Nitrification in activated sludge batch reactors is linked to protozoan grazing of the bacterial population

Protozoa feed upon free-swimming bacteria and suspended particles inducing flocculation and increasing the turnover rate of nutrients in complex mixed communities. In this study, the effect of protozoan grazing on nitrification was examined in activated sludge in batch cultures maintained over a 14-day period. A reduction in the protozoan grazing pressure was accomplished by using either a dilution series or the protozoan inhibitor cycloheximide. As the dilutions increased, the nitrification rate showed a decline, suggesting that a reduction in protozoan or bacterial concentration may cause a decrease in nitrification potential. In the presence of cycloheximide, where the bacterial concentration was not altered, the rates of production of ammonia, nitrite, and nitrate all were significantly lower in the absence of active protozoans. These results suggest that a reduction in the number or activity of the protozoans reduces nitrification, possibly by limiting the availability of nutrients for slow-growing ammonia and nitrite oxidizers through excretion products. Furthermore, the ability of protozoans to groom the heterotrophic bacterial population in such systems may also play a role in reducing interspecies competition for nitrification substrates and thereby augment nitrification rates.     

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.     

Fabrication of Bacteria Environment Cubes with Dry Lift-Off Fabrication Process for Enhanced Nitrification

We have developed a 3D dry lift-off process to localize multiple types of nitrifying bacteria in polyethylene glycol diacrylate (PEGDA) cubes for enhanced nitrification, a two-step biological process that converts ammonium to nitrite and then to nitrate. Ammonia-oxidizing bacteria (AOB) is responsible for converting ammonia into nitrite, and nitrite-oxidizing bacteria (NOB) is responsible for converting nitrite to nitrate. Successful nitrification is often challenging to accomplish, in part because AOB and NOB are slow growers and highly susceptible to many organic and inorganic chemicals in wastewater. Most importantly, the transportation of chemicals among scattered bacteria is extremely inefficient and can be problematic. For example, nitrite, produced from ammonia oxidation, is toxic to AOB and can lead to the failure of nitrification. To address these challenges, we closely localize AOB and NOB in PEGDA cubes as microenvironment modules to promote synergetic interactions. The AOB is first localized in the vicinity of the surface of the PEGDA cubes that enable AOB to efficiently uptake ammonia from a liquid medium and convert it into nitrite. The produced nitrite is then efficiently transported to the NOB localized at the center of the PEGDA particle and converted into non-toxic nitrate. Additionally, the nanoscale PEGDA fibrous structures offer a protective environment for these strains, defending them from sudden toxic chemical shocks and immobilize in cubes. This engineered microenvironment cube significantly enhances nitrification and improves the overall ammonia removal rate per single AOB cell. This approach—encapsulation of multiple strains at close range in cube in order to control their interactions—not only offers a new strategy for enhancing nitrification, but also can be adapted to improve the production of fermentation products and biofuel, because microbial processes require synergetic reactions among multiple species.     

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.     

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.     

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.     

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.     

泉州西湖沉积物中硝化细菌的分布及其作用

比较研究泉州西湖沉积物中氨氧化细菌(AOB)和亚硝酸盐氧化细菌(NOB)的分布及氨氧化潜力和亚硝酸盐(NO2?)氧化潜力。结果表明: 西湖沉积物中存在高浓度的有机质(OM)、总氮(TN)和氨氮。AOB生物量为1.1×106?6.4×106 个/g干土, 显著高于NOB生物量4.2×105?7.4×105 个/g 干土(配对t-检验, P<0.05)。对于NOB, 硝化杆菌属(Nitrobacter)和硝化螺菌属(Nitrospira)同时存在于西湖沉积物中, 以Nitrobacter为优势种群。AOB和NOB生物量的差异一定程度上导致西湖沉积物中氨氧化潜力显著高于NO2?氧化潜力(配对t-检验, P<0.05), NO2?氧化过程成为硝化作用的限制步骤。另外, 西湖沉积物中存在的较高浓度氨氮, 一方面促进了AOB的生长和活性, 导致较高速率的氨氧化过程, 另一方面却对亚硝酸盐氧化过程产生选择性抑制, 这也是导致NO2?氧化潜力较低的主要原因之一。     

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.     

Nitrification in hybrid bioreactors treating simulated domestic wastewater

Aim

To provide deeper insights into nitrification process within aerobic bioreactors containing supplemental physical support media (hybrid bioreactors).

Methods and Results

Three bench‐scale hybrid bioreactors with different media size and one control bioreactor were operated to assess how biofilm integrity influences microbial community conditions and bioreactor performance. The systems were operated initially at a 5‐day hydraulic retention time (HRT), and all reactors displayed efficient nitrification and chemical oxygen demand (COD) removal (>95%). However, when HRT was reduced to 2·5 days, COD removal rates remained high, but nitrification efficiencies declined in all reactors after 19 days. To explain reduced performance, nitrifying bacterial communities (ammonia‐oxidizing bacteria, AOB; nitrite‐oxidizing bacteria, NOB) were examined in the liquid phase and also on the beads using qPCR, FISH and DGGE. Overall, the presence of the beads in a reactor promoted bacterial abundances and diversity, but as bead size was increased, biofilms with active coupled AOB–NOB activity were less apparent, resulting in incomplete nitrification.

Conclusions

Hybrid bioreactors have potential to sustain effective nitrification at low HRTs, but support media size and configuration type must be optimized to ensure coupled AOB and NOB activity in nitrification.

Significance and Impact of the Study

This study shows that AOB and NOB coupling must be accomplished to minimize nitrification failure.     

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 thereactor'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.     

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.     

Partial nitrification in a sequencing batch reactor treating acrylic fiber wastewater

A sequencing batch reactor was employed to treat the acrylic fiber wastewater. The dissolved oxygen and mixed liquor suspended solids were 2–3 and 3,500–4,000 mg/L, respectively. The results showed ammonium oxidizing bacteria (AOB) had superior growth rate at high temperature than nitrite oxidizing bacteria (NOB). Partial nitrification could be obtained with the temperature of 28 °C. When the pH value was 8.5, the nitrite-N accumulation efficiency was 82 %. The combined inhibitions of high pH and free ammonium to NOB devoted to the nitrite-N buildup. Hydraulic retention time (HRT) was a key factor in partial nitrification control, and the optimal HRT was 20 h for nitrite-N buildup in acrylic fiber wastewater treatment. The ammonium oxidation was almost complete and the transformation from nitrite to nitrate could be avoided. AOB and NOB accounted for 2.9 and 4.7 %, respectively, corresponding to the pH of 7.0. When the pH was 8.5, they were 6.7 and 0.9 %, respectively. AOB dominated nitrifying bacteria, and NOB was actually washed out from the system.     

从典型硝化细菌到全程氨氧化微生物：发现及研究进展

生物硝化过程在全球氮循环中起关键性作用,被认为由氨氮氧化成亚硝酸盐和亚硝酸盐氧化成硝酸盐两个步骤组成,分别由氨氧化微生物(Ammonia oxidizing microorganisms,AOM)和硝化细菌(Nitrite oxidizing bacteria,NOB)催化完成。AOM包括氨氧化细菌(Ammonia oxidizing bacteria,AOB)和氨氧化古菌(Ammonia oxidizing archaea,AOA),AOB与AOA分布广泛,两者的相对丰度和氨氮浓度密切相关。2015年底,3个硝化螺菌属(Nitrospira)谱系Ⅱ的NOB被证实含有AOM的特征功能酶,包括氨单加氧酶(AMO)和羟胺脱氢酶(HAO),并证明NOB同时具有氨氧化和亚硝酸盐氧化的能力,命名为全程氨氧化微生物(Complete ammonia oxidizer,Comammox)。根据AMO的α亚基基因amoA的相似性将Comammox分为两大分支clade A和clade B。它们广泛分布于自然环境和人工系统,包括土壤(稻田、森林)、淡水(湿地、河流、湖泊沉积物、蓄水层)、污水处理厂和自来水厂等。本文综述了Comammox的发现及其最新的研究进展,并展望了Comammox作为氮循环关键功能菌群的研究方向和应用前景。     

短程硝化启动运行中功能菌群变化研究

【目的】短程硝化-厌氧氨氧化是可实现的最短生物脱氮工艺,短程硝化是实现该工艺的重要环节和必要条件。【方法】采用序批式反应器(SBR)来实现短程硝化过程的启动和稳定运行,并对该过程中的相关功能菌群变化进行检测分析。【结果】通过控制低DO浓度(<1 mg/L)和逐步提高氨氮进水负荷,可抑制氨氧化细菌(NOB)菌群增殖并促进亚硝酸氧化菌(AOB)菌群规模显著扩大,实现短程硝化过程的启动和稳定运行。在氨氮进水负荷为0.055 kg/(m3.d)时,平均氨氮去除容积负荷和污泥负荷可达到0.043kg/(m3.d)和0.16 kg/(kg.d),平均亚硝酸盐积累率可达到83.4%。在短程硝化启动和稳定运行过程中,NOB菌群密度从2.0×105CFU/mL降至1.5×104CFU/mL,相对丰度从5.51%降至2.14%;AOB菌群密度从4.5×104CFU/mL增加至1.5×107CFU/mL,相对丰度从0.18%增加至7.25%。【结论】AOB菌群规模的扩大是实现短程硝化和氨氮去除能力提高的主要原因,同时较高的进水氨氮浓度和负荷也会造成亚硝化活性的抑制。     

Concurrent nitrite oxidation and aerobic denitrification in activated sludge exposed to volatile fatty acids

The goal of this research was to investigate the simultaneous occurrence of nitrification and denitrification by activated sludge exposed to volatile fatty acids (VFAs) during aerobic wastewater treatment using a single-stage reactor. A mixture of VFAs was spiked directly into a continuous-stirred tank reactor (CSTR) to assess subsequent impacts on nitrite removal, nitrate formation, CO(2) fixation, total bacterial density, and dominant nitrite oxidizing bacteria (NOB) concentration (i.e., Nitrospira). The activity of the periplasmic nitrate reductase (NAP) enzyme and the presence of nap gene were also measured. A rapid decrease in the nitrate formation rate (>70% reduction) was measured for activated sludge exposed to VFAs; however, the nitrite removal rate was not reduced. The total bacterial density and Nitrospira concentration remained essentially constant; therefore, the reduction in nitrate formation rate was likely not due to heterotrophic uptake of nitrogen or to a decrease in the dominant NOB population. Additionally, VFA exposure did not impact microbial CO(2) fixation efficiency. The activity of NAP enzyme increased in the presence of VFAs suggesting that nitrate produced as a consequence of nitrite oxidation was likely further reduced to gaseous denitrification products via catalysis by NAP. Little, if any, nitrogen was discharged in the aqueous effluent of the CSTR after exposure to VFAs demonstrating that activated sludge treatment yielded compounds other than those typically produced solely by nitrification.     

Chemical inhibition of nitrification in activated sludge

Conventional aerobic nitrification was adversely affected by single pulse inputs of six different classes of industrially relevant chemical toxins: an electrophilic solvent (1-chloro-2,4-dinitrobenzene, CDNB), a heavy metal (cadmium), a hydrophobic chemical (1-octanol), an uncoupling agent (2,4-dinitrophenol, DNP), alkaline pH, and cyanide in its weak metal complexed form. The concentrations of each chemical source that caused 1 5, 25, and 50% respiratory inhibition of a nitrifying mixed liquor during a short-term assay were used to shock sequencing batch reactors containing nitrifying conventional activated sludge. The reactors were monitored for recovery over a period of 30 days or less. All shock conditions inhibited nitrification, but to different degrees. The nitrate generation rate (NGR) of the shocked reactors recovered overtime to control reactor levels and showed that it was a more sensitive indicator of nitrification inhibition than both initial respirometric tests conducted on unexposed biomass and effluent nitrogen species analyses. CDNB had the most severe impact on nitrification, followed by alkaline pH 11, cadmium, cyanide, octanol, and DNP. Based on effluent data, cadmium and octanol primarily inhibited ammonia-oxidizing bacteria (AOB) while CDNB, pH 11,and cyanide inhibited both AOB and nitrite-oxidizing bacteria (NOB). DNP initially inhibited nitrification but quickly increased the NGR relative to the control and stimulated nitrification after several days in a manner reflective of oxidative uncoupling. The shocked mixed liquor showed trends toward recovery from inhibition for all chemicals tested, but in some cases this reversion was slow. These results contribute to our broader effort to identify relationships between chemical sources and the process effects they induce in activated sludge treatment systems.     

Nitrite Removal Performance and Community Structure of Nitrite-Oxidizing and Heterotrophic Bacteria Suffered with Organic Matter

Altlhough ammonia oxidation and ammonia-oxidizing bacteria (AOB) have been extensively studied, nitrite oxidation and nitrite-oxidizing bacteria (NOB) are still not well understood. In this article, the effect of organic matter on NOB and heterotrophic bacteria was investigated with functional performance analysis and bacterial community shift analysis. The results showed that at low concentrations of initial sodium acetate [initial sodium acetate (ISA) = 0.5 or 1 g/L], the nitrite removal rate was higher than that obtained under autotrophic conditions and the bacteria had a single growth phase, whereas at high ISA concentrations (5 or 10 g/L), continuous aerobic nitrification and denitrification occurred in addition to higher nitrite removal rates, and the bacteria had double growth phases. The community structure of total bacteria strikingly varied with the different concentrations of ISA; the dominant populations shifted from autotrophic and oligotrophic bacteria (NOB, and some strains of Bacteroidetes, Alphaproteobacteria, Actinobacteria, and green nonsulfur bacteria) to heterotrophic and denitrifying bacteria (strains of Gammaproteobacteria, especially Pseudomonas stutzeri and P. nitroreducens). The reasons that nitrite removal rate increased with supplement of organic matters were discussed.     

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.