Comparing two start up strategies and the effect of temperature fluctuations on the performance of mainstream anammox reactors

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.

Feasibility of a return-sludge nursery concept for mainstream anammox biostimulation: creating optimal conditions for anammox to recover and grow in a parallel tank

To overcome limiting anammox activity, a return-sludge nursery concept is proposed. This concept blends reject water treated with partial nitritation with mainstream effluent to increase the temperature, N levels, and EC of the anammox nursery reactor, which sludge periodically passes through the return sludge line of the mainstream system. Various nursery frequencies were tested in two 2.5 L reactors, including 0.5-2 days of nursery treatment per 3.5-14 days of the total operation. Bioreactor experiments showed that nursery increased nitrogen removal rates during mainstream operation by 33-38%. The increased anammox activity can be partly (35-60%) explained by higher temperatures. Elevated EC, higher nitrogen concentrations, and a putative synergy and/or unknown factor were responsible for 15-16%, 12-14%, and 10-36%, respectively. A relatively stable microbial community dominated by "Candidatus Brocadia" was observed. This new concept boosted activity and sludge growth, which may facilitate mainstream anammox implementations based on partial nitritation/anammox or partial nitrification/denitratation/anammox.

A critical review on microbial ecology in the novel biological nitrogen removal process: Dynamic balance of complex functional microbes for nitrogen removal

The novel biological nitrogen removal process has been extensively studied for its high nitrogen removal efficiency, energy efficiency, and greenness. A successful novel biological nitrogen removal process has a stable microecological equilibrium and benign interactions between the various functional bacteria. However, changes in the external environment can easily disrupt the dynamic balance of the microecology and affect the activity of functional bacteria in the novel biological nitrogen removal process. Therefore, this review focuses on the microecology in existing the novel biological nitrogen removal process, including the growth characteristics of functional microorganisms and their interactions, together with the effects of different influencing factors on the evolution of microbial communities. This provides ideas for achieving a stable dynamic balance of the microecology in a novel biological nitrogen removal process. Furthermore, to investigate deeply the mechanisms of microbial interactions in novel biological nitrogen removal process, this review also focuses on the influence of quorum sensing (QS) systems on nitrogen removal microbes, regulated by which bacteria secrete acyl homoserine lactones (AHLs) as signaling molecules to regulate microbial ecology in the novel biological nitrogen removal process. However, the mechanisms of action of AHLs on the regulation of functional bacteria have not been fully determined and the composition of QS system circuits requires further investigation. Meanwhile, it is necessary to further apply molecular analysis techniques and the theory of systems ecology in the future to enhance the exploration of microbial species and ecological niches, providing a deeper scientific basis for the development of a novel biological nitrogen removal process.

Aeration strategies and temperature effects on the partial nitritation/anammox process for nitrogen removal: performance and bacterial community assessment

The partial nitritation/anammox process (PN/A) could be a promising alternative for nitrogen removal from high-strength wastewater. There is, however, a lack of information about suitable aeration and temperature for PN/A in single-stage reactors for high-strength wastewater, such as food waste (FW) digestate treatment. To this end, a laboratory-scale (10 L) partial nitritation/anammox sequencing batch reactor was operated for more than 230 days under four different intermittent aeration strategies and temperature variations (35°C and ambient temperature - 26-29°C) to investigate the feasibility of nitrogen removal from real FW digestate. High ammonium (NH4+-N) and total nitrogen (TN) removal median efficiencies of 81 and 63%, respectively (corresponding to median NH4+-N and TN loads removed of 76 and 67 g.m^-3.d^-1), were achieved when the aeration strategy comprised by 7 min/14 min off and an airflow rate of 0.050 L.min^-1.L_reactor^-1 was applied. Nitrogen removal efficiencies were not affected by temperature variations in southeastern Brazil. COD, chloride and organic nitrogen (520, 239 and 102.8 mg.L^-1, respectively) did not prevent PN/A. Changes of the bacterial community in response to aeration strategies were observed. Candidatus Brocadia dominated most of the time being more resistant to aeration and temperature changes than Candidatus Jettenia. This study demonstrated that optimizations of anoxic periods and airflow rate support PN/A with high nitrogen removal from FW digestate.

Challenges, solutions and prospects of mainstream anammox-based process for municipal wastewater treatment

Anaerobic ammonia oxidation (anammox) process has a promising application prospect for the mainstream deammonification of municipal wastewater due to its high efficiency and low energy consumption. In this paper, challenges and solutions of mainstream anammox-based process are summarized by analyzing the literature of recent ten years. Slow growth rate of anammox bacteria is a main challenge for mainstream anammox-based process, and enhancement of bacteria retention has been recognized to be necessary. Compared with directly increasing sludge retention time (SRT) with membrane bioreactors or sequencing batch reactors, culturing anammox bacteria in the form of biofilm or granule sludge is more promising for its feasibility of eliminating nitrite oxidizing bacteria (NOB). Besides, adding external electron donors or conductive materials and enriching the concentration of ammonia with absorption materials have also been proved helpful to improve the activity of anammox bacteria. Other challenges include the elimination of NOB and achieving ideal ratio of NH₄⁺ and NO₂^-. To solve these problems and achieve stable partial nitrification, composite control strategies based on low SRT and limited aeration are needed based on the special characteristics of ammonia oxidizing bacteria (AOB) and NOB. When treating actual wastewater, interference of low temperature and components in the influent is another problem. Relatively high activity of anammox bacteria has been realized after artificial acclimation at low temperature and the mechanism was also preliminary explored. Different pre-treatment sections have been designed to reduce the concentration of COD and S^2- from the influent. As for the nitrate produced by the anammox reaction, coupling processes are useful to reduce the concentration of nitrate in the effluent. In brief, suitable reactor and coupling process should be selected according to the temperature, influent quality and discharge targets of different regions. The future prospects of the mainstream anammox-based process are also put forward.

On anammox activity at low temperature: effect of ladderane composition and process conditions

The application of partial nitritation-anammox (PN/A) under mainstream conditions can enable substantial cost savings at wastewater treatment plants (WWTPs), but how process conditions and cell physiology affect anammox performance at psychrophilic temperatures below 15 °C remains poorly understood. We tested 14 anammox communities, including 8 from globally-installed PN/A processes, for (i) specific activity at 10-30 °C, (ii) composition of membrane lipids, and (iii) microbial community structure. We observed that membrane composition and cultivation temperature were closely related to the activity of anammox biomasses. The size of ladderane lipids and the content of bacteriohopanoids were key physiological components related to anammox performance at low temperatures. We also indicate that the adaptation of mesophilic cultures to psychrophilic regime necessitates months, but in some cases can take up to 5 years. Interestingly, biomass enriched in the marine genus "Candidatus Scalindua" displayed outstanding potential for nitrogen removal from cold streams. Collectively, our comprehensive study provides essential knowledge of cold adaptation mechanism, will enable more accurate modelling and suggests highly promising target anammox genera for inoculation and set-up of anammox reactors, in particular for mainstream WWTPs.

Evaluation of long-term preservation and reactivation efficiency of anaerobic ammonium oxidation (anammox) microorganisms based on activation energy

The preservation efficiency of mainstream (M-ANA) and sidestream anaerobic ammonium oxidation (anammox) (S-ANA) were evaluated based on their activation energy (E_a). The E_a of M-ANA cultivated under low nitrogen loads was lower than that of S-ANA, which greatly contributed to enhancing the viability of anammox during preservation at 4 °C. After preservation for 140 d, the decay rate (b_AN) of M-ANA ranged from 0.0012 to 0.0013/d; the b_AN of S-ANA was 0.0036-0.0041/d. The addition of hydrazine, which requires minimal energy to activate anammox metabolism, is highly beneficial for the viability of microorganisms. The low E_a of anammox contributes to efficient reactivation with rapid reactivation of heme c, and the addition of hydrazine makes the process more beneficial. Although the specific nitrogen removal rate of the M-SNA seed sludge was much lower than that of S-ANA, the rate of M-ANA became higher after 48 days of reactivation.

Low strength wastewater anammox start-up by stepwise decrement in influent nitrogen: Biofilm formation mechanism and mathematical modelling

The application of mainstream anammox process is hampered by its overlong start-up and instability under disturbance. A lab-scale mainstream anammox moving bed biofilm reactor (MBBR) was successfully started in 120 days with stepwise decrement in influent nitrogen concentration from sidestream to mainstream condition. The initial colonization by Candidatus Jettenia and filamentous fermenter Anaerolineaceae were potentially mediated by hydrophobic interaction and type IV pilus. Ca. Kuenenia with higher substrate affinity outcompeted Ca. Jettenia, and the predominant fermenters shifted to fermentative Ignavibacteriaceae in the mature biofilm. A novel mainstream anammox biofilm development (MABD) model was constructed to describe biofilm growth, population dynamics, and nitrogen removal performance. The simulation results suggested that higher inocula biomass (460-690 mgVSS·L^-1), relative abundance of low-affinity AnAOB in the inocula (e.g., Ca. Jettenia, 1.3-2%), and the early-stage solids retention time (45-68 days) were desired to form thicker biofilm and improve effluent quality during 120-day mainstream anammox MBBR start-up. The mechanistic insights into biofilm formation and predictive power of the newly developed MABD model are of importance to the design and operation of mainstream anammox processes towards more biofilm biomass and higher nitrogen removal efficiency.

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

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

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

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

Salinity effect on freshwater Anammox bacteria: Ionic stress and ion composition

The biggest challenge to apply Anammox to treat wastewater with elevated salt content is the inhibitory effect of salinity on freshwater Anammox bacteria (FAB). Most of the research into salinity inhibition has focused on the osmotic pressure effect, while the inhibitory effect and its mechanisms induced by ion composition are poorly understood. In this study, the individual and combined effect of NaCl, KCl and Na₂SO₄ on FAB (>99% belonging to Ca. Brocadia genera) were systematically investigated by batch tests. The corresponding responses of mRNA abundance of three functional genes (including nitrite reductase gene (nirS), hydrazine synthase gene (hzsB) and hydrazine dehydrogenase gene (hdh)) under different salt conditions were analyzed. The results indicated that NaCl, KCl and Na₂SO₄ have different inhibition effects, with the 50% inhibition at 0.106, 0.096 and 0.063 M, respectively. The combined inhibition of NaCl+KCl and NaCl+Na₂SO₄ on FAB were both synergistic; while the combined inhibition of NaCl+KCl+Na₂SO₄ was additive. The responses of mRNA (of genes: nirS, hzsB and hdh) suggested NaCl inhibited the transport of ammonium; Na₂SO₄ inhibited both nitrite and ammonium transport; high salinity inhibited functional enzyme activity. These results suggest both ionic stress and ion composition contributed to the observed inhibition.

Anammox Granules Acclimatized to Mainstream Conditions Can Achieve a Volumetric Nitrogen Removal Rate Comparable to Sidestream Systems

The implementation of mainstream anammox has gained increasing attention. In this study, the feasibility of using sidestream anammox granules to start up mainstream reactors was investigated by comparing two switching strategies. A maximum nitrogen removal potential of 3.6 ± 0.2 kg N m^-3 d^-1 was obtained for the reactor after direct switching to mainstream conditions (70 mg TN L^-1, 15 °C). Nevertheless, the reactor preacclimatized to 25 °C (Ma) exhibited a higher nitrogen removal potential of 7.0 ± 0.3 kg N m^-3 d^-1 at 15 °C, which is the highest volumetric nitrogen removal rate of mainstream anammox reactors to date. Candidatus Kuenenia stuttgartiensis was identified as the dominant anammox bacterium, and its relative abundance in two reactors remained stable throughout the whole operation (200 days). Moreover, with the aid of acclimatization, the activation energy was reduced and the specific growth rate became higher. These results indicated that the physiological evolution of the dominant anammox bacterium instead of interspecies selection was the main reason for the high potential during the switch to mainstream conditions. Therefore, using sidestream anammox granules as seed sludge to start up mainstream reactors was demonstrated to be feasible, and a switching strategy of acclimatization at 25 °C was recommended.

Inhibition of nitrite oxidizing bacterial activity based on low nitrite concentration exposure in an auto-recycling PN-Anammox process under mainstream conditions

For municipal wastewater with low temperature and ammonium, conventional oxygen-limited have difficulty achieving long-term stable inhibition of nitrite oxidizing bacteria (NOB) and stable nitritation. So a partial nitrification-anaerobic ammonium oxidation integrated reactor with independent partitions was used to investigate the feasibility of adding an auto-recycling system to promote low exposure of nitrite in the aerobic zone and to inhibit the NOB activity. The results showed that nitrite produced in the aerobic zone could be timely transported to the anaerobic zone for Anammox utilization, and the nitrite nitrogen concentration was diluted to keep within 1 mg/L in the aerobic zone by the effluent recycling. NOB growth was inhibited by nitrite deficiency. The maximum nitrogen removal rate of the reactor was 0.29 kg/(m³·d), and the nitrate nitrogen production rate of NOB was controlled within 0.04 kg/(m³·d). Nitrosomonas and Candidatus Kuenenia were found as functional species of ammonia-oxidizing bacteria and Anammox bacteria, respectively.

Effect of external hydrazine addition on anammox reactor start-up time

Hydrazine is an intermediate product of the anaerobic ammonium oxidation (Anammox) process where both ammonium and nitrite in wastewater are converted to nitrogen gas by bacteria. In this study the effect of external hydrazine addition (5, 10, 15, and 20 mg/L) on the start-up period of the Anammox process was studied using sequencing batch reactors (SBRs). The SBR with an addition of 10 mg/L hydrazine took only 7 weeks to stabilize and achieve the maximum removal of ammonium and nitrite, whereas the SBR without the addition of hydrazine took 12 weeks. The amount of Heme C extracted from the biomass indicated that externally added hydrazine accelerated the growth of Anammox bacteria and reduced the release of nitrous oxide gas from the reactors.

Nitrogen Removal Characteristics and Comparison of the Microbial Community Structure in Different Anaerobic Ammonia Oxidation Reactors

Nitrogen removal characteristics and the comparison of the microbial community structure were investigated in different anaerobic ammonia oxidation (Anammox) reactors: an anaerobic sequencing batch reactor (ASBR) and a biofilter reactor. The Anammox systems were inoculated with sludge from the second settling tank of a wastewater treatment plant in Guangzhou, China. After successful start up of Anammox, the microbial community structure of different Anammox reactors were studied through high-throughput sequencing. The results showed that anaerobic ammonium oxidation in the ASBR reactor could successfully start up after 134 days, while Anammox in the biofilter could start up after 114 days. In both systems, total nitrogen removal was at 80% after more than 200 days of operation. The diversity of denitrifying microorganisms was high in both reactors, with Planctomycetes as the main taxa. Anammox bacteria belonging to the genera Candidatus Anammoxoglobus and Kuenenia, were dominant in the ASBR, while all three genera of Candidatus, Anammoxoglobus, Kuenenia, and Brocadia, could be detected in the biofilter reactor. Therefore, the biofilter starts up faster than the ASBR, and contains richer species, which makes it more suitable to domesticate Anammox bacteria.