Free Nitrous Acid Concentration Research Articles

Direct and indirect effects of oxygen limitation on nitrification process applied to reject water treatment

The direct and indirect influence of temporary oxygen limitation on the nitrification of reject water was investigated using a continuous stirred tank bioreactor operated for 330 d at laboratory temperature (23 ± 1°C). A decrease in dissolved oxygen (DO) concentration from 3 to 0.7 mg L−1 lasted for 38 d and led to effective nitrite accumulation—more than 95% of total oxidized nitrogen was present as nitrite. The drop of DO concentration, at the same time, caused a decrease in the nitrogen oxidation rate from 1.36 to 0.73 kg N m−3 d−1. After a subsequent DO concentration increase to 3 mg L−1, the nitrite accumulation remained stable for another 90 d. This development was caused mainly by the indirect effects of DO limitation, consisting especially in the change of nitrogen species represented. A significant increase in free nitrous acid concentration induced by temporary DO limitation seems to be the key factor in this respect. The results of Fluorescence in situ hybridization analysis confirmed a washout of nitrite-oxidizing bacteria (NOB) during the period with high nitrite accumulation. The representation of NOB in total biomass decreased from 6.4% to less than 1% as a consequence of temporary DO limitation.

Integrating waste activated sludge (WAS) acidification with denitrification by adding nitrite (NO2−)

Adding nitrite (NO2−) to waste activated sludge (WAS) fermentation systems is an efficient approach to integrate fermentation with denitrification, and utilize volatile fatty acids (VFAs) to achieve a high nitrogen removal rate even at low C/N ratios. In this study, the effect of nitrite on the integrated WAS fermentation and denitrification (termed as WASFD) was investigated under acidic and alkaline conditions. The results indicated that adding nitrite achieved a most reduction of nitrite to N2 and improved the acidification of WAS with high VFAs production. Under acidic condition (pH = 5), the maximum VFAs produced with nitrite addition was 3.3 times that without nitrite addition. Higher concentration of free nitrous acid (FNA) at the pH of 5 increased WAS particulates, improved the hydrolysis of organic substrates, and finally promoted VFAs yields. Under alkaline condition (pH = 9), adding nitrite only increased the VFAs production by 1.5 times, indicating that acidic condition was preferable for acidification than alkaline condition.

AEROBIC AND ANAEROBIC AMMONIUM OXIDISING BACTERIA ENRICHMENT FROM MINED MUNICIPAL SOLID WASTE IN SHARONANAMMOX PROCESS

Ammoniacal nitrogen removal by novel processes like Single reactor system for high activity ammonia removal over nitrite (SHARON) and Anaerobic ammonium oxidation (ANAMMOX) processes are currently considered as advantageous than conventional processes. It requires aerobic ammonium oxidising bacteria (AOB) and anaerobic ammonium oxidising bacteria (AnAOB) to conduct the SHARON and ANAMMOX processes. This paper presents the feasibility of enriching the AOB and AnAOB using mined municipal solid waste (MSW) in batch reactors for a period of 37 days. In AOB reactor with nitrogen loading of 0.5 kg N/d showed Partial Nitritation Efficiency of 82.6% with 3.8 × 108 MPN/100mL of AOB population obtained. AnAOB reactor enriched with anammox biomass efficiently removed 78% of ammonia with the specific anammox activity reached up to 0.10 mg NH4-N/mg MLVSS/d. The nitrogen transformations along with the formation of intermediates (hydrazine and hydroxylamine), biomass development, free ammonia and free nitrous acid concentrations in the batch reactors confirmed the enrichment AOB and AnAOB biomass activity using mined MSW. (Keywords: Mined municipal solid waste, ammoniacal nitrogen removal, aerobic ammonium oxidising bacteria and anammox)

Enhanced lipid extraction from algae using free nitrous acid pretreatment

Lipid extraction has been identified as a major bottleneck for large-scale algal biodiesel production. In this work free nitrous acid (FNA) is presented as an effective and low cost pretreatment to enhance lipid recovery from algae. Two batch tests, with a range of FNA additions, were conducted to disrupt algal cells prior to lipid extraction by organic solvents. Total accessible lipid content was quantified by the Bligh and Dyer method, and was found to increase with pretreatment time (up to 48 h) and FNA concentration (up to 2.19 mg HNO2-N/L). Hexane extraction was used to study industrially accessible lipids. The mass transfer coefficient (k) for lipid extraction using hexane from algae treated with 2.19 mg HNO2-N/L FNA was found to be dramatically higher than for extraction from untreated algae. Consistent with extraction results, cell disruption analysis indicated the disruption of the cell membrane barrier.

Effect of process parameters and operational mode on nitrous oxide emissions from a nitritation reactor treating reject wastewater

Nitrous oxide (N2O) and methane emissions were monitored in a continuous granular airlift nitritation reactor from ammonium-rich wastewater (reject wastewater). N2O emissions were found to be dependent on dissolved oxygen (DO) concentration in the range of 1–4.5 mg O2/L, increasing within this range when reducing the DO values. At higher DO concentrations, N2O emissions remained constant at 2.2% of the N oxidized to nitrite, suggesting two different mechanisms behind N2O production, one dependent and one independent of DO concentration. Changes on ammonium, nitrite, free ammonia and free nitrous acid concentrations did not have an effect on N2O emissions within the concentration range tested. When operating the reactor in a sequencing batch mode under high DO concentration (>5 mg O2/L), N2O emissions increased one order of magnitude reaching values of 19.3 ± 7.5% of the N oxidized. Moreover, CH4 emissions detected were due to the stripping of the soluble CH4 that remained dissolved in the reject wastewater after anaerobic digestion. Finally, an economical and carbon footprint assessment of a theoretical scaled up of the pilot plant was conducted.

Nitrite (not free nitrous acid) is the main inhibitor of the anammox process at common pH conditions

Nitrite is a substrate but also an inhibitor of anaerobic ammonium oxidation (anammox).There is currently no consensus on whether ionized nitrite (INi) or free nitrous acid (FNA) is the actual inhibitor of the process. The inhibition by INi and FNA on the anammox process has been analysed using a wide range of INi and FNA concentrations and by altering the pH and total nitrite conditions. The inhibitory impacts of both species were quantified through a rational inhibition equation, considering INi and FNA as substrate inhibitor and non-competitive inhibitor, respectively. Inhibitory constants were calculated with strong statistical support as 561 mg INi-N l(-1) and 0.117 mg FNA-N l(-1). Based on the model, INi is the main inhibiting species of the anammox process at pH > 7.1, which are the most common conditions occurring in field applications of anammox.

Free Nitrous Acid (FNA)-Based Pretreatment Enhances Methane Production from Waste Activated Sludge

Anaerobic digestion of waste activated sludge (WAS) is currently enjoying renewed interest due to the potential for methane production. However, methane production is often limited by the slow hydrolysis rate and/or poor methane potential of WAS. This study presents a novel pretreatment strategy based on free nitrous acid (FNA or HNO2) to enhance methane production from WAS. Pretreatment of WAS for 24 h at FNA concentrations up to 2.13 mg N/L substantially enhanced WAS solubilization, with the highest solubilization (0.16 mg chemical oxygen demand (COD)/mg volatile solids (VS), at 2.13 mg HNO2-N/L) being six times that without FNA pretreatment (0.025 mg COD/mg VS, at 0 mg HNO2-N/L). Biochemical methane potential tests demonstrated methane production increased with increased FNA concentration used in the pretreatment step. Model-based analysis indicated FNA pretreatment improved both hydrolysis rate and methane potential, with the highest improvement being approximately 50% (from 0.16 to 0.25 d(-1)) and 27% (from 201 to 255 L CH4/kg VS added), respectively, achieved at 1.78-2.13 mg HNO2-N/L. Further analysis indicated that increased hydrolysis rate and methane potential were related to an increase in rapidly biodegradable substrates, which increased with increased FNA dose, while the slowly biodegradable substrates remained relatively static.

Inactivation kinetics of anaerobic wastewater biofilms by free nitrous acid

Recent studies have shown that free nitrous acid (FNA) is biocidal to a broad range of microorganisms. Microorganisms residing in anaerobic sewer biofilms were found to be inactivated after a short (6-24 h) exposure to FNA. In this study, we investigate the inactivation kinetics of anaerobic sewer biofilms grown in real wastewater. Microbial viability of biofilms was determined using LIVE/DEAD staining. A two-fraction kinetic model was developed to simulate the inactivation of mixed culture in biofilms. The kinetic parameters were estimated by using Bayesian statistics. Model simulation found that a fraction (85 %) of the biofilm community was highly sensitive to FNA with a high inactivation rate, and a fraction (15 %) was tolerant to FNA and persisted after FNA treatment. This different susceptibility to FNA treatment was likely due to the diverse microbial community and biofilm protection. The fact that nearly 85 % microbes were inactivated confirmed that FNA is a strong biocide to mixed-culture biofilms. It was found that the inactivation rate constant was not affected by pH levels. The kinetic model was successfully used to optimize FNA dosage for sulfide control in sewer biofilms. Also, results suggest that a high FNA concentration is preferred than long exposure time to reduce the total chemical consumption.

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.

Partial nitritation for subsequent Anammox to treat high-ammonium leachate

Nitrogen removal via autotrophic denitrification, an anaerobic ammonium oxidation (Anammox) process, requires an appropriate NO2-N/NH4-N ratio to provide nitrite as an intermediate. In this study, a laboratory-scale Hybrid Sequencing Batch Reactor (HSBR) was implemented for treating high-ammonium raw leachate to yield an appropriate NO2-N/NH4-N mixture as a pretreatment for subsequent Anammox. The optimal operating conditions were examined through the long-term operation of the HSBR. The experimental results showed that the prerequisite ratio of NO2-N/NH4-N was found with an initial ammonium concentration of 1200 mg/L, dissolved oxygen (DO) of 0.5–1.0 mg/L, sludge retention time (SRT) of 3 days and temperature of 31°C, which is essential for the subsequent Anammox process. Moreover, the inhibition of free ammonia (FA) and free nitrous acid (FNA) were also examined under a constant pH condition, and it was found that AOB (ammonium oxidation bacteria) had a great ability to adapt to a broad FA and FNA concentration, whereas NOB (nitrite-oxidizing bacteria) were inhibited by either FA or FNA concentration to a certain extent. It appears that partial nitritation could be implemented by facilitating FA and/or suppressing FNA concentration.

Two-Stage Start-Up to Achieve the Stable via-Nitrite Pathway in a Demonstration SBR for Anaerobic Codigestate Treatment

The supernatant effluent from full scale anaerobic codigestion of waste-activated sludge and the organic fraction of municipal solid waste was treated by a demonstration sequencing batch reactor for short-cut nitrogen removal. After inoculation with conventional municipal activated sludge, the strategy of two-stage start-up allowed us the fast speciation of ammonia oxidizing bacteria (achieved within 20 days), then the achievement of the maximal treatment potential (0.8 kgN m–3 d–1) for nitritation–denitritation. By automatic indirect control of the free ammonia and the free nitrous acid concentrations, the via-nitrite pathway was fully and stably achieved under aerobic conditions (DO of 1.5 mg L–1) and T of 15 °C. Under ordinary operation, the specific nitritation rates were 15–20 mgN gMLVSS–1h–1, while the denitritation rate was 45–50 mgN gMLVSS–1h–1. In-situ and ex-situ respirometry and gene-based molecular techniques demonstrated the stable presence of a dominant, restricted ammonia oxidizing bacteria...

Effect of temperature on AOB activity of a partial nitritation SBR treating landfill leachate with extremely high nitrogen concentration

This study investigates the effects of temperature on ammonia oxidizing bacteria activity in a partial nitritation (PN) sequencing batch reactor. Stable PN was achieved in a 250L SBR with a minimum operating volume of 111L treating mature landfill leachate containing an ammonium concentration of around 6000mg N-NH4+L−1 at both 25 and 35°C. A suitable influent to feed an anammox reactor was achieved in both cases. A kinetic model was applied to study the influence of free ammonia (FA), the free nitrous acid (FNA) inhibition, and the inorganic carbon (IC) limitation. NH4+ and NO2− concentrations were similar at 25 and 35°C experiments (about 2500mg N-NH4+L−1 and 3500mg N-NO2−L−1), FA and FNA concentrations differed due to the strong temperature dependence. FNA was the main source of inhibition at 25°C, while at 35°C combined FA and FNA inhibition occurred. DGGE results demonstrated that PN-SBR sludge was enriched on the same AOB phylotypes in both experiments.

Effect of pH, substrate and free nitrous acid concentrations on ammonium oxidation rate

Respirometric techniques have been used to determine the effect of pH, free nitrous acid (FNA) and substrate concentration on the activity of the ammonium oxidizing bacteria (AOB) present in an activated sludge reactor. With this aim, bacterial activity has been measured at different pH values (ranging from 6.2 to 9.7), total ammonium nitrogen concentrations (ranging from 0.1 to 10mgTANL−1) and total nitrite concentrations (ranging from 3 to 43mgNO2–NL−1). According to the results obtained, the most appropriate kinetic expression for the growth of AOB in activated sludge reactors has been established. Substrate half saturation constant and FNA and pH inhibition constants have been obtained by adjusting model predictions to experimental results. Different kinetic parameter values and different Monod terms should be used to model the growth of AOB in activated sludge processes and SHARON reactors due to the different AOB species that predominate in both systems.

Response of poly-phosphate accumulating organisms to free nitrous acid inhibition under anoxic and aerobic conditions

The response of free nitrous acid (FNA)-adapted poly-phosphate accumulating organisms (PAOs) to FNA inhibition under aerobic and anoxic conditions was studied. Anoxic P-uptake was 1–6 times more sensitive to the inhibition compared to aerobic P-uptake. The aerobic nitrite reduction rate increased with FNA concentration, accompanied by an equivalent decrease in the oxygen uptake rate, suggesting under high FNA concentration conditions, electrons were channeled to nitrite reduction from oxygen reduction. In contrast, the nitrite reduction rate decreased with increased FNA concentration under anoxic conditions. Anaerobic metabolism of PAO under both anoxic and aerobic conditions was observed at high FNA concentrations. Growth of PAOs decreased sharply with FNA concentration and stopped completely at FNA concentration of 10μgHNO2–N/L. This study, for the first time, investigated the function of nitrite/FNA in an aerobic denitrifying phosphate removal process by evaluating electron as well as energy balances, and provides explanation for FNA inhibition mechanisms.

Granulation of Nitrifying Bacteria in a Sequencing Batch Reactor for Biological Stabilisation of Source-Separated Urine

Biological stabilisation of human urine highly depends on the abundance and activities of nitrifying bacteria. However, it is quite difficult to enrich nitrifiers as bio-aggregation by self-immobilized biomass. In this study, granulation of nitrifying bacteria involving inoculation strategy was developed. Two sequencing batch reactors, the one inoculated with nitrifying bacteria and the other inoculated with aerobic granules, were operated in laboratory side by side with a feeding of urine solution. Aerobic nitrifying granules (ANG), with compact morphological structure and good nitrifying activity, were achieved in the reactor inoculated with aerobic granules. Enrichment of nitrifying bacteria favors the nutrient uptake, and hence, to obtain a high ammonia oxidation efficiency. Nonetheless, nitrite accumulation gradually dominated in reactor, partly attributes to a high concentration of free nitrous acid and free ammonia in bulk. The matured ANG had a rather stable microbial profile, as that a number of activated bacteria occupied the surface of granule. It was also found that ANG were much more impermeable than aerobic granules and activated sludge, which was demonstrated as smaller porosities, and therewith an excellent settleability. The results herein reveal that granulation of nitrifying bacteria could enrich the biomass to implement stabilisation of urine in biological way.

Optimization of intermittent, simultaneous dosage of nitrite and hydrochloric acid to control sulfide and methane productions in sewers

Free nitrous acid (FNA) was previously demonstrated to be biocidal to anaerobic sewer biofilms. The intermittent dosing of FNA as a measure for controlling sulfide and methane productions in sewers is investigated. The impact of three key operational parameters namely the dosing concentration, dosing duration and dosing interval on the suppression and subsequent recovery of sulfide and methane production was examined experimentally using lab-scale sewer reactors. FNA as low as 0.26 mg-N/L was able to suppress sulfide production after an exposure of 12 h. In comparison, 0.09 mg-N/L of FNA with 6-h exposure was adequate to restrain methanogenesis effectively. The recovery of sulfide production was well described by an exponential recovery equation. Model-based analysis revealed that 12-h dosage at an FNA concentration of 0.26 mg-N/L every 5 days can reduce the average sulfide production by >80%. Economic analysis showed that intermittent FNA dosage is potentially a cost-effective strategy for sulfide and methane control in sewers.

The effect of pH on N 2O production under aerobic conditions in a partial nitritation system

Ammonia-oxidising bacteria (AOB) are a major contributor to nitrous oxide (N 2O) emissions during nitrogen transformation. N 2O production was observed under both anoxic and aerobic conditions in a lab-scale partial nitritation system operated as a sequencing batch reactor (SBR). The system achieved 55 ± 5% conversion of the 1 g NH 4 +-N/L contained in a synthetic anaerobic digester liquor to nitrite. The N 2O emission factor was 1.0 ± 0.1% of the ammonium converted. pH was shown to have a major impact on the N 2O production rate of the AOB enriched culture. In the investigated pH range of 6.0–8.5, the specific N 2O production was the lowest between pH 6.0 and 7.0 at a rate of 0.15 ± 0.01 mg N 2O-N/h/g VSS, but increased with pH to a maximum of 0.53 ± 0.04 mg N 2O-N/h/g VSS at pH 8.0. The same trend was also observed for the specific ammonium oxidation rate (AOR) with the maximum AOR reached at pH 8.0. A linear relationship between the N 2O production rate and AOR was observed suggesting that increased ammonium oxidation activity may have promoted N 2O production. The N 2O production rate was constant across free ammonia (FA) and free nitrous acid (FNA) concentrations of 5–78 mg NH 3-N/L and 0.15–4.6 mg HNO 2-N/L, respectively, indicating that the observed pH effect was not due to changes in FA or FNA concentrations.

The strong biocidal effect of free nitrous acid on anaerobic sewer biofilms

Several recent studies showed that nitrite dosage to wastewater results in long-lasting reduction of the sulfate-reducing and methanogenic activities of anaerobic sewer biofilms. In this study, we revealed that the quick reduction in these activities is due to the biocidal effect of free nitrous acid (FNA), the protonated form of nitrite, on biofilm microorganisms. The microbial viability was assessed after sewer biofilms being exposed to wastewater containing nitrite at concentrations of 0–120 mg-N/L under pH levels of 5–7 for 6–24 h. The viable fraction of microorganisms was found to decrease substantially from approximately 80% prior to the treatment to 5–15% after 6–24 h treatment at FNA levels above 0.2 mg-N/L. The level of the biocidal effect has a much stronger correlation with the FNA concentration, which is well described by an exponential function, than with the nitrite concentration or with the pH level, suggesting that FNA is the actual biocidal agent. An increase of the treatment from 6 to 12 and 24 h resulted in only slight decreases in microbial viability. Physical disrupted biofilm was more susceptible to FNA in comparison with intact biofilms, indicating that the biocidal effect of FNA on biofilms was somewhat reduced by mass transfer limitations. The inability to achieve 2-log killing even in the case of disrupted biofilms suggests that some microorganisms may be more resistant to FNA than others. The recovery of biofilm activities in anaerobic reactors after being exposed to FNA at 0.18 and 0.36 mg-N/L, respectively, resembled the regrowth of residual sulfate-reducing bacteria and methanogens, further confirming the biocidal effects of FNA on microorganisms in biofilms.

Denitrifying phosphorus removal and impact of nitrite accumulation on phosphorus removal in a continuous anaerobic–anoxic–aerobic (A 2O) process treating domestic wastewater

A lab-scale anaerobic–anoxic–aerobic (A 2O) process was operated to investigate denitrifying phosphorus removal and nitritation–denitritation from domestic wastewater, especially regarding the impact of nitrite accumulation caused by nitritation on phosphorus removal. The results showed that mean total nitrogen (TN) removal was only about 47% and phosphorus removal was almost zero without the pre-anoxic zone and additional carbon source. Contrastively, with configuration of pre-anoxic zone, TN and phosphorus removal was increased to 75% and 98%, respectively, as well as denitrifying phosphorus removal of 66–91% occurred in the anoxic zone. Nitritation–denitritation was achieved through a combination of short aerobic actual hydraulic retention time and low dissolved oxygen levels (0.3–0.5 mg/L); however, phosphorus removal deteriorated with increase of nitrite accumulation rates. The free nitrous acid (FNA) concentration of 0.002–0.003 mg HNO 2–N/L in the aerobic zone inhibited phosphorus uptake, which was major cause of phosphorus removal deterioration. Through supplying the carbon sources to enhance denitrification and anaerobic phosphorus release, nitrite and FNA concentrations in the aerobic zone were reduced, and phosphorus removal was improved. Compared with nitrification–denitrification, nitritation–denitritation reduced the carbon requirement by 30% and performed biological nutrients removal well with mean TN and phosphorus removal of 85% and 96%, respectively.

Free nitrous acid (FNA) inhibition on denitrifying poly-phosphate accumulating organisms (DPAOs)

Free nitrous acid (FNA) has been identified to be a ubiquitous inhibitor of a wide range of microorganisms, including bacteria involved in wastewater treatment. The FNA-induced inhibition on the anoxic (nitrite as electron acceptor) metabolism of denitrifying poly-phosphate accumulating organisms (DPAOs) was investigated using sludge from a sequencing batch reactor performing carbon, nitrogen, and phosphorus removal from synthetic wastewater. We found that FNA had a much stronger inhibitory effect on phosphorus (P) uptake and glycogen production than on poly-beta-hydroxyalkanoate degradation and nitrite reduction. The intracellular adenosine triphosphate levels decreased sharply during the FNA incubation, and the decreasing rates were positively correlated with increasing FNA concentrations. The electron transport activity of DPAOs when exposed to FNA displayed a similar trend. Further, at FNA concentrations above 0.044 mg HNO(2)-N/L, the anaerobic metabolism of DPAOs was initiated despite of the presence of nitrite, as evidenced by the release of phosphorus and the consumption of glycogen. DPAO metabolism did not recover completely from FNA inhibition in the subsequent FNA-free environment. The recovery rate depended on the concentration of FNA applied in the previous anoxic period. These results suggest that the inhibitory effects are diverse and may be attributable to different mechanisms operating simultaneously.