Effects of pH on nitrite accumulation during wastewater denitrification

Distribution of nitrate/nitrite and toxic metals in the soil-potato system and its health risk assessment in Iran

Potato is one of the essential food products whose health quality is greatly influenced by soil contamination and properties. In the current study, we have investigated the physicochemical characteristics of agricultural areas and the accumulation of nitrite/nitrate and metals in potato products in Hamedan, Iran. After determining the physicochemical characteristics of soil samples from four agricultural regions of Hamedan, 48 potato samples were collected from these regions. The heavy metals and nitrate/nitrite content were determined by ICP-OES and calorimetric methods, respectively. A negative correlation was observed between soil pH changes with nitrite/nitrate content and the accumulation of some heavy elements in potatoes. Furthermore, a positive correlation was found between soil phosphorus content and lead accumulation in potato. In present study, the amounts of lead, nitrate, and nitrite in 83.3%, 56%, and 12% of the collected samples were higher than the permissible limit reported by the World Health Organization (WHO), respectively. The EDI range for nitrate and nitrite was determined to be 130-260 and 1.4-2.7 µg/kg/day, respectively, which is much lower than the RfD set by the US Environmental Protection Agency (USEPA) for nitrite and nitrate. Among metal pollutants, the toxic risk caused by lead in potato consumers was higher than the threshold limit. In conclusion, our findings showed that the physicochemical characteristics of the soil could effectively increase the availability of metal pollutants and nitrite/nitrate to the potato product and significantly reduce its health quality. Therefore, monitoring these pollutants in the soil-potato system, preventing the entry of industrial wastewater, and managing the use of agricultural fertilizers can effectively improve the health of this product for consumers.

Weak magnetic carriers reduce nitrite accumulation and boost denitrification at high nitrate concentrations by enriching functional bacteria and enhancing electron transfer

Biological denitrification is the dominant method for NO3- removal from wastewater, while high NO3- leads to NO2- accumulation and inhibits denitrification performance. In this study, different weak magnetic carriers (0, 0.3, 0.6, 0.9 mT) were used to enhance biological denitrification at NO3- of 50-2400 mg/L. The effect of magnetic carriers on the removal and mechanism of denitrification of high NO3- was investigated. The results showed that 0.6 and 0.9 mT carriers significantly enhanced the TN removal efficiency (>99%) and reduced the accumulation of NO2- (by > 97%) at NO3- of 1200-2400 mg/L 0.6 and 0.9 mT carriers stimulated microbial electron transport by improving the abundances of coenzyme Q-cytochrome C reductase (by 4.44-23.30%) and cytochrome C (by 2.90-16.77%), which contributed to the enhanced elimination of NO3- and NO2-. 0.6 and 0.9 mT carriers increased the activities of NAR (by 3.74-37.59%) and NIR (by 5.01-8.24%). The abundance of narG genes in 0.6 and 0.9 mT was 1.47-2.35 and 1.38-1.75 times that of R1, respectively, and the abundance of nirS genes was 1.49-2.83 and 1.55-2.39 times that of R1, respectively. Denitrifying microorganisms, e.g., Halomonas, Thauera and Pseudomonas were enriched at 0.6 and 0.9 mT carriers, which benefited to the advanced denitrification performance. This study suggests that weak magnetic carriers can help to enhance the biological denitrification of high NO3- wastewater.

Efficient nitrite accumulation in partial sulfide autotrophic denitrification (PSAD) system: insights of S/N ratio, pH and temperature

To provide the necessary nitrite for the Anaerobic Ammonium Oxidation (ANAMMOX) process, the effect of nitrite accumulation in the partial sulfide autotrophic denitrification (PSAD) process was investigated using an SBR reactor. The results revealed that the effectiveness of nitrate removal was unsatisfactory when the S/N ratio (mol/mol) fell below 0.6. The optimal conditions for nitrate removal and nitrite accumulation were achieved within the S/N ratio range of 0.7-0.8, resulting in an average Nitrate Removal Efficiency (NRE) of 95.84%±4.89% and a Nitrite Accumulation Rate (NAR) of 75.31%±6.61%, respectively. It was observed that the nitrate reduction rate was three times faster than that of nitrite reduction during a typical cycle test. Furthermore, batch tests were conducted to assess the influence of pH and temperature conditions. In the pH tests, it became evident that the PSAD process performed more effectively in alkaline environment. The highest levels of nitrate removal and nitrite accumulation were achieved at an initial pH of 8.5, resulting in a NRE of 98.30%±1.93% and a NAR of 85.83%±0.47%, respectively. In the temperature tests, the most favourable outcomes for nitrate removal and nitrite accumulation were observed at 22±1 ℃, with a NRE of 100.00% and a NAR of 81.03%±1.64%, respectively. Moreover, a comparative analysis of 16S rRNA sequencing results between the raw sludge and the sulfide-enriched culture sludge sample showed that Proteobacteria (49.51%) remained the dominant phylum, with Thiobacillus (24.72%), Prosthecobacter (2.55%), Brevundimonas (2.31%) and Ignavibacterium (2.04%) emerging as the dominant genera, assuming the good nitrogen performance of the system.

Using selectivity to evaluate aqueous- and resin-phase denitrification during biological ion exchange

An increased fertilizer application for agricultural purposes has resulted in increased nitrate (NO3-) levels in surface water and groundwater around the globe, highlighting demand for a low-maintenance NO3- treatment technology that can be applied to nonpoint sources. Ion exchange (IEX) is an effective NO3- treatment technology and research has shown that bioregeneration of NO3- laden resins has the potential to minimize operational requirements and brine waste production that often prevents IEX application for decentralized treatment. In this work, batch denitrification experiments were conducted using solutions with low IEX selectivity capable of supporting the growth of denitrifying bacteria, while minimizing NO3- desorption from resins, encouraging resin-phase denitrification. Although only 15% of NO3- was desorbed by the low selectivity solution, this initial desorption started a cycle in which desorbed NO3- was biologically transformed to NO2-, which further desorbed NO3- that could be biotransformed. Denitrification experiments resulted in a 43% conversion rate of initially adsorbed NO3-, but biotransformations stopped at NO2- due to pH limitations. The balance between adsorption equilibria and biotransformation observed in this work was used to propose a continuous-flow reactor configuration where gradual NO3- desorption might allow for complete denitrification in the short retention times used for IEX systems.

Microelectrolysis-integrated constructed wetland with sponge iron filler to simultaneously enhance nitrogen and phosphorus removal

Integrating sponge iron (SI) and microelectrolysis individually into constructed wetlands (CWs) to enhance nitrogen and phosphorus removal are challenged by ammonia (NH₄⁺-N) accumulation and limited total phosphorus (TP) removal efficiency, respectively. In this study, a microelectrolysis-assisted CW using SI as filler surrounding the cathode (e-SICW) was successfully established. Results indicated that e-SICW reduced NH₄⁺-N accumulation and intensified nitrate (NO₃^--N), the total nitrogen (TN) and TP removal. The concentration of NH₄⁺-N in the effluent from e-SICW was lower than that from SICW in the whole process with 39.2-53.2 % decrease, and as the influent NO₃^--N concentration of 15 mg/L and COD/N ratio of 3, the removal efficiencies of NO₃^--N, TN and TP in e-SICW achieved 95.7 ± 1.9 %, 79.8 ± 2.5 % and 98.0 ± 1.3 %, respectively. Microbial community analysis revealed that hydrogen autotrophic denitrifying bacteria of Hydrogenophaga was highly enriched in e-SICW.

Response of nitrogen removal performance and microbial community to a wide range of pH in thermophilic denitrification system

Thermophilic biological nitrogen removal would be a promising alternative to conventional approaches for the treatment of high-temperature wastewater. In this study, the response of thermophilic denitrification system (50 °C) to a wide range of pH (3-11) was investigated. The results showed that thermophilic denitrification could adapt to pH 5-11, but suffered from obvious nitrite and ammonia accumulation at pH 3. Microbial insights indicated that the enrichment of specific functional thermophiles has contributed to the tolerance towards unfavorable pH. Besides, the potential selecting advantage of nitrate reducing bacteria over nitrite reducing bacteria and the enrichment of dissimilatory nitrate reduction to ammonium (DNRA) bacteria could be responsible for the nitrite and ammonia accumulation at pH 3. Moreover, the functional gene prediction denoted higher narG/(nirK + nirS) and nrfA at pH 3, which could facilitate partial denitrification and DNRA. These findings could provide new insight into the application of thermophilic biological nitrogen removal.

Nitrogen dynamic in vitro using sludge of a sewage stabilization pond from Patagonia (Argentina)

A relevant and current aspect of wastewater treatment systems is related to the processes of the nitrogen cycle that results in its elimination in gaseous forms. In the present study, we report the first measurements of nitrate-reducing rate (NRR) at lab-scale, using the flow-through reactor technique with sludge of a sewage stabilization pond system located in Patagonia (Argentina). Sludge was collected from Inlet and Outlet areas, in winter and summer. The sludge was characterized by having high moisture content (>94%) and organic matter concentration greater than 37%. The nitrate reduction experimental dates fitted significantly to the Michaelis-Menten model, allowing the estimation of the parameters that regulate the NR kinetics. The maximum potential nitrate reduction rate (Rmax) showed great variability, registering a maximum of 131.6 μmol-N·gdw-1·h-1 (Outlet-Summer) and a minimum of 4.1 μmol-N·gdw-1·h-1 (Inlet-Winter). The lowest half saturation constant (Km) was recorded in the Inlet sludge during the winter (6.1 mg N-NO3-·L-1), which indicates a greater affinity for nitrate of this bacterial consortium. An unusually high activity of NR was registered, being higher with sludge from the Outlet zone and with summer temperature. In full-scale ponds, the NR activity could explain a relevant part of the nitrogen removal that involves the escape of gaseous forms.

NH4+-N/NO3--N ratio controlling nitrogen transformation accompanied with NO2--N accumulation in the oxic-anoxic transition zone

Although nitrogen (N) transformations have been widely studied under oxic or anoxic condition, few studies have been carried out to analyze the transformation accompanied with NO₂^--N accumulation. Particularly, the control of mixed N species in N-transformation remains unclear in an oxic-anoxic transition zone (OATZ), a unique and ubiquitous redox environment. To bridge the gap, in this study, OATZ microcosms were simulated by surface water and sediments of a shallow lake. The N-transformation processes and rates at different NH₄⁺-N/NO₃^--N ratios, and NO₂^--N accumulations in these processes were evaluated. N-transformation process exhibited a turning point. Simultaneous nitrification and denitrification occurred in its early stage (first 10 days, dissolved oxygen (DO) ≥ 2 mg/L) and then denitrification dominated (after 10 days, DO < 2 mg/L), which were not greatly affected by the NH₄⁺-N/NO₃^--N ratio, on the contrary, the transformation rates of NH₄⁺-N and NO₃^--N were distinctly affected. The NH₄⁺-N transformation rates were positively correlated with the NH₄⁺-N/NO₃^--N ratio. The highest NO₃^--N transformation rate was observed at an NH₄⁺-N/NO₃^--N ratio of 1:1 with organic carbon/NO₃^--N of 3.09. The NO₂^--N accumulation, which increased with the decrease in NH₄⁺-N/NO₃^--N ratio, was also controlled by organic carbon concentration and type. The peak concentration of NO₂^--N accumulation occurred only when the NO₃^--N transformation rate was particularly low. Thus, NO₂^--N accumulation may be reduced by adjusting the control parameters related to N and organic carbon sources, which enhances the theoretical insights for N-polluted aquatic ecosystem bioremediation.

Isolation and Characterization of Nitrate-Reducing Bacteria as Potential Probiotics for Oral and Systemic Health

Recent evidence indicates that the reduction of salivary nitrate by oral bacteria can contribute to prevent oral diseases, as well as increase systemic nitric oxide levels that can improve conditions such as hypertension and diabetes. The objective of the current manuscript was to isolate nitrate-reducing bacteria from the oral cavity of healthy donors and test their in vitro probiotic potential to increase the nitrate-reduction capacity (NRC) of oral communities. Sixty-two isolates were obtained from five different donors of which 53 were confirmed to be nitrate-reducers. Ten isolates were selected based on high NRC as well as high growth rates and low acidogenicity, all being Rothia species. The genomes of these ten isolates confirmed the presence of nitrate- and nitrite reductase genes, as well as lactate utilization genes, and the absence of antimicrobial resistance, mobile genetic elements and virulence genes. The pH at which most nitrate was reduced differed between strains. However, acidic pH 6 always stimulated the reduction of nitrite compared to neutral pH 7 or slightly alkaline pH 7.5 (p < 0.01). We tested the effect of six out of 10 isolates on in vitro oral biofilm development in the presence or absence of 6.5 mM nitrate. The integration of the isolates into in vitro communities was confirmed by Illumina sequencing. The NRC of the bacterial communities increased when adding the isolates compared to controls without isolates (p < 0.05). When adding nitrate (prebiotic treatment) or isolates in combination with nitrate (symbiotic treatment), a smaller decrease in pH derived from sugar metabolism was observed (p < 0.05), which for some symbiotic combinations appeared to be due to lactate consumption. Interestingly, there was a strong correlation between the NRC of oral communities and ammonia production even in the absence of nitrate (R = 0.814, p < 0.01), which indicates that bacteria involved in these processes are related. As observed in our study, individuals differ in their NRC. Thus, some may have direct benefits from nitrate as a prebiotic as their microbiota naturally reduces significant amounts, while others may benefit more from a symbiotic combination (nitrate + nitrate-reducing probiotic). Future clinical studies should test the effects of these treatments on oral and systemic health.

Recent advances in controlling denitritation for achieving denitratation/anammox in mainstream wastewater treatment plants

Denitratation (NO₃^-→NO₂^-)/anammox is a promising method for anammox application in mainstream wastewater treatment plants (WWTPs) to reduce oxygen and organic matter consumption. Achieving nitrite production via denitratation and controlling denitritation (NO₂^-→N₂) is the basis of the denitratation/anammox process. To control denitritation, the denitrifying biocommunity and growth rate are critically reviewed for biocommunity optimization. Then, the short-term and long-term effects of pH on denitritation were summarized and the possible mechanism was discussed, along with the effect of C/N ratio and organic matter type on denitritation. Meanwhile, the strategies for producing nitrite via controlling denitritation are discussed, as well as the processes for achieving nitrogen removal via denitratation/anammox in WWTPs. Finally, the practical application of denitratation/anammox in a full-scale mainstream WWTP is documented.

Effects of carbon availability in a woody carbon source on its nitrate removal behavior in solid-phase denitrification

Woody biomass is the most common natural carbon source applied in solid-phase denitrification (SPD). However, its denitrification ability is low in the SPD process due to its poor carbon availability. In this study, sawdust samples were pretreated to various degrees, and then filled into SPD bioreactors to reveal the relationship between carbon availability and denitrification behaviors. The behaviors include the denitrification process, internal effects of major factors (carbon availability, pH and temperature), and the presence of bacterial communities. Results shown that the long-term denitrification rate of pretreated sawdust was increased by 4.5-4.8 times over that of untreated sawdust (29.3 mg N L^-1 sawdust d^-1). However, despite improving the pretreatment degree of the sawdust in the bioreactor, the long-term denitrification rate shown no further increase. The denitrification rate was most influenced by the temperature, followed by the pH, and then the sawdust pretreatment degree. The denitrification rate increased with decreasing pH and rising temperature of the pretreated sawdust. The removed nitrate was rarely converted into nitrite or nitrous oxide, but ammonium was produced at high pH and temperature for the pretreated sawdust. The adverse effects of ammonium and dissolved organic carbon (DOC) reduced when the pH of the pretreated sawdust was lowered to 6.5. Hydrolytic and denitrifying bacteria formed the main SPD bioreactor bacteria, whose abundances increased with increasing sawdust pretreatment degree. The results were beneficial to reduce the hydrolytic retention time and adverse products for the SPD system using woody carbon source.

Nitrogen removal from coke making wastewater through a pre-denitrification activated sludge process

Under the Industrial Emissions Directive (IED), coke production wastewater must be treated to produce an effluent characterised by a total nitrogen (TN) <50 mg/L. An anoxic-aerobic activated sludge pilot-plant (1 m³) fed with coke production wastewater was used to investigate the optimal operational requirements to achieve such an effluent. The loading rates applied to the pilot-plant varied between 0.198-0.418 kg COD/m³.day and 0.029-0.081 kg TN/m³.day, respectively. The ammonia (NH₄⁺-N) removals were maintained at 96%, after alkalinity addition. Under all conditions, phenol and SCN^- remained stable at 96% and 100%, respectively with both being utilised as carbon sources during denitrification. The obtained results showed that influent soluble chemical oxygen demand (sCOD) to TN ratio of should be maintained at >5.7 to produce an effluent TN <50 mg/L. Furthermore, nitrite accumulation was observed under all conditions indicating a disturbance to the denitrification pathway. Overall, the anoxic-aerobic activated sludge process was shown to be a robust and reliable technology to treat coke making wastewater and achieve the IED requirements. Nevertheless, the influent to the anoxic tank should be monitored to ensure a sCOD:TN ratio >5.7 or, alternately, the addition of an external carbon source should be considered.

Dominance ofCandidatus saccharibacteriain SBRs achieving partial denitrification: effects of sludge acclimating methods on microbial communities and nitrite accumulation

Partial denitrification (NO₃⁻-N → NO₂⁻-N) was combined with anaerobic ammonium oxidation (ANAMMOX) to achieve nitrogen removal with a low C/N ratio and low energy consumption. Three different acclimation conditions, namely, R1 (sequencing batch reactor (SBR) under anoxic conditions), R2 (SBR under alternating anoxic/aerobic conditions), and R3 (SBR under low-intensity aeration), were investigated using glucose as an electron donor to achieve continuous accumulation of nitrite during a 120 d run. Subsequently, the denitrification performance and microbial community structure of the sludge were investigated. The results showed that the acclimatized sludge in reactors R2 and R3 achieved better partial denitrification performance than the sludge in R1 due to the presence of dissolved oxygen as a result of aeration. Notably, the R3 reactor had the optimal conditions for nitrite accumulation. The high-throughput sequencing analysis indicated that the dominant bacteria in R2 and R3 were Candidatus saccharibacteria with a relative abundance of 45.44% and 34.96%, respectively. This was the first time that Candidatus saccharibacteria was reported as the dominant bacteria in denitrifying sludge. The microbial diversity of the R1 reactor was much greater than that of R2 and R3, indicating that a larger proportion of denitrifying bacteria were present in the R2 and R3 reactors. In addition, the batch experiments showed that the higher the initial pH, the higher the nitrite accumulation rate was.

Partial denitrification (NO₃⁻-N → NO₂⁻-N) was combined with anaerobic ammonium oxidation (ANAMMOX) to achieve nitrogen removal with a low C/N ratio and low energy consumption.

Inhibition by free nitrous acid (FNA) and the electron competition of nitrite in nitrous oxide (N2O) reduction during hydrogenotrophic denitrification

Hydrogenotrophic denitrification is a promising technology for nitrate removal from organic-deficient wastewater or groundwater, and the attention of nitrous oxide (N₂O) emission during this process is required. Both nitrite and free nitrous acid (FNA or HNO₂) were reported to exert significant effects on N₂O reduction in heterotrophic denitrification, whereas, little knowledge has been obtained in hydrogenotrophic denitrification. In this study, we conducted a series of batch tests to comprehensively investigate the effects of nitrite, pH and FNA on N₂O production and reduction in a hydrogenotrophic denitrification process. The results showed that N₂O reduction rate decreased under both conditions of low pH and presence of nitrite, which would exert synergetic inhibition on N₂O reduction. The potential mechanisms that give rise to the results included electron competition and FNA inhibition. Electron competition between nitrite and N₂O reductases occurred when both nitrite and N₂O were added, which might contribute to the decrease in the N₂O reduction rate. The electron supply, which was obtained from the uptake of molecular hydrogen, declined with increasing FNA concentration according to a logarithmic model (R² = 0.9240). Additionally, the electron consumption rate of N₂O reductase to nitrite reductase ratio was initially stable and then decreased with increasing FNA concentration. The inhibition of N₂O reduction by FNA was determined to be reversible. The study suggested that both of the electron supply and N₂O reduction in hydrogenotrophic denitrification could be inhibited by FNA.

Effects of nitrate and sulfate on the performance and bacterial community structure of membrane-less single-chamber air-cathode microbial fuel cells

Membrane-less, single-chamber, air-cathode, microbial fuel cells (ML-SC MFCs) have attracted attention as being suitable for wastewater treatment. In this study, the effects of nitrate and sulfate on the performance of ML-SC MFCs and their bacterial structures were evaluated. The maximum power density increased after nitrate addition from 8.6 mW·m^-2 to 14.0 mW·m^-2, while it decreased after sulfate addition from 11.5 mW·m^-2 to 7.7 mW·m^-2. The chemical oxygen demand removal efficiencies remained at more than 90% regardless of the nitrate or sulfate additions. The nitrate was removed completely (93.0%) in the ML-SC MFC, while the sulfate removal efficiency was relatively low (17.6%). Clostridium (23.1%), Petrimonas (20.0%), and unclassified Rhodocyclaceae (6.2%) were dominant on the anode before the addition of nitrate or sulfate. After the addition of nitrate, Clostridium was still the most dominant on the anode (23.6%), but Petrimonas significantly decreased (6.0%) and unclassified Rhodocyclaceae increased (17.1%). After the addition of sulfate, the amount of Clostridium almost doubled in the composition on the anode (43.2%), while Petrimonas decreased (5.5%). The bacterial community on the cathode was similar to that on the anode after the addition of nitrate. However, Desulfovibrio was remarkably dominant on the cathode (32.9%) after the addition of sulfate. These results promote a deeper understanding of the effects of nitrate or sulfate on the ML-SC MFCs' performance and their bacterial community.

Diversity of culturable aerobic denitrifying bacteria in the sediment, water and biofilms in Liangshui River of Beijing, China

Aerobic denitrification is a process reducing the nitrate into gaseous nitrogen forms in the presence of oxygen gas, which makes the nitrification and denitrification performed simultaneously. However, little was known on the diversity of the culturable aerobic denitrifying bacteria in the surface water system. In this study, 116 strains of aerobic denitrifying bacteria were isolated from the sediment, water and biofilm samples in Liangshui River of Beijing. These bacteria were classified into 14 genera based on the 16 S rDNA, such as Pseudomonas, Rheinheimera, and Gemmobacter. The Pseudomonas sp., represented by the Pseudomonas stutzeri, Pseudomonas mendocina and Pseudomonas putida, composed the major culturable aerobic denitrifiers of the river, followed by Ochrobactrum sp. and Rheinheimera sp. The PCA plot showed the unclassified Pseudomonas sp. and Rheinheimera pacifica preferred to inhabit in biofilm phase while one unclassified Ochrobactrum sp. and Pseudomonas resinovorans had higher abundance in the sediment. In the overlying water, the Pseudomonas stutzeri and Ochrobactrum rhizosphaerae were found to have higher abundance, indicating these aerobic denitrifiers had different habitat-preferable characteristics among the 3 phases of river system. The findings may help select the niche to isolate the aerobic denitrifiers and facilitate the bioaugmentation-based purification of the nitrate polluted surface water.

Characteristic of nitrous oxide production in partial denitrification process with high nitrite accumulation

Nitrous oxide (N2O) production during the partial denitrification process with nitrate (NO3(-)-N) to nitrite (NO2(-)-N) transformation ratio of 80% was investigated in this study. Results showed that N2O was seldom observed before complete depletion of NO3(-)-N, but it was closely related to the reduction of NO2(-)-N rather than NO3(-)-N. High COD/NO3(-)-N was in favor of N2O production in partial denitrification with high NO2(-)-N accumulation. It was seriously enhanced at constant acidic pH due to the free nitrous acid (FNA) inhibition. However, the N2O production was much lower at initial pH of 5.5 and 6.5 due to the pH increase during denitrification process. Significantly, the pH turning point could be chosen as a controlled parameter to denote the end of NO3(-)-N reduction, which could not only achieve high NO2(-)-N accumulation but also decrease the N2O production significantly for practical application.

Biological nitrogen removal from sewage via anammox: Recent advances

Biological nitrogen removal from sewage via anammox is a promising and feasible technology to make sewage treatment energy-neutral or energy-positive. Good retention of anammox bacteria is the premise of achieving sewage treatment via anammox. Therefore the anammox metabolism and its factors were critically reviewed so as to form biofilm/granules for retaining anammox bacteria. A stable supply of nitrite for anammox bacteria is a real bottleneck for applying anammox in sewage treatment. Nitritation and partial-denitrification are two promising methods of offering nitrite. As such, the strategies for achieving nitritation in sewage treatment were summarized by reviewing the factors affecting nitrite oxidation bacteria growth. Meanwhile, the methods of achieving partial-denitrification have been developed through understanding the microorganisms related with nitrite accumulation and their factors. Furthermore, two cases of applying anammox in the mainstream sewage treatment plants were documented.

Effect of self-alkalization on nitrite accumulation in a high-rate denitrification system: Performance, microflora and enzymatic activities

The self-alkalization of denitrifying automatic circulation (DAC) reactor resulted in a large increase of pH up to 9.20 and caused a tremendous accumulation of nitrite up to 451.1 ± 49.0 mgN L(-1) at nitrate loading rate (NLR) from 35 kgN m(-3) d(-1) to 55 kgN m(-3) d(-1). The nitrite accumulation was greatly relieved even at the same NLR once the pH was maintained at 7.6 ± 0.2 in the system. Enzymatic assays indicated that the long-term bacterial exposure to high pH significantly inhibited the activity of copper type nitrite reductase (NirK) rather than the cytochrome cd1 type nitrite reductase (NirS). The terminal restriction fragment length polymorphism (T-RFLP) analysis revealed that the dominant denitrifying bacteria shifted from the NirS-containing Thauear sp. 27 to the NirK-containing Hyphomicrobium nitrativorans strain NL23 during the self-alkalization. The significant nitrite accumulation in the high-rate denitrification system could be therefore, due to the inhibition of Cu-containing NirK by high pH from the self-alkalization. The results suggest that the NirK-containing H. nitrativorans strain NL23 could be an ideal functional bacterium for the conversion of nitrate to nitrite, i.e. denitritation, which could be combined with anaerobic ammonium oxidation (Anammox) to develop a new process for nitrogen removal from wastewater.

Kinetics of autotrophic denitrification process and the impact of sulphur/limestone ratio on the process performance

Kinetics of sulphur-limestone autotrophic denitrification process in batch assays and the impact of sulphur/limestone ratio on the process performance in long-term operated packed-bed bioreactors were evaluated. The specific nitrate and nitrite reduction rates increased almost linearly with the increasing initial nitrate and nitrite concentrations, respectively. The process performance was evaluated in three parallel packed-bed bioreactors filled with different sulphur/limestone ratios (1:1, 2:1 and 3:1, v/v). Performances of the bioreactors were studied under varying nitrate loadings (0.05 - 0.80 gNO(-)(3) - NL⁻¹ d⁻¹) and hydraulic retention times (3-12 h). The maximum nitrate reduction rate of 0.66 g L⁻¹ d⁻¹ was observed at the loading rate of 0.80 g NO(-)(3) - N L⁻¹ d⁻¹ in the reactor with sulphur/limestone ratio of 3:1. Throughout the study, nitrite concentrations remained quite low (i.e. below 0.5 mg L⁻¹ NO(-)(2) -N. The reactor performance increased in the order of sulphur/limestone ratio of 3:1, 2:1 and 1:1. Denaturing gradient gel electrophoresis analysis of 16S rRNA genes showed quite stable communities in the reactors with the presence of Methylo virgulaligni, Sulfurimonas autotrophica, Sulfurovum lithotrophicum, Thiobacillus aquaesulis and Sulfurimonas autotrophica related species.

Long-term operation of a novel pilot-scale six tanks alternately operating activated sludge process in treating domestic wastewater

The performance of a new pilot-scale six tanks activated sludge process has been evaluated for 303 d, receiving real domestic wastewater with a flow rate of 15-24.4 L/h. Partial nitrification via nitrite and microbial community structure were investigated in this system. The result shows that the nitrite accumulation rate was achieved successfully over 94% in the last aerobic compartment through a combination of short hydraulic retention time and low dissolved oxygen (DO) level. Fluorescence in situ hybridization analysis was used to correlate ammonia-oxidizing bacteria (AOB) numbers with nutrient removal via nitrite. It was shown that in response to complete and partial nitrification modes, the numbers of AOB population were 7.7 x 10(7) cells/g mixed liquor suspended solids (MLSS) and 5.31 x 10(8) cells/g MLSS, respectively. The morphology of the sludge indicated that there is a small rod-shaped and spherical cluster which was mainly dominantly bacterial according to scanning electron microscope. Higher pollutant removal efficiencies of 86.2%, 98%, and 96.1%, for total nitrogen, NH4+ - N, and total phosphorus, respectively, were achieved by a long-term operation of the six tanks activated sludge process at a low DO concentration and low chemical oxygen demand to nitrogen ratio which were approximately equal to the complete nitrification-ldenitrification with the addition of an external carbon source at a concentration of 1.5-2.5 mg/L.