Mainstream double-anammox driven by nitritation and denitratation using a one-stage step-feed bioreactor with real municipal wastewater

Robustness of the anammox process at low temperatures and low dissolved oxygen for low C/N municipal wastewater treatment

Low water temperatures and ammonium concentrations pose challenges for anammox applications in the treatment of low C/N municipal wastewater. In this study, a 10 L-water bath sequencing batch reactor combing biofilm and suspended sludge was designed for low C/N municipal wastewater treatment. The nitrogen removal performance via partial nitrification anammox-(endogenous) denitrification anammox process was investigated with anaerobic-aerobic-anoxic mode at low temperatures and dissolved oxygen (DO). The results showed that with the decrease of temperature from 30 to 15℃, the influent and effluent nitrogen concentrations and nitrogen removal efficiencies were 73.7 ± 6.5 mg/L, 7.8 ± 2.8 mg/L, and 89.4 %, respectively, with aerobic hydraulic retention time of only 6 h and DO concentration of 0.2-0.5 mg/L. Among that, the stable anammox process compensated for the inhibitory effects of the low temperatures on the nitrification and denitrification processes. Notably, from 30 to 15℃, the anammox activity and relative abundance of the dominant Brocadia genus were increased from 39.7 to 45.5 mgN/gVSS/d and 7.3 to 12.0 %, respectively; the single gene expression level of the biofilm increased 9.0 times. The anammox bacteria showed a good adaptation to temperatures reduction. However, nitrogen removal by anammox was not improved by increasing DO (≥ 4 mg/L) at 8-4℃. Overall, the results of this study demonstrate the feasibility of the mainstream anammox process at low temperatures.

Recent advances of partial anammox by controlling nitrite supply in mainstream wastewater treatment through step-feed mode

At present, the step-feed process is a very active branch in practical application of mainstream wastewater treatment, and the anammox technology empowers the sustainable development and in-depth research of step-feed process. This review provides a systematically inspection of the realization and application of partial anammox process through step-feed mode, with a particular focus on controlling nitrite supply for anammox. The characteristics and advantages of step-feed mode in traditional management are reviewed. The unique organics utilization strategy by step-feed and indispensable intermittent aeration mode creates advantages for achieving nitritation (NH4+ → NO2-) and denitratation (NO3- → NO2-), providing flexible combination possibility with anammox. Additionally, the lab- or pilot-scale control strategies with different forms of anammox, including nitritation/anammox, denitratation/anammox, and double-anammox (combined nitritation/anammox and denitratation/anammox), are summarized. Finally, future directions and application perspectives on leveraging the relationship between flocs and biofilm, nitritation and denitratation, and different strains to maximize the anammox proportion in N-removal are proposed.

Increasing carbon to nitrogen ratio promoted anaerobic ammonia-oxidizing bacterial enrichment and advanced nitrogen removal in mainstream anammox system

The effects of fluctuating organic carbon to nitrogen (C/N) ratios on mainstream simultaneous partial nitrification, anammox, and denitrification (SNAD) process were studied over 376-day period. The nitrogen removal efficiency decreased from 85.0 ± 6.6 % to 75.8 ± 2.8 % as C/N ratio decreased (3.4 → 1.7), but increased to 82.0 ± 1.9 % when C/N ratio raised to 2.9 and to 78.4 ± 3.0 % when C/N ratio decreased again (2.9 → 2.1), indicating that high C/N ratios promoted nitrogen removal. As C/N ratio raised (1.7 → 2.9), anaerobic ammonia-oxidizing bacteria (AnAOB) abundance increased from 1.3 × 109 to 2.0 × 109 copies/L, which explained the improved nitrogen removal. With an elevated C/N ratio, partial nitrification and endogenous partial denitrification reactions were enhanced, providing more nitrite for AnAOB. Additionally, the aerobic_chemoheterotrophy function and particle sizes increased, forming more stable anoxic microenvironment for AnAOB. Overall, increasing C/N ratio promoted the stability of mainstream SNAD.

Analysis of full nitrification performance and optimization of reaction properties using N and O isotope fractionation

Isotopic fractionation properties have been successfully applied to identify the distribution and fate of nitrogen in ecosystems, revealing the dynamic response of N and O elements during nitrogen transport and transformation. However, only a few studies used the dual isotope technology in activated sludge treatment of domestic wastewater and many aspects of the process are unclear. Here, we use the dual isotope techniques to increase the understanding of the substrates required for nitrification reactions, nitrification performance, and process operation. Mixed sludge was successfully enriched with nitrifying bacteria in a continuous culture, and three dissolved oxygen (DO; 0.2-0.4, 3-4, and 7-8 mg/L) and three temperature levels (18 ± 1, 25 ± 1, and 33±1 °C) were tested for efficiency of nitrate nitrogen accumulation. Both δ15NNO3 and δ18ONO3 showed a gradual increase with an increase in DO or temperature, the increase in DO slowed down the fractionation effect of isotopes, and the increase in temperature reduced the variability in N and O utilization. The slope of δ15NNO3:δ18ONO3 gradually approached 1 with the increase in DO (<7 mg/L) or in temperature, and the optimal range of DO and temperature were accurately judged to strengthen the denitrification performance of nitrifying bacteria. δ18OH2O was successfully taken up to form NO2--N and NO3--N with 74 and 91% replacement rates, respectively, indicating that DO and H2O jointly completed the formation of nitrate nitrogen during the long nitrification process. In summary, the in situ dual isotope technology can help optimize the influence of environmental factors on nitrification performance to guide the long-term stable operation of nitrification reactions in sludge treatment and provide a reliable basis for complex activated sludge nitrification systems.

A critical review of improving mainstream anammox systems: Based on macroscopic process regulation and microscopic enhancement mechanisms

Full-scale anaerobic ammonium oxidation (anammox) engineering applications are vastly limited by the sensitivity of anammox bacteria to the complex mainstream ambience factors. Therefore, it is of great necessity to comprehensively summarize and overcome performance-related challenges in mainstream anammox process at the macro/micro level, including the macroscopic process variable regulation and microscopic biological metabolic enhancement. This article systematically reviewed the recent important advances in the enrichment and retention of anammox bacteria and main factors affecting metabolic regulation under mainstream conditions, and proposed key strategies for the related performance optimization. The characteristics and behavior mechanism of anammox consortia in response to mainstream environment were then discussed in details, and we revealed that the synergistic nitrogen metabolism of multi-functional bacterial genera based on anammox microbiome was conducive to mainstream anammox nitrogen removal processes. Finally, the critical outcomes of anammox extracellular electron transfer (EET) at the micro level were well presented, carbon-based conductive materials or exogenous electron shuttles can stimulate and mediate anammox EET in mainstream environments to optimize system performance from a micro perspective. Overall, this review advances the extensive implementation of mainstream anammox practice in future as well as shedding new light on the related EET and microbial mechanisms.

Coupled systems of pre-denitrification and partial nitritation/anammox improved functional microbial structure and nitrogen removal in treating swine manure digestate

This study evaluated the functional activity and microbial structure of a pre-denitrification and single-stage partial nitritation/anammox process (DB-SNAP) coupled system for effectively treating swine manure digestate (SMD). At influent ammonium concentrations of (1000 to 1500) mg/L, the pre-denitrification reactor increased the nitrogen removal efficiency (NRE) by 5%, resulting in an average NRE of 96%. The DB-SNAP and nitrogen-limited strategy facilitated the rapid adoption of anammox bacteria (AnAOB) in the SMD, maintaining a high specific rate of 0.3gN/gVSS/d. A high secretion of tightly bound extracellular polymeric substances (76 mg/gVSS to 102 mg/gVSS) promoted micro-granule aggregation and stability. Moreover, Ca. Kuenenia, an AnAOB genus, was highly enriched from 21% to (27 to 30) %, whereas Nitrospira, a nitrite-oxidizing bacteria, was significantly suppressed to (0 to 0.05) %. These findings will provide valuable guidance in implementing the anammox process in swine wastewater treatment.

Microbial ecological responses of partial nitritation/anammox granular sludge to real water matrices and its potential application

Granular sludges are commonly microbial aggregates used to apply partial nitritation/anammox (PN/A) processes during efficient biological nitrogen removal from ammonium-rich wastewater. Considering keystone taxa of anammox bacteria (AnAOB) in granules and their sensitivity to unfavorable environments, it is essential to investigate microbial responses of autotrophic PN/A granules to real water matrices containing organic and inorganic pollutants. In this study, tap water, surface water, and biotreated wastewater effluents were fed into a series of continuous PN/A granular reactors, respectively, and the differentiation in functional activity, sludge morphology, microbial community structure, and nitrogen metabolic pathways was analyzed by integrating kinetic batch testing, size characterization, and metagenomic sequencing. The results showed that feeding of biotreated wastewater effluents causes significant decreases in nitrogen removal activity and washout of AnAOB (dominated by Candidatus Kuenenia) from autotrophic PN/A granules due to the accumulation of heavy metals and formation of cavities. Microbial co-occurrence networks and nitrogen cycle-related genes provided evidence for the high dependence of symbiotic heterotrophs (such as Proteobacteria, Chloroflexi, and Bacteroidetes) on anammox metabolism. The enhancement of Nitrosomonas nitritation in the granules would be considered as an important contributor to greenhouse gas (N₂O) emissions from real water matrices. In a novel view on the application of microbial responses, we suggest a bioassay of PN/A granules by size characterization of red-color cores in ecological risk assessment of water environments.

Performance of anammox enchanced by pulsed electric fields under added organic carbon sources using integrated network and metagenomics analyses

This paper aims to investigate the function of a pulsed electric field (PEF) in the anaerobic ammonia oxidation (anammox) process after adding certain chemical oxygen demand (COD) through integrated network and metagenomics analyses. The findings showed that the presence of COD was detrimental to anammox, but PEF could significantly reduce the adverse effect. The total nitrogen removal in the reactor for applying PEF was 16.99% higher on average than the reactor for only dosing COD. Additionally, PEF upgraded the abundance of anammox bacteria subordinate to the phylum Planctomycetes by 9.64%. The analysis of molecular ecological networks promulgated that PEF resulted in an increase in network scale and topology complexity, thereby boosting the potential collaboration of the communities. Metagenomics analyses demonstrated that PEF dramatically promoted anammox central metabolism in the presence of COD, specifically enhancing pivotal N functional genes (hzs, hdh, amo, hao, nas, nor and nos).

Enhancing nitrogen removal of carbon-limited municipal wastewater in step-feed biofilm batch reactor through integration of anammox

The biological nitrogen removal of municipal wastewater was successfully improved by integrating anammox in a step-feed sequencing biofilm batch reactor. Despite fluctuating influent carbon to nitrogen ratio (1.9-5.1) and decreasing temperature (24.1-16.3 ℃), nitrogen removal efficiency of 95.9 ± 1.4 % and nitrogen removal rate of 0.23 ± 0.02 kg N/(m³·d) were successfully maintained without requirement of external carbon sources. The advanced removal performance was mainly attributed to the enhanced anammox. Anammox bacteria presented a high relative abundance (42.9% in biofilms, 1.5% in flocs) and anammox activity was as high as 5.42 ± 0.12 mg N/(g volatile suspended solids·h). Further analysis suggested that flexible control of influent organic and ammonium through step-feeding could provide multiple substrate supply for anammox reaction, potentially resulting in stable combination of anammox with hybrid-nitrite-shunt processes. Overall, this study provides a promising anammox-related application with simple-control step-feed strategy for enhanced and stable nitrogen removal from carbon-limited municipal wastewater.

Resilience of anammox application from sidestream to mainstream: A combined system coupling denitrification, partial nitritation and partial denitrification with anammox

Anaerobic ammonium oxidation (anammox) is a potential process to achieve the neutralization of energy and carbon. Due to the low temperature and variation of municipal sewage, the application of mainstream anammox is hard to be implemented. For spreading mainstream anammox in practice, several key issues and bottlenecks including the start-up, stable NO₂^--N supply, maintenance and dominance of AnAOB with high activity, prevention of NO₃^--N buildup, reduction of sludge loss, adaption to the seasonal temperature and alleviation of COD impacts on AnAOB are discussed and summarized in this review in order to improve its startup, stable operation and resilience of mainstream anammox. Hence a combined biological nitrogen removal (CBNR) system based on conventional denitrification, shortcut nitrification-denitrification, Partial Nitritation and partial Denitrification combined Anammox (PANDA) process through the management of organic matter and nitrate is proposed correspondingly aiming at adaptation to the variations of seasonal temperature and pollutants in influent.

Excellent anammox performance driven by stable partial denitrification when encountering seasonal decreasing temperature

Effluent quality deterioration caused by seasonal temperature reductions in wastewater treatment systems using partial anammox technology is a challenge that cannot be ignored. Here, relationships of denitrification and anammox under decreasing temperature were investigated in an anoxic moving bed biofilm reactor (MBBR). Compared with stable partial-denitrification (NO3- → NO2-), the NO2- reduction to N2 was considerably inhibited when the temperature decreased, conversely helping to the competition of NO2- for anammox. Namely, this transformation provided sufficient substrates for anammox bacteria. Although the TIN removal decreased slightly, anammox contribution was robustly maintained at 91.3 ± 6.6 %, even increased. High-throughput sequencing results revealed that anammox bacteria were enriched (0.56 % to 1.22 %). Moreover, qPCR results showed that increased ratio of hzsB/(nirK + nirS) further supported anammox gained an enhancement. This study demonstrated partial-denitrification/anammox process using anoxic MBBR could maintain stable autotrophic nitrogen removal contribution when encountering temperature decrease, providing a new perspective on the application of mainstream anammox.

Stable enhanced nitrogen removal from low COD/N municipal wastewater via partial nitrification-anammox in the traditional continuous anoxic/oxic process through bio-augmentation of partial nitrification sludge under decreasing temperatures

The application of partial nitrification-anammox (PNA) in continuous flow processes for treating low COD/N (C/N) sewage remains a critical challenge. Here, a traditional continuous anoxic/oxic (A/O) process was operated to investigate nitrogen removal from municipal wastewater by the bio-augmentation of partial nitrification sludge combined with the inoculation of biocarriers under decreasing temperatures. Stable enhanced nitrogen removal via PNA was achieved. The average total inorganic nitrogen in influent and effluent was 44.3 and 7.1 mg N/L under a low C/N ratio (3.4) and a short hydraulic retention time (8.2 h). The bio-augmentation of partial nitrification sludge enhanced the PNA process under low temperatures (16.9 ± 0.6 °C). The nitrogen removal efficiency remained stable at 83.3 ± 5.7 % as the temperature decreased from 29.1 to 16.3 °C, and the relative abundance of Ca. Brocadia in carrier biofilms increased from 2.22 % to 4.31 % and 3.27 % in two aerobic chambers after 70 days of operation.

Simultaneous nitrogen and phosphorus removal from municipal wastewater by Fe(III)/Fe(II) cycling mediated partial-denitrification/anammox

The efficient removal of nitrogen and phosphorus remains challenging for traditional wastewater treatment. In this study, the feasibility for enhancing the partial-denitrification and anammox process by Fe (III) reduction coupled to anammox and nitrate-dependent Fe (II) oxidation was explored using municipal wastewater. The nitrogen removal efficiency increased from 75.5 % to 83.0 % by adding Fe (III). Batch tests showed that NH₄⁺-N was first oxidized to N₂ or NO₂^--N by Fe (III), then NO₃^--N was reduced to NO₂^--N and N₂ by Fe (II), and finally, NO₂^--N was utilized by anammox. Furthermore, the performance of phosphorus removal improved by Fe addition and the removal efficiency increased to 78.7 %. High-throughput sequencing showed that the Fe-reducing bacteria Pseudomonas and Thiobacillus were successfully enriched. The abundance of anammox bacterial increased from 0.03 % to 0.22 % by multiple nitrite supply pathways. Fe addition presents a promising pathway for application in the anammox process.

Enhanced nitrogen removal from municipal wastewater via a novel combined process driven by partial nitrification/anammox (PN/A) and partial denitrification/anammox (PD/A) with an ultra-low hydraulic retention time (HRT)

Anaerobic ammonia oxidation (Anammox) is a highly productive research area in municipal wastewater treatment. A novel combined process driven by partial nitrification/anammox (PN/A) and partial denitrification/anammox (PD/A) was established in this paper using a sequencing batch reactor (SBR) and two up-flow sludge beds (USBs). Municipal wastewater after carbon removal pretreatment in SBR entered PN/A-USB. PN/A process was initiated and enhanced by optimizing the intermittent aeration mode under low dissolved oxygen (DO). After enhancing and stabilizing the PD/A process, PN/A effluent entered the PD/A-USB along with raw municipal wastewater at a ratio of 4:1 and the combined system was established. Through this, this study achieved a nitrogen removal efficiency (NRE) of 84.9 % from municipal wastewater at an ultra-low total hydraulic residence time (HRT) of 3.9 h. Candidatus Brocadia (1.8 % in PN/A, 1.0 % in PD/A) was the only functional anammox bacterium in the combined process.

A review on upgrading of the anammox-based nitrogen removal processes: Performance, stability, and control strategies

The anaerobic ammonia oxidation (anammox) process is a promising biological nitrogen removal technology. However, owing to the sensitivity and slow cell growth of anammox bacteria, long startup time and initially low nitrogen removal rate (NRR) are still limiting factors of practical applications of anammox process. Moreover, nitrogen removal efficiency (NRE) is often lower than 88 %. This review summarizes the most common methods for improving NRR by increasing microorganism concentration, and modifying reactor configuration. Recent integrated anammox-based systems were evaluated, including hydroxyapatite (HAP)-enhanced one-stage partial nitritation/anammox (PNA) process for a high NRR of over 2 kg N/m3/d at 25 °C, partial denitrification/anammox (PDA) process, and simultaneous partial nitrification, anammox, and denitrification process for a high NRE of up to 100 %. After discussing the challenges for the application of these systems critically, a combined system of anaerobic digestion, HAP-enhanced one-stage PNA and PDA is proposed in order to achieve a high NRR, high NRE, and phosphorus removal simultaneously.

Enhancing nitrogen removal through directly integrating anammox into mainstream wastewater treatment: Advantageous, issues and future study

Anaerobic ammonium oxidation (anammox) has great potential to be applied to the process of nitrogen removal from mainstream wastewater. However, directly applying complete anammox to the mainstream is typically hindered by low temperatures, a low ammonia concentration, and high organic matter concentrations. Directly integrating anammox into mainstream treatment by enhancing the in-situ enrichment of anammox bacteria in wastewater treatment plants (WWTPs) could effectively improve the nitrogen removal efficiency and reduce the treatment cost. A certain anammox bacteria abundance in full-scale WWTPs provides the feasibility of directly integrating anammox into mainstream treatment and realizing partial mainstream anammox. The technical development status of partial anammox and the mechanisms of achieving partial mainstream anammox by aeration and organic control are summarized. This review provides an enhanced understanding of this novel technical route of partial mainstream anammox treatment for improving the quality, performance, and prospects for this technology to be used in upgrading WWTPs.

Accelerated stabilization of high solid sludge by thermal hydrolysis pretreatment in autothermal thermophilic aerobic digestion (ATAD) process

Autothermal thermophilic aerobic digestion (ATAD) is a rapid biological treatment technology for sludge stabilization. To improve digestion efficiency and shorten stabilization time, thermal hydrolysis pretreatment was employed before ATAD of high solid sludge. The results showed that accelerated stabilization of high solid sludge (total solid = 10.1%) was achieved by thermal hydrolysis pretreatment with volatile solid removal efficiency of 40.3% after 8 days of ATAD, 11 days earlier than unpretreated sludge. The enhanced release and hydrolysis of intracellular organics resulted in a solubilization degree of 45.3%. The reduced sludge viscosity and improved fluidity after thermal hydrolysis facilitated mixing, aeration and organics degradation during ATAD. Excitation emission matrix analysis indicated that the fluorescence intensity of soluble microbial byproduct and tyrosine-like protein increased markedly after thermal hydrolysis and decreased after ATAD. The proportion of high molecular weight (MW > 10 kDa) substances in the supernatant increased significantly after thermal hydrolysis, while the low MW (MW < 1 kDa) substances decreased after ATAD. The significant difference in microbial composition between the pretreatment and control groups elucidated the accelerated sludge stabilization under thermal hydrolysis. This work provides an efficient and practical strategy to achieve rapid stabilization of high solid sludge.

Advanced nitrogen removal from real municipal wastewater by multiple coupling nitritation, denitritation and endogenous denitritation with anammox in a single suspended sludge bioreactor

Achieving advanced nitrogen removal based on anammox for treating mainstream municipal wastewater in a single suspended sludge bioreactor is a challenging research topic. In this study, multiple coupling nitritation, denitritation and endogenous denitritation with anammox (PNA-(E)PDA) was simultaneously achieved in a 10 L step-feed bioreactor, which enhanced stable nitrogen removal. After 223 days of operation, the total nitrogen concentrations of the influent and effluent were 70.7 ± 6.1 and 4.3 ± 1.8 mg/L, respectively, when treating municipal wastewater even at C/N ratio of 2.24 with only 5 h of aerobic time (DO: 0.5-0.8 mg/L). After the evolution of nitritation/anammox to PNA-(E)PDA, the contribution of anammox to nitrogen removal increased to 78.6% and the anammox activity increased from 4.3 ± 0.2 to 15.2 ± 0.7 mg NH₄⁺-N/gVSS/d. qPCR results showed that the abundance of anammox bacteria increased from 4.1 × 10⁹ to 4.5 × 10¹⁰ copies/ (g VSS). High-throughput sequencing further revealed that the relative abundance of Candidatus Brocadia, the dominant anammox genus, increased from 0.09 to 0.46%. Based on the strong competitiveness of anammox on nitrite, this novel PNA-(E)PDA process provides a potential strategy for enriching anammox bacteria in municipal wastewater treatment plants.

Mutual boost of granulation and enrichment of anammox bacteria in an anaerobic/oxic/anoxic system as the temperature decreases when treating municipal wastewater

Low temperature is an important factor affecting the municipal wastewater treatment systems. The aim of this study was tracking the variations in the abundance of anammox bacteria (AnAOB) and the sludge form as the temperature decreased. Mutual boost of granulation and enrichment of AnAOB was achieved even though the temperature dropped from 20.4 °C to 12.9 °C. The average particle size of the sludge increased from 128.5 μm to 245.6 μm. With low dissolved oxygen (DO) aeration (0.2-0.5 mg/L) and short oxic hydraulic retention time (HRT) (5 h), nitritation in the anaerobic/oxic/anoxic (AOA) system was stable enough to provide NO₂^- for AnAOB. Ca. Brocadia, a type of typical AnAOB, was enriched from 0.03% to 0.24% in the suspended sludge and reached 16.09% in the granular sludge. Overall, this study presents the prospects of anammox and granule technologies when treating municipal wastewater at a low temperature.

Metagenomics uncovers the effect of nitrogen-doped graphene on anammox consortia and microbial function

The effect of 50 mg/L nitrogen-doped graphene (N-G) on anammox microbial guild was studied by metagenomics in this paper. The continuous experiment results showed the average NRE improved by 17.57% with N-G addition. The metagenomic analysis revealed N-G significantly increased the relative abundance of dominant AnAOB (Candidatus Kuenenia) from 18.10% to 28.30%. And the FISH assay further manifested N-G promoted the growth of AnAOB biomass. Meanwhile, metagenomics indicated that N-G enriched the abundance of genes (Hzs, Hdh, NosZ, NorB, NirK, NirS and NrfA) involved in nitrogen metabolism to varying degrees. Furthermore, N-G not only improved the microbial functionality in terms of "Metabolism", but markedly upregulated the abundance of c-di-GMP synthesized genes and genes related to quinolone signal molecule, which contributed to more EPS content and better sludge settleability. In brief, this study provided a novel perspective for anammox biomass enrichment, which may be valuable for practical engineering applications of anammox.