Partial nitrification under limited dissolved oxygen conditions

Partial nitrification to nitrite is technically feasible and economically favourable, especially when wastewaters contained high ammonium concentrations or low C/N ratios. Partial nitrification can be obtained by selectively inhibiting nitrite-oxidizing bacteria (NOB) through appropriate regulation of the pH, temperature and dissolved oxygen (DO) concentrations. The effect of pH, DO levels and temperature on ammonia oxidation rate and nitrite accumulation was investigated in order to determine the optimal conditions for partial nitrification of synthetic wastewater with high ammonia concentration. The experiments performed at low DO levels to lower the total oxygen needed in the nitrification step, which means great saving in aeration. During the start-up stage pH and DO were set at 7.0–7.4 and 0.5 mg/l, respectively. The reactor was operated until complete partial nitrification was achieved. The effect of pH, DO on partial nitrification was studied, as pH was kept at 6.5, 7.5, 8.5, 9.5 and DO at 0.5±0.2, 1.5±0.2 and 2.5±0.2 mg/l, and temperature at 30 °C. The influence of temperature on k_a value was studied by keeping pH=7.5, DO=1.5 mg/l and temperature was controlled at 12, 20 and 30 °C, respectively. The results showed that partial nitrification to nitrite was steadily obtained and the optimal operational parameters were pH=7.5, DO=1.5 mg/l, T=30 °C based on ammonia oxidation rate and nitrite accumulation rate. The maximum k_a was achieved and to be 115.1×10⁻³ mg NH₄⁺–N (mg VSS h)⁻¹ under this condition.     

Capacities and limits of three different technologies for the biological treatment of leachate from solid waste landfill sites

Leachate from a municipal waste landfill site was treated using an activated sludge bioreactor, a fluidized bed biofilm reactor and a packed-bed column reactor (trickling filter). The leachate contained high organic matter (2.0–2.6 g/l of COD), high ammonium (300–700 mg/l) and sulphide (200–800 mg/l) concentrations, as well as low metal concentrations. The continuously operating reactors were employed to study the effects of TOC loading on the removal of TOC as well as on the nitrification and denitrification processes. Among the three biological treatment technologies investigated, the fluidized bed biofilm reactor was best with respect to removing ammonia and TOC. More than 90% of TOC and 99% of ammonia were removed when TOC loading was less than 0.5 kg/m³ × d. At a TOC loading of 4 kg/m³ × d, the removal of TOC and ammonia was 80% and 99%, respectively. In contrast, the treatment of leachate with the packed-bed reactor was successful in TOC removing only at TOC loading less than 0.3 kg/m³ × d (TOC elimination decreased from 86% at 0.06 kg/m³ × d to 60% at 0.3 kg/m³ × d). However, the reactor was active in nitrification even at a higher TOC loading (more than a 98% ammonia elimination at a TOC loading of 0.5 kg/m³ × d). Leachate was processed in the activated sludge reactor when TOC loading was less than 0.5 kg/m³ × d (with a removal of TOC and ammonia up to 83% and 99%, respectively). The activated sludge reactor was also effective in TOC removal at a higher TOC loading (e.g. a 74% TOC removal at a TOC loading of 1 kg/m³ × d), but for ammonia elimination, the activity continuously decreased (less than 60% ammonia removal at a TOC loading of 1 kg/m³ × d). Overloading in the activated sludge system was indicated by a high concentration of ammonia and nitrite in the effluent. In the packed bed reactor, overloading was characterized by a progressively incomplete TOC removal. No significant overloading was found in the fluidized bed reactor up to a TOC loading of 4 kg/m³ × d.     

Nitrate and nitrite in interstitial waters of eelgrass beds in relation to the rhizosphere

The distribution of nitrate and nitrite in the interstitial water of the sediment of eelgrass (Zostera marina) bed of Izembek Lagoon, Alaska, were investigated. Their concentrations were relatively high (0 to 9.8 μg-at.N·1^?1, average 4.8 for nitrate; 0 to 4.0 μ-at.N·1^?1, average 1.9 for nitrite) although the sediments were anoxic and contained hydrogen sulphide. The rates of bacterial denitrification measured by ¹⁵N tracer technique ranged from 0.49×10^?10 to 1.2 × 10^?9 g-atN·g^?1·h^?1. When a steady state is maintained, the loss of nitrate and nitrite must be balanced by their production by bacterial nitrification. Experimentally determined rate of nitrification in the sediment was of the same order. A model experiment demonstrated that oxygen is transported from leaves to rhizomes and roots of eelgrass and released into the sediment. The oxygen is used for nitrification in the rhizosphere in anoxic sediments.     

Impact of nitrite on aerobic phosphorus uptake by poly-phosphate accumulating organisms in enhanced biological phosphorus removal sludges

Impact of nitrite on aerobic phosphorus (P) uptake of poly-phosphate accumulating organisms (PAOs) in three different enhanced biological phosphorus removal (EBPR) systems was investigated, i.e., the enriched PAOs culture fed with synthetic wastewater, the two lab-scale sequencing batch reactors (SBRs) treating domestic wastewater for nutrient removal through nitrite-pathway nitritation and nitrate-pathway nitrification, respectively. Fluorescence in situ hybridization results showed that PAOs in the three sludges accounted for 72, 7.6 and 6.5 % of bacteria, respectively. In the enriched PAOs culture, at free nitrous acid (FNA) concentration of 0.47 × 10^?3 mg HNO₂-N/L, aerobic P-uptake and oxidation of intercellular poly-β-hydroxyalkanoates were both inhibited. Denitrifying phosphorus removal under the aerobic conditions was observed, indicating the existence of PAOs using nitrite as electron acceptor in this culture. When the FNA concentration reached 2.25 × 10^?3 mg HNO₂-N/L, denitrifying phosphorus removal was also inhibited. And the inhibition ceased once nitrite was exhausted. Corresponding to both SBRs treating domestic wastewater with nitritation and nitrification pathway, nitrite inhibition on aerobic P-uptake by PAOs did not occur even though FNA concentration reached 3 × 10^?3 and 2.13 × 10^?3 mg HNO₂-N/L, respectively. Therefore, PAOs taken from different EBPR activated sludges had different tolerance to nitrite.     

Studies in nitrification of municipal sewage in an upflow biofilter

The upflow aerated biofilter with polyurethane foam cubes as the supporting medium was used for the investigation of nitrification studies on municipal sewage (secondary treated as well as untreated domestic sewage). In case of secondary treated sewage effluent, a synthetic composition of NH₄ ⁺-N and COD of each 50?mg/l was studied for a HRT variation of 24, 12, 8 and 6 hours. The ammonium removal efficiencies were found to be in the range of 98 to 100% with the steady-state effluent concentrations of NH₄ ⁺-N and NO₂ ^?-N in the range of 1–4 mg/l and 0.1–0.2?mg/l respectively. In case of domestic sewage system, nitrification studies along with suspended solids removal study was carried-out on untreated sewage for a HRT variation of 24, 12 and 6 hours. The ammonium removal efficiencies of 100% were observed for all the three HRT values at very high COD/NH₄ ⁺-N ratio of 15. The suspended solids removal efficiencies of 95 to 98% were observed with the average effluent suspended solids concentration of 5.9 to 15.9?mg/l. The experiments were conducted in non-backwash conditions of the biofilter. The study has revealed the best use of the upflow biofilter system for nitrification applications and suspended solids removal.     

Assessment of Nitrification Potential in Ground Water Using Short Term, Single-Well Injection Experiments

Nitrification was measured within a sand and gravel aquifer on Cape Cod, MA, using a series of single-well injection tests. The aquifer contained a wastewater-derived contaminant plume, the core of which was anoxic and contained ammonium. The study was conducted near the downgradient end of the ammonium zone, which was characterized by inversely trending vertical gradients of oxygen (270 to 0 μM) and ammonium (19 to 625 μM) and appeared to be a potentially active zone for nitrification. The tests were conducted by injecting a tracer solution (ambient ground water + added constituents) into selected locations within the gradients using multilevel samplers. After injection, the tracers moved by natural ground water flow and were sampled with time from the injection port. Rates of nitrification were determined from changes in nitrate and nitrite concentration relative to bromide. Initial tests were conducted with ¹⁵N-enriched ammonium; subsequent tests examined the effect of adding ammonium, nitrite, or oxygen above background concentrations and of adding difluoromethane, a nitrification inhibitor. In situ net nitrate production exceeded net nitrite production by 3- to 6- fold and production rates of both decreased in the presence of difluoromethane. Nitrification rates were 0.02–0.28 μmol (L aquifer)⁻¹ h⁻¹ with in situ oxygen concentrations and up to 0.81 μmol (L aquifer)⁻¹ h⁻¹ with non-limiting substrate concentrations. Geochemical considerations indicate that the rates derived from single-well injection tests yielded overestimates of in situ rates, possibly because the injections promoted small-scale mixing within a transport-limited reaction zone. Nonetheless, these tests were useful for characterizing ground water nitrification in situ and for comparing potential rates of activity when the tracer cloud included non-limiting ammonium and oxygen concentrations.     

Effect of continuous Cd feeding on the performance of a nitrification reactor

The inhibitory effect of Cd on nitrification was investigated in a continuous-flow system with enriched nitrifying bacteria. The maximum specific ammonium utilization rate and the half-saturation constant were found as 671 mg NH₄–N/g VSS day and 0.48 mg/l, respectively. In the case of continuous Cd input at 1 and 2.5 mg/l, nitrification was inhibited by 30% and 47%, respectively. Inhibition ranged from 20% to 40% and no further increase in inhibition was exhibited in new runs except at 10 mg/l influent Cd. At 10 mg/l influent Cd, specific ammonium utilization and nitrate production rates were inhibited by 90%. On the contrary, a serious nitrite accumulation was not observed during this period. When Cd feeding was stopped, recovery from inhibition was observed after 37 day which was seen by the improvement in ammonium utilization and nitrate production rates. A shift in microbial population from the initial Nitrosomonas sp. to the Cd-tolerant Nitrosospira sp. was observed in the recovery period from severe Cd inhibition. After the domination of Nitrosospira species, redosing at 10 mg/l and then at 15 mg/l did not affect the performance as before.     

Free nitrous acid selectively inhibits and eliminates nitrite oxidizers from nitrifying sequencing batch reactor

In a complete nitrification sequencing batch reactor (CNSBR), where ammonium containing wastewater (200–1,000 mg N/L) is completely oxidized to nitrate up to 2.4 kg NH₄ ⁺–N/m³ d, both ammonia oxidizers and nitrite oxidizers were enriched in the sludge granules. Quantitative fluorescence in situ hybridization analyses of the sludge granules of the CNSBR showed that ammonia oxidizers and nitrite oxidizers occupied 31 and 4.2% of total bacteria, respectively. Most of the nitrite oxidizers were Nitrobacter species (95% of the nitrite oxidizers) and the remainder was Nitrospira species. The population of nitrite oxidizers was significantly higher than that of partial nitrification SBR (PNSBR) where most of the ammonium was oxidized to nitrite. The PNSBR had 37% (ammonia oxidizers) and 0.4% (nitrite oxidizers) of total bacteria. Comparative study with CNSBR and PNSBR revealed that free nitrous acid, rather than free ammonia, played a critical inhibition role to wash out nitrite oxidizers from the reactor. The concentrations of free ammonia and nitrite as well as free nitrous acid in the CNSBR selected Nitrobacter as the dominant nitrite oxidizers rather than Nitrospira.     

Heterotrophic nitrification by Arthrobacter sp. (strain 9006) as influenced by different cultural conditions,growth state and acetate metabolism

An Arthrobacter sp. (strain 9006), isolated from lake water, accumulated nitrite up to about 15 mg N/l, but no nitrate. In a mineral medium supplemented with tryptone, yeast extract, acetate and ammonium, the cells released nitrite into the medium parallel to growth or when growth had virtually ceased. The nitrite formed was proportional to the initial acetate concentration, indicating an involvement of acetate metabolism with nitrification. The organism grew with a wide variety of organic carbon sources, but washed cells formed nitrite from ammonium only in the presence of citrate, malate, acetate or ethanol. Magnesium ions were required for nitrification of ammonium and could not be replaced by other divalent metal ions. Analysis of the glyoxylate cycle key enzymes in washed suspensions incubated in a minimal medium revealed that isocitrate lyase and malate synthase were most active during the nitrification phase. Nitrite accumulation but not growth was inhibited by glucose, tryptone and yeast extract. A possible explanation for the different nitrification patterns during growth is based on the regulatory properties of glyoxylate cycle enzymes.Abbreviations IL Isocitrate lyase [threo-D_s-isocitrate glyoxylate-lase, E.C. 4.1.3.1.] - MS malate synthase [l-malate glyoxylate-lyase (CoA-acetylating), E.C. 4.1.3.2.]     

Simultaneous nitrification and denitrification with different mixed nitrogen loads by a hypothermia aerobic bacterium

Microorganism with simultaneous nitrification and denitrification ability plays a significant role in nitrogen removal process, especially in the eutrophic waters with excessive nitrogen loads. The nitrogen removal capacity of microorganism may suffer from low temperature or nitrite nitrogen source. In this study, a hypothermia aerobic nitrite-denitrifying bacterium, Pseudomonas tolaasii strain Y-11, was selected to determine the simultaneous nitrification and denitrification ability with mixed nitrogen source at 15 °C. The sole nitrogen removal efficiencies of strain Y-11 in simulated wastewater were obtained. After 24 h of incubation at 15 °C, the ammonium nitrogen fell below the detection limit from an initial value of 10.99 mg/L. Approximately 88.0 ± 0.33% of nitrate nitrogen was removed with the initial concentration of 11.78 mg/L and the nitrite nitrogen was not detected with the initial concentration of 10.75 mg/L after 48 h of incubation at 15 °C. Additionally, the simultaneous nitrification and denitrification nitrogen removal ability of P. tolaasii strain Y-11 was evaluated using low concentration of mixed NH₄⁺-N and NO₃^?–N/NO₂^?–N (about 5 mg/L-N each) and high concentration of mixed NH₄⁺–N and NO₃^?–N/NO₂^?–N (about 100 mg/L-N each). There was no nitrite nitrogen accumulation at the time of evaluation. The results demonstrated that P. tolaasii strain Y-11 had higher simultaneous nitrification and denitrification capacity with low concentration of mixed inorganic nitrogen sources and may be applied in low temperature wastewater treatment.     

Effect of dissolved organic carbon quality on microbial decomposition and nitrification rates in stream sediments

1. Microbial decomposition of dissolved organic carbon (DOC) contributes to overall stream metabolism and can influence many processes in the nitrogen cycle, including nitrification. Little is known, however, about the relative decomposition rates of different DOC sources and their subsequent effect on nitrification. 2. In this study, labile fraction and overall microbial decomposition of DOC were measured for leaf leachates from 18 temperate forest tree species. Between 61 and 82% (mean, 75%) of the DOC was metabolized in 24 days. Significant differences among leachates were found for labile fraction rates (P < 0.0001) but not for overall rates (P=0.088). 3. Nitrification rates in stream sediments were determined after addition of 10 mg C L^–1 of each leachate. Nitrification rates ranged from below detection to 0.49 μg N mL sediment^–1 day^–1 and were significantly correlated with two independent measures of leachate DOC quality, overall microbial decomposition rate (r=–0.594, P=0.0093) and specific ultraviolet absorbance (r=0.469, P=0.0497). Both correlations suggest that nitrification rates were lower in the presence of higher quality carbon. 4. Nitrification rates in sediments also were measured after additions of four leachates and glucose at three carbon concentrations (10, 30, and 50 mg C L^–1). For all carbon sources, nitrification rates decreased as carbon concentration increased. Glucose and white pine leachate most strongly depressed nitrification. Glucose likely increased the metabolism of heterotrophic bacteria, which then out‐competed nitrifying bacteria for NH₄⁺. White pine leachate probably increased heterotrophic metabolism and directly inhibited nitrification by allelopathy.     

Heterotrophic nitrification and aerobic denitrification inAlcaligenes faecalis strain TUD

Heterotrophic nitrification and aerobic and anaerobic denitrification byAlcaligenes faecalis strain TUD were studied in continuous cultures under various environmental conditions. Both nitrification and denitrification activities increased with the dilution rate. At dissolved oxygen concentrations above 46% air saturation, hydroxylamine, nitrite and nitrate accumulated, indicating that both the nitrification and denitrification were less efficient. The overall nitrification activity was, however, essentially unaffected by the oxygen concentration. The nitrification rate increased with increasing ammonia concentration, but was lower in the presence of nitrate or nitrite. When present, hydroxylamine, was nitrified preferentially. Relatively low concentrations of acetate caused substrate inhibition (K_I=109 M acetate). Denitrifying or assimilatory nitrate reductases were not detected, and the copper nitrite reductase, rather than cytochrome cd, was present. Thiosulphate (a potential inhibitor of heterotrophic nitrification) was oxidized byA. faecalis strain TUD, with a maximum oxygen uptake rate of 140–170nmol O₂·min^-1·mg prot^-1. Comparison of the behaviour ofA. faecalis TUD with that of other bacteria capable of heterotrophic nitrification and aerobic denitrification established that the response of these organisms to environmental parameters is not uniform. Similarities were found in their responses to dissolved oxygen concentrations, growth rate and ammonia concentration. However, they differed in their responses to externally supplied nitrite and nitrate.     

Calorimetric Glucose Determination by Glucose Oxidase —Methylene Blue

The reaction between glucose and methylene blue, catalyzed by glucose oxidase (GOD)was analysed calorimetrically. The amount of heat produced under saturating methylene blue concentrations ( > 10^?2 mol/1)was measured with glucose concentration and time as parameters (kinetic procedure) Kinetic constants (pseudo one substrate kinetics) were derived from the experimental data: K_M(glucose)= 1.18 × 10^?3 mol/l and V_max = 0.085 J/mg GOD min (3.89 · 10^?6 mol/mg GOD min) Comparison of caloric with optical measurements gave an enthalpy of reaction of 22.52 kJ/mol. Considering the observed substrate inhibition, glucose determinations are possible up to glucose concentrations of 0.1 mol/l.     

Health Risk Assessment of Chemical Mixtures Exposure to Residents in the Lake Taihu Region,China

This study assessed the mixture health risk for the residents of China's Lake Taihu region posed by a Persistent Organic Pollutants (POPs) mixture of dichloro-diphenyl-trichloroethane (DDT) and hexachlorocyclohexane (HCH). Multiple-pathway exposure models were used for exposure assessment in order to estimate the DDT and HCH exposure dose. The DDT and HCH PBPK models were developed and used for consequence assessment in order to analyze the pollutant distribution and accumulation process in human tissues. The tissue dose hazard index (HI) was used to estimate the mixture health risk. The results showed that the total exposure doses for male residents and female residents were 4.01 × 10^{? 4}～ 7.67 × 10^{? 3} mg/kg/day and 3.73 × 10^{? 4}～ 6.75 × 10^{? 3} mg/kg/day for DDT, respectively, and 3.78 × 10^{? 4}～ 5.14 × 10^{? 3} mg/kg/day and 3.53 × 10^{? 4}～ 4.66 × 10^{? 3} mg/kg/day for HCH, respectively. The maximum tissue concentrations in fat for male and female residents reached 110.51 mg/l and 97.21 mg/l for DDT, respectively, and 189.66 mg/l and 171.72 mg/l for HCH, respectively. The tissue dose hazard indexes for male and female residents were 0.1472 ～ 2.4990 and 0.1377 ～ 2.2230, respectively, and the probabilities of the risk exceeding the acceptable risk (HI = 1) for male and female were 24.60% and 16.51%, respectively.     

Inorganic nitrogen control in a novel zero-water exchanged aquaculture system integrated with airlift-submerged fibrous nitrifying biofilters

This work examined the feasibility of applying shrimp diets to establish nitrification on submerged fibrous biofilters. It also investigated the performance of a proposed zero-water exchanged aquaculture system, which integrated growing of aquatic stocks and operation of acclimated biofilters in the same environment. Addition of shrimp diets fully established nitrification within 3 weeks as indicated by continuous increase of nitrate and trivial levels of ammonium and nitrite. A series of batch experiment revealed an average ammonium degradation rate of 24.1 mg N m^?2 day^?1. Zero-water discharged tilapia cultivation could be carried out in the proposed aquaculture system for at least 44 days when daily inorganic loadings increased from 1.24 to 10.78 mg N l^?1 day^?1. The corresponding daily growth rates of tilapia from the proposed aquaculture systems integrated with acclimated biofilters varied from 3.01 to 3.35 g day^?1, which was approximately 7–16% better than numbers from the systems using non-acclimated biofilters.     

Production of nitric oxide in Nitrosomonas europaea by reduction of nitrite

Nitrosomonas europaea and Nitrosovibrio sp. produced NO and N₂O during nitrification of ammonium. Less then 15% of the produced NO was due to chemical decomposition of nitrite. Production of NO and especially of N₂O increased when the bacteria were incubated under anaerobic conditions at decreasing flow rates of air, or at increasing cell densities. Low concentrations of chlorite (10 M) inhibited the production of NO and N₂, but not of nitrite indicating that NO and N₂O were not produced during the oxidative conversion of ammonium to nitrite. NO and N₂O were produced during reduction of nitrite with hydrazine as electron donor in almost stoichiometric quantities indicating that reduction of nitrite was the main source of NO and N₂O.     

A novel bacterium carrying out anaerobic ammonium oxidation in a reactor for biological treatment of the filtrate of wastewater fermented sludge

A new genus and species of bacteria capable of ammonium oxidation under anaerobic conditions in the presence of nitrite is described. The enrichment culture was obtained from the Moscow River silt by sequential cultivation in reactors with selective conditions for anaerobic ammonium oxidation. Bacterial cells were coccoid, ～0.4 × 0.7 μm, with the intracellular membrane structures typical of bacteria capable of anaerobic ammonium oxidation (anammoxosome and paryphoplasm). The cells formed aggregates 5–25 μm in diameter (10 μm on average). They were readily adhered to solid surfaces. The cells were morphologically labile: they easily lost their content and changed their morphology during fixation for electron microscopy. The organism was capable of ammonium oxidation with nitrite. The semisaturation constants Ks for nitrite and ammonium were 0.38 mg N-NO₂/L and 0.41 mg N-NH₄/L, respectively. The maximal nitrite concentrations for growth were 90 and 75 mg N-NO₂/L for single and continuous application, respectively. The doubling time was 32 days, μ_max = 0.022 day^?1, the optimal temperature and pH were 20°C and 7.8–8.3, respectively. According to the results of 16S rRNA gene sequencing, the bacterium was assigned to a new genus and species within the phylum Planctomycetes. The proposed name for the new bacterium is Candidatus Anammoximicrobium moscowii gen. nov., sp. nov. (a microorganism carrying out anaerobic ammonium oxidation, isolated in the Moscow region).     

Effects of temperature,ph, salinity,and inorganic nitrogen on the rate of ammonium oxidation by nitrifiers isolated from wetland environments

Ammonium-oxidizing bacteria were examined in two wetland environments, a freshwater marsh and an estuarine bay, during a 2-year period. Two predominant types were consistently isolated, one from each environment. Both isolates were identified as species ofNitrosomonas. Using a closed culture, high cell density assay, the effects of temperature, pH, salinity, Na⁺, K⁺, nitrite, nitrate, and ammonium concentrations on ammonium oxidation were determined. Maximum activity was observed for the freshwater isolate at 35°C, pH 8.5, salinities of 0.3 to 0.5% Na⁺ and K⁺, and ammonium concentrations greater than 0.5 g/l. For the estuarine isolate, maximum activity was observed at 40°C, pH 8.0, salinities of 0.5 to 1.0%, 1.0% Na⁺ and K⁺, and 0.2 g/l ammonium. The estuarine isolate had a Na⁺ requirement which could be partially substituted by the K⁺, suggesting that the organism is a true estuarine bacterium. Nitrite inhibited both isolates at concentrations greater than 5 mg/l, whereas nitrate had no significant effect on either isolate.     

Nitrite effect on ammonium and nitrite oxidizing processes in a nitrifying sludge

Nitrite accumulation can be undesirable in nitrifying reactors used for the biological elimination of nitrogen from wastewaters because the ammonium oxidation process was seen to be inhibited. There is a need to better understand the effects of nitrite on both ammonium and nitrite oxidizing processes. In this paper, the effect of nitrite on the nitrifying activity of a sludge produced in steady-state nitrification was evaluated in batch cultures. At 25 mg N/l of added nitrite, nitrification was successfully carried out. Addition of higher nitrite concentrations to nitrifying cultures (100 and 200 mg N/l) provoked inhibitory effects on the nitrification respiratory process. Nitrite at 100 and 200 mg N/l induced a significant decrease in the values for nitrate yield (−20% and −34%, respectively) and specific rate of nitrate formation (−26% and −67%, respectively), while the ammonium consumption efficiency kept high and the specific rate of ammonium oxidation did not significantly change. This showed that the nitrite oxidizing process was more sensitive to the presence of nitrite than the ammonium oxidizing process. These results showed that as a consequence of nitrite accumulation in nitrification systems, the activity of the nitrite oxidizing bacteria could be more inhibited than that of the ammonium oxidizing bacteria, provoking a higher accumulation of nitrite in the medium.