Mainstream Conditions Research Articles

When innocence is no protection: governance failure of digitization and its impact on local level implementation

PurposeGovernance of e-government is rarely discussed in the initial digitization stage, especially in developing countries where the government’s focus is mainly to pursue rapid proliferation of digital adoption rather than to implement governance. This study aims to explore the consequences of this absence of governance at local level conditions.Design/methodology/approachAn in-depth exploratory case study is conducted at a municipal health government in a southern city in Kalimantan Island, Indonesia, examining the conditions of local actors in response to various nationwide health digitization imperatives. The postcolonial theory with the critical paradigm is used to interpret and conceptualize the empirical findings.FindingsThis study identifies two critical failures of digitization governance that represent the mainstream condition: horizontal sectoral ego and vertical asymmetry and misalignment. These failures have resulted in undesirable consequences at the subalterns indicated by diverse ambivalence and de-voiced constructs displayed by the local actors.Practical implicationsThis paper suggests that various issues that emerge from local level implementation in nationwide digitization agenda might not always be issues of local technology adoption, but rather negative impacts due to the absence of governance practice at the strategic level.Originality/valueThrough a critical perspective, this study unearths the underlying power and structural inequity responsible for generating the various issues and undesirable consequences that emerge at local levels related to the nationwide digitization agenda.

Comprehensively Investigating Performance and Potential Mechanism of Anammox-Based Nitrogen Removal Process with Different Microbial Morphologies Under Mainstream Conditions

Comprehensively Investigating Performance and Potential Mechanism of Anammox-Based Nitrogen Removal Process with Different Microbial Morphologies Under Mainstream Conditions

NOB suppression strategies in a mainstream membrane aerated biofilm reactor under exceptionally low lumen pressure

Integrating the aeration-efficient membrane aerated biofilm reactor (MABR) with anaerobic ammonium oxidation (anammox) could yield further reduction in energy in wastewater treatment facilities. However, nitrite oxidizing bacteria (NOB) suppression remained challenging due to the absence of intrinsic inhibition factors in mainstream conditions. This study investigated selective NOB suppression strategies in MABR under <5 kPa lumen pressure. Three MABRs were seeded from different seeding sludge, and operated under various ammonium loading rates, aeration pressure, and temporary inhibitory shock conditions. The three reactors were operated for 170–456 days depending on studied parameters. The results showed that higher ammonium loading could create a substrate-oxygen imbalance and quickly contain emergent NOB activity when aeration pressure was not excessive. In addition, lowering of aeration pressure reversed nitrite oxidizing activities without affecting ammonium oxidizing bacteria (AOB). Cultivating partial nitritation biofilm under zero positive aeration pressure slowed down the growth of NOB yet resulted in self-induced anammox activities. With the aid of temporary free ammonia (FA)/free nitrous acid (FNA) treatment, full-nitrifying biofilm could be transformed to stable partial nitritation biofilm. More than 84% nitrite accumulation ratio (NAR) was sustained during stable operation in each reactor together with an ammonium removal rate of more than 100 mg-N/L/d. Microbial analysis revealed that Nitrosomonas was the main AOB taxon in the three reactors while K-strategist Nitrospira showed presence despite low nitrite oxidizing activities. Under zero positive pressure, proliferation of Nitrospira was much slower while Candidatus Brocadia was self-induced. Furthermore, Nitrospira showed downturn after temporary inhibition treatment.

Verification of inhibition effects of anoxic/aerobic alternation on NOB in nitrosation system under mainstream conditions

Under the mainstream conditions, nitrosation is unstable due to the difficulty of Nitrite Oxidizing Bacteria (NOB) inhibition. Anoxic/aerobic alternation has been recommended for coupled control NOB strategies with low dissolved oxygen, but feasibility analysis is lacking. Therefore, the feasibility of NOB inhibition was investigated by inoculating different sludge and using this strategy. The results showed that the strategy had poor nitrosation effect on ordinary sludge and unstable nitrosation sludge, with the average ammonia convert ratio of 75.11% and 65.65% and Nitrite Accumulation Ratio (NAR) of 0, respectively. Even after inoculating nitrosation sludge, NAR will gradually decrease from 98.73% to 9.36%, indicating that the system cannot operate stably. Nitrospira abundance was higher than that in the initial stage, and could not be inhibited by this strategy. Therefore, the strategy of using intermittent aeration to inhibit NOB and realize nitrification under mainstream conditions is questioned, and comprehensive and in-depth research is needed.

Achieving high-rate partial nitritation with aerobic granular sludge at low temperatures.

Partial nitritation is necessary for the implementation of the mainstream anammox (anaerobic ammonium oxidation) process in wastewater treatment plants. However, the difficulty in outcompeting nitrite-oxidizing bacteria (NOB) at mainstream conditions hinders the performance of partial nitritation. The present work aimed to develop a high-rate partial nitritation process for low-ammonium wastewater treatment at low temperatures by seeding aerobic granules. Experimental results suggested that both stratified structure of nitrifiers developed in the granules and sufficient residual ammonium concentration (18-35mgN L-1) in the bulk liquid contributed to efficient NOB repression. With the hydraulic retention time progressively shortened from 1.0 to 0.17h, the influent nitrogen loading rate of the partial nitritation process reached 6.8 ± 0.4kgN m-3 d-1 even at 10-15°C. The high concentration (7.5 gVSS L-1) and activity (0.48gN g-1 VSS d-1 at 11°C) of granular sludge made the reactor possess an overcapacity evaluated by the ratio between the actual ammonium oxidation rate of the granules and their maximum potential. The overcapacity helped the reactor to face the adverse effect of decreasing temperatures. Overall, this work indicated the great potential of applying aerobic granules to achieve high-rate partial nitritation at mainstream conditions. Moreover, anammox bacteria with a relative abundance of 2.8% was also identified in the partial nitritation granules at the end of this study, suggesting that the granules provided a habitable niche for anammox bacteria growth. Note that these results cannot fully relate to the treatment of real domestic/municipal wastewater, they are a source of important information increasing the knowledge about low temperature partial nitrification.

In-situ fermentation coupling with partial-denitrification/anammox process for enhanced nitrogen removal in an integrated three-stage anoxic/oxic (A/O) biofilm reactor treating low COD/N real wastewater

Mainstream partial-denitrification with anammox (PD-anammox) process faced the challenge of complex organics involved in real sewage. Herein, PD-anammox coupled with in-situ fermentation was successfully achieved in a full biofilm system formed by three-stage anoxic/oxic reactor to treat real wastewater with low COD/N of 3.6. The total nitrogen (TN) removal efficiency was enhanced to 78.4% ± 3.6% with average TN and ammonium concentrations in effluent of 10.6 and 0.5 mg N/L, respectively. Batch tests confirmed that partial-denitrification was the major nitrite provider for anammox in the anoxic biofilm, while in-situ fermentation could decompose the complex organics to readily-biodegradable organics for full- or partial-denitrification. Additionally, a significant anammox bacteria (Candidatus Brocadia) population was detected in the second (3.53%) and third (4.46%) anoxic zones, while denitrifiers and fermentative bacteria were mainly enriched in the first anoxic zone. This study presents a feasible approach for PD-anammox process in practical application under mainstream condition.

Impact of the nitrifying community dynamics on the partial nitritation process performed by an AOB-enriched culture in a granular sludge airlift reactor

One of the main challenges for the full-scale implementation of the autotrophic biological nitrogen removal process is to achieve a stable long-term partial nitritation under the mainstream conditions of urban wastewater treatment plants. Here, the performance of partial nitritation process under mainstream conditions was investigated based on the change of the nitrifying community developed. A lab-scale airlift reactor was operated for 430 days with a granular sludge highly enriched in ammonia oxidizing bacteria (AOB) performing partial nitritation treating a mainstream-mimicked synthetic influent. The changes in solid retention time (SRT), dissolved oxygen (DO) concentration and temperature strongly affected to the microbial community developed, especially to the dynamics of the nitrite oxidizing bacteria (NOB) population, which triggered to instabilities in the partial nitritation process. The extremely high values of SRT applied (SRT = ∞) and long-term exposure to low temperature (10 °C) promoted the growth of NOB Nitrotoga genus and development of nitratation activity; and the subsequent variability in temperature (between 10 and 30 °C) and in DO concentration (between 0.4 ± 0.1 and 1.4 ± 0.3 mg O2 L−1) influenced the competitiveness between nitrifiers and the growth of species previously undetected in the culture, as Nitrospira genus. Anaerobic ammonia oxidizing bacteria were also identified during cultivation and assisted to the repression of the NOB activity at 25–15 °C. The results of the present study uphold that understanding the niche differentiation and competitiveness of NOB will be key to develop strategies for NOB repression in order to make possible full-scale implementation of partial nitritation process at mainstream conditions.

Influence of cooling air jets on the aerodynamic and aerothermal losses and cooling effectiveness of air-cooled radial flameholder

The flow losses of flameholder are associated with the combustion performance of augmented combustor. After introducing cooling air, the changing flow field and mixture temperature will significantly affect the flow losses and wall temperature of air-cooled flameholder. In this study, experimentally verified CFD simulation methods were adopted to investigate the influence of the cooling air jet angle and aerodynamic conditions on the flow losses and cooling effectiveness. Firstly, the impact of the cooling air jet angle combination and size on the injection and mixing losses, kinetic energy loss, thermal energy loss, and cooling effectiveness of air-cooled flameholder was analyzed. Then the cooling scheme with low flow losses and excellent cooling effectiveness was discussed under different cooling air and mainstream conditions. Results show that the angle combination affects the mixing losses slightly but the injection losses greatly; the angle size has a notable influence on both losses. When the cooling jet angle between cooling holes and the back wall of flameholder exceeds 60°, total pressure loss and thermal energy losses change lightly. Noticeably, there are apparent disparities of flow fields in the near-field wake region under different cooling air jets angle combinations and sizes, resulting in the different flame structures, wall temperature distributions, and cooling effectiveness. Overall, when all cooling air jets are injected perpendicular to the back wall, air-cooled flameholder has the minimum flow losses and the best cooling effectiveness, whereas it has the weakest flame stability. In addition, the total pressure losses increase with improving the flow rate and temperature of cooling air and the mainstream velocity, but that decreases with increasing mainstream temperature. The kinetic energy and thermal energy losses are raised with the increased coolant flow rate, mainstream temperature, and mainstream velocity but reduced with the enhanced cooling air temperature.

A successful start-up of an anaerobic membrane bioreactor (AnMBR) coupled mainstream partial nitritation-anammox (PN/A) system: A pilot-scale study on in-situ NOB elimination, AnAOB growth kinetics, and mainstream treatment performance

In this pilot-scale study, an innovative mainstream treatment process that couples the anaerobic membrane reactor (AnMBR) with a one-stage PN/A system was proposed for advancing the concept of carbon neutrality in the municipal wastewater treatment plant. This work demonstrates the start-up procedure of a pilot-scale one-stage PN/A system for mainstream treatment. The 255-day start-up of the one-stage PN/A system involved the cultivation of ammonium-oxidizing bacteria (AOB) from the activated sludge, suppression of nitrite-oxidizing bacteria (NOB), investigation of in-situ growth kinetics of anammox bacteria (AnAOB), and the 50-day operation of the pilot-scale AnMBR-PN/A process for natural mainstream treatment. It is verified in the pilot-scale system for the first time that the in-situ free ammonia (FA) and free nitrous acid (FNA) exposure could effectively eliminate the Nitrospira (the NOB genus) while retaining the Nitosonomas (the AOB genus) community in the suspended sludge. NOB community rebounding was not detected even at the mainstream conditions with low nitrogen concentrations (Influent ammonium concentration=38±6 mg-NH4+-N/L) by intermittent aeration to control the system dissolved oxygen (DO) below 0.5 mg/L. The results of the mainstream treatment showed that the average effluent total nitrogen (TN) in the coupled process was generally lower than 10 mg-N/L, which meets the discharge limits of most prefectures in Japan. The investigated results of the in-situ anammox bacteria (AnAOB) growth kinetics suggested that the promoted start-up strategy of taking advantage of the warm months with higher mainstream temperature to achieve the rapid in-situ growth of the AnAOB is applicable in the investigated regions. From the perspective of the removal performance of the TN and organic substance, the AnMBR-PN/A process has great potential as the layouts of the carbon-neutral mainstream wastewater treatment plants.

Microbial communities in an anammox reactor treating municipal wastewater at mainstream conditions: Practical implications of different molecular approaches

Biases on molecular techniques used to study microbial communities in the anammox process are not always considered. This work aimed to evaluate different molecular approaches to study bacterial communities in an anammox reactor treating real urban wastewater at mainstream conditions. 16S rRNA amplicon sequencing with different primer sets, metagenomics, and qPCR were evaluated. An enriched anammox reactor feed with synthetic wastewater was used as a positive control. We reported that, despite its in silico coverage, the universal primers 515F-909R had, in fact, very low coverage (1%) of the Planctomycetota phylum. The functional genes approach showed a taxonomic resolution at the species level, although genomic database updates are a bias to be considered. From the phylogenetic analysis, coherence among the approaches was obtained. Primers targeting anammox 16S rRNA and hydrazine synthase genes in qPCR showed consistency in the quantification, making them useful for comparing similar investigations. According to the results, recommendations on the biomolecular approach to be used were outlined.

Inhibition of nitrite-oxidizing bacteria in automatic recycling PN/ANAMMOX under mainstream conditions

At present, sustainable and stable partial nitrification has not been widely achieved in the mainstream PN/ANAMMOX process. Here, the feasibility of sustainable and stable partial nitrification was demonstrated in automatic recycling PN/ANAMMOX reactor under mainstream conditions using both simulation and experimental methods. Stable nitrite accumulation in the aerobic zone could be achieved via regulating dissolved oxygen (DO) concentrations and sludge retention time (SRT). The DO concentrations required for the repression of nitrite-oxidizing bacteria (NOB) were lower at longer SRTs. The DO concentrations and SRTs required for NOB repression were lower at lower temperatures. However, NOB repression was diminished by a persistent low DO and short SRT under mainstream conditions. With the introduction of automatic recycling, sustainable and stable partial nitrification was achieved. Effluent recycling could limit the nitrite-nitrogen required for NOB growth. Collectively, effluent recycling may serve as a feasible and useful strategy for NOB inhibition during the PN/ANAMMOX process.

Assessing the stability of one-stage PN/A process through experimental and modelling investigations

The mainstream partial nitritation/anammox (PN/A) process has been intensively studied but its stability remains a key challenge. It is shown here that biofilm thickness can exhibit a critical role in controlling the process stability of mainstream PN/A against dissolved oxygen (DO) variation. In a laboratory moving bed biofilm reactor (MBBR), PN/A performance was initially established in 200 days by controlling a low DO of 0.13 ± 0.07 mg O2/L in the bulk liquid, which deteriorated with an increase of DO (0.35 ± 0.13 mg O2/L) for over two months, and then rapidly recovered in a month with the initial low DO level re-applied. Biofilm thickness of PN/A carriers was measured during the experiment, which became significantly thinner (367 ± 146 μm) at mainstream conditions. The thin thickness primarily decreased the in-situ consumption rate of nitrite, rather than ammonium, when DO increased from 0.1 to 0.4 mg O2/L, due to that the thin thickness can only restrict anammox capacity. These results illustrated the role of biofilm thickness in regulating PN/A performance and microbial activities. Further investigation using an established model revealed the joint contribution of biofilm thickness and DO concentration to PN/A process, while particularly, the biofilm thickness can determine the optimal DO level for maximizing the nitrogen removal efficiency and system robustness against DO variation. These results highlight the need of considering biofilm thickness in PN/A process optimization and stability improvement in low-strength wastewater treatment.

Robustness of mainstream anammox activity at bench and pilot scale

New technologies and processes, such as mainstream anammox, aim to reduce energy requirements of wastewater treatment and improve effluent quality. However, in municipal wastewater (MWW) anammox system are often unstable due to process control disturbance, influent variability, or unwanted nitrite oxidizing bacteria (NOB). This study examines the anammox system by focusing on anammox activity and its robustness in a mainstream environment. An 8 m3 pilot-scale sequencing batch reactor (SBR) receiving pretreated MWW (with external nitrite addition) was seeded with pre-colonized carriers. Within six months at 12–20 °C an anammox activity of 200 gN·m−3·d−1 was achieved. After the startup an anammox activity of 260 ± 83 gN·m−3·d−1 was maintained over 450 days. The robustness of the anammox activity was analyzed through three disturbance experiments. Anammox biofilm on carriers were exposed to dissolved oxygen (DO = 1.6 mg·L−1, intermittent aeration), organic loading rate (OLR, C/N increased from 2:1 to 5:1) and temperature disturbances (20 °C to 12 °C) in triplicate 12 L bench scale reactors. The anammox activity and microbial community was monitored during these disturbances. The DO and OLR disturbance experiments were replicated at pilot scale to investigate upscaling effects. Bench and pilot scale anammox activity were unaffected by the DO disturbance. Similarly, an increase in OLR did not deteriorate the bench and pilot scale anammox activity, if nitrate was available. When, at bench scale, the reactor temperature was reduced from 20 °C to 12 °C overnight, anammox activity decreased significantly, this was not the case for the slow seasonal temperature changes (12–25 °C) at pilot scale where no strong temperature dependency was detected in winter. Metagenomic analysis revealed a broad range of Brocadiaceae species with no single dominant anammox species. Anammox thrive under mainstream conditions and can withstand typical process disruptions.

Uncertainty Quantification of Aero-Thermal Performance of a Blade Endwall Considering Slot Geometry Deviation and Mainstream Fluctuation

Abstract With the ever-increasing aerodynamic and thermal loads, the endwalls of modern gas turbines have become critical areas that are susceptible to manufacturing and operational uncertainties, making them highly prone to thermal failures. Therefore, it is of vital importance to quantify the impacts of input uncertainties on the aero-thermal performance of endwalls. First, based on the Kriging surrogate, an efficient uncertainty quantification (UQ) method suitable for expensive computational fluid dynamics (CFD) problems is proposed. By using this method, the impacts of slot geometry deviations (slot width, endwall misalignment) and mainstream condition fluctuations (turbulence intensity, inlet flow angle) on the aero-thermal performance of endwalls are quantified. Results show that the actual performance of endwalls has a high probability of deviating from its nominal value. The maximum deviations of aerodynamic loss, area-averaged film cooling effectiveness, and area-averaged Nusselt number reach 0.33%, 45%, and 5.0%, respectively. The critical regions that are most sensitive to the input uncertainties are also identified. Second, a global sensitivity analysis method is also performed to pick out the significant uncertain parameters and explore the relationship between input uncertainties and performance output. The inlet flow angle is proved to be the most significant parameter among the four input uncertain parameters. Besides, a positive incidence angle is found to be detrimental to both the aerodynamic performance and the thermal management of endwalls. Finally, the influence mechanisms of the inlet flow angle on endwall aero-thermal performance are clarified by a fundamental analysis of flow and thermal fields.

Role of air scouring in anaerobic/anoxic tanks providing nitrogen removal by mainstream anammox conversion in a hybrid biofilm/suspended growth full-scale WWTP in China.

A full-scale wastewater treatment plant in China experienced unintentional anammox bacterial enrichment on biofilm carriers placed in the anaerobic and anoxic zones of an anaerobic/anoxic/oxic process under ambient temperatures and without bioaugmentation. Here, we show that microaerophilic conditions resulting from air scouring needed for biofilm carrier suspension in the anaerobic/anoxic zones can support a robust nitritation/anammox process. Results from an in situ on/off air scouring test showed that air scouring strongly induced both ammonia and total inorganic nitrogen removal in the anaerobic/anoxic zones. Ammonium concentration in the anaerobic and anoxic tanks remained constant or even slightly increased when air scouring was off, indicating that air scouring made a noticeable difference in nitrogen profiles in the anaerobic/anoxic zones. Various batch tests further indicated that partial denitrification is not likely to generate nitrite for anammox bacteria. Robust nitritation, and anammox on the carriers, can occur at low dissolved oxygen conditions, as measured in the full-scale facility. The observations show that mainstream deammonification without sidestream bioaugmentation at moderate temperature is feasible and further optimization by a more dedicated design can result in improved nitrogen removal in cases when chemical oxygen demand is limited in mainstream wastewater treatment. PRACTITIONER POINTS: Microaerophilic conditions in a full-scale IFAS reactor caused mainstream anammox in moderate temperate area. Robust nitritation, and anammox on the carriers, can occur at low dissolved oxygen conditions in anaerobic/anoxic tanks with air scouring. Anammox can function well with conventional nitrification and denitrification process at mainstream conditions for stable nitrogen removal.

Effect of Mainstream Velocity on the Optimization of a Fan-Shaped Film-Cooling Hole on a Flat Plate

In this study, the effect of mainstream velocity on the optimization of a fan-shaped hole on a flat plate was experimentally investigated. The experiment was conducted by changing the forward expansion angle (βfwd), lateral expansion angle (βlat), and metering length ratio (Lm/D) of the film-cooling hole. A total of 13 cases extracted using the Box–Behnken method were considered to examine the effect of the shape parameters of the film-cooling hole under a 90 m/s mainstream velocity condition, and the results were compared with the results derived under a mainstream velocity of 20 m/s. One density ratio (DR = 2.0) and a blowing ratio (M) ranging from 1.0 to 2.5 were considered, and the pressure-sensitive paint (PSP) technique was applied for the film-cooling effectiveness (FCE). As a result of the experiment, the optimized hole showed a 49.3% improvement in the overall averaged FCE compared to the reference hole with DR = 2.0 and M = 2.0. As the blowing ratio increased, the hole exit area tended to increase, and this tendency was the same as that in the 20 m/s mainstream condition.

Bifurcation Analysis of an Impulsive System Describing Partial Nitritation and Anammox in a Hybrid Reactor.

Low-energy nitrogen removal under mainstream conditions is a technology that has received significant attention in recent years as the water industry drives toward long-term sustainability goals. Simultaneous partial nitritation-Anammox (PN/A) is one process that can provide substantial energy reduction and lower sludge yields. Mathematical modeling of the PN/A process offers engineers insights into the operating conditions necessary to maximize its potential. Laureni et al. (Laureni et al. Water Res. 2019, 14) have recently published a simplified mechanistic model of the process operated as a sequencing batch reactor that investigated the effect of three key operating parameters on performance (Anammox biofilm activity, dissolved oxygen concentration and fraction of solids wasted). The analysis of the model was limited, however, to simulation with relatively few discrete parameter sets. Here, we demonstrate through the use of bifurcation theory applied to an impulsive dynamical system that the parameter space can be partitioned into regions in which the system converges to different fixed points that represent different outcomes: either the washout of nitrite-oxidizing bacteria or their survival. Mapping process performance data onto these spaces allows engineers to target suitable operating regimes for specific objectives. Here, for example, we note that the nitrogen removal efficiency is maximized close to the curve that separates the regions in parameter space where nitrite-oxidizing bacteria washout from the region in which they survive. Further, control of solids washout and Anammox biofilm activity can also reduce oxygen requirements while maintaining an appropriate hydraulic retention time. The approach taken is significant given the possibility for using such a methodology for models of increasing complexity. This will enable engineers to probe the entire parameter space of systems of higher dimension and realism in a consistent manner.

Temperature modulates stress response in mainstream anammox reactors

Autotrophic nitrogen removal by anaerobic ammonium oxidizing (anammox) bacteria is an energy-efficient nitrogen removal process in wastewater treatment. However, full-scale deployment under mainstream conditions remains challenging for practitioners due to the high stress susceptibility of anammox bacteria towards fluctuations in dissolved oxygen (DO) and temperature. Here, we investigated the response of microbial biofilms with verified anammox activity to DO shocks under 20 °C and 14 °C. While pulse disturbances of 0.3 mg L−1 DO prompted only moderate declines in the NH4+ removal rates, 1.0 mg L−1 DO led to complete but reversible inhibition of the NH4+ removal activity in all reactors. Genome-centric metagenomics and metatranscriptomics were used to investigate the stress response on various biological levels. We show that temperature regime and strength of DO perturbations induced divergent responses from the process level down to the transcriptional profile of individual taxa. Community-wide gene expression differed significantly depending on the temperature regime in all reactors, and we found a noticeable impact of DO disturbances on genes involved in transcription, translation, replication and posttranslational modification at 20 °C but not 14 °C. Genome-centric analysis revealed that different anammox species and other key biofilm taxa differed in their transcriptional responses to distinct temperature regimes and DO disturbances.

Effective dampening of temperature effects in an anammox reactor treating real mainstream wastewater

The aim of the present study was to evaluate the capability of an up-flow anammox sludge bed reactor (13 L) of dampening a 10 °C temperature drop at mainstream conditions while operating at constant nitrogen loading rates (0.11 ± 0.01 g N L−1 d−1). The up-flow anammox sludge bed reactor was fed with an aerobically pre-treated mainstream wastewater. The reactor temperature was controlled as to mimic a (realistic) summer-to-winter transition from 20 to 10 °C plus a three months period at 10 °C. During the 350 days of operation, the average nitrogen removal rate remained high (0.10 ± 0.01 g N L−1 d−1), indicating that the 10 °C drop did not affect the overall efficiency of the reactor. This can be explained as anammox activity differed among the different sludge bed sections. At 20 °C, anammox activity was mainly located at the bottom of the sludge bed. At 10 °C, the anammox activity decrease of the bottom sludge bed was compensated by an activity increase within the upper sludge bed sections. The contribution of heterotrophic denitrifying activity to the total nitrogen removal rate was assessed to be 3-to-5 times lower than that of anammox, even at 10 °C. Microbial community of 16S rRNA gene-targeted sequencing analyses resulted in the identification of an uncharacterized Planctomycetes in high numbers. This study demonstrated that the low temperatures should not be an obstacle for the feasibility of anammox bacteria in the main water line of an urban wastewater treatment plant.

Spontaneous mainstream anammox in a full-scale wastewater treatment plant with hybrid sludge retention time in a temperate zone of China.

Spontaneous anammox bacteria enrichment at mainstream conditions was reported in a full-scale Wastewater Treatment Plant (WWTP) in a temperate zone of China. The mainstream anammox was observed after WWTP process retrofit, which constructed a hybrid sludge retention time (SRT) system by providing moving carriers in the anaerobic/anoxic tank and was initially designed to enhance the denitrification process in a conventional anaerobic/anoxic/oxic process. The hybrid SRT system achieved 86.0±4.6% total nitrogen (TN) removal via combined mainstream anammox and conventional denitrification. Autotrophic denitrification via mainstream anammox was confirmed by various shreds of evidence including high-throughput sequencing, specific anammox activity test, and 15 N isotopic tracing. Long-term anammox bacteria existence in the biofilm of the carrier in anoxic zones was detected in a much higher relative abundance compared with other spots. The contribution of anammox activity to TN removal was estimated at around 20%-30%. The reasons leading to spontaneous anammox enrichment were mainly attributed to the carriers for slow-growing bacteria growth and dissolved oxygen gradient in the anoxic tank (caused by intermittent aeration) for nitrite production. The insights of this full-scale case study provide important perspectives for future mainstream anammox application, and also the design of an energy-neutral WWTP process. PRACTITIONER POINTS: Spontaneous mainstream anammox in a full-scale WWTP after its retrofit in a temperate zone of China was reported. Anammox bacteria enrichment and long-term stability on moving carriers at mainstream conditions was achieved by modified hybrid SRT system. The hybrid SRT system achieve stable nitrogen removal even in cold winter and high BOD/N situation by combining mainstream anammox with conventional denitrification. Long term full-scale operation demonstrated excellent nitrogen removal with about 20%-30% contribution of mainstream anammox. This full-scale case study provided perspectives for future optimizing mainstream anammox application, and also energy-neutral WWTP process design.