Stratification of nitrifier guilds in granular sludge in relation to nitritation

Rapid start-up of an innovative pilot-scale staged PN/A continuous process for enhanced nitrogen removal from mature landfill leachate via robust NOB elimination and efficient biomass retention

The start-up and stable operation of partial nitritation-anammox (PN/A) treatment of mature landfill leachate (MLL) still face challenges. This study developed an innovative staged pilot-scale PN/A system to enhance nitrogen removal from MLL. The staged process included a PN unit, an anammox upflow enhanced internal circulation biofilm (UEICB) reactor, and a post-biofilm unit. Rapid start-up of the continuous flow PN process (full-concentration MLL) was achieved within 35 days by controlling dissolved oxygen and leveraging free ammonia and free nitrous acid to selectively suppress nitrite-oxidizing bacteria (NOB). The UEICB was equipped with an annular flow agitator combined with the enhanced internal circulation device of the guide tube, which achieved an efficient enrichment of Candidatus Kuenenia in the biofilm (relative abundance of 33.4 %). The nitrogen removal alliance formed by the salt-tolerant anammox bacterium (Candidatus Kuenenia) and denitrifying bacteria (unclassified SBR1031 and Denitratisoma) achieved efficient nitrogen removal of UEICB (total nitrogen removal percentage: 90.8 %) and at the same time effective treatment of the refractory organic matter (ROM). The dual membrane process of UEICB fixed biofilm combined with post-biofilm is effective in sludge retention, and can stably control the effluent suspended solids (SS) at a level of less than 5 mg/L. The post-biofilm unit ensured that effluent total nitrogen (TN) remained below the 40 mg/L discharge standard (98.5 % removal efficiency). Compared with conventional nitrification-denitrification systems, the staged PN/A process substantially reduced oxygen consumption, sludge production, CO2 emissions and carbon consumption by 22.8 %, 67.1 %, 87.1 % and 87.1 %, respectively. The 195-day stable operation marks the effective implementation of the innovative pilot-scale PN/A process in treating actual MLL. This study provides insights into strategies for rapid start-up, robust NOB suppression, and anammox biomass retention to advance the application of PN/A in high-ammonia low-carbon wastewater.

Achieving robust nitritation in a modified continuous-flow reactor: From micro-granule cultivation to nitrite-oxidizing bacteria elimination

In this study, a modified continuous-flow nitrifying reactor was successfully operated for rapid cultivation of micro-granules and achieving robust nitritation. Results showed that sludge granulation with mean size of ca. 100 µm was achieved within three weeks by gradually increasing settling velocity-based selection pressure from 0.48 to 0.9 m/hr. Though Nitrospira like nitrite-oxidizing bacteria (NOB) were enriched in the micro-granules with a ratio between ammonia-oxidizing bacteria (AOB) and NOB of 5.7%/6.5% on day 21, fast nitritation was achieved within one-week by gradually increasing of influent ammonium concentration (from 50 to 200 mg/L). Maintaining ammonium in-excess was the key for repressing NOB in the micro-granules. Interestingly, when the influent ammonium concentration switched back to 50 mg/L still with the residual ammonium of 15-25 mg/L, the nitrite accumulation efficiency increased from 90% to 98%. Experimental results suggested that the NOB repression was intensified by both oxygen and nitrite unavailability in the inner layers of micro-granules. Unexpectedly, continuous operation with ammonium in excess resulted in overproduction of extracellular polysaccharides and overgrowth of some bacteria (e.g., Nitrosomonas, Arenimonas, and Flavobacterium), which deteriorated the micro-granule stability and drove the micro-granules aggregation into larger ones with irregular morphology. However, efficient nitritation was stably maintained with extremely high ammonium oxidation potential (> 50 mg/g VSS/hr) and nearly complete washout of NOB was obtained. This suggested that smooth and spherical granule was not a prerequisite for achieving NOB wash-out and maintaining effective nitritation in the granular reactor. Overall, the micro-granules exhibited a great practical potential for high-rate nitritation.

A novel approach to estimate and control denitrification performance in activated sludge systems with respirogram technology

Respirogram technology has been widely applied for aerobic process, however, the response of respirogram to anoxic denitrification is still unclear. To reveal such response may help to design a new method for the evaluation of the performance of denitrification. The size distribution of flocs measured at different denitrification moments demonstrated a clear expansion of flocs triggered by denitrification, during which higher specific endogenous and quasi-endogenous respiration rates (SOUR_e and SOUR_q) were also observed. Furthermore, SOUR_q increases exponentially with the specific denitrification rate (SDNR), suggesting that there should be a maximum SDNR in conventional activated sludge systems. Based on these findings, an index R_q/t, defined as the ratio of quasi-endogenous (OUR_q) to maximum respiration rate (OUR_t), is proposed to estimate the denitrification capacity that higher R_q/t indicates higher denitrification potential, which can be readily obtained without complex measurement or analysis, and it offers a novel and promising respirogram-based approach for denitrification estimation and control by taking measures to extend anoxic time to maintain its value at a high level within a certain range.

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.

Mainstream short-cut N removal modelling: current status and perspectives

This work gives an overview of the state-of-the-art in modelling of short-cut processes for nitrogen removal in mainstream wastewater treatment and presents future perspectives for directing research efforts in line with the needs of practice. The modelling status for deammonification (i.e., anammox-based) and nitrite-shunt processes is presented with its challenges and limitations. The importance of mathematical models for considering N₂O emissions in the design and operation of short-cut nitrogen removal processes is considered as well. Modelling goals and potential benefits are presented and the needs for new and more advanced approaches are identified. Overall, this contribution presents how existing and future mathematical models can accelerate successful full-scale mainstream short-cut nitrogen removal applications.

Response of nitritation granules to anaerobically pre-treated municipal wastewater at low temperatures in a continuous-flow reactor

Achieving mainstream nitritation with aerobic granules is attractive based on increasing evidence but generally treating artificial low-ammonium wastewater. Real municipal wastewater is much more complex in composition, the behavior of the nitritation granules would be different when treating real municipal wastewater. Herein, the response of nitritation granules to influent shift from artificial low-ammonium (35-40 mg/L) wastewater to anaerobically pre-treated municipal wastewater (MWW_pre-treated) was investigated at low temperatures. Results showed that MWW_pre-treated caused the outgrowth of filamentous bacteria on the granule surface and developed into finger-like structures, which in turn resulted in the decrease of the overall granular sludge settleability. Batch-tests and microbial analysis indicated the functional and microbial differentiation between the newly formed fluffy exterior and the original compact granule. The fluffy exterior was dominated by genus Flavobacterium (66.6%) and primarily functioned as COD removal, whereas the nitrifiers (mainly Nitrosomonas) were still located in the compact core and performed nitritation. Moreover, the heterotrophs-dominated fluffy exterior hindered the oxygen transfer towards nitrifiers located in the compact granule and thereby facilitated the stable NOB repression in the granule particularly at low temperatures (<10 °C). Finally, gradual recovery of the granular sludge morphology and settleability occurred after the influent reverted to synthetic low-ammonium wastewater. Overall, this work demonstrated that the feeding of MWW_pre-treated only caused morphological changes of the nitritation granules, but its structural and functional stability could be maintained stably.

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.

Advanced nitrogen removal from anaerobically pre-treated potato wastewater via partial nitritation-anammox in a continuous fed SBR

Partial nitritation-anammox was carried out successfully in a continuous fed Sequencing Batch Reactor (cf-SBR), composed of 3 compartments operated in continuous mode. The reactor was operated with floccular biomass (flocs) and biofilm to remove nitrogen from the anaerobic effluent from the potato industry at different nitrogen loading rates (0.16 g TN L^-1 d^-1 - 0.8 g TN L^-1 d^-1). At the maximum nitrogen loading rate (NLR) evaluated the nitrogen removal and ammonia oxidation achieved were 62% and 74% respectively. During the evaluation of the NLR, it was observed an improvement of the characteristics of the sludge, improving the Sludge Volumetric Index (SVI) from 228 to 63 mL g^-1 MLSS. Moreover, molecular analysis (qPCR) confirmed the presence of anammox bacteria on the flocs and in the biofilm from the cf-SBR. The results showed the capability of the reactor to carry out the partial nitritation-anammox in the same reactor at pilot scale. The cf-SBR was presented as a suitable and feasible technology for advanced nitrogen removal under partial nitritation and anammox conditions.

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.

Recent advancements in the biological treatment of high strength ammonia wastewater

The estimated global population growth of 81 million people per year, combined with increased rates of urbanization and associated industrial processes, result in volumes of high strength ammonia wastewater that cannot be treated in a cost-effective or sustainable manner using the floc-based conventional activated sludge approach of nitrification and denitrification. Biofilm and aerobic granular sludge technologies have shown promise to significantly improve the performance of biological nitrogen removal systems treating high strength wastewater. This is partly due to enhanced biomass retention and their ability to sustain diverse microbial populations with juxtaposing growth requirements. Recent research has also demonstrated the value of hybrid systems with heterogeneous bioaggregates to mitigate biofilm and granule instability during long-term operation. In the context of high strength ammonia wastewater treatment, conventional nitrification-denitrification is hampered by high energy costs and greenhouse gas emissions. Anammox-based processes such as partial nitritation-anammox and partial denitrification-anammox represent more cost-effective and sustainable methods of removing reactive nitrogen from wastewater. There is also growing interest in the use of photosynthetic bacteria for ammonia recovery from high strength waste streams, such that nitrogen can be captured and concentrated in its reactive form and recycled into high value products. The purpose of this review is to explore recent advancements and emerging approaches related to high strength ammonia wastewater treatment.

Insight into how high dissolved oxygen favors the startup of nitritation with aerobic granules

To elucidate how high dissolved oxygen (DO) favors the startup of nitritation with aerobic granular sludge, two granular reactors were operated under low (1-2 mg O₂·L^-1) and high DO (3-5 mg O₂·L^-1) conditions with similar effluent ammonium concentrations (>20 mg N·L^-1). The results showed that though nitritation with an average nitrite accumulation ratio of above 95% was finally achieved in both reactors, a five-fold start-up time (eleven weeks) was required for the low DO reactor compared to the high DO reactor. Moreover, the nitritation performance was positively correlated with the extent of nitrifiers stratification in granules. The faster startup of nitritation under high DO conditions mainly resulted from the faster formation of well-stratified nitrifiers, with ammonium oxidizing bacteria (AOB) dominating granule surface. High DO operation combined with sufficient ammonium supply ensured the faster growth of AOB, which should provide a competitive advantage to AOB in competing for habitable space (i.e., granule surface). Besides, the lower porosity, larger size, and more active extracellular polymeric substances (particularly proteins) production of granules was observed under the high DO condition. Overall, these findings supported the proposition that the switch from mixed to stratified distribution of nitrifiers in granule was primarily driven by their competition for habitable space rather than by oxygen-limitation.

A framework based on fundamental biochemical principles to engineer microbial community dynamics

Microbial communities are complex but there are basic principles we can apply to constrain the assumed stochasticity of their activity. By understanding the trade-offs behind the kinetic parameters that define microbial growth, we can explain how local interspecies dependencies arise and shape the emerging properties of a community. If we integrate these theoretical descriptions with experimental 'omics' data and bioenergetics analysis of specific environmental conditions, predictions on activity, assembly and spatial structure can be obtained reducing the a priori unpredictable complexity of microbial communities. This information can be used to define the appropriate selective pressures to engineer bioprocesses and propose new hypotheses which can drive experimental research to accelerate innovation in biotechnology.

Performance and granular characteristics of salt-tolerant aerobic granular reactors response to multiple hypersaline wastewater

Aerobic granular sludge (AGS) technology has been recognized as a promising alternative to alleviate the osmotic stress of hypersaline wastewater. However, the response of AGS process to composite hypersaline wastewater on removal performance and populations was yet to be understood. In this work, two sequenced batch reactors were operated in parallel in absence (R0) and presence (R1) of high concentration sulfate as proxy for single and mixed salts (30 g salt·L^-1) respectively. Results demonstrated that the presence of sulfate in hypersaline wastewater enhanced chemical oxygen demand (COD) and total nitrogen (TN) removals of 95.3% and 65.5% respectively with lower accumulations of nitrite. High-throughput 16 S rRNA gene sequencing technique elucidated that Denitromonas (31.6%) and Xanthomarina (17.0%) were the more dominant genera in AGS response to mixed salts with high sulfate and laid the biological basis for strengthening removal performance. The enrichment of halophilic Luteococcus (23.5%) in the AGS surface indicated the potential role of mixed salts in shaping the physical properties and surface population structure of AGS. Our work could facilitate the potential applications of AGS technology for industrial hypersaline wastewater treatment with complicated compositions.

Optimization of the intermittent aeration to improve the stability and flexibility of a mainstream hybrid partial nitrification-anammox system

Intermittent aeration is favorable for maintaining a long-term sewage partial nitrification-anammox (PN/A) process but the underlying mechanism is not yet fully understood. In this study, mainstream PN/A was established in an integrated fixed film activated sludge (IFAS) PN/A reactor and nitrite oxidization bacteria (NOB) activity was continuously suppressed. The suppression of NOB was significantly affected by the dissolved oxygen (DO) concentration during the aeration period as well as the duration of anoxic period. NOB was more suppressed in the hybrid system under a low DO level (0.5 mg/L) than under a high DO level (1.5-1.8 mg/L). Meanwhile, shortening the anoxic time from 40 to 20 min and keeping low DO during the intermittent aeration cycle could still suppress NOB activity, increasing the nitrogen removal rate by 40%. Biomass segregation was also enhanced by low DO, which favors the NOB inhibition in IFAS PN/A system. Overall, under an optimized intermittent aeration, a stable and high nitrogen removal efficiency (80-89%) with a nitrogen removal rate of 0.101 kg-N/(m³·d). This study is useful to supports the application of PN/A in sewage treatment.

Characteristics of sludge granulation and EPS production in development of stable partial nitrification

In this study, sequencing batch reactor (SBR) and aerobic upflow sludge bed reactor (AeUSB) were used to investigate the effects of sludge granulation on establishing partial nitrification (PN) process and explore the potential mechanism. The particle size was positively related to the NO₂^--N accumulating performance, satisfactory nitrite accumulation rate (NAR) of 96.4% and 77.3% were obtained with the appearance of micro-granules in SBR and AeUSB after day 105 and 83, respectively. To our knowledge, it is the first report describing the importance of extracellular polymeric substances (EPS) content and components, such as aromatic protein-like substances, by-product-like substances and special protein secondary structures (α-helices and 3-turn helices) for sludge granulation and even PN process. Quantitative microbial analysis suggested that ammonium oxidizing bacteria (AOB) predominated in nitrifying-bacterium in micro-granules system, guaranteeing a highly efficient PN process.

Long-term stability of partial nitritation-anammox for treatment of municipal wastewater in a moving bed biofilm reactor pilot system

Nitrogen removal from the mainstream of municipal wastewater with partial nitritation-anammox (PNA) would be highly beneficial with regard to the uses of energy and organic carbon. However, the challenges of process instability, low nitrogen removal rates (NRR) and unwanted aerobic nitrite oxidation need to be solved to reach large-scale implementation. Here, we have operated pilot-scale moving bed biofilm reactors (MBBRs) for mainstream treatment, together with sidestream treatment of sludge liquor from anaerobic digestors, for over 900 days to investigate process stability, reactor performance and microbial community structure at realistic conditions. The MBBR biofilm contained stable and high relative abundances of anammox bacteria (10-32%) consisting of two major Brocadia sp. populations, and several populations of aerobic ammonia-oxidising bacteria (AOB) within Nitrosomonas sp. (0.2-3.1%), as assessed by 16S rDNA amplicon sequencing. In addition, nitrite-oxidising bacteria (NOB) consisting of Nitrospira sp. (0.4-0.8%) and Nitrotoga sp. (up to 0.4%) were present. Nitrogen was removed at a peak rate of 0.66 g N m^-2 d^-1 (0.13 kg N m^-3 d^-1) with a nitrate production over ammonium consumption of 15% by the NOB, at operation with continuous aeration at 15 °C. However, during most periods with continuous aeration, the NRR was lower (≈ 0.45 g N m^-2 d^-1), with larger relative nitrate production (≈40%), presumably due to problems to maintain stable residual ammonium concentrations during wet-weather mainstream flows. Changing reactor operation to intermittent aeration decreased the NRR but did not help in suppressing the NOB. The study shows that with MBBRs, stable mainstream PNA can be attained at realistic NRR, but with need for post-treatment of nitrate, since effective NOB suppression was hard to achieve.

Granules abrasion cause deterioration of nitritation in a mainstream granular sludge reactor with high loading rate

Biomass detachment generally occurred in granular sludge systems. However, little is known about the influence of biomass detachment on the granules performing nitritation. Here, a granular sludge reactor with high loading rates (6.8 ± 0.4 kg N·m^-3·d^-1) was achieved at mainstream conditions. Though the low ratio control strategy was maintained, the deterioration of nitritation performance was observed after the further increase of air supply rates to 3.4 ± 0.2 L min^-1. In parallel with that, the loss of AOB and the proliferation of NOB was observed. Additionally, with the decrease of granules size and biomass concentration, the incomplete stratification of nitrifiers in the granules was confirmed by batch tests. All these results suggested that granules abrasion under the high shear stress conditions caused the detachment of external AOB and hence resulted in the deteriorated stratified structure of nitrifiers, which subsequently contributed to the proliferation of the internal NOB and the deterioration of nitritation. These findings highlight that the granules abrasion should be well controlled in the development of high-rate nitritation process with granular sludge.

Towards mainstream deammonification of municipal wastewater: Partial nitrification-anammox versus partial denitrification-anammox

The mainstream deammonification has been believed as a viable technology for the energy-neutral municipal wastewater treatment, which can be realized through two approaches known as partial nitrification-anammox (PN/AMX) and partial denitrification-anammox (PDN/AMX). However, large-scale applications of these deammonification processes for municipal wastewater treatment have been rarely reported thus far. Given such a situation, this review examined the mainstream PN/AMX and PDN/AMX processes with the focus on their engineering feasibility, economic viability and potential challenges. It was revealed that soluble COD and stable nitrite production were the main challenges for mainstream deammonification. Pre-capture of COD was essential for mitigating the competition between denitrifiers and anammox bacteria on nitrite, while NOB suppression and partial denitrification control to nitrite stage were critical issues for stable nitrite production in PN and PDN processes respectively. Compared to nitrification-denitrification, the unit oxygen demand for nitrogen removal in PN/AMX and PDN/AMX could be reduced by 57.3% and 47.7%, while the sludge production could also be cut off by 83.7% and 66.3% in PN/AMX and PDN/AMX respectively. These clearly showed the greater economic viability and environmental sustainability of PN/AMX against PDN/AMX. Consequently, more effort is needed to improve the engineering feasibility of large-scale mainstream deammonification for municipal wastewater treatment.