Free Nitrous Acid Concentration Research Articles

Dynamic pH regulation drives Nitrosomonas for high-rate stable acidic partial nitritation

How to intensify the ammonia oxidation rate (AOR) is still a bottleneck impeding the technology development for the innovative acidic partial nitritation because the eosinophilic ammonia-oxidizing bacteria (AOB), such as Nitrosoglobus or Nitrosospira, were inhibited by the high-level free nitrous acid (FNA) accumulation in acidic environments. In this study, an innovative approach of dynamic acidic pH regulation control strategy was proposed to realize high-rate acidic partial nitritation driven by common AOB genus Nitrosomonas. The acidic partial nitrification process was carried out in a laboratory-scale sequencing batch moving bed biofilm reactor (SBMBBR) for long-term (700 days) to track the effect of dynamic acidic pH on nitrifying bacterial activity. The results indicated that the influent NH4+-N concentration was about 100 mg/L, the nitrite accumulation ratio was exceeding 90%, and the maximum AOR can reach 14.5 ± 2.6 mg N L−1h−1. Although the half-saturation inhibition constant of NOB (KI_FNA(AOB)) reached 0.37 ± 0.10 mg HNO2N/L and showed extreme adaptability in FNA, the inactivation effect of FNA (6.1 mg HNO2N/L) for NOB was much greater than that of AOB, with inactivation rates of 0.61 ± 0.08 h−1 and 0.06 ± 0.01 h−1, respectively. The effluent pH was gradually reduced to 4.5 by ammonia oxidation process and the periodic FNA concentration reached 6.5 mg HNO2N/L to inactivate nitrite-oxidizing bacteria (NOB) without negatively affecting Nitrosomonas during long-term operation. This result provides new insights for the future implementation of high-rate stabilized acidic partial nitritation by Nitrosomonas.

Impacts of free nitrous acid on stabilizing food waste and sewage sludge for anaerobic digestion

This work investigated the effectiveness of free nitrous acid (FNA) in enhancing organic waste solubilization to improve biogas production in anaerobic digestion (AD). The results indicated that FNA pretreatment can enhance soluble organic content and control H2S odor in tested organic wastes, including food waste, sewage sludge, and their combination. However, a significant decrease (>50 %) in FNA concentration was found in the reactors, possibly due to denitrifier-driven NO2– consumption. Biochemical methane potential (BMP) tests showed a 25 ± 8 % enhancement in CH4 production in the reactors fed with mixed substrate pretreated with 2.9 mg FNA-N/L. However, the presence of NO2– (325.6–2368.0 mg N/L) in some BMP reactors, due to carryover from FNA pretreatment, adversely affected CH4 production (>55 %) and prolonged lag time (>4.2 times). These findings are valuable for researchers and practitioners in waste management, offering insights for implementing FNA pretreatment to enhance the biodegradability of organic wastes in AD.

Enhancing Single- and Two-Stage Anaerobic Digestion of Thickened Waste-Activated Sludge through FNA-Heat Pretreatment

This study aimed to investigate the effect of combined Free Nitrous Acid (FNA)-Heat (i.e., FNH) pretreatment on single- and two-stage anaerobic digestion (AD) of thickened waste-activated sludge (TWAS). Single-stage AD was conducted in batches, while two-stage AD involved acidogenic fermentation under semi-continuous flow followed by batch methanogenesis. FNH pretreatment was applied before the acidogenic stage, using 1.4 mg HNO2-N/L FNA concentration at 25 °C, 37 °C, and 60 °C for 24 h. Among the scenarios, the most promising results were observed with two-stage AD fed with FNH-pretreated TWAS at 60 °C, showing higher COD solubilization and a reduction in volatile solids. Combined FNA-Heat pretreatment in two-stage AD yielded elevated methane production (363–415 mL CH4/g VS added) compared to single-stage digestion. Methane yields from FNA-Heat pretreated single-stage ranged from 332 to 347 mL CH4/g VS added, contrasting with 212 mL CH4/g VS added for untreated TWAS. Methane generation commenced early in both untreated and pretreated samples, attributed to soluble substrate abundance.

Low-concentration organics mitigate the inhibition of free nitrous acid on nitrification in biofilters for gaseous ammonia removal

The accumulation of free nitrous acid (FNA), a by-product of nitritation, restricts biofilter ammonia removal under medium-to-high inlet ammonia loading by inhibiting nitrifiers. Preliminarily testing under varying C/N ratios validated that the addition of low-concentration organics carbon (OC, C/N = 1.0–1.5) effectively alleviated FNA accumulation, thereby improving biofilter performance. Subsequently, three parallel ammonia removal biofilters with different inocula were set to investigate the underlying mechanisms. Upon elevating the inlet ammonia loading rate to 22.8 ± 1.2 gN m–3h−1, the biofilters removed only 60.3 ± 10.0 % of gaseous ammonia under inorganic carbon (IC) supplementation. However, the introduction of low-concentration OC (C/N = 1.0) facilitated nitrogen loss, enriched denitrifying bacteria (DNB), and bolstered the denitrification functional abundance. The successful coupling of denitrification significantly reduced FNA concentrations by 2 to 3 orders of magnitude. Therefore, the revival of nitrifiers contributed to the enhanced ammonia removal efficiency (94.2 ± 8.4 %). Nitrite-oxidizing bacteria (NOB) and DNB subcommunities were identified as the driving factors for the mitigation of FNA and improvement of biofilter performance by the Mantel test. Activated heterotrophs facilitated ammonia conversion as well when OC was supplied, supported by Pearson correlation analysis. This study elucidated the synergistic dynamics between nitrifiers and DNB, emphasizing the favorable cooperation arising from the low-concentration OC addition (C/N = 1.0–1.5). Overall, this study highlights the cooperative interaction between heterotrophic denitrifiers and autotrophic nitrifiers under low-concentration OC supply, which played a vital role in alleviating FNA inhibition and enhancing the biofilter ammonia removal efficiency.

Effect of free ammonia and free nitrous acid on nitrogen removal of heterotrophic nitrification and aerobic denitrification bacteria

Free ammonia (FA) and free nitrous acid (FNA) can inhibit nitrogen removal by nitrifying microorganisms. However, current studies on FA and FNA mainly focus on traditional nitrifying microorganisms, and little is known about the relationship between heterotrophic nitrification-aerobic denitrification (HN-AD) microorganisms and FA and FNA. This study selected Alcaligenes faecalis WT14, an HN-AD strain, as the research object. The effects of different FA and FNA concentrations on the nitrogen removal ability of strain WT14 were investigated under neutral (pH = 7) and alkaline (pH = 9) conditions. The results showed that FA and FNA affected the removal ability of strain WT14 to ammonia nitrogen (NH4+-N) and nitrite nitrogen (NO2−-N). The rapid reduction of NH4+-N removal efficiency was determined at different concentrations of 6.12 mg·L−1 (pH = 7) and 551.18 mg·L−1 (pH = 9) for FA, which was 0.483 mg·L−1 (pH = 7) and 0.005 mg·L−1 (pH = 9) for FNA, respectively. Strain WT14 preferred an alkaline environment for growth; the FA tolerance value was 1134.25 mg·L−1 in such an environment, which was 51.8 times that of a neutral environment. Furthermore, strain WT14 had a higher removal rate of NH4+-N and NO2−-N in an alkaline environment. Its highest removal rates of NH4+-N and NO2−-N were 15.03 mg·L−1·h−1 and 7.16 mg·L−1·h−1, respectively, which was much higher than the reported HN-AD microorganisms.

Decoupling the simultaneous effects of NO2−, pH and free nitrous acid on N2O and NO production from enriched nitrifying activated sludge

In the pursuit of energy and carbon neutrality, nitrogen removal technologies have been developed featuring nitrite (NO2−) accumulation. However, high NO2− accumulations are often associated with stimulated greenhouse gas (i.e., nitrous oxide, N2O) emissions. Furthermore, the coexistence of free nitrous acid (FNA) formed by NO2− and proton (pH) makes the consequence of NO2− accumulation on N2O emissions complicated. The concurrent three factors, NO2−, pH and FNA may play different roles on N2O and nitric oxide (NO) emissions simultaneously, which has not been systematically studied. This study aims to decouple the effects of NO2− (0-200 mg N/L), pH (6.5-8) and FNA (0-0.15 mg N/L) on the N2O and NO production rates and the production pathways by ammonia oxidizing bacteria (AOB), with the use of a series of precisely executed batch tests and isotope site-preference analysis. Results suggested the dominant factors affecting the N2O production rate were NO2− and FNA concentrations, while pH alone played a relatively insignificant role. The most influential factor shifted from NO2− to FNA as FNA concentrations increased from 0 to 0.15 mg N/L. At concentrations below 0.0045 mg HNO2-N/L, nitrite rather than FNA played a significant role stimulating N2O production at elevated nitrite concentrations. The inhibition effect of FNA emerged with further increase of FNA between 0.0045-0.015 mg HNO2-N/L, weakening the promoting effect of increased nitrite. While at concentrations above 0.015 mg HNO2-N/L, FNA inhibited N2O production especially from nitrifier denitrification pathway with the level of inhibition linearly correlated with the FNA concentration. pH and the nitrite concentration regulated the production pathways, with elevated pH promoting the nitrifier nitrification pathway, while elevated NO2− concentrations promoting the nitrifier denitrification pathway. In contrast to N2O, NO emission was less susceptible to FNA at concentrations up to 0.015 mg N/L but was stimulated by increasing NO2− concentrations. This study, for the first time, distinguished the effects of pH, NO2− and FNA on N2O and NO production, thereby providing support to the design and operation of novel nitrogen removal systems with NO2− accumulation.

Effect of free nitrous acid on extracellular polymeric substances production and membrane fouling in a nitritation membrane bioreactor

The membrane bioreactor (MBR) with nitritation based nitrogen removal processes has attracted growing interest in recent years, although membrane fouling in the nitritation MBR is a challenging issue. In this study, the inhibitory effect of free nitrous acid (FNA) on microbial extracellular polymeric substances (EPS) production and membrane fouling in a nitritation MBR was investigated. Results showed that EPS played a critical role in the biofouling process, and EPS production was affected by FNA concentration. As FNA concentration increased from 5.10 × 10−3 mg N/L to 1.34 × 10−2 mg N/L, protein (PN) and polysaccharide (PS) contents increased from 8.20 to 60.28 mg/g VSS and 4.74–30.46 mg/g VSS, respectively. However, when FNA concentration was 1.48 × 10−2 mg N/L, PN and PS reduced by 20.0% and 10.9%, respectively, indicating that the higher FNA concentration could reduce EPS production. The EPS reduction could be attributed to reduction in the loosely bound (LB) and tightly bound (TB) EPS but not the soluble microbial products (SMP). It was further revealed that higher FNA concentrations up to 1.48 × 10−2 mg N/L consequently mitigate trans-membrane pressure (TMP) rate in terms of dTMP/dt by 25.5% in the nitritation MBR. High throughput sequencing analysis revealed that the increase in FNA led to enrichment of Nitrosomonas but reduction in heterotrophic bacteria. This study showed that the appropriate FNA concentration affected EPS production and hence membrane fouling, leading to the possibility of membrane fouling mitigation by in-situ generated FNA in the nitritation MBR.

Free ammonia-free nitrous acid based partial nitrification in sequencing batch membrane aerated biofilm reactor.

Membrane aerated biofilm reactor (MABR) has attracted a lot of attention as an energy-efficient integrated nitrogen removing technology in recent years. However, it is lacking of understanding to realize stable partial nitrification in MABR because of its unique oxygen transfer mode and biofilm structure. In this study, free ammonia (FA) and free nitrous acid (FNA) based control strategies for partial nitrification with low NH4+-N concentration were proposed in a MABR of sequencing batch mode. The MABR was operated for over 500 days under different influent NH4+-N concentrations. With the influent NH4+-N of around 200mg-N/L, partial nitrification could be established with relatively low concentration of FA (0.4-2.2mg-N/L) which suppressed nitrite oxidizing bacteria (NOB) on the biofilm. With lower influent NH4+-N concentration of around 100mg-N/L, the FA concentration was lower and strengthened suppression strategies based on FNA were needed. With the final pH of operating cycles below 5.0, the FNA produced in the sequencing batch MABR could stabilize partial nitrification by eliminating NOB on the biofilm. Since the activity of ammonia oxidizing bacteria (AOB) was lower without the blow-off of dissolved carbon dioxide in the bubbleless MABR, longer hydraulic retention time was required to reach a low pH for high concentration of FNA to suppress NOB. After exposures to FNA, the relative abundance of Nitrospira was decreased by 94.6%, while the abundance of Nitrosospira increased greatly which became another dominant AOB genus in addition to Nitrosomonas.

Free nitrous acid prediction in ANAMMOX process using hybrid deep neural network model

Free nitrous acid (FNA) is a critical metric for stabilization of ANAMMOX but can not be directly and immediately measured by sensors or chemical measurement method, which hinders the effective management and operation for ANAMMOX. This study focuses on FNA prediction using hybrid model based on temporal convolutional network (TCN) combined with attention mechanism (AM) optimized by multiobjective tree-structured parzen estimator (MOTPE), called MOTPE-TCNA. A case study in an ANAMMOX reactor is carried out. Results show that nitrogen removal rate (NRR) is highly correlated with FNA concentration, indicating that it can forecast the operational status by predicting FNA. Then, MOTPE successfully optimizes the hyperparameters of TCN, helping TCN achieve a high prediction accuracy, and AM furtherly improves model accuracy. MOTPE-TCNA obtains the highest prediction accuracy, whose R2 value gets 0.992, increasing 1.71–11.80% compared to other models. As a deep neural network model, MOTPE-TCNA has more advantages than traditional machine learning methods in FNA prediction, which is beneficial to maintain the stable operation and easy control for ANAMMOX process.

Free nitrous acid-assisted asymmetrical alternating current electrochemistry (FNA-AACE) for multi-heavy metals decontamination in waste activated sludge

Heavy metal contamination of waste activated sludge (WAS) is a key factor limiting the land application of sludge for nutrients recovery. This study proposes a novel free nitrous acid (FNA)-assisted asymmetrical alternating current electrochemistry (FNA-AACE) process to achieve high-efficiency decontamination of multi-heavy metals (Cd, Pb, and Fe) in WAS. The optimal operating conditions, the heavy metal removal performance of FNA-AACE, and the related mechanisms for maintaining the high performance were systematically investigated. During the FNA-AACE process, FNA treatment was optimal with an exposure time of 13 h at a pH of 2.9 and an FNA concentration of 0.6 mg/g TSS. Then the sludge was washed with EDTA in a recirculating leaching system under asymmetrical alternating current electrochemistry (AACE). The 6-h working and the following electrode cleaning were defined as a working circle of AACE. After three cycles of working-cleaning periods in AACE treatment, the cumulative removal efficiency of the toxic metals Cd and Pb reached over 97% and 93%, respectively, whilst that of Fe was greater than 65%. This surpasses most previously reported efficiencies and possesses a shorter treatment duration and sustainable EDTA circulation. The mechanism analysis suggested that FNA pretreatment provoked the migration of heavy metals for leaching enhancement, as well as reduced the demand for EDTA eluent concentration and increased conductivity, which can improve the AACE efficiency. Meanwhile, the AACE process absorbed the anionic chelates of heavy metals and reduced them to zero-valent particles on the electrode, regenerating the EDTA eluent and maintaining its high extraction efficiency for heavy metals. In addition, FNA-AACE could provide different electric field operation modes, allowing it to have flexibility for the real application processes. This proposed process is expected to be coupled with anaerobic digestion in wastewater treatment plants (WWTPs) for high efficiency of heavy metal decontamination, sludge reduction, and resource/energy recovery.

Rapid start-up and stable maintenance of the mainstream nitritation process based on the accumulation of free nitrous acid in a pilot-scale two-stage nitritation-anammox system

Two-stage partial nitritation (PN) and anammox (AMX) systems showed promising results for applying autotrophic nitrogen removal under mainstream conditions. In this study, a pilot-scale (600 L per reactor) two-stage PN/AMX system was installed in a municipal wastewater treatment plant (WWTP) provided with a high-rate activated sludge (HRAS) system for organic carbon removal. The PN/AMX system was operated without temperature control (ranging from 11 to 28 °C) and was subjected to the same variations in wastewater characteristics as the WWTP (22 to 63 mg NH4+- N/L). The developed strategy is simple, does not require the addition of chemicals and is characterised by short start-up periods. The PN process was established by applying a high hydraulic load and maintained by in situ accumulated free nitrous acid (FNA) of 0.015–0.2 mg HNO2-N/L. Based on pH value, a controlled aeration strategy was applied to achieve the target nitrite to ammonium ratio in the effluent (1.1 g NO2–-N/g NH4+-N) to feed the AMX reactor. Although NOB were not fully washed out from the system, nitrite accumulation remained (>99 %) stable with no evidence of NOB activity. In the AMX reactor, an overall nitrogen removal efficiency of 80 % was achieved. Regarding effluent quality, 12 ± 3 mg TN/L was obtained, but 5 mg NO3–-N/L was already in the HRAS effluent. The relative abundance of NOB showed a strong negative correlation with the FNA concentration, providing a good strategy for establishing PN under mainstream conditions.

Nitric oxide and Nitrous oxide accumulation, oxygen production during nitrite denitrification in an anaerobic/anoxic sequencing batch reactor: exploring characteristics and mechanism.

Nitrite denitrification has received increasing attention due to its high efficiency, low energy consumption, and sludge yield. However, the nitric oxide (NO) and nitrous oxide (N2O) which are harmful to the environment, microorganisms, and humans are produced in this process. In order to mitigate NO and N2O production, the biological mechanisms of NO and N2O accumulation, as well as NO detoxification during nitrite denitrification in a sequencing batch reactor were studied. Results showed that the peak of NO accumulation increased from 0.29 [Formula: see text] 0.01 to 3.12 [Formula: see text] 0.34mg L-1 with the increase of carbon to nitrogen ratio (COD/N), which is caused by the sufficient electron donor supply for NO2--N reduction process at high COD/N. Furthermore, the result suggested that NO accumulation with no pH adjustment was 12 times higher than that with pH adjustment. It is related to the inhibition on NO reductase caused by the high free nitrous acid (FNA) and NO concentration with no pH adjustment. The pathways of NO detoxification included NO emission, reduction, and dismutation, and the more NO produced, the high proportion of NO dismutation pathway. Result showed that the maximum of oxygen production during NO dismutation reached to 1.39mg L-1. N2O accumulation was mainly associated with FNA and NO inhibition, COD/N. The peak of N2O accumulation presented a completely opposite trend at pH adjustment and no pH adjustment, it is because that the higher FNA and NO concentration at high COD/N without pH adjustment will inhibit the N2O reductase activity, resulting in the N2O reduction was hindered during nitrite denitrification.

Free nitrous acid (FNA) pretreatment enhances biomethanation of lignocellulosic agro-waste (wheat straw)

Agro-residues having lignocellulosic biomass are considered the most effective source (heating value 16 MJ/kg) for energy production through anaerobic digestion (AD). However, complex lignocellulosic fraction in agro-residue obstructs its biotransformation and is a rate-limiting step of the AD process. This study presents a novel free nitrous acid (FNA) pretreatment to enhance the biomethanation of wheat straw (WS). The effect of FNA pretreatment was evaluated through anaerobic digestion test and model-based evaluation performed in two phases, i.e., mono-digestion (phase I) and co-digestion of wheat straw with cattle manure and food waste (phase II). For phase I, the first-order, transference, and dual pool first-order models showed the best model fit with R2 > 0.98, but the dual pool first-order model (DP-FOM) showed the least root mean square error (RMSE) of 1.88–62.10. The DP-FOM analysis of assays from anaerobic digestion test showed the biogas yields of 576, 564, and 565 mL/g VSadded at 1.77, 3.54, and 5.31 mg HNO2–N/L FNA concentrations and 12 h reaction time, respectively. The corresponding increment in biogas yields was 10.87, 8.85 and 8.80%. In phase II, DP-FOM analysis revealed the best model fit with R2 > 0.99 and the least RMSE of 8.88–13.04. The DP-FOM analysis from assays from anaerobic digestion test for 12 h FNA pretreatment showed the peak biogas yields of 715, 648, and 654mL/gVSadded at 1.77, 3.54 and 5.31 mg HNO2–N/L FNA concentrations, respectively. The corresponding improvement in biogas yields was 32.24, 19.81, and 20.90%. The FNA pretreatment can potentially improve the biogas yield from wheat straw effectively under mono- and co-digestion conditions. Even co-digestion could achieve enhanced biogas yield over mono-digestion.

Anammox in a biofilter reactor to treat wastewater of high strength nitrogen

An anammox biofilter reactor (A-BFR) under high total nitrogen (TN) concentration and high nitrogen loading rate (NLR) was operated for 283 days and the biomass characteristics were investigated at the end of the experiment to discuss the origin of strong impact resistance of A-BFR. The result showed TN removal rate of A-BFR reached 81.76 % averagely under the influent concentration of 525 mg/L and the NLR of 6.30 kg N/m3/d. Furthermore, A-BFR was partitioned as the core zone and the polish zone spontaneously according to the water quality along the height of reactor due to the characteristics of plug flow. Although the core zone bore an actual NLR of 17.71 kg N/m3/d, and an average free ammonia (FA) concentrations of 13.35 mg/L as well as a mean free nitrous acid (FNA) concentrations of 36.89 μg/L, it still obtained the TN removal of >80 %, which could be attributed to the high biomass of 30.67 gVSS/L comprised of biofilm and granular sludge aggregates, the high specific anammox activity up to 49.24 mg N/g VSS/d, and high extracellular polymeric substances of 148.69 mg/gVSS. In addition, the enrichment of anammox bacteria in the core zone attained over 50 %, and the dominant bacteria were unclassified_Candidatus_Brocadiaceae and Candidatus Kuenenia after the operation of 283 days. These findings revealed the unique characteristics of biomass in the core zone decided the robust capacity of A-BFR to resist high NLR and the inhibition of FA and FNA from high TN concentration, which would enrich the knowledge and understanding of anammox with plug flow reactor under higher FA and FNA concentrations, implying BFR was a robust and resilient reactor for anammox.

Investigating the long and short-term effect of free ammonia and free nitrous acid levels on nitritation biomass of a sequencing batch reactor treating thermally pre-treated sludge reject water

This work examined the short and long-term effects of different free ammonia (FA) and free nitrous acid (FNA) levels on (i) acclimatized biomass treating sludge reject water via nitrite in a sequencing batch reactor (SBR) and (ii) non-aclimatized biomass treating municipal wastewater via nitrate in the activated sludge process. In the acclimatized biomass, the threshold for the transition from nitrification to nitritation was the FA increase to 10–20 mgNH3-N/L while the SBR unit showed no inhibition on the ammonia uptake rate (AUR) at FA levels up to 65 mgNH3-N/L. Short-term exposure of the acclimatized biomass on FNA showed that AUR inhibition could be more than 50 % for FNA concentration >10 μgHNO2-N/L. The FNA inhibition results were simulated using non-competitive inhibition kinetics that showed that the inhibition constant corresponding to the FNA concentration that inhibits the process by 50 % (i.e. KiFNA) was much higher in the acclimatized biomass.

Free Nitrous Acid Inhibits Atenolol Removal during the Sidestream Partial Nitritation Process through Regulating Microbial-Induced Metabolic Types.

Limited studies have attempted to evaluate pharmaceutical removal during the sidestream partial nitritation (PN) process. In this work, atenolol biodegradation by PN cultures was investigated by maintaining ammonium and pH at different levels. For the first time, free nitrous acid (FNA), other than ammonium, pH, and free ammonia, was demonstrated to inhibit atenolol removal, with biodegradation efficiencies of ∼98, ∼67, and ∼28% within 6 days at average FNA levels of 0, 0.03, and 0.19 mg-N L-1, respectively. Ammonia-oxidizing bacteria (AOB)-induced metabolism was predominant despite varying FNA concentrations. In the absence of ammonium/FNA, atenolol was mostly biodegraded via AOB-induced metabolism (65%) and heterotroph-induced metabolism (33%). AOB-induced metabolism was largely inhibited (down to 29%) at 0.03 mg-N L-1 FNA, while ∼27 and ∼11% were degraded via heterotroph-induced metabolism and AOB-induced cometabolism, respectively. Higher FNA (0.19 mg-N L-1) substantially reduced atenolol biodegradation via heterotroph-induced metabolism (4%), AOB-induced metabolism (16%), and AOB-induced cometabolism (8%). Newly identified products and pathways were related to metabolic types and FNA levels: (i) deamination and decarbonylation (AOB-induced cometabolism, 0.03 mg-N L-1 FNA); (ii) deamination from atenolol acid (heterotrophic biodegradation); and (iii) nitro-substitution (reaction with nitrite). This suggests limiting FNA to realize simultaneous nitrogen and pharmaceutical removal during the sidestream process.

Determining Factors for Nitrite Accumulation in an Acidic Nitrifying System: Influent Ammonium Concentration, Operational pH, and Ammonia-Oxidizing Community.

Acidic nitrification is attracting wide attention because it can enable robust suppression of nitrite-oxidizing bacteria (NOB) in wastewater treatment. This study reports a comprehensive assessment of the novel acidic nitrification process to identify the key factors that govern stable nitrite accumulation. A laboratory-scale moving-bed biofilm reactor receiving low-alkalinity wastewater was continuously operated under acidic conditions (pH < 6) for around two years, including nine stages varying influent and operational conditions. The results revealed that nitrite accumulation was related to three factors, i.e., influent ammonium concentration, operating pH, and ammonia-oxidizing microbial community. These three factors impact nitrite accumulation by altering the in situ concentration of free nitrous acid (FNA), which is a potent inhibitor of NOB. The critical FNA concentration is approximately one part per million (ppm, ∼1 mg HNO2-N/L), above which nitrite accumulation is stably maintained in an acidic nitrifying system. The findings of this study suggest that stable nitrite accumulation via acidic ammonia oxidation can be maintained under a range of influent and operational conditions, as long as a ppm-level of FNA is established. Taking low-strength mainstream wastewater (40-50 mg NH4+-N/L) with limited alkalinity as an example, stable nitrite accumulation was experimentally demonstrated at a pH of 4.35, under which an in situ FNA of 2.3 ± 0.6 mg HNO2-N/L was attained. Under these conditions, Candidatus Nitrosoglobus became the only ammonia oxidizer detectable by 16S rRNA gene sequencing. The results of this study deepen our understanding of acidic nitrifying systems, informing further development of novel wastewater treatment technologies.

Ammonia oxidation intermediates-induced abiotic transformation of 17α-ethinylestradiol in nitrification process of wastewater biological treatment

17α-Ethinylestradiol (EE2) has aroused growing concern due to its increasing worldwide prevalence and high ecotoxicological risk. It has been reported that the removal of EE2 in the nitrification process of wastewater treatment plants (WWTPs) is attributed to co-metabolism induced by ammonia oxidizers. However, the effect of ammonia oxidation intermediates produced by ammonia oxidizers on the abiotic transformation of EE2 is unclear. In this study, the effect of several ammonia oxidation intermediates, including free nitrous acid (FNA), hydroxylamine (NH2OH), and nitric oxide (NO) on the abiotic transformation of EE2 was evaluated comprehensively. The dynamics of EE2 transformation induced by FNA were affected by the FNA concentration and the initial EE2 concentration. During EE2 nitration induced by FNA, 2,4-nitro-EE2 would be the final nitration product of EE2. FNA-mediated transformation of EE2 was effective for reducing estrogenicity and the transformation was stable under decreased FNA levels or during refrigeration at 4 °C. The key reactive nitrogen species derived from the FNA-induced EE2 nitration reaction have been verified to be nitrogen dioxide (NO2) and/or NO by using a scavenger. Under acidic conditions, NO2 was inferred to be the key reactive nitrogen species inducing the nitration reaction of EE2. As an ammonia oxidation intermediate or FNA-derived species, NO can transform EE2 under alkaline conditions. Unlike FNA and NO, NH2OH could not transform EE2. These findings provide a deeper understanding of the abiotic transformation mechanism of EE2 induced by ammonia oxidation intermediates, exhibiting a significant value in the removal of micropollutants during ammonia oxidation in WWTPs.

Recovery of ammonium nitrate solution from urine wastewater via novel free nitrous acid (FNA)-mediated two-stage processes

Nitrogen (N) recovery from urine wastewater has been an important route to overcome intensive energy consumption in the conventional N-removal process. Hereby, this study reports a new process for recovering N in the form of ammonium nitrate (AN, i.e. NH4NO3), a common, stabilized type of fertilizer in agriculture. The process is achieved through two-stage biological–chemical reactions by using free nitrous acid (FNA) as an intermediate. In the first stage, a biological reactor was operated for 170 days, with ammonium-rich human urine wastewater as influent and gradually increased influent ammonium loading rates. Then a closed chemical reactor was designed to generate AN from the effluent of the biological reactor. Experimental results demonstrate that the two-stage biological-chemical process is technically and economically viable. Comprehensive modelling analyses show that FNA is the key to maintain stable partial nitritation in the biological reactor, and also critically affects the reaction rate of the chemical nitrite oxidation. pH adjustment of the bioreactor effluent is needed to increase an FNA concentration up to 250 mg HNO2-N/L in the biological reactor effluent. As such, chemical nitrite oxidation can reach a substantially high rate, i.e. at 15 kg N/(m3·d), compared with the nitrite production rate of ∼0.5 kg N/(m3·d) achieved in the biological reactor. This result also indicates that the biological stage is a rate-limiting step in the overall process. Overall, the proposed FNA-based approach provides a promising option for nitrogen recovery from urine wastewater.

A novel stable nitritation process: Treating sludge by alternating free nitrous acid/heat shock

The feasibility of stable nitritation of sludge alternately treated by free nitrous acid (FNA) and heat shock in a sequencing batch reactor (SBR) was investigated in this study. The linear regression method was used to determine the optimal treatment conditions. Results revealed that an FNA concentration of 2.20 mg HNO2-N/L, exposure time of 24 h, and treatment ratio of 20% could inhibit nitrite oxidizing bacteria (NOB) activity to the greatest extent while maintaining the maximum ammonium oxidizing bacteria (AOB) activity; after heat shock at 60 °C for 20 min, NOB were inhibited while AOB still had certain activity. In the long-term continuous-flow experiment, the single FNA or heat shock treatments easily allowed adapt NOB to affect the stability of nitritation. The alternating FNA/heat shock treatment can achieve long-term stability of nitritation. Microbial community analysis revealed that the alternating FNA/heat shock treatment could inhibit NOB while maintaining high AOB abundance.