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PubMed2006

Community structures and activities of nitrifying and denitrifying bacteria in industrial wastewater-treating biofilms.

The bacterial community structure, in situ spatial distributions and activities of nitrifying and denitrifying bacteria in biofilms treating industrial wastewater were investigated by combination of the 16S rRNA gene clone analysis, fluorescence in situ hybridization (FISH) and microelectrodes. These results were compared with the nitrogen removal capacity of the industrial wastewater treatment plant (IWTP). Both nitrification and denitrification occurred in the primary denitrification (PD) tank and denitrification occurred in the secondary denitrification (SD) tank. In contrast, nitrification and denitrification rates were very low in the nitrification (N) tank. 16S rRNA gene clone sequence analysis revealed that the bacteria affiliated with Alphaproteobacteria, followed by Betaproteobacteria, were numerically important microbial groups in three tanks. The many clones affiliated with Alphaproteobacteria were closely related to the denitrifying bacteria (e.g., Hyphomicrobium spp., Rhodopseudomonas palustris, and Rhodobacter spp.). In addition, Methylophilus leisingeri affiliated with Betaproteobacteria, which favorably utilized methanol, was detected only in the SD-tank to which methanol was added. Nitrosomonas europaea and Nitrosomonas marina were detected as the ammonia-oxidizing bacteria affiliated with Betaproteobacteria throughout this plant, although the dominant species of them was different among three tanks. Nitrifying bacteria were mainly detected in the upper parts of the PD-biofilm whereas their populations were low in the upper parts of the N-biofilm. The presence of denitrifying bacteria affiliated with Hyphomicrobium spp. in SD- and N-biofilms was verified by FISH analysis. Microelectrode measurements showed that the nitrifying bacteria present in the N- and PD-biofilms were active and the bacteria present in the SD-biofilm could denitrify.

Bacteria↗

PubMed2003

Dual isotope and isotopomer ratios of N2O emitted from a temperate grassland soil after fertiliser application.

The N2O and N2 fluxes emitted from a temperate UK grassland soil after fertiliser application (equivalent to 25 and 75 kg N ha(-1)) were simultaneously measured, using a new automated soil incubation system, which replaces soil atmosphere (N2 dominated) with a He+O2 mixture. Dual isotope and isotopomer ratios of the emitted N2O were also determined. Total N2O and N2 fluxes were significantly lower (P<0.001) in the control (0 kg N) than in the 25 and 75 kg N treatments. The total N2O flux was significantly higher (P<0.001) in the 75 kg N than in the 25 kg N treatment. The general patterns of N2O and N2 fluxes were similar for both fertiliser treatments. The total gaseous N loss in the control treatment was nearly all N2, whereas in the fertiliser treatment more N2O than N2 was emitted from the soil. The ratio N2O/N2 fluxes as measured during the experiment suggested three phases in N2O production, in phase 1 nitrification>denitrification, in phase 2 denitrification>nitrification, and in phase 3 denitrification (and total denitrification)>>nitrification. Dual delta15N and delta18O isotope and isotopomer (delta15Nalpha and delta15Nbeta) value ratios of emitted N2O also pointed towards an increasing dominance of the production of N2O by denitrification and total denitrification. The site preference value from the soil-emitted N2O was lower than the troposphere value. This confirmed that the enhanced troposphere N2O site preference could result from back injection of N2O from the stratosphere. The measurements of N2O/N2 flux ratio and the isotopic content of emitted N2O pointed, independently, to similar temporal trends in N2O production processes after fertiliser application to grassland soil. This confirmed that both measurements are suitable diagnostic tools to study the N2O production process in soils.

Agriculture↗

PubMed2004

Distinguishing sources of N2O in European grasslands by stable isotope analysis.

Nitrifiers and denitrifiers are the main producers of the greenhouse gas nitrous oxide (N(2)O). Knowledge of the respective contributions of each of these microbial groups to N(2)O production is a prerequisite for the development of effective mitigation strategies for N(2)O. Often, the differentiation is made by the use of inhibitors. Measurements of the natural abundance of the stable isotopes of N and O in N(2)O have been suggested as an alternative for the often unreliable inhibition studies. Here, we tested the natural abundance incubation method developed by Tilsner et al.1 with soils from four European grasslands differing in long-term management practices. Emission rates of N(2)O and stable isotope natural abundance of N(2)O and mineral N were measured in four different soil incubations: a control with 60% water-filled pore space (WFPS), a treatment with 60% WFPS and added ammonium (NH(4) (+)) to support nitrifiers, a control with 80% WFPS and a treatment with 80% WFPS and added nitrate (NO(3) (-)) to support denitrifiers. Decreases in NH(4) (+) concentrations, linked with relative (15)N-enrichment of residual NH(4) (+) and production of (15)N-depleted NO(3) (-), showed that nitrification was the main process for mineral N conversions. The N(2)O production, however, was generally dominated by reduction processes, as indicated by the up to 20 times larger N(2)O production under conditions favouring denitrification than under conditions favouring nitrification. Interestingly, the N(2)O concentration in the incubation atmospheres often levelled off or even decreased, accompanied by increases in delta(15)N and delta(18)O values of N(2)O. This points to uptake and further reduction of N(2)O to N(2), even under conditions with small concentrations of N(2)O in the atmosphere. The measurements of the natural abundances of (15)N and (18)O proved to be a valuable integral part of the natural abundance incubation method. Without these measurements, nitrification would not have been identified as essential for mineral N conversions and N(2)O consumption could not have been detected.

Europe↗

PubMed2005

A novel dual-isotope labelling method for distinguishing between soil sources of N2O.

We present a novel 18O-15N-enrichment method for the distinction between nitrous oxide (N2O) from nitrification, nitrifier denitrification and denitrification based on a method with single- and double-15N-labelled ammonium nitrate. We added a new treatment with 18O-labelled water to quantify N2O from nitrifier denitrification. The theory behind this is that ammonia oxidisers use oxygen (O2) from soil air for the oxidation of ammonia (NH3), but use H2O for the oxidation of the resulting hydroxylamine (NH2OH) to nitrite (NO2-). Thus, N2O from nitrification would therefore be expected to reflect the 18O signature of soil O2, whereas the 18O signature of N2O from nitrifier denitrification would reflect that of both soil O2 and H2O. It was assumed that (a) there would be no preferential removal of 18O or 16O during nitrifier denitrification or denitrification, (b) the 18O signature of the applied 18O-labelled water would remain constant over the experimental period, and (c) any O exchange between H(2)18O and NO3- would be negligible under the chosen experimental conditions. These assumptions were tested and validated for a silt loam soil at 50% water-filled pore space (WFPS) following application of 400 mg N kg-1 dry soil. We compared the results of our new method with those of a conventional inhibition method using 0.02% v/v acetylene (C2H2) and 80% v/v O2 in helium. Both the 18O-15N-enrichment and inhibitor methods identified nitrifier denitrification to be a major source of N2O, accounting for 44 and 40%, respectively, of N2O production over 24 h. However, compared to our 18O-15N-method, the inhibitor method overestimated the contribution from nitrification at the expense of denitrification, probably due to incomplete inhibition of nitrifier denitrification and denitrification by large concentrations of O2 and a negative effect of C2H2 on denitrification. We consider our new 18O-15N-enrichment method to be more reliable than the use of inhibitors; it enables the distinction between more soil sources of N2O than was previously possible and has provided the first direct evidence of the significance of nitrifier denitrification as a source of N2O in fertilised arable soil.

Journal Article↗

PubMed1990

Bioenergetic examination of the heterotrophic nitrifier-denitrifier Thiosphaera pantotropha.

The heterotrophic nitrifying-denitrifying bacterium Thiosphaera pantotropha is remarkable as it nitrifies and denitrifies simultaneously. With respect to nitrogenous compounds, whether nitrification or denitrification results in energy conservation is of interest. Proton translocation studies were performed to determine if energy was conserved by the bacterium during heterotrophic nitrification and denitrification. Hydrazine (N2H5+) was employed as the heterotrophic nitrification substrate while nitrate, nitrite and nitrous oxide were used as denitrification substrates. Analysis of the data indicate that the bacterium does not conserve energy when hydrazine was the substrate. Conversely, energy was conserved when either nitrate, nitrite or nitrous oxide functioned as the oxidants during denitrification-dependent proton translocation experiments. Thiosphaera pantotropha thus is similar to other heterotrophic nitrifiers-denitrifiers in that it conserves energy while denitrifying but has not been observed to do so when heterotrophically nitrifying.

Gram-Negative Chemolithotrophic Bacteria↗

PubMed1994

Biological nitrogen removal from wastewater.

A review of the research on the kinetics of nitrification and denitrification is presented including an explanation of reaction engineering models. The results of laboratory scale experiments with high rate nitrification processes are discussed using kinetic results for oxygen limitation as well as for substrate limitation and inhibition. It can be demonstrated that reaction engineering models are helpful for a better understanding of the processes and for the design of reactors. Pilot scale investigations from the last 15 years show remarkable advances in the increase in nitrification efficiency and in the stabilization of the process. The time is ripe for nitrogen removal from industrial effluents in full scale processes!

Models, Theoretical↗

PubMed2001

Nitrate Regeneration Coupled to Degradation of Different Size Fractions of DON by the Picoplankton in the Elbe Estuary.

Bacterial conversion of high (HMW > 3,000 Da) and low (LMW < 3,000 Da) molecular weight DON (dissolved organic nitrogen) was studied along the freshwater section of the Elbe estuary during the summer of 1997. Indigenous populations of picoplankton were incubated in a flow-through chamber that allowed a constant exchange of sterile, filtered Elbe water as the culture medium for the microorganisms, which remained within the chamber. Nitrogen conversion was followed by changes in the concentrations of total and low molecular weight DON and dissolved inorganic nitrogen compounds, the uptake of O2, and bacterial growth. Along the Elbe estuary, total DON concentrations varied between 0.69 and 1.1 mg N L-1, of which between 64 and 79% was LMW-DON. Ammonium was a minor nutrient present in the Elbe at concentrations below 0.26 mg N L-1. During incubation in the laboratory between 27 and 64% of the LMW-DON was consumed at rates ranging from 24 to 51 µg N L-1 h-1. HMW-DON was used only when the degradable LMW-DON pool became exhausted and accounted for between 60 and 100% of the HMW-DON. This produced an increase in the DON consumption rate between 43 and 79 µg N L-1 h-1. Nitrification rates were independent of the external ammonium concentration until it decreased to below 1 µg N-NH4 L-1. Most of the N in the nitrification process originated as NH4 regenerated from DON. Between 75 and 100% of the LMW-DON and ammonium consumed was rapidly converted to nitrate. This amount decreased to between 65 and 85% when HMW-DON was consumed in addition to the LMW-DON. DON and ammonium consumption supported nitrification rates up to 71 µg N L-1 h-1. The amount of DOC (dissolved organic carbon) degraded was not equivalent to the C:N ratio of the total dissolved organic matter. Calculations based on oxygen consumption for respiration and ammonium regeneration revealed that the substrates used during the incubations contained C:N at ratios of about 3:1. These results suggest that the nitrogen-rich compounds had been removed from the dissolved organic matter and subsequently consumed by bacteria, while the carbon skeleton remained mostly unaffected by the degradation processes.

Journal Article↗

PubMed2002

Oxygen-limited autotrophic nitrification/denitrification by ammonia oxidisers enables upward motion towards more favourable conditions.

The hypothesis is formulated that in case of oxygen limitation in the sediment, nitrifiers switch from nitrification to oxygen-limited autotrophic nitrification-denitrification (OLAND) in order to survive and maintain activity. During OLAND, ammonium is oxidised using nitrite as e-acceptor to form dinitrogen gas. As an additional advantage they benefit from the gaseous N(2) formed as a means of transport. In this way, the nitrifiers can move out of the sediment and rise through the water column towards more favourable conditions. At the surface, the bacteria could take up oxygen, and recommence nitrification. In order to test this hypothesis, nitrifying sediment with an overlaying water column was simulated in lab-scale columns. Nitrogen transformations and material transport through the water column were followed after addition of different forms of nitrogen under oxygen-limited conditions. (15)N-labelling experiments showed a large contribution of OLAND to the observed nitrogen deficits. Nitrifier enumerations, fluorescent in situ hybridisation and 16S rRNA gene analysis revealed increased populations of ammonia oxidising nitrifiers in the upper water layers. The results presented support the proposed hypothesis of transport using OLAND. Nitrifying activity in the sediment immediately recovered almost completely from prolonged oxygen-limited incubation when oxygen concentrations were increased.

Anaerobiosis↗

PubMed2004

Biological nitrogen and organic matter removal from tannery wastewater in pilot plant operations in Ethiopia.

The objective of this study was to set-up a pilot plant and to evaluate its effectiveness for biological nitrogen and organic matter removal from tannery wastewater in Ethiopia. A pilot wastewater treatment plant consisting of a predenitrification-nitrification process was constructed and operated for 6 months. This was fed with a raw tannery wastewater obtained from the Modjo Tannery located 70 km south of the capital, Addis Ababa. Up to 98% total nitrogen and chemical oxygen demand, and 95% ammonium nitrogen removal efficiencies were achieved in the system. The average effluent ammonium nitrogen ranged from 8.4 mg l(-1) to 86.0 mg l(-1), whereas the average effluent for nitrate nitrogen ranged from 2.9 mg l(-1) to 4.4 mg l(-1). The average values of denitrification and nitrification rates determined by nitrate and ammonium uptake rates (NUR and AUR) were 8.0 mg NO3-N [g volatile suspended solids (VSS)](-1) h(-1) and 5.4 mg NH4-N (g VSS)(-1) h(-1), respectively, demonstrating that the treatment processes of the pilot plant were effective. Further studies of the effect of chromium III on AUR showed 50% inhibition at a concentration of 85 mg l(-1), indicating that this metal was not causing process inhibition during performance operations. Thus, the predenitrification-nitrification process was found to be efficient for simultaneous removal of nitrogen and organic substrates from tannery wastewaters.

Chromium↗

PubMed2004

Patterns of nitrogen accumulation and cycling in riparian floodplain ecosystems along the Green and Yampa rivers.

Patterns of nitrogen (N) accumulation and turnover in riparian systems in semi-arid regions are poorly understood, particularly in those ecosystems that lack substantial inputs from nitrogen fixing vegetation. We investigated sources and fluxes of N in chronosequences of riparian forests along the regulated Green River and the free-flowing Yampa River in semi-arid northwestern Colorado. Both rivers lack significant inputs from N-fixing vegetation. Total soil nitrogen increased through time along both rivers, at a rate of about 7.8 g N m(-2) year(-1) for years 10-70, and 2.7 g N m(-2)year(-1) from years 70-170. We found that the concentration of N in freshly deposited sediments could account for most of the soil N that accumulated in these floodplain soils. Available N (measured by ion exchange resin bags) increased with age along both rivers, more than doubling in 150 years. In contrast to the similar levels of total soil N along these rivers, N turnover rates, annual N mineralization, net nitrification rates, resin-N, and foliar N were all 2-4 times higher along the Green River than the Yampa River. N mineralization and net nitrification rates generally increased through time to steady or slightly declining rates along the Yampa River. Along the Green River, rates of mineralization and nitrification were highest in the youngest age class. The high levels of available N and N turnover in young sites are not characteristic of riparian chronosequences and could be related to changes in hydrology or plant community composition associated with the regulation of the Green River.

Desert Climate↗

PubMed2006

Functional and community-level soil microbial responses to zinc addition may depend on test system biocomplexity.

The effect of zinc on soil nitrification and composition of the microbial community in soil was investigated using a full factorial experiment with five zinc concentrations and four levels of biological complexity (microbes only, microbes and earthworms (Eisenia fetida), microbes and Italian ryegrass (Lolium multiflorum var. Macho), and microbes, ryegrass and earthworms). After 6 weeks of exposure, the activity of soil nitrifying bacteria was measured and the microbial community structure was characterized by phospholipid fatty acid (PLFA) analysis. Soil nitrification and several PLFA markers were significantly influenced by either zinc addition and/or the presence of earthworms or ryegrass, and one of the most pronounced changes was the increase of fungi and decrease of bacteria with increasing concentrations of zinc. Of particular interest, however, was the potential interaction between the presence of plants and/or earthworms and the effect of zinc, which the factorial study design allowed us to explore. Such an effect was observed in two cases: Earthworms reduced the positive effect of zinc on the fungal biomass (ANOVA, p=0.03), and the effect of earthworms on the soil nitrification activity depended on zinc concentration (ANOVA, p<0.05). The effect of earthworm presence was not very large, but it does show that multispecies tests might give information about metal toxicity or bioavailability that cannot be predicted from single-species tests.

Analysis of Variance↗

PubMed2005

Presence of Nitrosospira cluster 2 bacteria corresponds to N transformation rates in nine acid Scots pine forest soils.

The relation between environmental factors and the presence of ammonia-oxidising bacteria (AOB), and its consequences for the N transformation rates were investigated in nine Scots pine (Pinus sylvestris L.) forest soils. In general, the diversity in AOB appears to be strikingly low compared to other ecosystems. Nitrosospira cluster 2, as determined by temporal temperature gradient electrophoresis and sequencing, was the only sequence cluster detected in the five soils with high nitrification rates. In the four soils with low nitrification rates, AOB-like sequences could not be detected. Differences in nitrification rates between the forest soils correlated to soil C/N ratio (or total N) and atmospheric N deposition.

Ammonia↗

PubMed2005

Optimization of the aerobic biological treatment of thermophilically treated refractory wastewater.

A pilot scale conventional activated sludge was operated for over 600 days to study its effectiveness at further remediating the effluent of an existing industrial site's thermophilic biological treatment stage. During the course of the study, the activated sludge was able to further biodegrade the contaminants in the incoming industrial wastewater in terms of both BOD and nitrogen reductions at varying hydraulic and solids retention times, despite elevated concentrations of soluble copper being present. A limiting hydraulic retention time (HRT) for BOD removal of 1.5 days was observed as well as the loss of nitrification occurred at a solids retention time (SRT) of approximately 6 days. Biokinetic coefficients were determined with the maximum rate of substrate utilization per unit mass of microorganisms, k, of 0.14 mgVSS/(mgsBOD-d) and the Monod half velocity constant, K(s), of 9.4 mgsBOD/L. Simultaneous nitrification and denitrification (SND) of the nitrogenous compounds found in this wastewater was observed throughout the majority of the experimentation while the bulk DO in the system was greater than 1 mg/L. The activated sludge was estimated to contain soluble copper on the order of 1 mg/L throughout the course of operation with no apparent detriment to nitrification. Additionally, the activated sludge was able to biologically remove the main solvents found in the influent wastewater. The removals of trace levels of N-nitrosodimethylamine (NDMA) were also observed.

Biodegradation, Environmental↗

PubMed2006

Simultaneous removal of phenol, ammonium and thiocyanate from coke wastewater by aerobic biodegradation.

A laboratory-scale activated sludge plant composed of a 20 L volume aerobic reactor followed by a 12 L volume settling tank and operating at 35 degrees C was used to study the biodegradation of coke wastewater. The concentrations of ammonium nitrogen (NH(4)(+) -N), phenols, chemical oxygen demand (COD) and thiocyanate (SCN(-)) in the wastewater ranged between 504 and 2,340, 110 and 350, 807 and 3,275 and 185 and 370 mg/L, respectively. The study was undertaken with and without the addition of bicarbonate. The addition of this inorganic carbon source was necessary to favour nitrification, as the alkalinity of the wastewater was very low. Maximum removal efficiencies of 75%, 98% and 90% were obtained for COD, phenols and thyocianates, respectively, without the addition of bicarbonate. The concentration of ammonia increased in the effluent due to both the formation of NH(4)(+) as a result of SCN(-) biodegradation and to organic nitrogen oxidation. A maximum nitrification efficiency of 71% was achieved when bicarbonate was added, the removals of COD and phenols being almost similar to those obtained in the absence of nitrification. Batch experiments were performed to study the influence of pH and alkalinity on the biodegradation of phenols and thiocyanate.

Aerobiosis↗

PubMed2006

Improving the performance of sequencing batch reactor (SBR) by the addition of zeolite powder.

Two types of operation means "SBR reactor alone (control reactor)" and "adding zeolite powder into SBR reactor (test reactor)" were used to treat municipal wastewater. The test results revealed that zeolite powder addition could improve the activity of the activated sludge. It was investigated the specific oxygen utilization rate (SOUR) of the tested zeolite sludge were about double times that of the control activated sludge, and the nitrification rate and settling property of zeolite-activated sludge were both improved. Due to the combination of zeolite adsorption for NH(4)(+)-N and enhanced simultaneous nitrification and de-nitrification (SND), a higher nitrogen removal was observed in test reactor compared to the control reactor, and the addition of zeolite powder is helpful to inhabit sludge bulking. In addition, through long-term parallel shock load test, it was found that the zeolite powder addition could enhance the ability of activated sludge in resisting the shock load of organics and ammonium. Compared to the control activated sludge, zeolite powder added activated sludge could remove COD, NH(4)(+)-N, TN and TP significantly in a shorter cycle time. At the same operational time period, the test SBR could treat wastewater quantity 1.22 times that treated in control SBR.

Environmental Restoration and Remediation↗

PubMed2026

Detoxification-driven recovery of nitrogen removal under high linear alkylbenzene sulfonate stress by immobilized Pseudomonas sp. LM2.

Linear alkylbenzene sulfonate (LAS) is a widely used anionic surfactant that can inhibit microbial activity and destabilize biological wastewater treatment systems under high loading conditions. In this study, a sequencing batch reactor (SBR) was exposed to increasing LAS concentrations (0-100 mg/L) to define the collapse trajectory of activated sludge and evaluate recovery following bioaugmentation with immobilized Pseudomonas sp. LM2. The system remained stable at 20 mg/L LAS, deteriorated after prolonged exposure to 50 mg/L, and developed severe sludge disintegration with near-complete nitrification failure at 100 mg/L. In the LM reactor, LAS removal increased from 25.4% to 53.1% after bioaugmentation, together with improved nitrogen-removal performance. Blank carriers did not restore reactor performance. Genome annotation showed that LM2 encoded genes associated with LAS degradation and denitrification but lacked nitrification genes. Thus, the observed nitrification recovery was likely indirect and associated with lower LAS stress and recovery of indigenous nitrifiers. Quantitative PCR and amplicon analyses showed enrichment of Nitrospira and Thauera and a shift toward more deterministic community assembly after bioaugmentation. The carrier-only control showed taxonomic recovery but persistent functional inhibition under high residual LAS exposure. Overall, the results indicate that immobilized bioaugmentation can support partial recovery of nitrogen removal under severe LAS stress.

Bioaugmentation↗

PubMed2003

Characterization of nitrifying granules produced in an aerobic upflow fluidized bed reactor.

Since nitrification is the rate-determining step in the biological nitrogen removal from wastewater, many research studies have been conducted on the immobilization of nitrifying bacteria. In this research, granulation of nitrifying bacteria in an aerobic upflow fluidized bed (AUFB) reactor in a nitrification process for inorganic wastewater containing 500 g/m(3) of NH(4)(+)-N was investigated. It was observed that spherical, pseudocubic and elliptical granules with a diameter of 346 microm were produced at the bottom of the reactor after 300 days. Denaturing gradient gel electrophoresis analysis revealed that Nitrosomonas-like bacteria were the dominant ammonia-oxidizing species in the granules. Many colonies of Nitrosomonas-like bacteria were found in the outer part of the granules based on the spatial distribution analysis by fluorescence in situ hybridization. By stepwise reduction of the hydraulic retention time, the ammonia removal rate of the AUFB reactor containing these nitrifying granules finally reached 1.5 kg-N/m(3)/day. Results suggested that the use of granules realizes the retention of a large amount of nitrifying bacteria in the reactor, which guarantees a highly efficient nitrification.

Ammonia↗

PubMed2004

Micro-profiles of activated sludge floc determined using microelectrodes.

The microbial activity within activated sludge floc is a key factor in the performance of the activated sludge process. In this study, the microenvironment of activated sludge flocs from two wastewater treatment plants (Mill Creek Wastewater Treatment Plant and Muddy Creek Wastewater Treatment Plant, with aeration tank influent CODs of 60-120 and 15-35 mg/L, respectively) were studied by using microelectrodes. Due to microbial oxygen utilization, the aerobic region in the activated sludge floc was limited to the surface layer (0.1-0.2mm) of the sludge aggregate at the Mill Creek plant. The presence of an anoxic zone inside the sludge floc under aerobic conditions was confirmed in this study. When the dissolved oxygen (DO) in the bulk liquid was higher than 4.0mg/L, the anoxic zone inside the activated sludge floc disappeared, which is helpful for biodegradation. At the Muddy Creek plant, with its lower wastewater pollutant concentrations, the redox potential and DO inside the sludge aggregates were higher than those at the Mill Creek plant. The contaminant concentration in the bulk wastewater correlates with the oxygen utilization rate, which directly influences the oxygen penetration inside the activated sludge floc, and results in redox potential changes within the floc. The measured microprofiles revealed the continuous decrease of nitrate concentration inside the activated sludge floc, even though significant nitrification was observed in the bulk wastewater. The oxygen consumption and nitrification rate analyses reveal that the increase of ammonia flux under aerobic conditions correlates with nitrification. Due to the metabolic mechanisms of the microorganisms in activated sludge floc, which varies from one treatment plant to another, the oxygen flux inside the sludge floc changes accordingly.

Ammonia↗