological denitrification is maintained. Based on a previous study it was established that steel wool, with its relatively low specific surface area, exhibited the least propensity to abiotically reduce nitrate. It was also established that to achieve acceptable end-product distribution, the steel wool concentration in the reactive porous media has to be lowered even below the lowest value, i.e., 4.0?g steel wool/m3 of sand, used during that study. It was further hypothesized that to counter any detrimental effect of lower steel wool concentration on biological nitrate removal rate, increase of the retention time in porous media to values higher than 13 days, the maximum value investigated in that study, may be necessary. In the present study, experiments were conducted in batch reactors containing denitrifying microorganisms and various concentrations of steel wool and in semibatch reactors containing sand seeded with denitrifying microorganisms and various concentrations of steel wool. Based on the results of the semibatch experiments, it appears that to achieve acceptable end-product distribution, the steel wool concentration in the reactive porous media has to be maintained around 2.0?g steel wool/m3 sand and the corresponding retention time in the reactive media must be around 26 days.     

The kinetics of dissolution of sphalerite in ferric chloride solution

The dissolution of sphalerite in acidic ferric chloride solution was investigated in the temperature range 320 to 360 K. Both sized particles from three sources and polished flat surfaces were used as samples. The effect of stirring rate, temperature, ferric and ferrous ion concentration, purity, and particle size on the dissolution rate were determined. During the initial stages of the process chemical reaction at the mineral surface is rate controlling while during the later stages diffusion through the product sulfur layer is rate controlling. Overall the process follows the mixed-control model embodying both chemical reaction and diffusion. The activation energy for the dissolution of sphalerite particles was found to be 46.9 kJ/mol.     

Leaching kinetics of copper from natural chalcocite in alkaline Na4EDTA solutions

The leaching behavior of copper from natural chalcocite (Cu₂S) particles in alkaline Na₄EDTA solutions containing oxygen was examined at atmospheric pressure. The EDTA leaching process took place with consecutive reactions, where the solid product of the first reaction, covellite (CuS), became the reactant for the second. The copper leached into the alkaline solutions was immediately consumed by the chelation of copper (II) with EDTA, and the mineral sulfur was completely oxidized to sulfate ion. The experimental data for the leaching rate of copper were analyzed with a familiar shrinking-particle model for reaction control. The conversion rate of chalcocite to covellite was found to be about 10 times as high as the dissolution rate of covellite. The time required for complete dissolution of covellite was directly proportional to the initial particle size and was inversely proportional to the square root of the product of the hydroxide ion concentration and the oxygen partial pressure, but it was independent of the Na4EDTA concentration in the presence of excess Na₄EDTA. The observed effects of the relevant operating variables on the dissolution rate were consistent with a kinetic model for electrochemical reaction control. The kinetic model was developed by applying the Butler-Volmer equation to the electrochemical process, in which the anodic reaction involves the oxidation of covellite to copper (II) ion and sulfate ion and the cathodic reaction involves the reduction of oxygen in alkaline solution. The rate equation allowed us to predict the time required for the complete leaching of copper from chalcocite in the alkaline Na4EDTA solutions.