Process optimization by decoupled control of key microbial populations: distribution of activity and abundance of polyphosphate-accumulating organisms and nitrifying populations in a full-scale IFAS-EBPR plant

This study investigated the abundance and distribution of key functional microbial populations and their activities in a full-scale integrated fixed film activated sludge-enhanced biological phosphorus removal (IFAS-EBPR) process. Polyphosphate accumulating organisms (PAOs) including Accumulibacter and EBPR activities were predominately associated with the mixed liquor (>90%) whereas nitrifying populations and nitrification activity resided mostly (>70%) on the carrier media. Ammonia oxidizer bacteria (AOB) were members of the Nitrosomonas europaea/eutropha/halophila and the Nitrosomonas oligotropha lineages, while nitrite oxidizer bacteria (NOB) belonged to the Nitrospira genus. Addition of the carrier media in the hybrid activated sludge system increased the nitrification capacity and stability; this effect was much greater in the first IFAS stage than in the second one where the residual ammonia concentration becomes limiting. Our results show that IFAS-EBPR systems enable decoupling of solid residence time (SRT) control for nitrifiers and PAOs that require or prefer conflicting SRT values (e.g. >15 days required for nitrifiers and <5 days preferred for PAOs). Allowing the slow-growing nitrifiers to attach to the carrier media and the faster-growing phosphorus (P)-removing organisms (and other heterotrophs, e.g. denitrifiers) to be in the suspended mixed liquor (ML), the EBPR-IFAS system facilitates separate SRT controls and overall optimization for both N and P removal processes.     

Total nitrogen removal in a hybrid, membrane-aerated activated sludge process

The hybrid (suspended and attached growth) membrane biofilm process (HMBP) is a novel method to achieve total nitrogen removal from wastewater. Air-filled hollow-fiber membranes are incorporated into an activated sludge tank, and a nitrifying biofilm develops on the membranes, producing nitrite and nitrate. By suppressing bulk aeration, the bulk liquid becomes anoxic, and the nitrate/nitrite can be reduced with influent BOD. The key feature that distinguishes the HMBP from other membrane-aerated processes is that it is hybrid; heterotrophic bacteria are kept mainly in suspension by maintaining low bulk liquid BOD concentrations. We investigated the HMBP's performance under a variety of BOD and ammonium loadings, and determined the dominant mechanisms of nitrogen removal. Suspended solids increased with the BOD loadings, maintaining low bulk liquid BOD concentrations. As a result, nitrification rates were insensitive to the BOD loadings, remaining at 1gNm(-2)day(-1) for BOD loadings ranging from 4 to 17gBODm(-2)day(-1). Nitrification rates decreased during short-term spikes in bulk liquid BOD concentrations. Shortcut nitrogen removal was confirmed using microsensor measurements, showing that nitrite was the dominant form of oxidized nitrogen produced by the biofilm. Fluorescence in situ hybridization (FISH) showed that ammonia oxidizing bacteria (AOB) were dominant throughout the biofilm, while nitrite oxidizing bacteria (NOB) were only present in the deeper regions of the biofilm, where the oxygen concentration was above 2mg/L. Denitrification occurred mainly in the suspended phase, instead of in the biofilm, decreasing the potential for biofouling. When influent BOD concentrations were sufficiently high, full denitrification occurred, with total nitrogen (TN) removal approaching 100%. These results suggest that the process is well-suited for achieving concurrent BOD and TN removal in activated sludge.     

Effects of free cyanide on microbial communities and biological carbon and nitrogen removal performance in the industrial activated sludge process

The changes in process performance and microbial communities under free cyanide (CN⁻) were investigated in a lab-scale activated sludge process treating industrial wastewater. The performance of phenol degradation did not appear to be adversely affected by increases in CN⁻ concentrations. In contrast, CN⁻ was found to have an inhibitory effect on SCN⁻ biodegradation, resulting in the increase of TOC and COD concentrations. Nitratation also appeared to be inhibited at CN⁻ concentrations in excess of 1.0 mg/L, confirming that nitrite-oxidizing bacteria (NOB) is more sensitive to the CN⁻ toxicity than ammonia oxidizing bacteria (AOB). After CN⁻ loads were stopped, SCN⁻ removal, denitrification, and nitrification inhibited by CN⁻ were recovered to performance efficiency of more than 98%. The AOB and NOB communities in the aerobic reactor were analyzed by terminal restriction fragment length (T-RFLP) and quantitative real-time PCR (qPCR). Nitrosomonas europaea lineage was the predominant AOB at all samples during the operation, but an obvious change was observed in the diversity of AOB at the shock loading of 30 and 50 mg/L CN⁻, resulting in Nitrosospira sp. becoming dominant. We also observed coexisting Nitrospira and Nitrobacter genera for NOB. The increase of CN⁻ loading seemed to change the balance between Nitrospira and Nitrobacter, resulting in the high dominance of Nitrobacter over Nitrospira. Meanwhile, through using the qPCR, it was observed that the nitrite-reducing functional genes (i.e., nirS) were dominant in the activated sludge of the anoxic reactor, regardless of CN⁻ loads.     

Exploring the relationship between viscous bulking and ammonia-oxidiser abundance in activated sludge: A comparison of conventional and IFAS systems

This study investigated the nature of viscous sludge bulking within a molasses-fed integrated fixed-film activated sludge (IFAS) and conventional activated sludge (AS) plant by routinely measuring the total carbohydrate and protein fractions of the mixed liquor (ML). The impacts of sludge settleability and plant performance on the relative abundance of ammonia-oxidising bacteria (AOB) (Nitrosomonas oligotropha-cluster) were also investigated using quantitative polymerase chain reaction (qPCR). Results showed that sludge volume index (SVI) correlated positively with the amount of ML total carbohydrate in both the IFAS and traditional AS plants, highlighting the influential role that ML polysaccharide concentration plays on sludge settleability in these reactors. Results also revealed a negative relationship between the AOB/total Bacteria ratio and SVI, demonstrating that a poor settling sludge generally coincided with periods of relatively low AOB abundance. The existence of these relationships suggests that readily available organic carbon (molasses) was likely to have been present in excess in these systems. Our qPCR results also showed that concentrations of both AOB and total Bacteria genomic copies detected within the ML of the IFAS and conventional AS plants were remarkably similar. For the IFAS system, results showed that the ML supported an equivalent number of AOB (per gram of biomass) to that detected on the plastic IFAS media carriers, suggesting that the suspended biomass fraction plays an equally important role in the overall nitrification performance of these systems. Interestingly, large observed variations in AOB and AOB/total Bacteria ratio measured within both the ML and IFAS media carriers had no measurable impact on the apparent nitrification performance of these systems; indicating the presence of some excess or ‘reserve’ nitrifying capacity above that which is required for effective plant performance. Results presented here also constitute the first known side-by-side comparison of the distribution of AOB in IFAS and conventional racetrack-like AS plants at the full-scale level.     

Simulation of closed double‐sludge retention time anoxic‐oxic process in wastewater treatment: case study for a utility model process

The ‘closed double‐sludge retention time anoxic‐oxic (SRT AO) process’ is a utility model designed by the Shanghai Urban Construction Design and Research Institute. It can quantitatively control the nitrification level by adjusting wastewater distribution and mixed sludge return during wastewater treatment, and can thus considerably reduce construction investment and operation costs. However, mixed sludge return from the sedimentation tank may dilute the concentration of nitrobacteria because the heterotrophic bacteria propagate faster. In this paper, the closed double‐SRT AO process was modelled and simulated based on its application at the Zhuyuan Second Wastewater Treatment Plant (WWTP). The distribution ratio was found to have a significant influence on the nitrobacteria's concentration but does not eliminate the existence of nitrobacteria in the system. Extension of sludge age enhanced the heterotrophic bacteria concentration and to a greater extent the nitrobacteria concentration, thus attenuated the dilution of nitrobacteria. Mixed liquid recycling showed little effect on nitrobacteria concentration. The closed double‐SRT AO process in Zhuyuan Second WWTP had enough capacity for complete nitrification, but the shortage of organic matter in the influent impeded the nitrogen removal.     

硝化菌在不同载体中的富集效率研究

利用MBBR型、纤维球、细菌球三种载体在污水厂生化池中进行硝化菌群的富集,通过测定反应活性速率及微生物多样性对载体富集硝化菌群进行研究。结果表明,三种载体均在富集30 d左右时效果最佳。此时,细菌球载体富集硝化菌群中氨氧化菌(AOB)和亚硝酸盐氧化菌(NOB)的反应比速率分别达到了2.72、1.68 mg/(gVSS·h),通常作为限制性因素的AOB比速率相较于活性污泥提高了42.41%;且载体中富集的AOB/NOB值最高可达2.10,相较于活性污泥(AOB/NOB值约为1),载体选择性富集了更多的AOB。因此,按50%的填充体积投加细菌球挂膜载体,其AOB和NOB的反应比速率可分别提高71.2%和44.7%。高通量测序结果表明,细菌球载体中硝化菌数量占比高达7.40%,为活性污泥中硝化菌含量的2.1倍。另外,菌群种属分析结果表明,载体生物膜中的菌群比活性污泥更加多样化,增加了系统的稳定性和抗冲击性。     

Diversity study of nitrifying bacteria in full-scale municipal wastewater treatment plants

We hypothesize that activated-sludge processes having stable and complete nitrification have significant and similar diversity and functional redundancy among its ammonia- and nitrite-oxidizing bacteria, despite differences in temperature, solids retention time (SRT), and other operating conditions. To evaluate this hypothesis, we examined the diversity of nitrifying bacterial communities in all seven water-reclamation plants (WRPs) operated by Metropolitan Water Reclamation District of Greater Chicago (MWRDGC). These plants vary in types of influent waste stream, plant size, water temperature, and SRT. We used terminal restriction fragment length polymorphism (T-RFLP) targeting the 16S rRNA gene and group-specific ammonia-monooxygenase functional gene (amoA) to investigate these hard-to-culture nitrifying bacteria in the full-scale WRPs. We demonstrate that nitrifying bacteria carrying out the same metabolism coexist in all WRPs studied. We found ammonia-oxidizing bacteria (AOB) belonging to the Nitrosomonas europaea/eutropha, Nitrosomonas oligotropha, Nitrosomonas communis, and Nitrosospira lineages in all plants. We also observed coexisting Nitrobacter and Nitrospira genera for nitrite-oxidizing bacteria (NOB). Among the factors that varied among the WRPs, only the seasonal temperature variation seemed to change the nitrifying community, especially the balance between Nitrosospira and Nitrosomonas, although both coexisted in winter and summer samples. The coexistence of various nitrifiers in all WRPs is evidence of functional redundancy, a feature that may help maintain the stability of the system for nitrification.     

Achieving nitrogen removal via nitrite in a pilot-scale continuous pre-denitrification plant

Nitrogen removal via nitrite (the nitrite pathway) is beneficial for carbon-limited biological wastewater treatment plants. However, partial nitrification to nitrite has proven difficult in continuous processes treating domestic wastewater. The nitrite pathway is achieved in this study in a pilot-scale continuous pre-denitrification plant (V = 300 L) treating domestic wastewater by controlling the dissolved oxygen (DO) concentration at 0.4-0.7 mg/L. It is demonstrated that the nitrite pathway could be repeatedly and reliably achieved, with over 95% of the oxidized nitrogen compounds at the end of the aerobic zone being nitrite. The nitrite pathway improved the total nitrogen (TN) removal by about 20% in comparison to the nitrate pathway, and also reduced aeration costs by 24%. FISH analysis showed that the nitrite oxidizing bacteria (NOB) population gradually reduced at low DO levels, and reached negligible levels when stable nitrite pathway was established. It is hypothesized that NOB was washed out due to its relatively lower affinity with oxygen. A lag phase was observed in the establishment of the nitrite pathway. Several sludge ages were required for the onset of the nitrite pathway after the application of low DO levels. However, nitrite accumulation increased rapidly after that. A similar lag phase was observed for the upset of the nitrite pathway when a DO concentration of 2-3 mg/L was applied. The nitrite pathway negatively impacted on the sludge settleability. A strong correlation between the sludge volume index and the degree of nitrite accumulation was observed.     

Diversity of nitrifying bacteria in full-scale chloraminated distribution systems

Chloramination for secondary disinfection of drinking water often promotes the growth of nitrifying bacteria in the distribution system due to the ammonia introduced by chloramine formation and decay. This study involved the application of molecular biology techniques to explore the types of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) present in several full-scale chloraminated systems. The results of AOB community characterization indicated the ubiquitous detection of representatives from the Nitrosomonas genus, with Nitrosospira constituting a negligible or small fraction of the AOB community in all but one sample. Cloning and sequencing demonstrated the presence of AOB representatives within the Nitrosomonas oligotropha cluster, a phylogenetic subgroup of AOB from which isolates demonstrate a high affinity for ammonia. For the NOB communities, Nitrospira were detected in most of the samples, while Nitrobacter were only detected in a few samples. These results provide insight into the types of AOB responsible for nitrification episodes in full-scale chloraminated systems, which should help direct future studies aimed at characterizing relevant AOB growth and inactivation properties. Furthermore, the detection of NOB in most of the samples suggests a need to evaluate the contribution of biological nitrite oxidation relative to chemical oxidation in these systems.     

Fate of beta blockers and psycho-active drugs in conventional wastewater treatment

The removal of beta blockers and psycho-active drugs was investigated in a representative conventional German WWTP by long-term measurement campaigns along different biological treatment processes. The activated sludge treatment with an elevated SRT of 18 d was the only process which led to a significant removal of certain beta blockers and psycho-active drugs. The removal efficiency was below 60% for all compounds except for the natural opium alkaloids codeine and morphine being removed by more than 80%. Primary biological transformation and sorption onto sludge as the main removal mechanisms were examined in lab-scale batch experiments. Sorption onto activated sludge was found to be negligible (<3%). The biological transformation could be described by pseudo-first order kinetics and the transformation constants k_biol were used to predict the removal of beta blockers and psycho-active drugs in an activated sludge unit with a model. For most compounds the removal efficiencies measured on the full-scale WWTP were within the 95% confidence intervals predicted by the model. The results from full-scale measurements and modeling indicate that biological transformation in the nitrification tank together with parameters such as the sludge retention time and the temperature is crucial regarding the biological transformation of beta blockers and psycho-active drugs in conventional WWTPs.     

Suspended biofilm carrier and activated sludge removal of acidic pharmaceuticals

Removal of seven active pharmaceutical substances (ibuprofen, ketoprofen, naproxen, diclofenac, clofibric acid, mefenamic acid, and gemfibrozil) was assessed by batch experiments, with suspended biofilm carriers and activated sludge from several full-scale wastewater treatment plants. A distinct difference between nitrifying activated sludge and suspended biofilm carrier removal of several pharmaceuticals was demonstrated. Biofilm carriers from full-scale nitrifying wastewater treatment plants, demonstrated considerably higher removal rates per unit biomass (i.e. suspended solids for the sludges and attached solids for the carriers) of diclofenac, ketoprofen, gemfibrozil, clofibric acid and mefenamic acid compared to the sludges. Among the target pharmaceuticals, only ibuprofen and naproxen showed similar removal rates per unit biomass for the sludges and biofilm carriers. In contrast to the pharmaceutical removal, the nitrification capacity per unit biomass was lower for the carriers than the sludges, which suggests that neither the nitrite nor the ammonia oxidizing bacteria are primarily responsible for the observed differences in pharmaceutical removal. The low ability of ammonia oxidizing bacteria to degrade or transform the target pharmaceuticals was further demonstrated by the limited pharmaceutical removal in an experiment with continuous nitritation and biofilm carriers from a partial nitritation/anammox sludge liquor treatment process.     

Ammonia-oxidizing archaea involved in nitrogen removal

Ammonia oxidation is critical to global nitrogen cycling and is often thought to be driven only by ammonia-oxidizing bacteria. The recent finding of new ammonia-oxidizing organisms belonging to the archaeal domain challenges this perception. Two major microbial groups are now believed to be involved in ammonia oxidation: chemolithotrophic ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). Candidatus “Nitrosopumilus maritimus”, the first isolated ammonia-oxidizing archaeon from a tropical marine aquarium tank, representative of the ubiquitous marine group 1 Crenarchaeota, contains putative genes for all three subunits (amoA, amoB, and amoC) of ammonia monooxygenase, the key enzyme responsible for ammonia oxidation. In this article, important concepts of the nitrogen cycle, ammonia oxidation processes, ammonia-oxidizing organisms, and their physiology are described. AOA are found to thrive in various habitats including hot/thermal springs, marine and fresh waters, soils, and wastewater treatment systems, where they may outnumber their counterpart, AOB. Various molecular tools have been applied to study AOB and AOA and determine their abundance and community structure changes from natural and engineered systems. The presence of AOA in activated sludge opens new opportunities for elucidating its role of ammonia removal in wastewater treatment plants and wetlands. Several significant questions related to AOA research have been raised to evoke reader involvement for broadening future studies.     

Nitrification efficiency and nitrifying bacteria abundance in combined AS-RBC and A2O systems

This study makes a comparison between the nitrification performance of TNCU-I (a combined activated sludge-rotating biological contactor process) and A2O systems by the use of a pilot plant and batch experiments. The nitrifier abundance in both systems was determined, using cloning-denaturing gradient gel electrophoresis (DGGE) and fluorescent in-situ hybridization (FISH), to investigate the role of rotating biological contactor in the TNCU-I process. The stability of the nitrification performance and the specific nitrification rate were found to be greater in TNCU-I system than in the A2O system. RBC biofilm promoted nitrifying activity that contributed to the nitrification performance, especially at a low SRT. By using the cloning-DGGE method, the genera Nitrosospira and Nitrospira were found to be present in all the samples, while the genus Nitrosomonas was observed only in the TNCU-I RBC biofilm. In addition, the proportions of ammonia oxidizer in the TNCU-I RBC biofilm, the TNCU-I activated sludge and the A2O activated sludge were 11.4%, 13.2%, and 4.1%, respectively, higher than the nitrite oxidizer fractions of 3.3%, 5.7% and 2.1%, respectively, according to the cloning-DGGE method. On the other hand, the proportions of ammonia oxidizers in the afore-mention materials were 10.3%, 13.7%, and 5.2%, higher than the nitrite oxidizer fractions of 2.5%, 3.6% and 2.3%, according to the FISH experiments. This implies that the proportion of ammonia oxidizer in the TNCU-I process was 3.2 and 2.6 times that in the A2O process, determined by the cloning-DGGE and FISH methods, respectively. These amounts are also close to the ammonia oxidization rate of 2.9 times. All the data show that RBC added to the aerobic zone of TNCU-I process would increase the nitrifier abundance and enhance the nitrification performance of the system.     

Bacterial response to a shock load of nanosilver in an activated sludge treatment system

The growing release of nanosilver into sewage systems has increased the concerns on the potential adverse impacts of silver nanoparticles (AgNPs) in wastewater treatment plants. The inhibitory effects of nanosilver on wastewater treatment and the response of activated sludge bacteria to the shock loading of AgNPs were evaluated in a Modified Ludzack-Ettinger (MLE) activated sludge treatment system. Before shock-loading experiments, batch extant respirometric assays determined that at 1 mg/L of total Ag, nitrification inhibitions by AgNPs (average size = 1-29 nm) and Ag⁺ ions were 41.4% and 13.5%, respectively, indicating that nanosilver was more toxic to nitrifying bacteria in activated sludge than silver ions. After a 12-h period of nanosilver shock loading to reach a final peak silver concentration of 0.75 mg/L in the MLE system, the total silver concentration in the mixed liquor decreased exponentially. A continuous flow-through model predicted that the silver in the activated sludge system would be washed out 25 days after the shock loading. Meanwhile, a prolonged period of nitrification inhibition (>1 month, the highest degree of inhibition = 46.5%) and increase of ammonia/nitrite concentration in wastewater effluent were observed. However, nanosilver exposure did not affect the growth of heterotrophs responsible for organic matter removal. Microbial community structure analysis indicated that the ammonium-oxidizing bacteria and nitrite-oxidizing bacteria, Nitrospira, had experienced population decrease while Nitrobacter was washed out after the shock loading.     

污泥减量工艺:HA-A/A-MCO的好氧脱氮机制分析

针对污泥减量技术存在对氮、磷去除能力低的问题,开发了一种具有强化脱氮除磷功能并可实现污泥减量化的HA-A/A-MCO工艺。在该工艺取得同步脱氮除磷和污泥减量优异效果的条件下,采用其处理校园生活污水,当进水TN平均为47 mg/L时,出水TN为10.9 mg/L,系统的总脱氮率为76.8%,其中好氧脱氮量占总脱氮量的50%,缺氧脱氮量占26%;HA-A/A-MCO系统存在着在好氧条件下具有反硝化能力的菌属,对好氧脱氮有一定贡献,且DO浓度对其反硝化能力没有抑制作用;好氧池中的DO浓度梯度有利于在污泥絮体内形成缺氧环境,从而促进同步硝化反硝化(SND)的发生,但减小污泥絮体尺寸会削弱絮体内部缺氧区域比例、降低SND的脱氮效率。     

Monochloramine cometabolism by Nitrosomonas europaea under drinking water conditions

Chloramine is widely used in United States drinking water systems as a secondary disinfectant, which may promote the growth of nitrifying bacteria because ammonia is present. At the onset of nitrification, both nitrifying bacteria and their products exert a monochloramine demand, decreasing the residual disinfectant concentration in water distribution systems. This work investigated another potentially significant mechanism for residual disinfectant loss: monochloramine cometabolism by ammonia-oxidizing bacteria (AOB).Monochloramine cometabolism was studied with the pure culture AOB Nitrosomonas europaea (ATCC 19718) in batch kinetic experiments under drinking water conditions. Three batch reactors were used in each experiment: a positive control to estimate the ammonia kinetic parameters, a negative control to account for abiotic reactions, and a cometabolism reactor to estimate the cometabolism kinetic constants. Kinetic parameters were estimated in AQUASIM with a simultaneous fit to all experimental data. The cometabolism reactors showed a more rapid monochloramine decay than in the negative controls, demonstrating that cometabolism occurs. Cometabolism kinetics were best described by a pseudo first order model with a reductant term to account for ammonia availability. Monochloramine cometabolism kinetics were similar to those of ammonia metabolism, and monochloramine cometabolism was a significant loss mechanism (30–60% of the observed monochloramine decay). These results suggest that monochloramine cometabolism should occur in practice and may be a significant contribution to monochloramine decay during nitrification episodes in drinking water distribution systems.     

Impact of inocula and growth mode on the molecular microbial ecology of anaerobic ammonia oxidation (anammox) bioreactor communities

The composition of distinctly inoculated granular anammox and biofilm-based completely autotrophic nitrogen removal over nitrite (CANON) bioreactors was investigated from start-up through continuous long-term operation via denaturing gradient gel electrophoresis (DGGE) and sequencing. The granular anammox reactor was seeded with sludge from an operational anammox reactor in Strass, Austria. The CANON reactor was seeded with activated sludge from a local wastewater treatment plant in New York City. The principal anammox bacteria (AMX) shifted from members related to Kuenenia stuttgartiensis present in the initial inoculum to members related to Candidatus Brocadia fulgida during pre-enrichment (before this study) and to members related to Candidatus Brocadia sp. 40 (during this study) in the granular reactor. AMX related to C. Brocadia sp. 40 were also enriched from activated sludge in the CANON reactor. The estimated doubling times of AMX in the granular and CANON reactors were 5.3 and 8.9 days, respectively, which are lower than the value of 11 days, reported previously. Both the granular anammox and CANON reactors also fostered significant amounts of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). The fractions of AMX and two groups of NOB were generally similar in the granular anammox and CANON reactors. However, the diversity and fractions of AOB in the two reactors was markedly different. Therefore, it is suggested that the composition of the feed and extant substrate concentrations in the reactor likely select for the microbial community composition more than the inocula and reactor configuration. Further, such selection is not equivalent for all resident communities.     

Alumina nanoparticles-induced effects on wastewater nitrogen and phosphorus removal after short-term and long-term exposure

Alumina nanoparticles (Al₂O₃ NPs) have been widely used in many fields, which causes a growing concern about their potential health and environmental risks. However, their possible impacts on wastewater nitrogen and phosphorus removal have not yet been reported. In this study, both short-term and long-term effects of Al₂O₃ NPs on wastewater nutrient removal were investigated. Scanning electron microscope (SEM) analysis showed that most of Al₂O₃ NPs were adsorbed onto activated sludge, but these NPs had no adverse effects on the surface integrity and viability of activated sludge. It was found that short-term exposure to 1 and 50 mg/L Al₂O₃ NPs induced marginal influences on wastewater nitrification, denitrification and phosphorus removal. Nevertheless, the prolonged exposure to 50 mg/L Al₂O₃ NPs was observed to decrease the total nitrogen (TN) removal efficiency from 80.4% to 62.5% due to the suppressed denitrification process, although biological phosphorus removal and the transformations of intracellular polyhydroxyalkanoates and glycogen were not affected. Quantitative PCR assays indicated that compared with the control, 50 mg/L Al₂O₃ NPs decreased the abundance of denitrifying bacteria in activated sludge. Further enzyme activity tests showed that the activities of key denitrifying enzymes (nitrate reductase and nitrite reductase) were inhibited, which might be responsible for the negative effects of 50 mg/L Al₂O₃ NPs on wastewater nitrogen removal after long-term exposure.     

Dynamic and distribution of ammonia-oxidizing bacteria communities during sludge granulation in an anaerobic-aerobic sequencing batch reactor

The structure dynamic of ammonia-oxidizing bacteria (AOB) community and the distribution of AOB and nitrite-oxidizing bacteria (NOB) in granular sludge from an anaerobic-aerobic sequencing batch reactor (SBR) were investigated. A combination of process studies, molecular biotechniques and microscale techniques were employed to identify and characterize these organisms. The AOB community structure in granules was substantially different from that of the initial pattern of the inoculants sludge. Along with granules formation, the AOB diversity declined due to the selection pressure imposed by process conditions. Denaturing gradient gel electrophoresis (DGGE) and sequencing results demonstrated that most of Nitrosomonas in the inoculating sludge were remained because of their ability to rapidly adapt to the settling-washing out action. Furthermore, DGGE analysis revealed that larger granules benefit more AOB species surviving in the reactor. In the SBR were various size granules coexisted, granule diameter affected the distribution range of AOB and NOB. Small and medium granules (d < 0.6 mm) cannot restrict oxygen mass transfer in all spaces of the sludge. Larger granules (d > 0.9 mm) can result in smaller aerobic volume fraction and inhibition of NOB growth. All these observations provide support to future studies on the mechanisms responsible for the AOB in granules systems.     

Application of calorimetric measurements for biokinetic characterisation of nitrifying population in activated sludge

A preliminary investigation is described on the application of calorimetry as a sensitive technique to evaluate nitrifying activity in activated sludge. Calorimetric profiles (thermograms) related to heat dissipation due to biological nitrification reactions (ammonia or nitrite consumption) have been interpreted. Correlations between calorimetric data and the main process variables, i.e. ammonia and nitrite concentration and oxygen uptake, have been verified, and confirm the potential of calorimetry to investigate, monitor and control even weakly exothermic biological processes like autotrophic nitrification. Heat yields (Y(Q/i)) for ammonia, nitrite, and oxygen, defined as the heat released per unit amount of converted reactant, have been separately evaluated. Moreover, calorimetric experiments on activated sludge from a full-scale nitrogen removal wastewater treatment plant have been carried out and kinetic parameters for both ammonia and nitrite oxidising bacteria have been estimated.