NOB suppression strategies in a mainstream membrane aerated biofilm reactor under exceptionally low lumen pressure

Establishing mainstream partial nitrification in the membrane aerated biofilm reactor by limiting the oxygen concentration in the biofilm

The proliferation of nitrite oxidizing bacteria (NOB) still remains as a major challenge for nitrogen removal in mainstream wastewater treatment process based on partial nitrification (PN). This study investigated different operational conditions to establish mainstream PN for the fast start-up of membrane aerated biofilm reactor (MABR) systems. Different oxygen controlling strategies were adopted by employing different influent NH4+-N loads and oxygen supply strategies to inhibit NOB. We indicated the essential for NOB suppression was to reduce the oxygen concentration of the inner biofilm and the thickness of aerobic biofilm. A higher NH4+-N load (7.4 g-N/(m2·d)) induced higher oxygen utilization rate (14.4 g-O2/(m2·d)) and steeper gradient of oxygen concentration, which reduced the thickness of aerobic biofilm. Employing closed-end oxygen supply mode exhibited the minimum concentration of oxygen to realize PN, which was over 46% reduction of the normal open-end oxygen mode. Under the conditions of high NH4+-N load and closed-end oxygen supply mode, the microbial community exhibited a comparative advantage of ammonium oxidizing bacteria over NOB in the aerobic biofilm, with a relative abundance of Nitrosomonas of 30-40% and no detection of Nitrospira. The optimal fast start-up strategy was proposed with open-end aeration mode in the first 10 days and closed-end mode subsequently under high NH4+-N load. The results revealed the mechanism of NOB inhibition on the biofilm and provided strategies for a quick start-up and stable mainstream PN simultaneously, which poses great significance for the future application of MABR.

Lumen air pressure regulated multifunctional microbiotas in membrane-aerated biofilm reactors for simultaneous nitrogen removal and antibiotic elimination from aquaculture wastewater

In this study, two membrane-aerated biofilm reactors (MABRs) were constructed: one solely utilizing biofilm and another hybrid MABR (HMABR) incorporating both suspended-sludge and biofilm to treat low C/N aquaculture wastewater under varying lumen air pressure (LAP). Both HMABR and MABR demonstrated superior nitrogen removal than conventional aeration reactors. Reducing LAP from 10 kPa to 2 kPa could enhance denitrification processes without severely compromising nitrification, resulting in an increase in total inorganic nitrogen (TIN) removal from 50.2±3.1 % to 71.6±1.0 %. The HMABR exhibited better denitrification efficacy than MABR, underscoring its potential for advanced nitrogen removal applications. A decline in LAP led to decreased extracellular polymeric substance (EPS) production, which could potentially augment reactor performance by minimizing mass transfer resistance while maintaining microbial matrix stability and function. Gene-centric metagenomics analysis revealed decreasing LAP impacted nitrogen metabolic potentials and electron flow pathways. The enrichment of napAB at higher LAP and the presence of complete ammonia oxidation (Comammox) Nitrospira at lower LAP indicated aerobic denitrification and Comammox processes in nitrogen removal. Multifunctional microbial communities developed under LAP regulation, diversifying the mechanisms for simultaneous nitrification-denitrification. Increased denitrifying gene pool (narGHI, nirK, norB) and enzymatic activity at a low LAP can amplify denitrification by promoting denitrifying genes and electron flow towards denitrifying enzymes. Sulfamethoxazole (SMX) was simultaneously removed with efficiency up to 80.2 ± 3.7 %, mainly via biodegradation, while antibiotic resistome and mobilome were propagated. Collectively, these findings could improve our understanding of nitrogen and antibiotic removal mechanisms under LAP regulation, offering valuable insights for the effective design and operation of MABR systems in aquaculture wastewater treatment.

High-rate nitrogen removal by partial nitritation/anammox with a single-stage membrane-aerated biofilm reactor

In this study, a membrane aerated biofilm reactor (MABR) coupled partial nitritation/anammox (PN/A) system was established for high-rate nitrogen removal. Results showed that the nitrogen removal efficiency of 90.34% was finally obtained when influent ammonia increased from 150 mg L-1 to 300 mg L-1. Based on the fluorescence spectroscopy technology, the raised hydrophobicity tryptophan in extracellular polymeric substances (EPS) promoted biofilm formation and bacteria aggregation. 16S rRNA gene amplicon sequencing revealed that the relative abundance of AOB and AnAOB was also enhanced by ammonia (Nitrosomonas and Candidatus Brocadia increased by 6.02 % and 10.06 % in biofilm, respectively), which further facilitated nitrogen removal efficiency. Furthermore, the key functional genes involved in partial nitritation and anammox, especially hao and nirK, up-regulated by 1.31 and 1.26 times, respectively, accelerating the electron generation and consumption. Therefore, raising influent ammonia content intensified microbial electron transfer behavior and high-rate nitrogen metabolism.

How to Provide Nitrite Robustly for Anaerobic Ammonium Oxidation in Mainstream Nitrogen Removal

Innovation in decarbonizing wastewater treatment is urgent in response to global climate change. The practical implementation of anaerobic ammonium oxidation (anammox) treating domestic wastewater is the key to reconciling carbon-neutral management of wastewater treatment with sustainable development. Nitrite availability is the prerequisite of the anammox reaction, but how to achieve robust nitrite supply and accumulation for mainstream systems remains elusive. This work presents a state-of-the-art review on the recent advances in nitrite supply for mainstream anammox, paying special attention to available pathways (forward-going (from ammonium to nitrite) and backward-going (from nitrate to nitrite)), key controlling strategies, and physiological and ecological characteristics of functional microorganisms involved in nitrite supply. First, we comprehensively assessed the mainstream nitrite-oxidizing bacteria control methods, outlining that these technologies are transitioning to technologies possessing multiple selective pressures (such as intermittent aeration and membrane-aerated biological reactor), integrating side stream treatment (such as free ammonia/free nitrous acid suppression in recirculated sludge treatment), and maintaining high activity of ammonia-oxidizing bacteria and anammox bacteria for competing oxygen and nitrite with nitrite-oxidizing bacteria. We then highlight emerging strategies of nitrite supply, including the nitrite production driven by novel ammonia-oxidizing microbes (ammonia-oxidizing archaea and complete ammonia oxidation bacteria) and nitrate reduction pathways (partial denitrification and nitrate-dependent anaerobic methane oxidation). The resources requirement of different mainstream nitrite supply pathways is analyzed, and a hybrid nitrite supply pathway by combining partial nitrification and nitrate reduction is encouraged. Moreover, data-driven modeling of a mainstream nitrite supply process as well as proactive microbiome management is proposed in the hope of achieving mainstream nitrite supply in practical application. Finally, the existing challenges and further perspectives are highlighted, i.e., investigation of nitrite-supplying bacteria, the scaling-up of hybrid nitrite supply technologies from laboratory to practical implementation under real conditions, and the data-driven management for the stable performance of mainstream nitrite supply. The fundamental insights in this review aim to inspire and advance our understanding about how to provide nitrite robustly for mainstream anammox and shed light on important obstacles warranting further settlement.

Unraveling microbial characteristics of simultaneous nitrification, denitrification and phosphorus removal in a membrane-aerated biofilm reactor

This study describes the simultaneous removal of carbon, ammonium, and phosphate from domestic wastewater by a membrane-aerated biofilm reactor (MABR) which was operated for 360 days. During the operation, the maximum removal efficiencies of chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) reached 93.1%, 83.98%, and 96.41%, respectively. Statistical analysis showed that the MABR could potentially treat wastewater with a high ammonium concentration and a relatively low C/N ratio. Dissolved oxygen and multiple pollutants, including ammonium, carbon, phosphate, and sulfate, shaped the structure of the microbial community in the MABR. High throughput sequencing uncovered the crucial microbiome in ammonium transformation in MABR. Phylogenetic analysis of the ammonia monooxygenase (amoA) genes revealed an important role for comammox Nitrospira in the nitrification process. Diverse novel phosphate-accumulating organisms (Thauera, Bacillus, and Pseudomonas) and sulfur-oxidizing bacteria (Thiobacillus, Thiothrix and Sulfurimonas) were potentially involved in denitrification in MABR. The results from this study suggested that MABR could be a feasible system for the simultaneous removal of nitrogen, carbon, phosphorus, and sulfur from sewage water.

Challenges of suppressing nitrite-oxidizing bacteria in membrane aerated biofilm reactors by low dissolved oxygen control

Membrane aerated biofilm reactor (MABR) and shortcut nitrogen removal are two types of solutions to reduce energy consumption in wastewater treatment, with the former improving the aeration efficiency and the latter reducing the oxygen demand. However, integrating these two solutions, i.e., achieving shortcut nitrogen removal in MABR, is challenging due to the difficulty in suppressing nitrite-oxidizing bacteria (NOB). In this study, four MABRs were established to demonstrate the feasibility of initiating, maintaining, and restoring NOB suppression using low dissolved oxygen (DO) control, in the presence and absence of anammox bacteria, respectively. Long-term results revealed that the strict low DO (< 0.1 mg/L) in MABR could initiate and maintain stable NOB suppression for more than five months with nitrite accumulation ratio above 90 %, but it was unable to re-suppress NOB once they prevailed. Moreover, the presence of anammox bacteria increased the threshold of DO level to maintain NOB suppression in MABRs, but it was still incapable to restore the deteriorated NOB suppression in conjunction with low DO control. Mathematical modelling confirmed the experimental results and further explored the differences of NOB suppression in conventional biofilms and MABR biofilms. Simulation results showed that it is more challenging to maintain stable NOB suppression in MABRs compared to conventional biofilms, regardless of biofilm thickness or influent nitrogen concentration. Kinetic mechanisms for NOB suppression in different types of biofilms were proposed, suggesting that it is difficult to wash out NOB developed in the innermost layer of MABR biofilms because of the high oxygen level and low sludge wasting rate. In summary, this study systematically demonstrated the challenges of NOB suppression in MABRs through both experiments and mathematical modelling. These findings provide valuable insights into the applications of MABRs and call for more studies in developing effective strategies to achieve stable shortcut nitrogen removal in this energy-efficient configuration.

Free ammonia-free nitrous acid based partial nitrification in sequencing batch membrane aerated biofilm reactor

Membrane aerated biofilm reactor (MABR) has attracted a lot of attention as an energy-efficient integrated nitrogen removing technology in recent years. However, it is lacking of understanding to realize stable partial nitrification in MABR because of its unique oxygen transfer mode and biofilm structure. In this study, free ammonia (FA) and free nitrous acid (FNA) based control strategies for partial nitrification with low NH₄⁺-N concentration were proposed in a MABR of sequencing batch mode. The MABR was operated for over 500 days under different influent NH₄⁺-N concentrations. With the influent NH₄⁺-N of around 200 mg-N/L, partial nitrification could be established with relatively low concentration of FA (0.4-2.2 mg-N/L) which suppressed nitrite oxidizing bacteria (NOB) on the biofilm. With lower influent NH₄⁺-N concentration of around 100 mg-N/L, the FA concentration was lower and strengthened suppression strategies based on FNA were needed. With the final pH of operating cycles below 5.0, the FNA produced in the sequencing batch MABR could stabilize partial nitrification by eliminating NOB on the biofilm. Since the activity of ammonia oxidizing bacteria (AOB) was lower without the blow-off of dissolved carbon dioxide in the bubbleless MABR, longer hydraulic retention time was required to reach a low pH for high concentration of FNA to suppress NOB. After exposures to FNA, the relative abundance of Nitrospira was decreased by 94.6%, while the abundance of Nitrosospira increased greatly which became another dominant AOB genus in addition to Nitrosomonas.

Membrane-aerated biofilm reactor (MABR): recent advances and challenges

Membrane-aerated biofilm reactor (MABR) has been considered as an innovative technology to solve aeration issues in conventional bioreactors. MABR uses a membrane to supply oxygen to biofilm grown on the membrane surface. MABR can perform bubbleless aeration with high oxygen transfer rates, which can reduce energy requirements and expenses. In addition, a unique feature of counter-diffusion creates a stratified biofilm structure, allowing the simultaneous nitrification–denitrification process to take place in a single MABR. Controlling the biofilm is crucial in MABR operation, since its thickness significantly affects MABR performance. Several approaches have been proposed to control biofilm growth, such as increasing shear stress, adding chemical agents (e.g., surfactant), using biological predators to suppress microorganism growth, and introducing ultrasound cavitation to detach biofilm. Several studies also showed the important role of membrane properties and configuration in biofilm development. In addition, MABR demonstrates high removal rates of pollutants in various wastewater treatments, including in full-scale plants. This review presents the basic principles of MABR and the effect of operational conditions on its performance. Biofilm formation, methods to control its thickness, and membrane materials are also discussed. In addition, MABR performance in various applications, full-scale MBRs, and challenges is summarized.