Mainstream partial nitritation-anammox in municipal wastewater treatment: status, bottlenecks, and further studies

Achieving stable nitrogen removal performance of mainstream PN-ANAMMOX by combining high-temperature shock for selective recovery of AOB activity

This paper describes the new concept of the mainstream partial nitritation (PN)-anaerobic ammonium oxidation (ANAMMOX) combined with a high-temperature shock strategy for the selective recovery of ammonia-oxidizing bacteria (AOB) activity. In the preliminary test, the temperature shock condition for PN was optimized (60 °C and > 20 min). Based on this, the implementation strategy in a continuous stirred tank reactor (CSTR) system was studied further, and the polyvinyl alcohol (PVA)/sodium alginate carrier exposure ratio (ER) and dissolved oxygen (DO) concentration were considered as primary variables. The AOB activity was recovered selectively when the ER of the carrier ranged from 20 to 40%, and the DO was higher than 2.3 mg O₂/L. This was not the case for nitrite-oxidizing bacteria (NOB) (AOB: 1.17±0.1 gNH₄⁺-N/L_Carrier/d, NOB: 0.34±0.1 gNO₃^--N/L_Carrier/d). As a result, the activity of AOB was recovered selectively with a decrease in Nitrospira spp., which was verified by kinetic and microbial analyses for the AOB (K_{S, DO} = 3.89 mgO₂/L) and NOB (K_{S, DO} = 1.14 mgO₂/L). Eventually, the mainstream PN-ANAMMOX was achieved with a nitrogen removal efficiency of 81.5±3.3% for 95 days. The findings provide insight to establishing a stable mainstream PN-ANAMMOX process using a high-temperature shock strategy.

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 (18-35 mg N L^-1) in the bulk liquid contributed to efficient NOB repression. With the hydraulic retention time progressively shortened from 1.0 to 0.17 h, the influent nitrogen loading rate of the partial nitritation process reached 6.8 ± 0.4 kg N m^-3 d^-1 even at 10-15 °C. The high concentration (7.5 gVSS L^-1) and activity (0.48 g N 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.

An effective strategy for in situ start-up of mainstream anammox process treating domestic sewage

The feasibility of in situ start-up of mainstream anammox process was investigated in three parallel sequencing batch biofilm reactors (SBBRs) inoculated with nitrification sludge, partial nitrification sludge, and denitrifying phosphorus removal sludge, respectively. The SBBRs were operated under alternate anaerobic/aerobic/anoxic pattern at ambient temperature (16.5-26.8 °C). The influent organic and nitrogen loading rates were increased stepwise. Anammox bacteria grew exponentially with relative abundance and overall bacterial activity increasing from 0 to 0.004% to 0.29-0.40% and 'not detected' to 6-7 mg N/L/h, respectively. Desirable nitrogen removal efficiency of about 86% was obtained in 3-4 months for the influent nitrogen of 40.5-73.6 mg N/L. Anammox was the primary nitrogen transformation pathway. For the anammox bacterial enrichment, biofilm, alternate anaerobic/aerobic/anoxic pattern, and limited aeration played important roles. Seed sludge with high ammonium oxidizing bacterial activity further promoted the start-up of anammox process. The in situ start-up strategy could promote the full-scale application of mainstream anammox.

Advanced nitrogen removal from low C/N municipal wastewater by combining partial nitrification-anammox and endogenous partial denitrification-anammox (PN/A-EPD/A) process in a single-stage reactor

In this study, an innovative partial nitrification-anammox (PN/A) and endogenous partial denitrification-anammox (EPD/A) process was developed in a single-stage integrated fixed film activated sludge sequencing batch reactor (IFAS-SBR) treating real municipal wastewater with C/N ratio below 3.2. Enhanced efficiency of total nitrogen (TN) removal reached 90.1% with low HRT of 12 h and DO of 0.4 ± 0.1 mg/L. Detailed nitrogen removal mechanism analysis of typical cycle revealed that 89.9% of TN was eliminated through anammox pathway. Anammox bacteria (Candidatus Brocadia) and endogenous denitrifying bacteria (Candidatus Competibacter) were abundant both in biofilms and suspended sludge, meanwhile ammonium-oxidizing bacteria has outcompeted nitrite-oxidizing bacteria, which all favored the synergistic effect of anammox with PN and EPD and contributed to the improvement of nitrogen removal. Overall, the above results confirmed that combined PN/A and EPD/A process is a reliable and efficient alternative for mainstream anammox process.

Incorporation of the complete ammonia oxidation (comammox) process for modeling nitrification in suspended growth wastewater treatment systems

The newly discovered process complete ammonia oxidation (comammox) has changed the traditional understanding of nitrification. In this study, three possible concepts of comammox were developed and incorporated as part of an extended two-step nitrification model. For model calibration and validation, two series of long-term biomass washout experiments were carried out at 12 °C and 20 °C in a laboratory sequencing batch reactor. The inoculum biomass was withdrawn from a large biological nutrient removal wastewater treatment plant. The efficiency of the examined models was compared based on the behaviors of ammonia, nitrite, and nitrate in the studied reactor. Predictions of the conventional approach to comammox, assuming the direct oxidation of ammonia to nitrate, were slightly better than the two other approaches. Simulation results revealed that comammox could be responsible for the conversion of >20% of the influent ammonia load. Therefore, the role of commamox in the nitrogen mass balance in activated sludge systems should not be neglected and requires further investigation. Furthermore, sensitivity and correlation analysis revealed that the maximum growth rates (μ), oxygen half-saturation (K_O), and decay rates (b) of the canonical nitrifiers and comammox were the most sensitive factors, and the highest correlation was found between μ and b among all considered kinetic parameters. The estimated μ values by the best model were 0.57, 0.11, and 0.15 d^-1 for AOB, NOB, and comammox bacteria, respectively.

Efficiency of hybrid systems enhanced with different sludge ratios in improving resistance to short-term low temperatures

Complete autotrophic nitrogen removal over nitrite (CANON) is used in wastewater treatment. However, the performance of the CANON system significantly decreases at low temperatures; thus, a new strategy to improve the resistance of the CANON system is required. To investigate the impact of sludge ratio control (high-granule, equivalent, and high-floc systems) on the resistance of CANON to low temperatures, and their recovery after restoring to normal temperature, the nitrogen removal performance of hybrid systems with different ratios was evaluated. The equivalent system had the lowest nitrite accumulation rate and highest nitrogen removal rate. Anaerobic ammonia oxidation was the rate-limiting step of each system, and hzs was the rate-limiting gene. The higher anaerobic ammonium oxidizing bacteria (AAOB) abundance and hzs expression levels resulted in an equivalent system with better resistance and recovery to short-term low temperatures at the gene level.

Activities and metabolic versatility of distinct anammox bacteria in a full-scale wastewater treatment system

Anaerobic ammonium oxidation (anammox) is a key N₂-producing process in the global nitrogen cycle. Major progress in understanding the core mechanism of anammox bacteria has been made, but our knowledge of the survival strategies of anammox bacteria in complex ecosystems, such as full-scale wastewater treatment plants (WWTPs), remains limited. Here, by combining metagenomics with in situ metatranscriptomics, complex anammox-driven nitrogen cycles in an anoxic tank and a granular activated carbon (GAC) biofilm module of a full-scale WWTP treating landfill leachate were constructed. Four distinct anammox metagenome-assembled genomes (MAGs), representing a new genus named Ca. Loosdrechtii, a new species in Ca. Kuenenia, a new species in Ca. Brocadia, and a new strain in "Ca. Kuenenia stuttgartiensis", were simultaneously retrieved from the GAC biofilm. Metabolic reconstruction revealed that all anammox organisms highly expressed the core metabolic enzymes and showed a high metabolic versatility. Pathways for dissimilatory nitrate reduction to ammonium (DNRA) coupled to volatile fatty acids (VFAs) oxidation likely assist anammox bacteria to survive unfavorable conditions and facilitate switches between lifestyles in oxygen fluctuating environments. The new Ca. Kuenenia species dominated the anammox community of the GAC biofilm, specifically may be enhanced by the uniquely encoded flexible ammonium and iron acquisition strategies. The new Ca. Brocadia species likely has an extensive niche distribution that is simultaneously established in the anoxic tank and the GAC biofilm, the two distinct niches. The highly diverse and impressive metabolic versatility of anammox bacteria revealed in this study advance our understanding of the survival and application of anammox bacteria in the full-scale wastewater treatment system.

Achieving advanced nitrogen removal in a novel partial denitrification/anammox-nitrifying (PDA-N) biofilter process treating low C/N ratio municipal wastewater

For achieving mainstream anammox, a novel partial denitrification/anammox-nitrifying (PDA-N) biofilter process to treat municipal wastewater was developed. This process achieved a total inorganic nitrogen (TIN) removal efficiency of 81%, with an average effluent TIN of 7.31 mg·L^-1, when the ratio of influent chemical oxygen demand (COD) to TIN was 3.2. Approximately 97% of the TIN was removed by anammox in the PDA biofilter. Nitrite was provided by partial denitrification for anammox. Partial denitrification was driven by Thaurea in the middle and lower regions of the PDA biofilter, while anammox was mainly driven by Candidatus Brocadia in the middle and upper regions. When treating real municipal wastewater, the TIN was efficiently removed in the PDA-N biofilter, with the effluent TIN of 5.96 mg·L^-1. Anammox played a primary role, achieving approximately 98% of the TIN removal. Compared to the traditional nitrification/denitrification process, this process can economize organic carbon demand and oxygen consumption.

Rapidly achieving partial nitrification of municipal wastewater in enhanced biological phosphorus removal (EBPR) reactor: Effect of heterotrophs proliferation and microbial interactions

Stable nitritation is the major challenge for short-cut nitrogen removal from municipal wastewater. This paper demonstrated a rapid achievement of partial nitrification (PN) in an enhanced biological phosphorus removal (EBPR) reactor treating domestic wastewater. Polyphosphate accumulating organisms (PAOs) were enriched operated at a short aerobic HRT (2.0 h) and SRT (10 d), with satisfactory phosphorus removal efficiency (95.9%). Both of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) were elutriated simultaneously. Interestingly, AOB recovered much faster than NOB by a subsequent extension of aerobic HRT and SRT, resulting in a rapid development of PN within 15 days. Ammonia oxidation rates of AOB significantly increased by 44.2%, facilitating a high nitrite accumulation rate (NAR) of 95.8%. Genus Tetrasphaera, Halomonas, Paracoccus and Candidatus_Accumulibacter belonging to PAOs accounted for 4.6%. The proliferation of heterotrophs, typically as PAOs, maximized the microbial competition against NOB by favoring AOB activity and synergy with functional bacteria.

Enhancing autotrophic nitrogen removal with a novel dissolved oxygen-differentiated airlift internal circulation reactor: Long-term operational performance and microbial characteristics

Autotrophic nitrogen removal (ANR) processes have not been widely applied in wastewater treatment due to their long start-up time and unstable performance. In this study, a novel dissolved oxygen-differentiated airlift internal circulation reactor was developed to enhance ANR from wastewater. During 200 days of continuous operation, the reactor start-up was achieved within 30 days; a high total nitrogen removal efficiency of 80% was achieved and stably maintained under an aeration rate of 0.90 L/min and hydraulic retention time of 6 h. Additionally, the color of sludge went from a light yellow to dark red, and the amount and size of the micro-granules increased obviously. Medium-sized (1.0-2.5 mm) micro-granules accounted for 72.4% on day 190. The specific anammox activity increased from 0.53 to 1.43 g-N/g-VSS/d, while the SNOA decreased from 0.93 to 0.08 g-N/g-VSS/d. Furthermore, the microbial analysis showed that the Nitrosomonas (4.2%) and Candidatus Brocadia (22.6%) were enriched and formed the micro-granules after the reactor's long-term operation. The results indicate that novel configuration realizes the partitioning of dissolved oxygen (DO), optimizes nitritation and anammox reactions, and accelerates biochemical reactions, thereby enhancing ANR performance. This study provides a practical alternative to enhance ANR performance and a scientific basis for the development and application of novel nitrogen removal reactors.

Using anammox biofilms for rapid start-up of partial nitritation-anammox in integrated fixed-film activated sludge for autotrophic nitrogen removal

Integrated fixed-film activated sludge (IFAS) reactors are suitable for partial nitritation-anammox (PNA) for autotrophic nitrogen removal; however, its start-up and biofilm formation are slow and difficult. In this study, a new sludge seeding strategy was developed for the start-up of PNA-IFAS by using the pre-cultivated anammox biofilms. Two bioreactors were used in the experimental study, including a reactor that was started conventionally with the pre-acclimated suspended PNA sludge and bare biocarriers (PA-S) and a reactor that used the new seeding method with anammox biofilms pre-acclimated on biocarriers and ammonia-oxidizing bacteria (AOB) sludge in the suspension (PA-B). The use of anammox biofilms as the seed biomass greatly shortened the start-up period of the PNA-IFAS reactor to 1 month or so. Moreover, reactor PA-B achieved a higher nitrogen removal rate (707.3 mg N/(L·d)), better nitrogen removal efficiency (86.8 ± 2.8%), and lower nitrate yield (9.4%) than reactor PA-S. The biofilm development in PA-B was accelerated and its biofilm content was nearly 10 times higher than that of PA-S. The initial segregation of anammox in the biofilm and AOB in the suspended sludge provided an environment that not only accelerated the start-up of PNA-IFAS but also helped suppress the enrichment of unwanted nitrite-oxidizing bacteria (NOB) in the bioreactor, as evidenced by the lower NOB abundance in PA-B (<0.5%) than in PA-S (>2.2%) according to microbial community analysis.

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.

Centralized iron-dosing into returned sludge brings multifaceted benefits to wastewater management

Iron salts (i.e. FeCl₃) are the most used chemicals in the urban wastewater system. Iron is commonly dosed into sewage or the mainstream system, which provides multiple benefits such as enhanced phosphorus removal and improved sludge settleability/dewaterability. This study reported and demonstrated a new approach that dosed FeCl₃ into returned sludge in order to bring two more benefits to wastewater management: short-cut nitrogen removal via the nitrite pathway and less biomass production. This approach is achieved based on our findings that with similar amount of FeCl₃, centralized iron dosing into a sidestream sludge unit generated iron concentration two orders of magnitude higher than the common mainstream dosing (e.g. 10-40 mg Fe/L-wastewater), leading to sludge acidification (pH = 2.1) with Fe (III) hydrolysis. Together with accumulated nitrite in the supernatant of the sludge, ppm-level of free nitrous acid was generated and thus enabled sludge disintegration, cell lysis, and selective elimination of nitrite-oxidizing bacteria (NOB). Long-term effects on nitrifying bacteria and overall reactor performance were evaluated using two laboratory reactor experiments for over one year. The experimental reactor showed stable nitrite accumulation with an average NO₂^-/(NO₂^- + NO₃^-) ratio above 80% and ∼30% observed biomass yield reduction compared to those in control reactors. In addition, the centralized sludge dosing strategy still provided benefits such as improved settleability and dewaterability of sludge and enhanced phosphorus removal.

Nitrate removal by anammox biomass with intracellular carbon source as electron donors via DNRA pathway

In this work, a novel nitrate (NO₃^-) reduction pathway by anaerobic ammonium oxidation (anammox) biomass was firstly discovered with the intracellular carbon sources as the only electron donors. And the possible reaction mechanism was deduced to be intracellular dissimilatory nitrate reduction to ammonium (DNRA) pathway according to the experimental results. In batch experiments, without any external electron donors, NO₃^--N (about 50 mg/L) was reduced to N₂ within 48 h, and a small amount of NO₂^--N was detected (the maximum of 2 mg/L) with the anammox biomass concentration of 4400 mg/L. Acetylene (4.46 mmol/L) addition resulted in obvious NH₄⁺ accumulation during NO₃^- degradation by anammox biomass, since acetylene mainly interfered in hydrazine (N₂H₄) generation from NH₄⁺ and NO. Without HCO₃^- addition, the NO₃^--N degradation rate was slower than that with HCO₃^- addition. Simultaneously, glycogen contents inside anammox biomass decreased to 133.22 ± 1.21 mg/g VSS and 129.79 ± 1.21 mg/g VSS with and without HCO₃^-, respectively, from 142.20 ± 0.61 mg/g VSS. In the long-term experiment, anammox biomass stably degraded NO₃^--N without external electron donors addition, and the maximum removal efficiency of NO₃^--N reached 55.4%. The above results indicated the anammox bacteria utilized the DNRA pathway to reduce NO₃^- to NO₂^- and further NH₄⁺, then normal anammox metabolism would continue to convert the produced NO₂^- and NH₄⁺ to N₂. The intracellular stored carbon sources (e.g., glycogen) were supposed to be electron donors for NO₃^- degradation. This capability would enhance the viability and living space of anammox bacteria in different natural ecosystems, and make it plausible that complete nitrogen removal could be implemented only by the anammox process.

Operational mode affects the role of organic matter in granular anammox process

In the presence of organic matter, the granular anammox system under sequencing batch mode showed more robust anammox performance than that under completely mixed mode, which was attributed to the better biomass retention with high settling ability and stability of granular sludge. Based on the specific anammox activity test, stratified and mixed distribution of heterotrophic bacteria was found under completely mixed and sequencing batch mode, respectively. The stratified microbial distribution resulted in low enzyme activity of anammox bacteria and sludge disintegration by hindering substrate transfer with a large accumulation of EPS on the granular surface. Whereas the heterotrophic bacteria mixed in granules (mixed microbial distribution) act as a "skeleton", which increased the particle size, density, and stability of granular sludge. Compared with biokinetic-based selection, diffusion-based selection with high substrate penetration depth more likely resulted in the mixed granular structure and strong resistance to organic inhibition under sequencing batch mode.

Enhancing the in-situ enrichment of anammox bacteria in aerobic granules to achieve high-rate CANON at low temperatures

In this study, a high-rate CANON (Complete Autotrophic Nitrogen-removal Over Nitrite) process was started up successfully by enhancing the in-situ enrichment of anammox bacteria in aerobic granules at conditions relevant for mainstream wastewater treatment. Firstly, to provide nitrite for anammox bacteria growth efficient nitrite-oxidizing bacteria (NOB) repression was rapidly achieved and stably maintained. Both low dissolved oxygen (DO) and ammonium concentrations ratio (DO/NH₄⁺ <0.15) and selective washing-out of NOB-preferred smaller particles at short hydraulic retention time (HRT, 25-15 min) contributed to the NOB repression. Then the stepwise down-regulating DO concentrations from 2.8 to 1.2 mg/L enhanced the enrichment of anammox bacteria in the aerobic granules. The enriched anammox species was dominated by Ca. Brocadia sapporoensis with the estimated growth rate of 0.008-0.013 d^-1 at 15 °C. Chloroflexi and Chlorobi-affiliated bacteria were also significantly enriched in the granules, which may benefit the anammox bacteria activity and growth. At the end of this study, the average total nitrogen removal rate and efficiency of the granular CANON process respectively reached 1.26 kg N·m^-3·d^-1 and 68% treating low-strength ammonium (∼50 mg N·L^-1) wastewater under such aggressive conditions (DO = 0.8-1.5 mg/L, HRT< 1.0 h, and T = 15 °C). Overall, the aerobic granules provided a habitable niche for the proliferation and almost complete retention of the anammox bacteria. This study provides a roadmap for in-situ starting up of high-rate CANON process for mainstream wastewater treatment with aerobic granules as inoculum.

Development of a single-stage mainstream anammox process using a sponge-bed trickling filter

Anaerobic ammonia oxidation to nitrogen gas using nitrite as the electron acceptor (anammox process) is considered a cost-effective solution for nitrogen removal after an anaerobic pre-treatment process. In this study, we conducted a laboratory-scale experiment to develop a single-stage partial nitritation-anammox process in a sponge-based trickling filter (STF) reactor, inoculated with anammox sludge, simulating the treatment of anaerobically pretreated concentrated domestic sewage without mechanical oxygen control. The influent ammonia concentration was 100 mg-N·L^-1. The K_La of the STF reactor was higher than those observed for conventional activated sludge processes. The STF reactor performed at 89.8 ± 8.2% and 42.7 ± 16.9% ammonia and TN removal efficiency, respectively, with a nitrogen loading rate of 0.55 ± 0.20 kg-N·m^-3·day^-1 calculated based on sponge volume. Microbial community analysis of the STF-retained sludge indicated that both autotrophic and heterotrophic nitrogen removal occurred in the reactor.

Recent technologies for nutrient removal and recovery from wastewaters: A review

Water scarcity and its pollution has become a concern in recent times. The disposal of nutrient-rich (nitrogen and phosphorous) wastewater is also one of the main cause of water pollution through eutrophication, reduced dissolved oxygen that poses threat to aquatic ecosystems. As a result, nutrient removal has become a mandate apart from the removal of organics. However, the removal of nutrients from sewage is a challenging task. Conversely, conventional biological treatment processes provide little relief in nutrient removal. The treated effluents from conventional biological processes do not achieve the stringent nutrient removal disposal standard limits and become primary cause of pollution in the receiving water bodies. This has stressed upon the need for eco-friendly, low-energy and cost-efficient nutrient removal treatment technologies. Various biological treatment combinations or variants are in use for the efficient removal of nutrients. The biological processes in itself or in combination with chemical processes are preferred over technologies based solely on physico-chemical processes for its treatment performance at lower cost. This review summarizes the existing treatment processes and their possible up-gradation with the aim to accomplish the marked effluent standards for the nutrients. The concept of conventional systems and advanced systems for nutrients (nitrogen and phosphorous) removal which are already developed or under development are deeply discussed. Further, the challenges of each treatment systems are abridged. Finally, the possible suggestions for the modification/retrofitting of existing treatment systems for achieving stringent disposal standards are pointed out.

Activity-Based Cell Sorting Reveals Resistance of Functionally Degenerate Nitrospira during a Press Disturbance in Nitrifying Activated Sludge

Managing and engineering activated sludge wastewater treatment microbiomes for low-energy nitrogen removal requires process control strategies to stop the oxidation of ammonium at nitrite. Our ability to out-select nitrite-oxidizing bacteria (NOB) from activated sludge is challenged by their metabolic and physiological diversity, warranting measurements of their in situ physiology and activity under selective growth pressures. Here, we examined the stability of nitrite oxidation in activated sludge during a press disturbance induced by treating a portion of return activated sludge with a sidestream flow containing free ammonia (FA) at 200 mg NH₃-N/liter. The nitrite accumulation ratio peaked at 42% by day 40 in the experimental bioreactor with the press disturbance, while it did not increase in the control bioreactor. A subsequent decrease in nitrite accumulation within the experimental bioreactor coincided with shifts in dominant Nitrospira 16S rRNA amplicon sequence variants (ASVs). We applied bioorthogonal noncanonical amino acid tagging (BONCAT) coupled with fluorescence-activated cell sorting (FACS) to investigate changes in the translational activity of NOB populations throughout batch exposure to FA. BONCAT-FACS confirmed that the single Nitrospira ASV washed out of the experimental bioreactor had reduced translational activity following exposure to FA, whereas the two Nitrospira ASVs that emerged after process acclimation were not impacted by FA. Thus, the coexistence of functionally degenerate and physiologically resistant Nitrospira populations provided resilience to the nitrite-oxidizing function during the press disturbance. These results highlight how BONCAT-FACS can resolve ecological niche differentiation within activated sludge and inform strategies to engineer and control microbiome function.

IMPORTANCE Nitrogen removal from activated sludge wastewater treatment systems is an energy-intensive process due to the large aeration requirement for nitrification. This energy footprint could be minimized with engineering control strategies that wash out nitrite-oxidizing bacteria (NOB) to limit oxygen demands. However, NOB populations can have a high degree of physiological diversity, and it is currently difficult to decipher the behavior of individual taxa during applied selective pressures. Here, we utilized a new substrate analog probing approach to measure the activity of NOB at the cellular translational level in the face of a press disturbance applied to the activated sludge process. Substrate analog probing corroborated the time series reactor sampling, showing that coexisting and functionally degenerate Nitrospira populations provided resilience to the nitrite oxidation process. Taken together, these results highlight how substrate analog approaches can illuminate in situ ecophysiologies within shared niches, and can inform strategies to improve microbiome engineering and management.

Unravelling adaptation of nitrite-oxidizing bacteria in mainstream PN/A process: Mechanisms and counter-strategies

Stable suppression of nitrite-oxidizing bacteria (NOB) is still a major challenge for the implementation of partial nitritation and anammox (PN/A) in mainstream treatment. Despite numerous suppression strategies demonstrated, it is increasingly recognized that NOB could develop resistance to these strategies, threatening the long-term stability of the mainstream PN/A process. This study aims to understand adaption mechanisms and develop counter-strategies to overcome the adaptation. To this end, three previously-demonstrated suppression strategies, including NOB inactivation via side stream sludge treatment with free ammonia (FA), the use of low dissolved oxygen (DO), and the use of anammox to scavenge nitrite, were stepwise applied, over a period of 800 days, to a laboratory-scale reactor treating effluent from a high-rate activated sludge (HRAS) plant. FA sludge treatment alone sustained nitrite accumulation for about two months, after which NOB adaptation occurred causing PN to fail. The FA adaptation was induced by a shift in the NOB community from Nitrospira to Ca. Nitrotoga. The latter was found to have higher resistance to FA and also a higher maximum specific growth rate. Low DO at 0.2-0.4 mg O₂ L^-1 was then applied, in conjunction with FA treatment, which successfully eliminated Ca. Nitrotoga and re-established PN. However, new and unidentified NOB with a higher apparent oxygen affinity emerged in three months, again leading to PN failure. Lastly, as the third strategy for NOB suppression, anammox was introduced as an in-situ nitrite-scavenger. The combo-strategy delivered reliable NOB suppression with no further adaptation in the remaining experimental period (eight months). The resulted one-stage PN/A reactor achieved a nitrogen removal efficiency of 84.2 ± 5.37%. A control reactor, operated in parallel under the same conditions but without FA treatment, only achieved 10.4 ± 4.6% nitrogen removal, with anammox completely outcompeted by NOB in the last phase.

Responses of anammox process to elevated Fe(III) stress: Reactor performance, microbial community and functional genes

The aim of present study was to re-evaluate the impacts of elevated Fe(III) stress on anaerobic ammonium oxidation (anammox) process. The results indicated that long-term low concentration Fe(III) (5 and 10 mg/L) exposure significantly improved the nitrogen removal efficiency of anammox process, while high concentration Fe(III) (50 and 100 mg/L) significantly deteriorated the reactor performance. Batch assays showed that the specific anammox activity, heme c content and hydrazine dehydrogenase activity were significantly increased and decreased under low and high concentration Fe(III) exposure, respectively, indicating an enhancement and inhibition of anammox activity. Moreover, the presence of high concentration Fe(III) significantly shifted the anammox community structure. Ca. Brocadia was the predominant anammox genus, whose abundance decreased from 14.26% to 8.13% as Fe(III) concentration increased from 0 to 100 mg/L. In comparison, the abundance of denitrifiers progressively increased from 3.70% to 6.68% with increasing Fe(III) concentration. These suggested that different functional bacteria differed in their responses to Fe(III) stress. Furthermore, long-term Fe(III) exposure significantly up-regulated the abundances of genes associated with nitrogen metabolism and Fe(III) reduction. Overall, the obtained findings are expected to advances our understanding of the responses of anammox process to elevated Fe(III) stress.

Achieving superior nitrogen removal performance in low-strength ammonium wastewater treatment by cultivating concentrated, highly dispersive, and easily settleable granule sludge in a one-stage partial nitritation/anammox-HAP reactor

In low-strength ammonium wastewater (LSAWW) treatment, the application of anammox-based process is still limited due to extreme instability and the poor nitrogen removal rate (NRR). In this work, granule sludge, comprised of functional microbes and hydroxyapatite (HAP), was inoculated and cultivated in a one-stage partial nitritation/anammox (PNA) reactor for LSAWW treatment. The results showed that at the hydraulic retention time (HRT) of about 1.0 h and the influent ammonium concentration of 63.0 mg/L, an average NRR of 1.28 kg/m³/d was achieved, which far exceeds that reported in similar studies. The main inorganic matter in sludge was identified as HAP through the X-ray diffractometer and Raman spectrum analysis. The tomographic images of wet granule created through computed tomography revealed that the interior density of the granules was uneven and many hollow structures existed in the granule interior. Combined with the Scanning Electron Microscope images of dry granules, it was found that the granules were comprised of hollow sub-granules. Since the biomass in the reactor increased with no obvious increase in the granule size, it was inferred that the hollow sub-granules had fragile connections with each other and that granules division occurred easily, resulting in the high dispersity of sludge. Florescence in situ hybridization results also showed that the ammonium-oxidizing bacteria and anammox bacteria were mainly distributed in the two sides of the sub-granule shells and the HAP in the middle. This kind of structure raised the density of granules and improved the settleability of sludge, which made it possible to achieve a high biomass in the reactor at a short HRT. Therefore, the sludge formed in the reactor was concentrated, highly dispersive and easily settleable. These factors appear to be crucial for achieving the desired nitrogen removal performance. This study marks a big leap in LSAWW treatment through the one-stage PNA process and has great potential in actual applications.

Adaptation of flocculent anammox culture to low temperature by cold shock: long-term response of the microbial population

Partial nitritation-anammox (PN/A) process will substantially reduce the costs for the removal of nitrogen in the mainstream of municipal sewage. However, one of the mainstream PN/A challenges is to reduce the time necessary for the adaptation of anammox bacteria to lower temperatures in mild climates. In this study, we exposed anammox flocculent culture to cold shocks [35°C → 5°C (8 h) → 15°C] and evaluated long-term cold shock response. Over a post-shock period of 40 d at 15°C, the nitrogen removal rates in the shocked culture were significantly higher compared to control, with maximum rates up to 0.082 and 0.033 kg-N/kg-VSS/d or 0.164 and 0.076 kg-N/m3/d, for shocked culture and control, respectively. In the corresponding semi-batch cycles, the shocked culture was on average 136 ± 101% more active than the control, due to the negative effect of cold shock on side populations and more active anammox cells. Per FISH, Ca. Brocadia anammoxidans and Ca. Scalindua survived the shock and remained present throughout. Thus, both anammox microorganisms seem to respond favourably to cold shocks. In sum, we provide further evidence that cold shocks accelerate the adaptation of anammox to the mainstream of municipal WWTPs. Further, for the first time, we report the long-term adaptive response of anammox to cold shocks.

Achieving Efficient and Stable Deammonification at Low Temperatures—Experimental and Modeling Studies

The short-term effects of temperature on deammonification sludge were evaluated in a laboratory-scale sequencing batch reactor (SBR). Mathematical modeling was used for further evaluations of different intermittent aeration strategies for achieving high and stable deammonification performance at decreasing temperatures. As for the biomass cultivated at high temperatures (e.g., 30 °C), a higher temperature dependency (the adjusted Arrhenius coefficient θ for 11–17 °C = 1.71 vs. θ for 17–30 °C = 1.12) on the specific anammox growth rates was found at lower temperatures (11–17 °C) in comparison with higher temperatures (17–30 °C). Further evaluations of recovering the nitrogen removal efficiency at decreasing temperatures with the mathematical model by modifying the intermittent aeration strategies (aeration frequency (F) and the ratio (R) between non-aerated (non-aer) phase and aerated (aer) phase durations) indicated that intermittent aeration with a prolonged non-aerated phase (e.g., R ≥ 4 regardless of F value) would help to maintain high and stable deammonification performance (~80%) at decreasing temperatures (14–22 °C). Extending the non-aerated phases (increasing R) and reducing the frequency (F) of off/on phase changes have a positive effect on increasing energy savings, leading to increasing interest in this method.

Insights into two stable mainstream deammonification process and different microbial community dynamics at ambient temperature

How to achieve stable mainstream deammonification is still a huge challenge. In this work, satisfactory nitrogen removal were achieved in a deammonification granular sludge reactor (R1, 0.42 ± 0.03 kg N / (m³·d)) and a mixed flocculent with granular sludge reactor (R2, 0.39 ± 0.04 kg N / (m³·d)) at ambient temperature (21-28 ℃) . The good adaptability of anammox bacteria (Candidatus Jettenia) to ambient temperature ensured its efficient activity (0.84-1.54 mg N/(g VSS·h)). The overexpression ammonia monooxygenase gene abundances in ammonia oxidizing bacteria (Nitrosomonas) was also predicted. The inhibition of hydrazine and the competition of denitrifying bacteria (Denitratisoma) to nitrite nitrogen, leading to a low Nitrospira relative abundances (0.2%-2.1%) . It was also found that R1 was more resistant to the unfavorable condition. For R2, higher Denitratisoma abundances (9.2%-18.5%) and predicted metabolic pathway abundances related to carbon metabolism were observed.

Generalized temperature dependence model for anammox process kinetics

Temperature is a key operational factor influencing the anammox process kinetics. In particular, at temperatures below 15 °C, the specific anammox activity (SAA) considerably decreases. This study aimed to describe the temperature dependence of the anammox process kinetics in the temperature range from 10 to 55 °C, including the specific characteristics of "cold anammox". The commonly used Arrhenius and extended and modified Ratkowsky equations were examined. The Ratkowsky equations yielded a strong correlation (coefficient of determination, R² = 0.93-0.96) between the measured and predicted data over the analyzed temperature range (10-55 °C). However, these equations could not correctly reflect the anammox temperature dependence at temperatures below 15 °C (R² = 0.36-0.48). Therefore, a new generalized temperature model was proposed. The generalized temperature equation (GTE) considered the division of the analyzed temperature range into three temperature ranges: 10-15 °C, 15-35 °C and 35-55 °C. The ranges correspond to "cold anammox", "(low) mesophilic anammox" and "thermophilic anammox". The applied approach yielded a strong correlation between the measured and predicted SAA (R² = 0.97) over the temperature range from 10 to 55 °C and over the low-temperature range from 10 to 15 °C (R² = 0.99). Overall, the GTE could enhance the predictions of the temperature dependence of the anammox process kinetics. The GTE can help examine anammox-based bioaugmentation systems operating at both high temperatures (sidestream reactors) and low temperatures (mainstream reactors).

Research progress on enhancing the performance of autotrophic nitrogen removal systems using microbial immobilization technology

The autotrophic nitrogen removal process has great potential to be applied to the biological removal of nitrogen from wastewater, but its application is hindered by its unstable operation under adverse environmental conditions, such as those presented by low temperatures, high organic matter concentrations, or the presence of toxic substances. Granules and microbial entrapment technology can effectively retain and enrich microbial assemblages in reactors to improve operating efficiency and reactor stability. The carriers can also protect the reactor's internal microorganisms from interference from the external environment. This article critically reviews the existing literature on autotrophic nitrogen removal systems using immobilization technology. We focus our discussion on the natural aggregation process (granulation) and entrapment technology. The selection of carrier materials and entrapment methods are identified and described in detail and the mechanisms through which entrapment technology protects microorganisms are analyzed. This review will provide a better understanding of the mechanisms through which immobilization operates and the prospects for immobilization technology to be applied in autotrophic nitrogen removal systems.

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.

Unveiling performance stability and its recovery mechanisms of one-stage partial nitritation-anammox process with airlift enhanced micro-granules

The performance stability and its recovery mechanisms of a partial nitritation-anammox process were investigated. A one-stage airlift enhanced micro-granules (AEM) system was operated for 650 days continuously to treat 50 mg-NH₄/L wastewater. During the stable stage, a high nitrogen removal efficiency of 72.7 ± 8.4% lasting for 230 days was successfully achieved under 0.28 L/min aeration rate and 0.10-0.20 mg/L dissolved oxygen (DO) concentration. A microbial consortium with good granularity appeared in red. The specific activity of anammox and ammonia oxidation increased to 1.02 and 0.93 g-N/g-VSS/d, respectively. Meanwhile, the microbial analysis showed the AEM system shifted the dominant microflora from Proteobacteria to Planctomycetes in which Candidatus Brocadia abundance reached a high of 35.0%. The results reveal that the long-term airlift-aeration promoted granulation and further enhanced activities, the abundances of anammox bacteria, and suppressed nitrite-oxidizing bacteria. Optimizing the DO control is also critical for stability increment and process recovery.

A continuous plug-flow anaerobic/aerobic/anoxic/aerobic (AOAO) process treating low COD/TIN domestic sewage: Realization of partial nitrification and extremely advanced nitrogen removal

The realization of stable partial nitrification and advanced nitrogen removal are not acquired effectively in conventional pre-denitrification biological nitrogen removal processes treating domestic sewage. Herein, a novel anaerobic/aerobic/anoxic/aerobic (AOAO) continuous plug-flow reactor, characterized with double sludge reflux and a bypass of anaerobic mixed liquor conveyed to anoxic zone, was first constructed to realize stable partial nitrification in treating domestic sewage. The alternating anoxic/aerobic conditions and longer anoxic sludge retention time might be responsible for the partial nitrification. Nitrite accumulation ratio reached 89.3 ± 3.3% with the maximum activity ratio of AOB to NOB increasing from 0.72 to 8.17. A content total inorganic nitrogen (TIN) removal efficiency (93.7 ± 2.2%) and effluent TIN concentration (2.9 ± 0.9 mg N/L) were obtained after 238 days' operation. Specifically, nitrogen balance of the typical cycle showed that about 30.1% of TIN was removed through simultaneous partial nitrification and denitrification (SND) in aerobic zone and 48.2% by endogenous denitrification in anoxic zone. The AOAO process is an economic treatment for domestic sewage with aerobic hydraulic retention time (HRT) of 4 h.

Insights into Nitrous Oxide Mitigation Strategies in Wastewater Treatment and Challenges for Wider Implementation

Nitrous oxide (N₂O) emissions account for the majority of the carbon footprint of wastewater treatment plants (WWTPs). Many N₂O mitigation strategies have since been developed while a holistic view is still missing. This article reviews the state-of-the-art of N₂O mitigation studies in wastewater treatment. Through analyzing existing studies, this article presents the essential knowledge to guide N₂O mitigations, and the logics behind mitigation strategies. In practice, mitigations are mainly carried out by aeration control, feed scheme optimization, and process optimization. Despite increasingly more studies, real implementation remains rare, which is a combined result of unclear climate change policies/incentives, as well as technical challenges. Five critical technical challenges, as well as opportunities, of N₂O mitigations were identified. It is proposed that (i) quantification methods for overall N₂O emissions and pathway contributions need improvement; (ii) a reliable while straightforward mathematical model is required to quantify benefits and compare mitigation strategies; (iii) tailored risk assessment needs to be conducted for WWTPs, in which more long-term full-scale trials of N₂O mitigation are urgently needed to enable robust assessments of the resulting operational costs and impact on nutrient removal performance; (iv) current mitigation strategies focus on centralized WWTPs, more investigations are warranted for decentralised systems, especially decentralized activated sludge WWTPs; and (v) N₂O may be mitigated by adopting novel strategies promoting N₂O reduction denitrification or microorganisms that emit less N₂O. Overall, we conclude N₂O mitigation research is reaching a maturity while challenges still exist for a wider implementation, especially in relation to the reliability of N₂O mitigation strategies and potential risks to nutrient removal performances of WWTPs.

Stoichiometric and kinetic characterization of an acid-tolerant ammonia oxidizer 'Candidatus Nitrosoglobus'

Recently, acidic (i.e. pH<5) nitrification in activated-sludge is attracting attention because it enables stable nitritation (NH₄⁺ → NO₂^-), and enhances sludge reduction and stabilization. However, the key acid-tolerant ammonia oxidizers involved are poorly understood. In this study, we performed stoichiometric and kinetic characterization of a new acid-tolerant ammonia-oxidizing bacterium (AOB) belonging to gamma-proteobacterium, Candidatus Nitrosoglobus. Ca. Nitrosoglobus was cultivated in activated-sludge in a laboratory membrane bioreactor over 200 days, with a relative abundance of 55.1 ± 0.5% (indicated by 16S rRNA gene amplicon sequencing) at the time of the characterization experiments. Among all known nitrifiers, Ca. Nitrosoglobus bears the highest resistance to nitrite, low pH, and free nitrous acid (FNA). These traits define Ca. Nitrosoglobus as an adversity-strategist that tends to prosper in acidic activated-sludge, where the low pH (< 5.0) and high levels of FNA (at parts per million levels) sustained and inhibited all other nitrifiers. In contrast, in the conventional pH-neutral activated-sludge process, Ca. Nitrosoglobus is less competitive with canonical AOB (e.g. Nitrosomonas) due to the relatively slow specific growth rate and low affinities to both oxygen and total ammonia. These results advance our understanding of acid-tolerant ammonia oxidizers, and support further development of the acidic activated-sludge process in which Ca. Nitrosoglobus can play a critical role.

Highly enriched anammox within anoxic biofilms by reducing suspended sludge biomass in a real-sewage A2/O process

This study proposes a novel strategy of stably enriching anammox in mainstream, based on the competitive difference to NO₂^- between anoxic biofilms and suspended sludge. A modified anaerobic-anoxic-oxic (A²/O) process run for 500 days with actual municipal wastewater. Microbial analysis revealed that anoxic-carrier biofilms had a significantly higher abundance of anammox (qPCR: 0.74% - 4.34%) than suspended sludge (P< 0.001). Batch tests showed that anammox within anoxic-carrier biofilms contributed to significant nitrogen removal, coupled with partial-denitrification (NO₃^- → NO₂^-). The anammox genus, Ca. Brocadia, was highly enriched when suspended sludge was accidentally lost. Further batch tests found that reducing suspended biomass helped anammox enrichment in anoxic-carrier biofilms, because the suspended sludge had strong NO₂^- competition (NO₂^- → N₂) with anammox (increased nirK). Metagenomic sequencing revealed that Ca. Brocadia dominates in the anoxic-carrier biofilms, and is the most important narG contributor to NO₃^- → NO₂^-, which could have promoted the competition of NO₂^- with heterotrophic bacteria. For this A²/O process, the low effluent total nitrogen (8.9 mg ± 1.0 mg N/L) was attributed to partial-denitrification coupling with anammox, demonstrating that this process is applicable to the general influent N-concentration range (30 mg - 50 mg NH₄⁺-N/L) of municipal wastewater treatment plants (WWTPs). Based on the special competitive preference of anammox for NO₂^-, this study provides a promising and practical alternative for enriching anammox bacteria in municipal WWTPs.

Microbial diversity reveals the partial denitrification-anammox process serves as a new pathway in the first mainstream anammox plant

A full-scale sewage treatment plant in Xi'an city is discovered as the first mainstream anaerobic ammonia oxidation (anammox) treatment process in China. Whether its biological mechanism is the nitritation-anammox or partial denitrification (PD)-anammox brought violent controversy between two groups. As a third party, here we uncovered the mystery of the moving-bed biofilm reactor (MBBR) as a PD-anammox process by analyzing the diversity and phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) of microbes in anoxic pond. Anammox bacteria was found in the MBBR anoxic tank, which abundance is 8.9 times of that in the common anaerobic-anoxic-oxic process, confirming the existence of anammox process. The denitrifying bacteria (DNB) content in the anoxic tank is 5.9 times of the content of ammonia oxidizing bacteria (AOB), thus the DNB-anammox system is proved at the microbial composition level. The PICRUSt analysis found that ammonium nitrogen is mainly derived from the deamination of urea. The functional genes NAR and AMO of DNB and AOB are 910.84 and 5.80 rpms, respectively. The NAR gene content is 157.0 times of the AMO gene content and it is proved at the genetic level that the nitrite in the anoxic pool is mainly derived from denitrification. This study demonstrated the feasibility and advantages of the PD-anammox in the anammox process, which is different from the traditional nitritation-anammox demonstrated in Strass Wastewater Treatment Plant, Austria and Changi Water Reclamation Plant, Singapore and provided an alternative option for the mainstream application of anammox.

Deciphering nitrogen removal mechanism through marine anammox bacteria treating nitrogen-laden saline wastewater under various phosphate doses: Microbial community shift and phosphate crystal

The effect of phosphate on marine anammox bacteria (MAB)-dominated anammox process in nitrogen-laden saline wastewater was first investigated. The activity of MAB was enhanced by dosing low concentrations of phosphate (5-30 mg/L PO₄^3--P), and the time of complete ammonium removal was shortened by 0.5 h. When PO₄^3--P exceeded 160 mg/L, the calcium magnesium phosphate precipitation was formed in the reactor. The contact between substrates and biomass was hindered by the sediments, and the nitrogen removal performance of MAB was also worsened. At 400 mg/L PO₄^3--P, the ammonium removal rate and nitrite removal rate decreased to 0.45 and 0.43 kg/(m³⋅d), respectively. During the 158-day operation, MAB was still the dominant strain, but its relative abundance decreased by 15.4% at 400 mg/L PO₄^3--P. Besides, the presence of sediments stimulated the production of extracellular polymeric substances and the maximum yield reached 11.25 mg/g⋅wet weight at 200 mg/L PO₄^3--P.

A novel SAD process: Match of anammox and denitrification

Anammox biotechnology has been widely applied for its attractive advantages, but its application has been seriously limited due to the instinctive drawback of nitrate production. In this work, a novel Sequential Anammox and Denitrification (SAD) system was developed for the advanced nitrogen removal by using solid carbon source (SCS) and coupling anammox with denitrification. The long-term operation results demonstrated that the SAD system could remove the total nitrogen (TN) efficiently, with the effluent TN concentration of 1.4 ± 0.5 mg N/L, the TN removal efficiency (NRE) of 99.3 ± 0.2%, and the TN removal rate (NRR) of 1.7 ± 0.1 kg/(m³·d). The determination results showed that SCS had a good property for sustained release of COD, with a dissolved organic yield (by COD) of 1.1 g-COD/g-rice. When the addition rate was set at 6 g-rice/7-days, the COD release rate of 0.9 kg-COD/(m³·d) from SCS matched the nitrate production rate of 1.2 × 10^-1 kg-N/(m³·d) from anammox with consumption ratio of 7.5. The analysis on the microbial community revealed that Candidatus_Brocadia and Denitratisoma were the dominant functional bacteria for anammox and denitrification, which contributed to about 92.7% and 6.6% of the total nitrogen removal, respectively. This work is helpful for the innovation and application of anammox-based technology.

Development of Strategies for AOB and NOB Competition Supported by Mathematical Modeling in Terms of Successful Deammonification Implementation for Energy-Efficient WWTPs

Novel technologies such as partial nitritation (PN) and partial denitritation (PDN) could be combined with the anammox-based process in order to alleviate energy input. The former combination, also noted as deammonification, has been intensively studied in a frame of lab and full-scale wastewater treatment in order to optimize operational costs and process efficiency. For the deammonification process, key functional microbes include ammonia-oxidizing bacteria (AOB) and anaerobic ammonia oxidation bacteria (AnAOB), which coexisting and interact with heterotrophs and nitrite oxidizing bacteria (NOB). The aim of the presented review was to summarize current knowledge about deammonification process principles, related to microbial interactions responsible for the process maintenance under varying operational conditions. Particular attention was paid to the factors influencing the targeted selection of AOB/AnAOB over the NOB and application of the mathematical modeling as a powerful tool enabling accelerated process optimization and characterization. Another reviewed aspect was the potential energetic and resources savings connected with deammonification application in relation to the technologies based on the conventional nitrification/denitrification processes.

Effects of granular activated carbon and Fe-modified granular activated carbon on anammox process start-up

In this study, granular activated carbon (GAC) and Fe-modified granular activated carbon (FeGAC) prepared by ultrasonic impregnation method were added into respective up-flow anaerobic sludge blanket (UASB) reactors to explore their effects on the anammox process start-up. The results showed that the time of anammox system start-up could be reduced from 108 d in R1 (control group) to 94 d in R2 (GAC reactor) and to 83 d in R3 (FeGAC reactor). After 120 days of operation, the nitrogen removal rates (NRR) of all reactors could reach more than 0.8 kg-N m⁻³ d⁻¹. Extracellular polymeric substance (EPS) amount, heme c content and the anammox bacterial functional gene copy numbers gradually increased in all reactors with the passage of culture time, and manifested the superiority in R3 especially. High throughput sequencing revealed that Candidatus Kuenenia was the dominant species in all reactors in the end. It was also demonstrated that FeGAC markedly strengthened the growth and aggregation of anammox bacteria, which is promising for the practical application of the anammox process.

In this study, granular activated carbon (GAC) and Fe-modified granular activated carbon (FeGAC) prepared by ultrasonic impregnation method were added into respective up-flow anaerobic sludge blanket (UASB) reactors to explore their effects on the anammox process start-up.

Possible mechanism of efficient mainstream partial nitritation/anammox (PN/A) in hybrid bioreactors (IFAS)

An explanation of possible mechanism of efficient PN/A in hybrid bioreactors was presented. The bottleneck process is nitritation. Surplus nitrite production by ammonium oxidizing bacteria (AOB) is required for assuring the activity of anammox bacteria and eliminating nitrite oxidizing bacteria (NOB). It will be possible if nitrogen removal rate by AOB (r_{N_AOB}) is higher than NOB (r_{N_NOB}). It was shown that in biofilm AnAOB bacteria should out-compete NOB, whereas nitrogen transformation rates by AOB are usually lower than NOB. However, the growth of r-AOB in activated sludge allows out-selecting NOB. Impact of ammonium-, nitrite-nitrogen and suspended biomass concentration in hybrid PN/A systems on nitrogen removal rates in the temperature ranges from 10°C to 25°C was presented and discussed. Because bulk liquid ammonium nitrogen concentration can be higher in SBR bioreactors (after certain period of time after aeration starts) or in the initial zones of plug-flow systems than in fully mixed systems, conditions for running efficient PN/A are more favourable in intermittently aerated 'IFAS-SBR' or 'IFAS-plug flow' bioreactors.

Free ammonia resistance of nitrite-oxidizing bacteria developed in aerobic granular sludge cultivated in continuous upflow airlift reactors performing partial nitritation

Free ammonia (FA) inhibition has been taken advantage as a strategy to suppress the growth of nitrite-oxidizing bacteria (NOB) in aerobic granules stabilized in a continuous upflow airlift reactor to achieve partial nitritation. However, after nearly 18 months of continuous exposure of aerobic granules to FA in the reactor, the FA inhibition of NOB was proven ineffective, and the partial nitritation gradually shifted to partial nitrification even though the long-term granule structural stability remained excellent under the continuous-flow mode. The extent of NOB resistance to FA inhibition was quantified based on the kinetic response of NOB to various FA concentrations in the form of an uncompetitive inhibition coefficient. It was confirmed that the NOB immobilized in larger granules under longer term exposure to FA tend to become more resistant to FA. Thereby, it was concluded that NOB can develop strong resistance to FA after continuous exposure, and thus, FA inhibition is not a reliable strategy to achieve partial nitritation in mainstream wastewater treatment. PRACTITIONER POINTS: Nitrifying aerobic granules can remain structurally stable in continuous-flow bioreactors. NOB developed free ammonia resistance after 6-month continuous exposure. Larger aerobic granules tended to develop stronger free ammonia resistance. Free ammonia inhibition is not a reliable strategy for mainstream anammox.

Key factors governing the performance and microbial community of one-stage partial nitritation and anammox system with bio-carriers and airlift circulation

A one-stage airlift internal circulation biofilm reactor was continuously operated for 668 days to treat 50 mg/L of ammonia wastewater to pursue the long-term stability of partial nitritation and anammox (PNA) process. The operational performance and microbial community structure of the biofilm and the flocs were investigated. A nitrogen removal efficiency (NRE) of 70% was obtained successfully at a dissolved oxygen (DO) of 0.05-0.15 mg/L by regulating aeration rate. The microbial analysis indicated Candidatus Brocadia (29.5%) and Nitrosomonas (6.8%) were dominant in both biofilms and flocs. It was found that DO control and aeration rate were the key factors in performance stability, and a stable performance could be recovered and maintained under oxygen-limiting conditions. Further, the achievement of activated ammonia oxidation bacteria (AOB), dominated anammox bacteria (AMX), suppressed NOB, and controlled heterotrophic bacteria (HB) in the biofilms played a major role in the long-term stable operation.

Deciphering correlation between permeability and size of anammox granule: "pores as medium"

Anammox granular sludge bed technology has been widely applied for its attractive advantages. Efficient mass transfer is an important factor for the anammox granules to play their role. In this study, steady-state anammox granules were used to investigate the correlation between the permeability and granule size with the granule pore as pivot. The results of size distribution showed that the anammox granules could be divided into 6 groups: 200-500 µm (I), 500-1000 µm (II), 1000-1500 µm (III), 1500-2000 µm (IV), 2000-3000 µm (V) and ≥3000 µm (VI). The results of settling experiment demonstrated that the permeability of anammox granules was negatively correlated with the granule size. The fluid collection efficiency declined from 39.4% to 9.3% for granule group I to III, and further to 0 for granule group IV to VI (granule size was larger than 1.5 mm). The observation of micro-CT revealed that the pore structure of anammox granules varied significantly with the increase of granule size, forming a denser surface layer and sparser interior. The chemical analysis and microscopic observation indicated that the pore plugging of surface layer by cell proliferation and EPS secretion was the main cause for the permeability deterioration. The findings of this study will help to understand the mass transfer of anammox granules and promote the development of anammox processes.

Hydroxylamine and the nitrogen cycle: A review

Aerobic ammonium oxidizing bacteria were first isolated more than 100 years ago and hydroxylamine is known to be an intermediate. The enzymatic steps involving hydroxylamine conversion to nitrite are still under discussion. For a long time it was assumed that hydroxylamine was directly converted to nitrite by a hydroxylamine oxidoreductase. Recent enzymatic evidences suggest that the actual product of hydroxylamine conversion is NO and a third, yet unknown, enzyme further converts NO to nitrite. More recently, ammonium oxidizing archaea and complete ammonium oxidizing bacteria were isolated and identified. Still the central nitrogen metabolism of these microorganisms presents to researchers the same puzzle: how hydroxylamine is transformed to nitrite. Nitrogen losses in the form of NO and N₂O have been identified in all three types of aerobic ammonium oxidizing microorganisms and hydroxylamine is known to play a significant role in the formation. Yet, the pathways and the factors promoting the greenhouse gas emissions are to be fully characterized. Hydroxylamine also plays a yet poorly understood role on anaerobic ammonium oxidizing bacteria and is known to inhibit nitrite oxidizing bacteria. In this review, the role of this elusive intermediate in the metabolism of different key players of the nitrogen cycle is discussed, as well as the putative importance of hydroxylamine as a key nitrogen metabolite for microbial interactions within microbial communities and engineered systems. Overall, for the first time putting together the acquired knowledge about hydroxylamine and the nitrogen cycle over the years in a review, setting potential hypothesis and highlighting possible next steps for research.

Light as a Novel Inhibitor of Nitrite-Oxidizing Bacteria (NOB) for the Mainstream Partial Nitrification of Wastewater Treatment

Conventional biological nutrient removal processes in municipal wastewater treatment plants are energy-consuming, with oxygen supply accounting for 45–75% of the energy expenditure. Many recent studies examined the implications of the anammox process in sidestream wastewater treatment to reduce energy consumption, however, the process did not successfully remove nitrogen in mainstream wastewater treatment with relatively low ammonia concentrations. In this study, blue light was applied as an inhibitor of nitrite-oxidizing bacteria (NOB) in a photo sequencing batch reactor (PSBR) containing raw wastewater. This simulated a biological nitrogen removal system for the investigation of its application potential in nitrite accumulation and nitrogen removal. It was found that blue light illumination effectively inhibited NOB rather than ammonia-oxidizing bacteria due to their different sensitivity to light, resulting in partial nitrification. It was also observed that the NOB inhibition rates were affected by other operational parameters like mixed liquor suspended solids (MLSS) concentration and sludge retention time (SRT). According to the obtained results, it was concluded that the process efficiency of partial nitrification and anammox (PN/A) could be significantly enhanced by blue light illumination with appropriate MLSS concentration and SRT conditions.

Comammox Nitrospira Species Dominate in an Efficient Partial Nitrification-Anammox Bioreactor for Treating Ammonium at Low Loadings

Bacteria capable of complete ammonia oxidation (comammox) are widespread and contribute to nitrification in wastewater treatment facilities. However, their roles in partial nitrification-anaerobic ammonium oxidation (anammox) systems remain unclear. In this study, a bench-scale bioreactor with continuous stirring was operated for more than 1000 days with limited oxygen supply to achieve efficient nitrogen removal (70.1 ± 2.7%) at a low ammonium loading of 35.2 mg-N/L/day. High-throughput amplicon sequencing analysis of the comammox ammonia monooxygenase subunit A (amoA) gene revealed seven sequence types from two clusters in clade A of comammox Nitrospira. Quantitative polymerase chain reaction analyses suggested that the comammox species dominated the ammonia-oxidizing community, with an abundance as high as 89.2 ± 7.9% in total prokaryotic amoA copies. Multiple linear regression further revealed the substantial contribution of the comammox Nitrospira to ammonia oxidation in the bioreactor. The investigation with bioreactor and batch experiments consistently showed that activities of comammox Nitrospira were inhibited by free ammonia far more severely than other ammonia-oxidizing microbes. Overall, this study provided new insight into the ecology of comammox Nitrospira under hypoxic conditions and suggested comammox-associated partial nitrification-anammox as a potential method for treating low-strength ammonium-containing wastewater.

Robust Nitritation Sustained by Acid-Tolerant Ammonia-Oxidizing Bacteria

Oxidation of ammonium to nitrite rather than nitrate, i.e., nitritation, is critical for autotrophic nitrogen removal. This study demonstrates a robust nitritation process in treating low-strength wastewater, obtained from a mixture of real mainstream sewage with sidestream anaerobic digestion liquor. This is achieved through cultivating acid-tolerant ammonia-oxidizing bacteria (AOB) in a laboratory nitrifying bioreactor at pH 4.5-5.0. It was shown that nitrite accumulation with a high NO₂^-/(NO₂^- + NO₃^-) ratio of 95 ± 5% was stably maintained for more than 300 days, and the obtained volumetric NH₄⁺ removal rate (i.e., 188 ± 14 mg N L^-1 d^-1) was practically useful. 16S rRNA gene sequencing analyses indicated the dominance of new AOB, "Candidatus Nitrosoglobus," in the nitrifying guild (i.e., 1.90 ± 0.08% in the total community), with the disappearance of typical activated sludge nitrifying microorganisms, including Nitrosomonas, Nitrospira, and Nitrobacter. This is the first identification of Ca. Nitrosoglobus as key ammonia oxidizers in a wastewater treatment system. It was found that Ca. Nitrosoglobus can tolerate low pH (<5.0), and free nitrous acid (FNA) at levels that inhibit AOB and nitrite-oxidizing bacteria (NOB) commonly found in wastewater treatment processes. The in situ inhibition of NOB leads to accumulation of nitrite (NO₂^-), which along with protons (H⁺) also produced in ammonium oxidation generates and sustains FNA at 3.0 ± 1.4 mg HNO₂-N L^-1. As such, robust PN was achieved under acidic conditions, with a complete absence of NOB. Compared to previous nitritation systems, this acidic nitritation process is featured by a higher nitric oxide (NO) but a lower nitrous oxide (N₂O) emission level, with the emission factors estimated at 1.57 ± 0.08 and 0.57 ± 0.03%, respectively, of influent ammonium nitrogen load.

Sludge ratio affects the start-up performance and functional bacteria distribution of a hybrid CANON system

To investigate the effect of sludge ratio on the hybrid CANON system, autotrophic nitrogen removal sludge was inoculated with different granule/floc ratios to initiate the CANON system, and maintained the sludge ratio during the operation process. The start-up performances were compared, and the distribution characteristics of functional bacteria were investigated. The results show that the Equivalent system (granules:flocs = 1:1-1:1.5) successfully started-up on day 19, and the nitrogen removal rate (NRR) reached 0.299 kgN m^-3·d^-1 on day 63. At the same time, it was less affected by the load shock than High-granules and High-flocs systems. Therefore, the Equivalent system had the strongest start-up performance. The activities of the functional bacteria conformed to spatial heterogeneity, unlike the abundance. With the increased floc proportion, the difference in the activity and abundance of anaerobic ammonium-oxidizing bacteria (AAOB) between the granules and flocs increased, while there was a decrease in the difference in aerobic ammonium-oxidizing bacteria (AOB). However, the abundance of Nitrosomonas in the granules was higher than in the flocs when the proportion of flocs was higher than 50%. This study provides new ideas and insights for the fast start-up of the CANON system and can conform to the varying needs of engineering applications.

Analyzing the effect of organic carbon on partial nitrification-anammox process based on metagenomics and quorum sensing

The effects of adding organic carbon on the performance of different partial nitrification-anammox (PNA) process (the activated sludge process and biofilm process) were studied, especially nitrogen removal, functional microbial activity, and microbial community structure. The potential influences of quorum sensing (QS) on the nitrogen metabolism were also analyzed. The results showed that the addition of organic carbon in biofilm systems could reduce total nitrogen (TN) removal percentages, while in activated systems it could increase TN removal percentages. The TN removal percentages in SBBR-CN (the biofilm system with addition of organic carbon) and SBR-CN (the activated sludge system with addition of organic carbon) were 15% and 45%, respectively, and those in SBBR-N (the biofilm system without addition of organic carbon) and SBR-N (the activated sludge system without addition of organic carbon) were 75% and 21%, respectively. Batch experiments have proved that organic carbon inhibited the activities of nitrite-oxidizing bacteria (NOB) and anaerobic ammonia oxidation (anammox) bacteria, and organic carbon could promote the activity of denitrifying bacteria in activated sludge systems. Compared with activated sludge systems, biofilm systems could protect the activity of anammox bacteria. The relative abundances of ammonia oxidizing bacteria (AOB) and anammox bacteria were decreased, while the relative abundances of denitrifying bacteria (Thauera) were increased with the addition of organic carbon. The biofilm systems were more conducive to the growth of anammox bacteria. Metagenomics revealed that the same bacteria might be involved in different nitrogen metabolism, and nitrogen metabolism was achieved through the complex cooperation among functional bacteria. Besides, functional bacteria involving in the nitrogen metabolism had genes related to QS, indicating that QS might affect the nitrogen metabolism by regulating the functional bacteria activity. PRACTITIONER POINTS: PNA was achieved through SBBR and complete nitrification was achieved through SBR under the low ammonia nitrogen concentration condition. The effect of organic carbon on biofilm and activated sludge PNA process was different under the low ammonia nitrogen concentration condition. QS and QQ may affect the nitrogen removal performance by regulating the expression of nitrogen metabolism-related genes.