Nitrification-denitrification via nitrite accumulation for nitrogen removal from wastewaters

The biological nitrification-denitrification process is used extensively for removal of ammonia nitrogen from wastewaters. Saves in aeration, organic matter (for denitrification) and surplus sludge are achievable if nitrite accumulation is possible in the nitrification step. In this paper, operational parameters were studied for each process for maximum nitrite accumulation in the nitrification step and nitrite adaptation in the denitrification step. Nitrite accumulation during nitrification can be controlled by the dissolved oxygen (DO) concentration, presenting a maximum of 65% at around 0.7 mg DO/L. Denitrification can be adapted to nitrite and the process is stable if nitrite in the reactor is keep low. The performance of a continuous stirred tank reactor (CSTR) and an up flow sludge blanket reactor (USB) were compared. Once the operational parameters were established, a CSTR for nitrification and an USB reactor for denitrification were operated in series for 25 days. The process was stable and a steady state was maintained for 20 days, and 93.5% of overall nitrogen removal was achieved in the nitrification-denitrification via the nitrite process.     

Effects of aeration cycles on nitrifying bacterial populations and nitrogen removal in intermittently aerated reactors

The effects of the lengths of aeration and nonaeration periods on nitrogen removal and the nitrifying bacterial community structure were assessed in intermittently aerated (IA) reactors treating digested swine wastewater. Five IA reactors were operated in parallel with different aeration-to-nonaeration time ratios (ANA). Populations of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were monitored using 16S rRNA slot blot hybridizations. AOB species diversity was assessed using amoA gene denaturant gradient gel electrophoresis. Nitrosomonas and Nitrosococcus mobilis were the dominant AOB and Nitrospira spp. were the dominant NOB in all reactors, although Nitrosospira and Nitrobacter were also detected at lower levels. Reactors operated with the shortest aeration time (30 min) showed the highest Nitrosospira rRNA levels, and reactors operated with the longest anoxic periods (3 and 4 h) showed the lowest levels of Nitrobacter, compared to the other reactors. Nitrosomonas sp. strain Nm107 was detected in all reactors, regardless of the reactor's performance. Close relatives of Nitrosomonas europaea, Nitrosomonas sp. strain ENI-11, and Nitrosospira multiformis were occasionally detected in all reactors. Biomass fractions of AOB and effluent ammonia concentrations were not significantly different among the reactors. NOB were more sensitive than AOB to long nonaeration periods, as nitrite accumulation and lower total NOB rRNA levels were observed for an ANA of 1 h:4 h. The reactor with the longest nonaeration time of 4 h performed partial nitrification, followed by denitrification via nitrite, whereas the other reactors removed nitrogen through traditional nitrification and denitrification via nitrate. Superior ammonia removal efficiencies were not associated with levels of specific AOB species or with higher AOB species diversity.     

Achieving nitrite accumulation in a continuous system treating low-strength domestic wastewater: switchover from batch start-up to continuous operation with process control

Although biological nitrogen removal via nitrite is recognized as one of the cost-effective and sustainable biological nitrogen removal processes, nitrite accumulation has proven difficult to achieve in continuous processes treating low-strength nitrogenous wastewater. Partial nitrification to nitrite was achieved and maintained in a lab-scale completely stirred tank reactor (CSTR) treating real domestic wastewater. During the start-up period, sludge with ammonia-oxidizing bacteria (AOB) but no nitrite-oxidizing bacteria (NOB) was obtained by batch operation with aeration time control. The nitrifying sludge with the dominance of AOB was then directly switched into continuous operation. It was demonstrated that partial nitrification to nitrite in the continuous system could be repeatedly and reliably achieved using this start-up strategy. The ratio of dissolved oxygen to ammonium loading rate (DO/ALR) was critical to maintain high ammonium removal efficiency and nitrite accumulation ratio. Over 85% of nitrite accumulation ratio and more than 95% of ammonium removal efficiency were achieved at DO/ALR ratios in an optimal range of 4.0–6.0 mg O₂/g N d, even under the disturbances of ammonium loading rate. Microbial population shift was investigated, and fluorescence in situ hybridization analysis indicated that AOB were the dominant nitrifying bacteria over NOB when stable partial nitrification was established.     

Unravelling the reasons for disproportion in the ratio of AOB and NOB in aerobic granular sludge

In this study, we analysed the nitrifying microbial community (ammonium-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB)) within three different aerobic granular sludge treatment systems as well as within one flocculent sludge system. Granular samples were taken from one pilot plant run on municipal wastewater as well as from two lab-scale reactors. Fluorescent in situ hybridization (FISH) and quantitative PCR (qPCR) showed that Nitrobacter was the dominant NOB in acetate-fed aerobic granules. In the conventional system, both Nitrospira and Nitrobacter were present in similar amounts. Remarkably, the NOB/AOB ratio in aerobic granular sludge was elevated but not in the conventional treatment plant suggesting that the growth of Nitrobacter within aerobic granular sludge, in particular, was partly uncoupled from the lithotrophic nitrite supply from AOB. This was supported by activity measurements which showed an approximately threefold higher nitrite oxidizing capacity than ammonium oxidizing capacity. Based on these findings, two hypotheses were considered: either Nitrobacter grew mixotrophically by acetate-dependent dissimilatory nitrate reduction (ping-pong effect) or a nitrite oxidation/nitrate reduction loop (nitrite loop) occurred in which denitrifiers reduced nitrate to nitrite supplying additional nitrite for the NOB apart from the AOB.     

Impact of protozoan grazing on nitrification and the ammonia- and nitrite-oxidizing bacterial communities in activated sludge

In activated sludge, protozoa feed on free-swimming bacteria and suspended particles, inducing flocculation and increasing the turnover rate of nutrients. In this study, the effect of protozoan grazing on nitrification rates under various conditions in municipal activated sludge batch reactors was examined, as was the spatial distribution of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) within the activated sludge. The reactors were monitored for ammonia, nitrite, nitrate, and total nitrogen concentrations, and bacterial numbers in the presence and absence of cycloheximide (a protozoan inhibitor), allylthiourea (an inhibitor of ammonia oxidation), and EDTA (a deflocculating agent). The accumulations of nitrate, nitrite, and ammonia were lower in batches without than with protozoa grazing. Inhibition of ammonia oxidation also decreased the amount of nitrite and nitrate accumulation. Inhibiting protozoan grazing along with ammonia oxidation further decreased the amounts of nitrite and nitrate accumulated. Induction of deflocculation led to high nitrate accumulation, indicating high levels of nitrification; this effect was lessened in the absence of protozoan grazing. Using fluorescent in situ hybridization and confocal laser scanning microscopy, AOB and NOB were found clustered within the floc, and inhibiting the protozoa, inhibiting ammonia oxidation, or inducing flocculation did not appear to lower the number of AOB and NOB present or affect their position within the floc. These results suggest that the AOB and NOB are present but less active in the absence of protozoa.     

Identification of novel small molecule enhancers of protein production by cultured mammalian cells

Shortcut nitrogen removal, that is, removal via formation and reduction of nitrite rather than nitrate, has been observed in membrane-aerated biofilms (MABs), but the extent, the controlling factors, and the kinetics of nitrite formation in MABs are poorly understood. We used a special MAB reactor to systematically study the effects of the dissolved oxygen (DO) concentration at the membrane surface, which is the biofilm base, on nitrification rates, extent of shortcut nitrification, and microbial community structure. The focus was on anoxic bulk liquids, which is typical in MAB used for total nitrogen (TN) removal, although aerobic bulk liquids were also studied. Nitrifying MABs were grown on a hollow-fiber membrane exposed to 3 mg N/L ammonium. The MAB intra-membrane air pressure was varied to achieve different DO concentrations at the biofilm base, and the bulk liquid was anoxic or with 2 g m(-3) DO. With 2.2 and 3.5 g m(-3) DO at the biofilm base, and with an anoxic bulk-liquid, the ammonium fluxes were 0.75 and 1.0 g N m(-2) day(-1), respectively, and nitrite was the main oxidized nitrogen product. However, with membrane DO of 5.5 g m(-3), and either zero or 2 g m(-3) DO in the bulk, the ammonium flux was around 1.3 g N m(-2) day(-1), and nitrate flux increased significantly. For all experiments, the cell density of ammonium oxidizing bacteria (AOB) was relatively uniform throughout the biofilm, but the density of nitrite oxidizing bacteria (NOB) decreased with decreasing biofilm DO. Among NOB, Nitrobacter spp. were dominant in biofilm regions with 2 g m(-3) DO or greater, while Nitrospira spp. were dominant in regions with less than 2 g m(-3) DO. A biofilm model, including AOB, Nitrobacter spp., and Nitrospira spp., was developed and calibrated with the experimental results. The model predicted the greatest extent of nitrite formation (95%) and the lowest ammonium oxidation flux (0.91 g N m(-2) day(-1)) when the membrane DO was 2 g m(-3) and the bulk liquid was anoxic. Conversely, the model predicted the lowest extent of nitrite formation (40%) and the highest ammonium oxidation flux (1.5 g N m(-2) day(-1)) when the membrane-DO and bulk-DO were 8 g m(-3) and 2 g m(-3), respectively. The estimated kinetic parameters for Nitrospira spp., revealed a high affinity for nitrite and oxygen. This explains the dominance of Nitrospira spp. over Nitrobacter spp. in regions with low nitrite and oxygen concentrations. Our results suggest that shortcut nitrification can effectively be controlled by manipulating the DO at the membrane surface. A tradeoff is made between increased nitrite accumulation at lower DO, and higher nitrification rates at higher DO.     

Nitrite-oxidizing bacteria guild ecology associated with nitrification failure in a continuous-flow reactor

Nitrification is an important process for nitrogen removal in many wastewater treatment plants, which requires the mutualistic oxidation of ammonia to nitrate by ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). However, this process can be quite unpredictable because both guilds are conditionally sensitive to small changes in operating conditions. Here, dynamics are examined within the NOB guild in two parallel chemostats operated at low and high dilution rates (0.10 and 0.83 day(-1), respectively) during periods of varying nitrification performance. NOB and AOB guild abundances and nitrogen-oxidation efficiency were relatively constant over time in the 0.10 day(-1) reactor; however, the 0.83 day(-1) reactor had two major disturbance episodes that caused destabilization of the NOB guild, which ultimately led to nitrification failure. The first episode caused the extinction of Nitrospira spp. from the system, resulting in chronic incomplete ammonia oxidation and nitrite accumulation. The second episode caused complete loss of nitrification activity, likely resulting from metal toxicity and the previous extinction of Nitrospira spp. from the system. These results exemplify the types of changes that can occur within the NOB guild that result in process impairment or failure, and provide one possible explanation for why nitrification is often unstable at higher dilution rates.     

In situ characterization of nitrifiers in an activated sludge plant: detection of Nitrobacter Spp

Aims: The purpose of this work was to investigate microbial ecology of nitrifiers at the genus level in a typical full-scale activated sludge plant. Methods and Results: Grab samples of mixed liquor were collected from a plug-flow reactor receiving domestic wastewater. Fluorescent in situ hybridization technique (FISH) was used to characterize both ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) in combination with Confocal Scanning Laser Microscope (CSLM). Fluorescently labelled, 16S rRNA-targeted oligonucleotide probes were used in this study. Both Nitrosomonas and Nitrosospira genera as AOB and Nitrobacter and Nitrospira genera as NOB were sought with genus specific probes Nsm156, Nsv443 and NIT3 and NSR1156, respectively. Conclusions: It was shown that Nitrosospira genus was dominant in the activated sludge system studied, although Nitrosomonas is usually assumed to be the dominant genus. At the same time, Nitrobacter genus was detected in activated sludge samples. Significance and Impact of the Study: Previous studies based on laboratory scale pilot plants employing synthetic wastewater suggested that only Nitrospira are found in wastewater treatment plants. We have shown that Nitrobacter genus might also be present. We think that these kinds of studies may not give a valid indication of the microbial diversity of the real full-scale plants fed with domestic wastewater.     

Long-term effect of dissolved oxygen on partial nitrification performance and microbial community structure

In this study, the performance of partial nitrification via nitrite and microbial community structure were investigated and compared in two sequencing batch reactors (SBR) with different dissolved oxygen (DO) levels. Both reactors achieved stable partial nitrification with nitrite accumulation ratio of above 95% by using real-time aeration duration control. Compared with high DO (above 3 mg/l on average) SBR, simultaneous nitrification and denitrification (SND) via nitrite was carried out in low DO (0.4–0.8 mg/l) SBR. The average efficiencies of SND in high DO and low DO reactor were 7.7% and 44.9%, and the specific SND rates were 0.20 and 0.83 mg N/(mg MLSS h), respectively. Low DO did not produce sludge with poorer settling properties but attained lower turbidities of the effluent than high DO. Fluorescence in situ hybridization (FISH) analysis in both the reactors showed that ammonia-oxidizing bacteria (AOB) were the dominant nitrifying bacteria and nitrite-oxidizing bacteria (NOB) did not be recovered in spite of exposing nitrifying sludge to high DO. The morphology of the sludge from both two reactors according to scanning electron microscope indicated that small rod-shaped and spherical clusters were dominant, although filamentous bacteria and few long rod-shaped coexisted in the low DO reactor. By selecting properly DO level and adopting process control method is not only of benefit to the achievement of novel biological nitrogen removal technology, but also favorable to sludge population optimization.     

Effect of temperature and free ammonia on nitrification and nitrite accumulation in landfill leachate and analysis of its nitrifying bacterial community by FISH

The cause of seasonal failure of a nitrifying municipal landfill leachate treatment plant utilizing a fixed biofilm was investigated by wastewater analyses and batch respirometric tests at every treatment stage. Nitrification of the leachate treatment plant was severely affected by the seasonal temperature variation. High free ammonia (NH3-N) inhibited not only nitrite oxidizing bacteria (NOB) but also ammonia oxidizing bacteria (AOB). In addition, high pH also increased free ammonia concentration to inhibit nitrifying activity especially when the NH4-N level was high. The effects of temperature and free ammonia of landfill leachate on nitrification and nitrite accumulation were investigated with a semi-pilot scale biofilm airlift reactor. Nitrification rate of landfill leachate increased with temperature when free ammonia in the reactor was below the inhibition level for nitrifiers. Leachate was completely nitrified up to a load of 1.5 kg NH4-N m(-3)d(-1) at 28 degrees C. The activity of NOB was inhibited by NH3-N resulting in accumulation of nitrite. NOB activity decreased more than 50% at 0.7 mg NH3-N L(-1). Fluorescence in situ hybridization (FISH) was carried out to analyze the population of AOB and NOB in the nitrite accumulating nitrifying biofilm. NOB were located close to AOB by forming small clusters. A significant fraction of AOB identified by probe Nso1225 specifically also hybridized with the Nitrosomonas specific probe Nsm156. The main NOB were Nitrobacter and Nitrospira which were present in almost equal amounts in the biofilm as identified by simultaneous hybridization with Nitrobacter specific probe Nit3 and Nitrospira specific probe Ntspa662.     

Estimation of nitrifier abundances in a partial nitrification reactor treating ammonium-rich saline wastewater using DGGE, T-RFLP and mathematical modeling

The bacterial community in a partial nitrification reactor was analyzed on the basis of 16S rRNA gene by cloning–sequencing method, and the percentages of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) in the activated sludge were quantified by three independent methods, namely, denaturing gradient gel electrophoresis (DGGE), terminal restriction fragment length polymorphism (T-RFLP) and Double Monod modeling. The clone library results suggested that there were only a dominant AOB and a dominant NOB species in the reactor, belonging to Nitrosomonas genus and Nitrospira genus, respectively. The percentages of NOB in total bacterial community increased from almost 0% to 30% when dissolved oxygen (DO) levels were changed from 0.15 mg/L to 0.5 mg/L, coinciding with the accumulation and conversion of nitrite, while the percentages of AOB changed little in the two phases. The results confirmed the importance of low DO level for inhibiting NOB to achieve partial nitrification. Furthermore, the percentages of AOB and NOB in the total bacteria community were estimated based on the results of batch experiments using Double Monod model, and the results were comparable with those determined according to profiles of DGGE and T-RFLP.     

Nitrate removal in a packed bed reactor using volatile fatty acids from anaerobic acidogenesis of food wastes

A packed bed reactor (PBR) was fed with nitrate containing synthetic wastewater or effluent from a sequencing batch reactor used for nitrification. The C source introduced into the PBR consisted of volatile fatty acids (VFAs) produced from anaerobic acidogenesis of food wastes. When nitrate loading rates ranged from 0.50 to 1.01 kg N/m³·d, the PBR exhibited 100∼98.8% NO₃ ⁻-N removal efficiencies and nitrite concentrations in the effluent ranged from 0 to 0.6 NO₂ ⁻-N mg/L. When the PBR was further investigated to determine nitrate removal activity along the bed height using a nitrate loading rate less than 1.01 kg N/m³·d, 100% nitrate removal efficiency was observed. Approximately 83.2% nitrate removal efficiency was observed in the lower 50% of the packed-bed height. When reactor performance at a C/N ratio of 4 and a C/N ratio of 5 was compared, the PBR showed better removal efficiency (96.5%) of nitrate and less nitrite concentration in the effluent at the C/N ratio of 5. VFAs were found to be a good alternative to methanol as a carbon source for denitrification of a municipal wastewater containing 40 mg-N/L.     

Combined removal of sulfur compounds and nitrate by autotrophic denitrification in bioaugmented activated sludge system

An autotrophic denitrification process using reduced sulfur compounds (thiosulfate and sulfide) as electron donor in an activated sludge system is proposed as an efficient and cost effective alternative to conventional heterotrophic denitrification for inorganic (or with low C/N ratio) wastewaters and for simultaneous removal of sulfide or thiosulfate and nitrate. A suspended culture of sulfur-utilizing denitrifying bacteria was fast and efficiently established by bio-augmentation of activated sludge with Thiobacillus denitrificans. The stoichiometry of the process and the key factors, i.e. N/S ratio, that enable combined sulfide and nitrogen removal, were determined. An optimum N/S ratio of 1 (100% nitrate removal without nitrite formation and low thiosulfate concentrations in the effluent) has been obtained during reactor operation with thiosulfate at a nitrate loading rate (NLR) of 17.18 mmol N L(-1) d(-1). Complete nitrate and sulfide removal was achieved during reactor operation with sulfide at a NLR of 7.96 mmol N L(-1) d(-1) and at N/S ratio between 0.8 and 0.9, with oxidation of sulfide to sulfate. Complete nitrate removal while working at nitrate limiting conditions could be achieved by sulfide oxidation with low amounts of oxygen present in the influent, which kept the sulfide concentration below inhibitory levels.     

Analysis and succession of nitrifying bacteria community structure in sequencing biofilm batch reactor

To reveal the succession procedure of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) community structure in sequencing biofilm batch reactor (SBBR), the molecular biological techniques of denaturing gradient gel electrophoresis (DGGE), cloning, and real-time PCR were applied. DGGE showed that the structural diversity of the bacterial community increased during the biofilm formation period, and some kinds of populations had been highly preponderant consistently. The results of cloning and sequencing revealed that Nitrosomonas was the dominant species. The real-time PCR analysis indicated that the amount of the AOB increased significantly after the cultivation period, and the NOB gradually decreased. The AOB content on the 25th day was 17 times that of the 6th day. It also showed the biofilm formed successfully with accumulating nitrite and prepared to achieve the achievement of simultaneous nitrification and denitrification in SBBR. Furthermore, the ammonia-oxidizing rate was in correspondence with the NH₄ ⁺-N removal efficiency.     

硝化作用研究的新发现:单步硝化作用与全程氨氧化微生物

硝化作用是氨被微生物氧化为硝酸盐的过程,分别由氨氧化微生物(AOB和AOA)和亚硝酸盐氧化细菌(NOB)主导完成.一个世纪以来,我们把这个分步硝化过程当成唯一的硝化途径来学习和研究.虽然根据动力学理论推测,环境中应该存在单步硝化作用,即由一种微生物单独完成整个硝化过程,将NH₃氧化为NO₃^-,但一直没有研究能直接证明该种微生物的存在.直到2015年底,3个科研团队分别在不同环境中发现了3种不同的经过纯培养的细菌(Candidatus Nitrospira nitrosa、Candidatus Nitrospira nitrificans和Candidatus Nitrospira inopinata)和一种未经过纯培养的细菌(类Nitrospira),它们都具备单独将氨氧化为硝酸盐的能力,这些微生物被定义为全程氨氧化微生物(Comammox).单步硝化作用和全程氨氧化微生物的发现终结了传承百年的理论,并引发了众多关于全球氮素循环的重要科学问题,如这些微生物在环境中的生态位点及其在硝化作用中的相对贡献等.本文就单步硝化作用及全程氨氧化微生物的发现作了简要概述.     

Biological treatment of ammonium-rich wastewater by partial nitritation and subsequent anaerobic ammonium oxidation (anammox) in a pilot plant

In wastewater treatment plants with anaerobic sludge digestion, 15-20% of the nitrogen load is recirculated to the main stream with the return liquors from dewatering. Separate treatment of this ammonium-rich digester supernatant would significantly reduce the nitrogen load of the activated sludge system. Some years ago, a novel biological process was discovered in which ammonium is converted to nitrogen gas under anoxic conditions with nitrite as the electron acceptor (anaerobic ammonium oxidation, anammox). Compared to conventional nitrification and denitrification, the aeration and carbon-source demand is reduced by over 50 and 100%, respectively. The combination of partial nitritation to produce nitrite in a first step and subsequent anaerobic ammonium oxidation in a second reactor was successfully tested on a pilot scale (3.6 m(3)) for over half a year. This report focuses on the feasibility of nitrogen removal from digester effluents from two different wastewater treatment plants (WWTPs) with the combined partial nitritation/anammox process. Nitritation was performed in a continuously stirred tank reactor (V=2.0 m(3)) without sludge retention. Some 58% of the ammonium in the supernatant was converted to nitrite. At 30 degrees C the maximum dilution rate D(x) was 0.85 d(-1), resulting in nitrite production of 0.35 kg NO(2)-N m(-3)(reactor) d(-1). The nitrate production was marginal. The anaerobic ammonium oxidation was carried out in a sequencing batch reactor (SBR, V=1.6 m(3)) with a nitrogen elimination rate of 2.4 kg N m(-3)(reactor) d(-1) during the nitrite-containing periods of the SBR cycle. Over 90% of the inlet nitrogen load to the anammox reactor was removed and the sludge production was negligible. The nitritation efficiency of the first reactor limited the overall maximum rate of nitrogen elimination.     

Distinctive microbial ecology and biokinetics of autotrophic ammonia and nitrite oxidation in a partial nitrification bioreactor

Biological nitrogen removal (BNR) based on partial nitrification and denitrification via nitrite is a cost-effective alternate to conventional nitrification and denitrification (via nitrate). The goal of this study was to investigate the microbial ecology, biokinetics, and stability of partial nitrification. Stable long-term partial nitrification resulting in 82.1 +/- 17.2% ammonia oxidation, primarily to nitrite (77.3 +/- 19.5% of the ammonia oxidized) was achieved in a lab-scale bioreactor by operation at a pH, dissolved oxygen and solids retention time of 7.5 +/- 0.1, 1.54 +/- 0.87 mg O(2)/L, and 3.0 days, respectively. Bioreactor ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) populations were most closely related to Nitrosomonas europaea and Nitrobacter spp., respectively. The AOB population fraction varied in the range 61 +/- 45% and was much higher than the NOB fraction, 0.71 +/- 1.1%. Using direct measures of bacterial concentrations in conjunction with independent activity measures and mass balances, the maximum specific growth rate (micro(max)), specific decay (b) and observed biomass yield coefficients (Y(obs)) for AOB were 1.08 +/- 1.03 day(-1), 0.32 +/- 0.34 day(-1), and 0.15 +/- 0.06 mg biomass COD/mg N oxidized, respectively. Corresponding micro(max), b, and Y(obs) values for NOB were 2.6 +/- 2.05 day(-1), 1.7 +/- 1.9 day(-1), and 0.04 +/- 0.02 mg biomass COD/mg N oxidized, respectively. The results of this study demonstrate that the highly selective partial nitrification operating conditions enriched for a narrow diversity of rapidly growing AOB and NOB populations unlike conventional BNR reactors, which host a broader diversity of nitrifying bacteria. Further, direct measures of microbial abundance enabled not only elucidation of mixed community microbial ecology but also estimation of key engineering parameters describing bioreactor systems supporting these communities.     

Competition between Nitrospira spp. and Nitrobacter spp. in nitrite-oxidizing bioreactors

In this work the question was addressed if in nitrite-oxidizing activated sludge systems the environmental competition between Nitrobacter spp. and Nitrospira spp., which only recently has been discovered to play a role in these systems, is affected by the nitrite concentrations. Two parallel chemostats were inoculated with nitrifying-activated sludge containing Nitrospira and operated under identical conditions. After addition of Nitrobacter to both chemostats, the nitrite concentration in the influent of one of the chemostats was increased such that nitrite peaks in the bulk liquid of this reactor were detected. The other chemostat served as control reactor, which always had a constant nitrite influent concentration. The relative cellular area (RCA) of Nitrospira and Nitrobacter was determined by quantitative fluorescence in situ hybridization (FISH). The nitrite perturbation stimulated the growth of Nitrobacter while in the undisturbed control chemostat Nitrospira dominated. Overall, the results of this experimental study support the hypothesis that Nitrobacter is a superior competitor when resources are abundant, while Nitrospira thrive under conditions of resource scarcity. Interestingly, the dominance of Nitrobacter over Nitrospira, caused by the elevated nitrite concentrations, could not be reverted by lowering the available nitrite concentration to the original level. One possible explanation for this result is that when Nitrobacter is present at a certain cell density it is able to inhibit the growth of Nitrospira. An alternative explanation would be that the length of the experimental period was not long enough to observe an increase of the Nitrospira population.     

Analysis of the characteristics of short-cut nitrifying granular sludge and pollutant removal processes in a sequencing batch reactor

Aerobic granular sludge is a new type of microbe auto-immobilization technology; in this paper, short-cut nitrification and denitrification were effectively combined with the granular sludge technology. Simultaneous nitrification and denitrification granules were developed in a sequencing batch reactor (SBR) using synthetic wastewater with a high concentration of ammonia nitrogen at 25 °C with a dissolved oxygen concentration above 2.0 mg/L and a 15 days sludge retention time. The characteristics of the sludge and the removal efficiency were studied, and the removal mechanisms of the pollutants and the process of short-cut nitrification were analyzed. The average granule diameter of the granular sludge was 704.0 μm. The removal rates of pollutants and the accumulation rate of nitrite in the SBR were studied. During treatment of wastewater with a high concentration of ammonia nitrogen, simultaneous nitrification, and denitrification and the stripping process could contribute to the removal of total nitrogen. The high pH value, the high concentration of free ammonia, and the delamination of granular sludge were the main factors contributing to the short-cut nitrification property of granular sludge in the reaction process.     

Activated sludge under free ammonia treatment using gel immobilization technology for long-term partial nitrification with different initial biomass

To achieve stable partial nitrification, activated sludge from a wastewater treatment plant using free ammonia (FA) inhibition was immobilized in a polyvinyl alcohol carrier. After FA treatment at 16.44 mg L⁻¹ for 1 day, due to the increased growth rate gap between ammonium-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), AOB enrichment and NOB inhibition were achieved within 12 days, with AOB and NOB accounting for 65.61 and 0.05%, respectively. Subsequently, with dissolved oxygen concentrations of 4−5 mg L⁻¹, pH of 7.6–7.8 and temperature of 25 ± 1 °C, the immobilized carrier made of activated sludge achieved more than 90% and more than 86% of nitrite accumulation rate at the influent ammonia concentration of 90−110 mg L⁻¹ and 35−50 mg L⁻¹, respectively. After 50 days operation, the NOB content was 0.10%, indicating the immobilized carrier provided favorable conditions for maintaining the low NOB content. Furthermore, due to the low NOB content in the inoculum and the oxygen-limited environment formed by the increase in the AOB numbers in the carrier, immobilized carrier with different initial biomass (1, 2.5 and 5%) can achieve stable partial nitrification.