S0-driven partial denitrification coupled with anammox (S0PDA) enables highly efficient autotrophic nitrogen removal from wastewater

This study proposed a novel strategy that integrates S0 particles (diameter: 2-3 mm) and granular sludge to establish S0-driven partial denitrification coupled with anammox (S0PDA) process for autotrophic nitrogen removal from NH4+- and NO3--containing wastewaters. This process was evaluated using an up-flow anoxic sludge bed bioreactor, operating continuously for 240 days. The influent concentrations of NH4+ and NO3- were 29.9 ± 2.7 and 50.2 ± 2.7 mg-N/L, respectively. Throughout the operation, the hydraulic retention time was shortened from 4.0 h to 2.0 h, while the effluent concentrations of NH4+ and NO3- were maintained at a desirable level of 1.45-1.51 mg-N/L and 4.46-6.52 mg-N/L, respectively. Despite an autotrophic process, the nitrogen removal efficiency and rate reached up to 88.5 ± 2.0 % and 1.75 ± 0.07 kg-N/(m3·d), respectively, indicating the remarkable robustness of the S0PDA process. Autotrophic anammox and sulfur-oxidizing bacteria (Candidatus Brocadia and Thiobacillus) were the predominant bacterial genera involved in the S0PDA process. Candidatus Brocadia was primarily enriched in the granular sludge, with a relative abundance of 6.70 %. Thiobacillus occupied a unique niche on the S0 particles, with a relative abundance as high as 57.6 %, of which Thiobacillus thioparus with partial denitrification function (reducing NO3- to NO2- without further reduction to N2) accounted for 78.0 %. These findings challenge the stereotype of low efficiency in autotrophic nitrogen removal from wastewater, shedding fresh light on the applications of autotrophic processes.

Efficient nitrogen removal in a total floc sludge system from domestic wastewater with low C/N: High anammox nitrogen removal contribution driven by endogenous partial denitrification

Since unsustainable partial nitrification prone to unstable nitrogen removal rates, cultivation and enrichment of AnAOB, further improve autotrophic nitrogen removal contribution have been challenges in the mainstream anammox process. This study proposed a new strategy to enrich AnAOB motivated by endogenous partial denitrification (EPD) in total floc sludge system through the AOA process with sustainable nitrification. The results showed that in the presence of NH₄⁺ and NO₃^- at the anoxic stage of N-EPDA, Ca. Brocadia was enriched (0.005%→0.92%) in floc sludge via internal carbon source metabolism of EPD. The C/N and temperature of N-EPDA were also optimized to achieve higher activities of EPD and anammox. The N-EPDA was operated at low C/N ratio (3.1) with anammox nitrogen removal contribution of 78% during the anoxic stage, Eff.TIN of 8.3 mg/L and NRE of 83.5% during phase III, achieved efficient autotrophic nitrogen removal and AnAOB enrichment in the absence of partial nitrification.

Intensifying single-stage denitrogen by a dissolved oxygen-differentiated airlift internal circulation reactor under organic matter stress: Nitrogen removal pathways and microbial interactions

Current research focuses on efficient single-stage nitrogen removal from organic matter wastewater using the partial nitritation-anammox (PNA) process. In this study, we constructed a single-stage partial nitritation-anammox and denitrification (SPNAD) system using a dissolved oxygen-differentiated airlift internal circulation reactor. The system was operated continuously for 364 days at 250 mg/L NH₄⁺-N. During the operation, the COD/NH₄⁺-N ratio (C/N) was increased from 0.5 to 4 (0.5, 1, 2, 3, and 4), and the aeration rate (AR) gradually increased. The results showed that the SPNAD system maintained efficient and stable operation at C/N = 1-2 and AR = 1.4-1.6 L/min, with an average total nitrogen removal efficiency of 87.2%. The removal pathways of pollutants in the system and the interactions between microbes were revealed by analyzing the changes in sludge characteristics and microbial community structure at different phases. As the influent C/N increased, the relative abundance of Nitrosomonas and Candidatus Brocadia decreased, and that of denitrifying bacteria, such as Denitratisoma, increased to 44%. The nitrogen removal pathway of the system gradually changed from autotrophic nitrogen removal to nitrification-denitrification. At the optimum C/N, the SPNAD system synergistically removed nitrogen through PNA and nitrification-denitrification. Overall, the unique reactor configuration facilitated the formation of dissolved oxygen compartments, providing a suitable environment for different microbes. An appropriate organic matter concentration maintained the dynamic stability of microbial growth and interactions. These enhance microbial synergy and enable efficient single-stage nitrogen removal.

Ecological insight into deterioration of one-stage partial nitritation and anammox system during environmental disturbance

This study investigated the relationship between the performance and the ecological features of a one-stage partial nitritation and anammox disturbed by oxygen. The disturbance caused an irreversible deterioration of the nitrogen removal rate from 0.8 to 0.05 kg N/(m³∙d) although the anammox genera increased from 1% to 1.4%. Meanwhile, the richness and evenness reduced from 455 and 4.00 to 429 and 3.81, respectively, following a similar pattern to the community complexity. The community drifted and formed three distinct clusters during and after the disturbance. Furthermore, 234 of 634 operational taxonomic units in the community were depleted despite recovered diversity and complexity during long-term stable operation. In conclusion, the ecological fluctuation of the microbial community with decreasing resilience was the driving force that fatally collapsed the system performance. This study suggests that ecological features are conducive to the diagnosis, prediction, and optimization of a partial nitritation and anammox system.

Nitrogen removal of thermal hydrolysis-anaerobic digestion liquid: A review

Thermal hydrolysis (TH) pretreatment, as an anaerobic digestion (AD) pretreatment, has not only been verified in the laboratory but also frequently employed in actual engineering. However, the properties of anaerobic digestion liquid (ADL), such as high organic matter concentration, high ammonia nitrogen (NH₄⁺-N) concentration, and low carbon-nitrogen ratio (C/N), have posed some difficulties in the follow-up treatment. To address the above issues, the autotrophic nitrogen removal (ANR) process is developed to treat ADL. Due to the NH₄⁺-N, organic materials, toxic and harmful substances in the ADL that might directly impact the activity of functional bacteria, the ADL should be treated before being fed into the ANR process. This paper provided a focused review of the thermal hydrolysis-anaerobic digestion process (TH-ADP) mechanism and the ANR mechanism, summarized the existing difficulties in the treatment of thermal hydrolysis-anaerobic digestion liquid (TH-ADL), assessed the research status thoroughly, and offered the potential solutions to the problems.

Rapid start-up of PN/A process and efficient enrichment of functional bacteria: A novel aerobic-biofilm/anaerobic-granular nitrogen removal system (OANRS)

Reactor configuration, control strategy and inoculation method were key factors affecting the rapid start-up of partial nitrification/anammox (PN/A) process and the efficient enrichment of functional bacteria (anammox and ammonia oxidizing bacteria). At present, PN/A process was generally operated through single factor rather than forming a system. In this study, a novel aerobic-biofilm/anaerobic-granular nitrogen removal system (OANRS) was constructed, which consisted of a multi-stage aerobic-biofilm/anaerobic-granular baffle reactor (MOABR) and a control strategy on pH/aeration time. PN process was started within 10d, and PN/A process was started on the basis of stable PN process within 41d. The simultaneous enrichment of functional bacteria was achieved by combining the advantages of single-stage and two-stage PN/A process. The results of high-throughput sequencing showed that Candidatus Kuenenia (20.42 ± 15.88%) was highly enriched in each compartment at day 98, and the relative abundance of Candidatus Kuenenia in the anaerobic compartment R4 was as high as 43.13%.

Rapid cultivation and enrichment of anammox bacteria solely using traditional activated sludge as inoculum and biocarrier in low-strength real sewage treatment

In low-ammonia sewage anammox process, cultivation and enrichment of anammox bacteria (AnAOB) is a challenge especially from traditional activated sludge. To this end, a novel strategy solely using activated sludge as inoculum and biocarrier in a dynamic fixed-bed reactor was proposed in this study. During 115-day operation, excellent performance was achieved with influent total inorganic nitrogen (TIN) and effluent TIN of 55.3 mg·L-1 and 4.1 mg·L-1, respectively. Rapid enrichment of AnAOB (doubling time: 8.5 days) was demonstrated by augmented specific anammox activity (trace value to 1.85 mg N·g VSS-1·h-1) and increased hzsB gene number (106 to 109 copies·g-1 dry sludge), with predominance of Candidatus_Brocadia. Large-flocs aggregate was the primary habitat for AnAOB with highest abundance and capacity. The distinctive sludge properties, symbiotic microbial interactions and dynamic operation scheme facilitated AnAOB growth and retention. This study provides a simple, economic and workable approach for the start-up of mainstream anammox process.

Model-based analysis of sulfur-based denitrification in a moving bed biofilm reactor

In this study, a biofilm model was developed for sulfur-based denitrification in a moving bed biofilm reactor (MBBR), including mass transport as well as the conversion kinetics of sulfur-oxidizing bacteria (SOB). The experimental reactor simulated received a synthetic wastewater containing nitrate, sulfide and thiosulfate. The substrate affinity of SOB for intermediary elemental sulfur (S⁰) was found the most sensitive parameter. After estimating this single parameter, the model could adequately describe the steady state performance of the experimental MBBR. The experimental and simulated mass balances indicated that a fraction of influent sulfur accumulated into intermediate S⁰. Furthermore, the simulations showed that SOB were active over the entire thickness of a 200 µm biofilm. The simulation results allowed to quantify the extent of diffusion and substrate limitation. Scenario analyses indicated that the specific nitrogen loading rate could be increased from 0.05 to 0.20 kg N.kg^-1 VSS.day^-1 (corresponding to 0.22-0.86 kg N.m^-2.day^-1 expressed per biofilm surface area) while maintaining nitrogen removal efficiencies above 70%. An increasing specific nitrogen loading rate in this range resulted in an almost linearly increasing specific nitrogen removal rate, independent from whether it was realized through a decreasing HRT, carrier filling ratio or biofilm thickness.

Rapid initiation of a single-stage partial nitritation-anammox process treating low-strength ammonia wastewater: Novel insights into biofilm development on porous polyurethane hydrogel carrier

Media-supported biofilm is a powerful strategy for growth and enrichment of slow-growing microorganisms. In this study, a single-stage nitritation-anammox process treating low-strength wastewater was successfully started to investigate the biofilm development on porous polyurethane hydrogel carrier. Suspended biomass migration into the carrier and being entrapment by its internal interconnected micropores dominated the fast initial colonization stage. Both surface-attached growth and embedded growth of microbes occurred during the following accumulation stage. Fluorescence in situ hybridization analysis of mature biofilm indicated that ammonium-oxidizing bacteria located at the outer layers featured a surface-attached growth, while anammox microcolonies housed in the inner layers proliferated as an embedded-like growth. In this way, the growth rate of anammox bacteria (predominated by Candidatus Kuenenia) could be 0.079 d^-1. The anammox potential of the biofilm reactor reached 1.65 ± 0.3 kg/m³/d within two months. This study provides novel insights into nitritation-anammox biofilm formation on the porous polyurethane hydrogel carrier.

Dual inner circulation and multi-partition driving single-stage autotrophic nitrogen removal in a bioreactor

The single-stage autotrophic nitrogen removal (ANR) process is impeded by a long start-up cycle and unstable operation performance. In this study, an airlift inner-circulation partition bioreactor (AIPBR) was operated continuously for 215 days to explore methods of strengthening the performance and stable operation of the single-stage ANR system. AIPBR start-up period took around 38 days, the total nitrogen removal efficiency was > 85% on day 35. With the decrease of hydraulic retention time and the increase of aeration rate, the nitrogen removal rate increased to 0.85 ± 0.02 kg-N/m³/day. The sludge morphology gradually changed into dark-red floc-coupled granular sludge. Nitrosomonas (9.95%) and Candidatus Brocadia (6.41%) were dominant in the sludge. During long-term operation, AIPBR achieved the dual inner circulation of sewage and sludge and then formed effective dissolved oxygen and sludge partitions to provide a suitable growth environment for various functional bacteria, promote synergy between them, and strengthen the ANR performance.

Integrated self-forming dynamic membrane (SFDM) and membrane-aerated biofilm reactor (MABR) system enhanced single-stage autotrophic nitrogen removal

The membrane aerated biofilm reactor (MABR) is a novel bioreactor technology, facilitating single-stage autotrophic nitrogen removal. Two laboratory-scale MABRs equipped with non-woven fabrics were operated simultaneously without and with a self-forming dynamic membrane (SFDM) filtration module. After 87 days of operation (system start-up), the reactor incorporated with SFDM filtration showed better performance in terms of total nitrogen removal (>80%) and effluent suspended solid (less than1 mg/L) than the MABR in the up flow anaerobic sludge blanket (UASB) configuration (i.e., without SFDM). The incorporation of SFDM has the ability to retain more slow growing biomass (anammox) inside the reactor. Microbial characterization by 16S rRNA-based amplicon sequencing shows that the abundance and composition of microbial communities in two MABR systems were different, i.e., the genusRhodanobacterwas abundant in UASB-MABR, while Calorithrixwas dominant in SFDM-MABR. PCA-based statistical analysis demonstrated a positive association between reactor performance, membrane characteristics and microbial communities.

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.

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.

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.

Performance and microbial community structure of a full-scale ANITATMMox bioreactor for treating reject water located in Finland

The aim of this work was to study the operational performance and the microbial community dynamics during the start-up of ANITA^TMMox technology implemented at full-scale wastewater treatment plant in Finland to treat reject water from anaerobic digesters. The average ammonium removal in the studied setup reached around 90%, withstanding ammonium loads up to 0.13 g N m^-2h^-1. The nitrite concentration in the effluent did not exceed 10 mg L^-1, and there was a slight accumulation of NO₃^--N during the operation which was controlled. Thus, the result showed a robust success to high ammonium loading in presence of organic matter. The sequencing showed a heterogeneous microbial population where Methanosaeta, WCHA1-57 genus, Sphingobacteriia, Chlorobia and diverse unknown fungi were found as dominant phylotypes. Moreover, members of the Brocadiaceae family were dominant in the adhered biomass, mostly represented by Candidatus Scalindua, rarely reported in WWTPs. Overall, the results demonstrated a drastic effect of region-specific operational conditions on carrier biofilm microbial communities as it was demonstrated by the microbial studies.

Evaluating the roles of coexistence of sludge flocs on nitrogen removal and nitrous oxide production in a granule-based autotrophic nitrogen removal system

Certain levels of sludge flocs would always coexist in granule-based reactors due to the biomass detachment from granules. Such inevitable coexistence could affect both total nitrogen (TN) removal and nitrous oxide (N₂O) production in autotrophic nitrogen removal systems. This work utilized a mathematical approach to systematically study the influence of the coexisting sludge flocs on TN removal and N₂O production in a granular nitritation-anaerobic ammonium oxidation (Anammox) process for the first time, based on a 2-pathway N₂O production model concept. The modelling results reveal that the highest TN removal efficiency decreases from ca. 87-88% to ca. 41-49% as the fraction of sludge flocs in the system increases from 10% to 40%, while the N₂O production rate gradually increases with such increase. Meanwhile, both bulk dissolved oxygen (DO, 0.05-0.3 mg/L) and the size of granule (200-400 μm) could also influence the TN removal efficiency and N₂O production. As the fraction of sludge flocs increases from 10% to 40%, the contribution of granular biomass to total N₂O production is reduced due to increase of N₂O-producing ammonia-oxidizing bacteria (AOB) in the sludge flocs, and the increase of granule size could intensify such decrease. In addition, the hydroxylamine oxidation pathway dominates the nitrifier denitrification pathway in both granules and sludge flocs under various testing conditions, whereas the increasing contribution of the latter would occur at a certain DO range, higher fraction of sludge flocs and smaller granule size. These results disclose an important influence of the coexisting sludge flocs on the performance of granular nitritation-Anammox systems.

Steady-state and dynamic analysis of the single-stage anammox granular sludge reactor show that bulk ammonium concentration is a critical control variable to mitigate feeding disturbances

One of the challenges for the application of the ANAMMOX (anaerobic ammonium oxidation) process for nitrogen removal in the mainstream municipal wastewater treatment is the disturbances in the feeding influent, in terms of the organic material, ammonium concentration, flow rate and temperature. Past experimental and mathematical simulation researches were mainly carried out under stable feeding condition. It is not clear how the ANAMMOX responds to the feeding disturbances. In this study, a 1D (one-dimensional) biofilm model was used to examine the behavior of feeding disturbances in the single-stage granule PN-A (partial nitrification -ANAMMOX) process. The results indicated that the feeding disturbances could be mitigated by maintaining a small amount of ammonium in the bulk liquid. A cascade control strategy based on DO (dissolved oxygen) manipulation to derive the preset bulk ammonium set-point value (i.e., 2 mg N/L) was shown to be successful in achieving the maximum TN (total nitrogen) removal percentage of above 95% under dynamic feeding conditions. The long-term simulation showed that the small bulk ammonium accumulation could also suppress the NOB (nitrite-oxidizing bacteria) growth.

Two-stage granular sludge partial nitritation/anammox process for the treatment of digestate from the anaerobic digestion of the organic fraction of municipal solid waste

The increasing amount of source separated organic fraction of municipal solid wastes (OFMSW) treated by anaerobic digestion for energy recovery requires the implementation of cost-efficient processes for the treatment of the produced digestate, especially in terms of nitrogen removal. The autotrophic nitrogen removal process, based on the coupling of two biological processes, partial nitritation (PN) and anammox (A), appears as a suitable solution due to important savings in operational costs compared to conventional treatment processes. However, its application could be hampered by the high salinity and inhibitory potential of this kind of digestate. In this contribution, two lab-scale granular sludge reactors performing the PN and anammox processes, respectively, were used to treat (opportunely diluted) real OFMSW digestate originating from full-scale biogas plants with the aim of assessing their treatment feasibility in a two-stage PN/A configuration. The PN process was implemented in an air-lift granular sludge reactor and was able to treat a nitrogen loading rate of about 1 g N L^-1 d^-1 at 30 ± 0.5 °C; moreover, its effluent was suitable for the subsequent anammox treatment, with an appropriate effluent NO₂^-/NH₄⁺ ratio and marginal inhibiting effects. In the anammox granular sludge reactor, the anammox activity was affected by high salinity levels, nonetheless a stable reactor performance at a nitrogen removing rate of 0.83 ± 0.20 and 0.31 ± 0.04 g N L^-1 d^-1 at 35 ± 0.5 °C, were achieved when treating 50% and 30% diluted real wastewaters at a conductivity in the reactor of 9.1 and 11.2 mS cm^-1, respectively.

Performance of partial nitritation-anammox processes at mainstream conditions in an IFAS system

The partial nitritation-anammox processes implementation in the main line of wastewater treatment plants would lead them closer to the energy autarky. With this purpose, an integrated fixed film activated sludge (IFAS) reactor was operated at pilot scale. Efficient nitrogen removal (72 ± 11%) was achieved for anaerobically pre-treated municipal wastewater at low temperature (21 - 15 °C), with a nitrogen removal rate of 37 ± 3 g N/(m³·d) at 15 °C. The ammonium oxidizing bacteria were more abundant in the activated sludge, while anammox bacteria were primarily located in biofilm attached onto the carriers surface. Nitrite oxidizing bacteria (NOB) activity was similar between both fractions and its specific activity decreased more than that of other populations when the operating temperature was reduced. Furthermore, the IFAS operational strategy (aerobic/anoxic periods) allowed an efficient NOB activity suppression inside the reactor, which accounted only for the 10 - 20% of the maximum potential activity.

Effective nitrogen removal in a two-stage partial nitritation-anammox reactor treating municipal wastewater - Piloting PN-MBBR/AMX-IFAS configuration

A novel partial nitritation-anammox (PNA) reactor configuration was piloted for 250 days. Primary effluent from full-scale municipal wastewater treatment plant was treated in a two-stage biofilm system incorporating innovative process control for cold partial nitritation. Partial nitritation was combined with carbon removal in a moving bed biofilm reactor (MBBR) to achieve high-rate treatment and nitritation was obtained with dissolved oxygen to total ammonium nitrogen (DO/TAN) ratio control and free ammonia (FA) for inhibition of nitratation. Effluent from MBBR was directed to an integrated fixed-film activated sludge (IFAS) reactor where nitrogen was removed via anammox. MBBR achieved partial nitritation at 2.0 ± 0.3 g-N m^-2 d^-1 and nitrogen removal in the IFAS reactor reached 0.45 ± 0.1 g-N m^-2 d^-1 (55 g-N m^-3 d^-1). The process performed well at 19 ± 3 °C with an average effluent total inorganic nitrogen (TIN) concentration of 11 ± 4 mg L^-1.

Anammox bacteria exhibit capacity to withstand long-term starvation stress: A proteomic-based investigation of survival mechanisms

Although anammox bacteria are commonly exposed to long-term starvation during transportation and preservation process, physiological changes in these organisms during long-term starvation are not well understood, nor are the molecular bases of their starvation survival strategies. To reveal survival mechanisms during long-term anaerobic and anoxic starvation (60 days at 20 ± 1 °C), metaproteomic technology was utilized to identify differentially expressed proteins in Candidatus Kuenenia stuttgartiensis. Our results showed that Candidatus Kuenenia stuttgartiensis exhibits a capacity to withstand long-term starvation stress. Although activity decay rates of 0.0129 d^-1 and 0.0049 d^-1 were observed for anammox sludge in anoxic and anaerobic starvation, the relative abundance of Candidatus Kuenenia stuttgartiensis, the shape of anammox granules, and the fraction of viable cells remained constant under both anaerobic and anoxic starvation conditions. Metaproteomics results illustrated that Candidatus Kuenenia stuttgartiensis maintained stable levels of most intracellular proteins, especially enzymes involved in principal metabolic pathways after 60-d of anaerobic or anoxic starvation, thereby allowing cells to regain metabolic activities once substrates became available. Induction of starvation proteins could be a survival strategy employed by Candidatus Kuenenia stuttgartiensis to resist long-term starvation stresses. During anaerobic starvation, 34 proteins were upregulated, five of which were associated with carbohydrate catabolism and oxidation of organic compounds, thereby increasing potential for utilization of endogenous carbon sources to produce energy. During anoxic starvation, only two proteins were upregulated, which may be attributed to insufficient energy for the synthesis of starvation-induced proteins.

The effect and biological mechanism of granular sludge size on performance of autotrophic nitrogen removal system

The autotrophic process for nitrogen removal has attracted worldwide attention in the field of wastewater treatment, and the performance of this process is greatly influenced by the size of granular sludge particles present in the system. In this work, the granular sludge was divided into three groups, i.e. large size (> 1.2 mm), medium size (0.6-1.2 mm) and small size (< 0.6 mm). The medium granular sludge was observed to dominate at high volumetric nitrogen loading rates, while offering strong support for good performance. Its indispensable contribution was found to originate from improved settling velocity (0.84 ± 0.10 cm/s), high SOUR-A (specific oxygen uptake rate for ammonia oxidizing bacteria, 25.93 mg O₂/g MLVSS/h), low SOUR-N (specific oxygen uptake rate for nitrite oxidizing bacteria, 3.39 mg O₂/g MLVSS/h), and a reasonable microbial spatial distribution.

Performance of autotrophic nitrogen removal from digested piggery wastewater

The performance of an autotrophic nitrogen removal process to treat digested piggery wastewater (DPW) was investigated by gradually shortening the HRT and enhancing the DPW concentration during 390days of operation. The results showed that the total nitrogen removal rate and efficiency reached 3.9kg-Nm^-3day^-1 and 73%, which were significantly higher than the levels reported previously. A high relative abundance of Nitrosomonas (4.2%) and functional microbes (12.15%) resulted in a high aerobic ammonium oxidizing activity (1.25±0.1g-NgVSS^-1d^-1), and a good settling ability (SVI, 78.42mLg^-1SS) resulted in a high sludge concentration (VSS, 11.01gL^-1), which laid a solid foundation for the excellent performance. High-throughput pyrosequencing indicated that, compared with synthetic wastewater, the DPW decreased the relative abundances of every functional group of nitrogen removal microbes, and increased relative abundances of anaerobes (15.7%), sulfur-oxidizing bacteria (9.4%) and methanogens (40.8%).

High-throughput sequencing-based microbial characterization of size fractionated biomass in an anoxic anammox reactor for low-strength wastewater at low temperatures

The microbial characterization of three size-fractionated sludge obtained from a suspended-growth anoxic anammox reactor treating low-strength wastewater at low temperatures were investigated by using high-throughput sequencing. Particularly, the spatial variability in relative abundance of microorganisms involved in nitrogen metabolism were analyzed in detail. Results showed that population segregation did occur in the reactor. It was found, for the first time, that the genus Nitrotoga was enriched only in large granules (>400μm). Three anammox genus including Candidatus Jettenia, Brocadia and Kuenenia were detected. Among them, Candidatus Brocadia and Kuenenia preferred to grow in large-sized granules (>400μm), whereas Candidatus Jettenia dominated in small- and moderate-sized sludge (<400μm). The members of genus Candidatus Jettenia appeared to play the vital role in nitrogen removal, since sludge with diameters smaller than 400μm accounted for 81.55% of the total biomass. However, further studies are required to identify the activity of different-size sludge.

Short-term and long-term effects of Zn (II) on the microbial activity and sludge property of partial nitrification process

Autotrophic nitrogen removal was an innovative and economical nitrogen removal technology with less oxygen and no organics consumption, in which partial nitrification (PN) is the key component. It is necessary to clear the impact of metal ions on PN since the development of industry increased their opportunity for entering into wastewater. In this study, PN process was successfully started-up in an SBR, the short-term and long-term effects of Zn (II) on microbial bioactivity and the sludge adsorption ability for Zn (II) were investigated. Results suggested that low Zn (II) were favorable for AOB bioactivity, while the long-term effect also induced NOB bioactivity. The suppression threshold of Zn (II) on AOB in short-term effect was 10mgL^-1, which rose to 50mgL^-1 in the long-term effect due to the self-adaption. The PN sludge presented prominent absorbability for zinc and performed a quadratic relation with the Zn (II) concentration.

Effects of dissolved oxygen on microbial community of single-stage autotrophic nitrogen removal system treating simulating mature landfill leachate

The performance of four identical sequencing biofilm batch reactors (SBBR) for autotrophic nitrogen removal was investigated with 2000mg/L ammonia-containing mature landfill leachate at 30°C. The main objective of this study was to evaluate the effects of dissolved oxygen (DO) on the performance and microbial community of single-stage nitrogen removal using anammox and partial nitritation (SNAP) system. At an applied load of 0.5kgNm(-3)d(-1), average total nitrogen removal efficiency (TNRE) above 90% was long-term achieved with an optimal DO concentration of 2.7mg/L. The microelectrode-measured profiles showed the microenvironments inside the biofilms. 16S ribosomal Ribonucleic Acid (rRNA) amplicon pyrosequencing and denaturing gradient gel electrophoresis (DGGE) were used to analyze the microbial variations of different DO concentrations and different positions inside one reactor.

Suspended sludge and biofilm shaped different anammox communities in two pilot-scale one-stage anammox reactors

The abundance and diversity of anammox bacteria was investigated in two pilot-scale integrated fixed-film activated sludge (IFAS) reactors treating high ammonium wastewater. Reactor A was inoculated with nitrifying sludge, while Reactor B was inoculated with suspended anammox sludge with the dominant anammox bacteria of Candidatus 'Kuenenia'. After 180days' operation, the predominate anammox bacteria was Candidatus 'Brocadia' (65%) in the biofilm, while Candidatus 'Kuenenia' (86%) outcompeted with other anammox bacteria in suspended sludge in Reactor A. Candidatus 'Kuenenia' were dominated in suspended sludge through the entire experiment in Reactor B. In contrast, the predominated species shifted from Candidatus 'Kuenenia' (89%) into Candidatus 'Brocadia' (66%) in the biofilm of Reactor B. This study indicated that Candidatus 'Brocadia' preferred to grow in the biofilm, while Candidatus 'Kuenenia' would dominant over other anammox bacteria in the suspended sludge. Further studies are required to identify the internal factors affecting the distribution of anammox bacteria.

Impact of methionine on a partial-nitritation biofilter

It has been demonstrated that an anaerobic digestion process cannot attain an efficient removal of several amino acids, with methionine being one of the most persistent of these. Thus, the effect that methionine amino acid has over the partial-nitritation process with fixed-biofilm configuration in terms of performance and bacterial community dynamics has been investigated. With respect to the performance with no addition, 100 mg/L methionine loading decreased ammonium oxidation efficiency in 60% and 100% at concentrations of 300 and 500 mg/L methionine, respectively. Bacterial biomass sharply increased by 30, 65, and 230% with the addition of 100, 300, and 500 mg/L methionine, respectively. Bacterial community analysis showed that methionine addition supported the proliferation of a diversity of heterotrophic genera, such as Lysobacter and Micavibrio, and reduced the relative abundance of ammonium oxidizing genus Nitrosomonas. This research shows that the addition of methionine affects the performance of the partial-nitritation process. In this sense, amino acids can pose a threat for the of partial-nitritation process treating anaerobic digester supernatant at full-scale implementation.

Startup and oxygen concentration effects in a continuous granular mixed flow autotrophic nitrogen removal reactor

The startup and performance of the completely autotrophic nitrogen removal over nitrite (CANON) process was tested in a continuously fed granular bubble column reactor (BCR) with two different aeration strategies: controlling the oxygen volumetric flow and oxygen concentration. During the startup with the control of oxygen volumetric flow, the air volume was adjusted to 60mL/h and the CANON reactor had volumetric N loadings ranging from 7.35 to 100.90mgN/Ld with 36-71% total nitrogen removal and high instability. In the second stage, the reactor was operated at oxygen concentrations of 0.6, 0.4 and 0.2mg/L. The best condition was 0.2 mgO2/L with a total nitrogen removal of 75.36% with a CANON reactor activity of 0.1149gN/gVVSd and high stability. The feasibility and effectiveness of CANON processes with oxygen control was demonstrated, showing an alternative design tool for efficiently removing nitrogen species.

Environmental sustainability of an energy self-sufficient sewage treatment plant: improvements through DEMON and co-digestion

It is still not proven that treatment of sewage in a wastewater treatment plant (WWTP) is (in every case) environmentally friendly. To address this matter, we have applied a state-of-the-art life cycle assessment (LCA) to an energy self-sufficient WWTP in Strass (Austria), its supply chain and the valorization of its 'products': produced electricity out of biogas from sludge digestion and the associated stabilized digestate, applied as agricultural fertilizer. Prominent aspects of our study are: a holistic environmental impact assessment, measurement of greenhouse gas emissions (including N2O), and accounting for infrastructure, replacement of conventional fertilizers and toxicity of metals present in the stabilized digestate. Additionally, the environmental sustainability improvement by implementing one-stage partial nitritation/anammox (e.g. DEMON(®)) and co-digestion was also assessed. DEMON on the digesters reject water leads to a considerable saving of natural resources compared to nitritiation/denitritation (about 33% of the life cycle resource input), this through the lowering of sludge consumption for N-removal, and thus increasing electricity production via a higher sludge excess. However, its N2O emission could be restrained through further optimization as it represents a large share (30-66%) of the plants' damaging effect on human health, this through climate change. The co-substrate addition to the digester resulted in no significant improvement of the digestion process but induced net electricity generation. If respective amounts of conventional fertilizers are replaced, the land application of the stabilized digestate is environmentally friendly through prevention of natural resource consumption and diversity loss, but possibly not regarding human health impact due the presence of toxic heavy metals, mainly Zn, in the digestate. The outcomes show that the complete life cycle results in a prevention of resource extraction from nature and a potential mitigation of diversity loss (though for some impact categories no quantification of associated diversity loss is possible) but it also leads to a damaging effect on human health, mainly via climate change and heavy metal toxicity. Since it is for now impossible to aggregate the impact to these different aspects in a sound manner, it is not yet possible to consider in this case the studied system as environmentally friendly. Generally, the field of LCA needs further development to present a better and single outcome.

Autotrophic nitrogen removal by a two-step SBR process applied to mixed agro-digestate

The aim of this research was to evaluate the applicability of partial-nitritation/anammox processes for biological N removal from a centrifuge supernatant coming from a full scale anaerobic digester fed on a mixture of piggery manure, poultry manure, and agro-wastes. Stable partial nitritation was achieved at pilot-scale (650L SBR), obtaining a suitable influent for the anammox lab-scale SBR reactor (3L). The anammox lab scale reactor was fed with increasing fractions of the partial nitritation effluent, blended with synthetic wastewater. In the last 100days no dilution was used. The nitrogen loading rate applied to the anammox reactor was 0.5-0.6gNL(-)(1)d(-)(1) and the average nitrogen removal was 91±10%. During the first days of operation with undiluted supernatant, the maximum anammox activity in the SBR decreased, but recovered afterwards, suggesting the ability of the anammox biomass to acclimate to the wastewater. N2O emissions in both reactors were also measured.

Simultaneous partial nitritation and anammox at low temperature with granular sludge

Autotrophic nitrogen removal in the main stream appears as a prerequisite for the implementation of energy autarchic wastewater treatment plants. To investigate autotrophic nitrogen removal a lab-scale gas-lift sequencing batch reactor with granular sludge was operated for more than 500 days. The reactor was operated at temperatures between 20 and 10 °C on autotrophic medium with ammonium (60 and 160 mg-N L(-1)) as only nitrogen compound at an HRT of 0.23-0.3 d. The dissolved oxygen (DO) concentration was shown to be an effective control parameter for the suppression of the undesired nitratation process. DO control guaranteed the effective suppression of the nitratation both at 20 and 15 °C, allowing nitrogen removal rates of 0.4 g-NTot L(-1) d(-1) at nitrogen removal efficiencies of 85-75%. Prolonged operation at 10 °C caused a slow but unrestrainable decrease in anammox activity and process efficiency. This study represents a proof of concept for the application of the autotrophic nitrogen removal in a single reactor with granular sludge at main stream conditions.

Starved anammox cells are less resistant to NO₂⁻ inhibition

Anaerobic ammonium oxidizing (anammox) bacteria are be inhibited by their terminal electron acceptor, nitrite. Serious nitrite inhibition of the anammox bacteria occurs if the exposure coincides with the absence of the electron donating substrate, ammonium and pH < 7.2. Starvation of biomass occurs during underloading of bioreactors or biomass storage. This work investigated the effect of starvation on the sensitivity of anammox bacteria to nitrite exposure. Batch activity tests were carried out evaluating the response of anammox biomass subjected to different levels of starvation upon exposure to nitrite in the presence and absence of ammonium (active- and resting-cells, respectively). The response of the bacteria was evaluated by measuring the specific anammox activity and the evolution of the ATP content in the biomass over time. The 50% inhibitory concentrations of nitrite in starved- and fresh-resting-cells was 7 mg N L(-1) and 52 mg N L(-1), respectively. By contrast, only moderate nitrite inhibition occurred to starved anammox biomass when exposed to nitrite and ammonium simultaneously. Maximum ATP levels were observed in fresh cells. The ATP content in starved resting cells peaked 2-3 h after addition of NO2(-)(-). The response was hindered in cells starved for long periods. These findings agreed with a bioreactor study in which underloading of anammox biomass (0.10 g N L(-1) d(-1)) decreased its tolerance to a nitrite (only) exposure (101 mg NO2(-)-N L(-1)) and completely disrupted the N removal capacity of the biomass. A similar accumulation of 108 mg NO2(-)-N L(-1) after operation at 0.95 g N L(-1) d(-1) did not cause observable inhibition of the bacteria. The results taken as a whole demonstrate that starved anammox biomass is highly sensitive to nitrite toxicity. An explanation is proposed based on energy requirements to translocate nitrite in the cell.

High specific activity for anammox bacteria enriched from activated sludge at 10°C

Anammox in the water line of a waste water treatment plant (WWTP) saves energy for aeration and allows for recovering biogas from organic material. Main challenges for applying the anammox process in the water line are related to the low temperature of <20°C, causing a significant drop in the specific anammox activity. The aim of this research was to enrich a cold-adapted anammox species, with a high specific activity. This was achieved in a 4.2L reactor operated at 10°C, fed with 61 mg (NH4+NO2)-N/L and inoculated with activated sludge from two selected municipal WWTPs. Candidatus Brocadia fulgida was the dominant species in the enriched biomass, with a specific activity was 30-44 mg N/(g VS d). This is two times higher than previously reported at 10°C, which is beneficial for full scale application. Biomass yield was 0.046 g biomass/g N converted, similar to that at higher temperatures.

Effect of particle size on the performance of autotrophic nitrogen removal in the granular sludge bed reactor and microbiological mechanisms

The effect of particle size on the performance of autotrophic nitrogen removal in the granular sludge bed reactor (GSB-ANR) and microbiological mechanisms were investigated. The results indicated that performance of GSB-ANR process decreased gradually with the increase of the granular sludge size. Indeed small granules ranging between 0.5 and 0.9mm had a higher nitrogen removal capacity than large ones. The reasons of this effect were that (i) the aerobic ammonium oxidizing capacity of microorganisms was the bottle neck of nitrogen removal in GSB-ANR process, and the increase of aerobic ammonium oxidizing activity enhances nitrite production in nitrification and promotes subsequent nitrite consumption during anaerobic ammonia oxidation; (ii) the aerobic/anaerobic zone separation in granular sludge was the key factor affecting the aerobic ammonium oxidizing capacity of microorganisms. The small granules had a larger aerobic functional zone (75.1%) which was profitable for up-regulating the expression level of functional gene in aerobic ammonium oxidizing microorganisms.

Start-up of the completely autotrophic nitrogen removal process using low activity anammox inoculum to treat low strength UASB effluent

The start-up of the completely autotrophic nitrogen removal process was examined in a sequencing batch reactor (SBR) using low activity anoxic ammonium oxidation (anammox) inoculum. The SBR received effluent from an upflow anaerobic sludge blanket (UASB) that treated low strength wastewater. The volumetric nitrogen loading rate (vNLR) was first 0.24 ± 0.11 kg Nm(-3)d(-1) and then reduced to 0.10 ± 0.02 kg Nm(-3)d(-1). The average specific anammox activity was 2.27 ± 1.31 mg N (gVSS h)(-1), at 30°C representing an increase of 161% compared to the inoculum. The decrease in vNLR did not significantly affect anammox activity, but resulted in a decrease of denitrifying heterotrophic activity to very low levels after the first 30 days owing to the decrease of organic loading rate (OLR). Fluorescence in situ hybridization (FISH) analysis confirmed the stable presence of anammox bacteria in biomass. Numerous filamentous microorganisms were present, several of which were in a state of endogenous respiration.

The enhancement of completely autotrophic nitrogen removal over nitrite (CANON) by N2H4 addition

The long-term addition of N2H4 to completely autotrophic nitrogen removal over nitrite (CANON) sequencing batch reactors (SBRs) recovered and enhanced their autotrophic nitrogen removal capacity while simultaneously reducing their production of NO3(-). The total nitrogen (TN) removal rate and TN removal efficiency of the process increased from 0.202±0.011 to 0.370±0.016 kg N/m(3)/d and from 65.1±3.75% to 77.4±3.8%, respectively, and the molar ratio of NO3(-) production to NH4(+) removal (MRNN) decreased to 0.058. The most effective concentration of N2H4 addition was approximately 3.99 mg/L. N2H4 could increase the specific growth rate of anaerobic ammonium-oxidizing bacteria (AnAOB) and inhibit aerobic ammonia oxidation. The electrons released from the oxidation of additional N2H4 using hydrazine dehydrogenase (HDH), which substituted the electrons from NO2(-) oxidation to NO3(-), replenished the consumption of AnAOB anabolism and significantly reduced the consequent NO3(-) production.