Application of gas permeable membranes for auxiliary oxygenation of sequencing batch reactors

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.

Long-term operation assessment of a full-scale membrane-aerated biofilm reactor under Nordic conditions

Membrane-aerated biofilm reactor (MABR) technology is an exciting alternative to conventional activated sludge, with promising results in bench and pilot-scale systems. Nevertheless, there is still a lack of long-term and full-scale data under different operational conditions. This study aims to report the performance of a full-scale hybrid MABR located in the North of Europe. Influent, effluent, and exhaust data were collected for 1 year (September 2019 to September 2020) using online sensors/gas-analyzers and off-line laboratory analysis. Next, oxygen transfer rate (OTR), oxygen transfer efficiency (OTE), and nitrification rates (NR) were quantified as process indicators. Finally, multivariate methods were used to find patterns among monitored variables. Observations revealed that lower airflows achieved higher OTE at the same values of OTR and OTR was strongly correlated to ammonia/um concentration in the MABR tank (NH_x,eff). The dynamics between oxygen concentration in the exhaust (O_2,exh) and NH_x,eff indicated that a nitrifying biofilm was established within 3 weeks. Average NR were calculated using four different methods and ranged between 1 and 2 g N m^-2d^-1. Principal component analysis (PCA) explained 81.4% of the sample variance with the first three components and cluster analysis (CA) divided the yearly data into five distinctive periods. Hence, it was possible to identify typical Nordic episodes with high frequency of heavy rain, low temperature, and high variations in pollution load. The study concludes that nitrification capacity obtained with MABR is robust during cold weather conditions, and its volumetric value is comparable to other well-established biofilm-based technologies. Moreover, the aeration efficiency (AE) obtained in this study, 5.8 kg O₂ kW h^-1, would suppose an average reduction in energy consumption of 55% compared to fine pore diffused aeration and 74% to the existing surface aeration at the facility.

Enrichment, Isolation, and Characterization of High-Affinity N2O-Reducing Bacteria in a Gas-Permeable Membrane Reactor

The recent discovery of nitrous oxide (N₂O)-reducing bacteria suggests a potential biological sink for the potent greenhouse gas N₂O. For an application toward N₂O mitigation, characterization of more isolates will be required. Here, we describe the successful enrichment and isolation of high-affinity N₂O-reducing bacteria using a N₂O-fed reactor (N₂OFR). Two N₂OFRs, where N₂O was continuously and directly supplied as the sole electron acceptor to a biofilm grown on a gas-permeable membrane, were operated with acetate or a mixture of peptone-based organic substrates as an electron donor. In parallel, a NO₃^- -fed reactor (NO₃FR), filled with a nonwoven sheet substratum, was operated using the same inoculum. We hypothesized that supplying N₂O vs NO₃^- would enhance the dominance of distinct N₂O-reducing bacteria. Clade II type nosZ bacteria became rapidly enriched over clade I type nosZ bacteria in the N₂OFRs, whereas the opposite held in the NO₃FR. High-throughput sequencing of 16S rRNA gene amplicons revealed the dominance of Rhodocyclaceae in the N₂OFRs. Strains of the Azospira and Dechloromonas genera, canonical denitrifiers harboring clade II type nosZ, were isolated with high frequency from the N₂OFRs (132 out of 152 isolates). The isolates from the N₂OFR demonstrated higher N₂O uptake rates (V_max: 4.23 × 10^-3-1.80 × 10^-2 pmol/h/cell) and lower N₂O half-saturation coefficients (K_m,N2O: 1.55-2.10 μM) than a clade I type nosZ isolate from the NO₃FR. Furthermore, the clade II type nosZ isolates had higher specific growth rates on N₂O than nitrite as an electron acceptor. Hence, continuously and exclusively supplying N₂O in an N₂OFR allows the enrichment and isolation of high-affinity N₂O-reducing strains, which may be used as N₂O sinks in bioaugmentation efforts.

A membrane aerated biofilm reactor for sulfide control from anaerobically treated wastewater

A upflow anaerobic sludge blanket reactor was operated combined to a membrane aerated biofilm reactor for sulfate removal and for elemental sulfur reclamation. A commercial silicon tube was used as an oxygen delivery diffuser. The process achieved high rates of sulfide removal from the liquid phase (90%). The hydrogen sulfide removal was influenced by the pH value and at pH value of 7.5, 98% of the H₂S was removed. The elemental sulfur was observed inside the membrane, with content in the biomass of 21%. Through the massive sequencing of the samples, the microbial community diversity and the stratification of biomass inside the silicon tube was demonstrated, confirming the presence of sulfide-oxidizing bacteria on the membrane wall. The most important genera found related to the sulfur cycle were Sulfuricurvum, Geovibrio, Flexispira and Sulforospirillum.

Sulfide-oxidizing bacteria establishment in an innovative microaerobic reactor with an internal silicone membrane for sulfur recovery from wastewater

A novel bioreactor, employing a silicone membrane for microaeration, was studied for partial sulfide oxidation to elemental sulfur. The objective of this study was to assess the feasibility of using an internal silicone membrane reactor (ISMR) to treat dissolved sulfide and to characterize its microbial community. The ISMR is an effective system to eliminate sulfide produced in anaerobic reactors. Sulfide removal efficiencies reached 96 % in a combined anaerobic/microaerobic reactor and significant sulfate production did not occur. The oxygen transfer was strongly influenced by air pressure and flow. Pyrosequencing analysis indicated various sulfide-oxidizing bacteria (SOB) affiliated to the species Acidithiobacillus thiooxidans, Sulfuricurvum kujiense and Pseudomonas stutzeri attached to the membrane and also indicated similarity between the biomass deposited on the membrane wall and the biomass drawn from the material support, supported the establishment of SOB in an anaerobic sludge under microaerobic conditions. Furthermore, these results showed that the reactor configuration can develop SOB under microaerobic conditions and can improve and reestablish the sulfide conversion to elemental sulfur.

Effect of oxygen partial pressure and chemical oxygen demand loading on the biofilm properties in membrane-aerated bioreactors

Membrane-aerated biofilms with oxygen and nutrients diffusing from the opposite sides possess distinct properties, including the ability to couple aerobic and anaerobic processes. The objective of this study was to examine the effects of oxygen partial pressure and chemical oxygen demand (COD) loading on biofilm properties. Two laboratory-scale membrane-aerated bioreactors were operated for a total of 283 days, with one reactor operated at 42, 60, and 89 kPa (0.41, 0.59, and 0.88 atm) oxygen, and the other reactor at 25 kPa (0.25 atm) oxygen (air control). The biofilm detached at the oxygen partial pressures of 60 and 89 kPa (0.59 and 0.88 atm) at a COD loading of 11.3 kg COD/1000 m2/d, but was sustained at the oxygen partial pressures of 25 and 42 kPa (0.25 and 0.41 atm), with a porous structure at the membrane interface at the COD loading of 11.3 kg COD/1000 m2/d. Biofilm formation was improved at a higher COD loading. It is proposed that the loss of extracellular polymeric substances at the biofilm bottom is the cause for the biofilm detachment subjected to a higher oxygen partial pressure.

Membrane-aerated biofilms for high rate biotreatment: performance appraisal, engineering principles, scale-up, and development requirements

Diffusion of the electron acceptor is the rate controlling step in virtually all biofilm reactors employed for aerobic wastewater treatment. The membrane-aerated biofilm reactor (MABR) is a technology that can deliver oxygen at high rates and transfer efficiencies, thereby enhancing the biofilm activity. This paper provides a comparative performance rate analysis of the MABR in terms of its application for carbonaceous pollutant removal, nitrification/denitrification and xenobiotic biotreatment. We also describe the mechanisms influencing process performance in the MABR and the inter-relationships between these factors. The challenges involved in scaling-up the process are discussed with recommendations for prioritization of research needs.

Carbon membrane-aerated biofilm reactor for synthetic wastewater treatment

A carbon membrane-aerated biofilm reactor (CMABR) was developed to treat synthetic wastewater. Such membrane exhibited a high degree of adhesion and good permeability. Continuous experiments showed that COD and NH4(+)-N removal efficiency were 90 +/- 2 and 92 +/- 4% at removal rates of 35.6 +/- 3.8 g COD/m2 per day and 9.3 +/- 0.6 g NH4(+)-N/ m2 per day, respectively. After 108 days, effluent total nitrogen (TN) kept at 35 +/- 4 mg/L when influent NH4(+)-N increased to 144-164 mg/L and removal efficiency of TN reached 78 +/- 3%. Furthermore, Stoichiometric analysis revealed that 70-90% of oxygen supplied was consumed by nitrifier. Scanning electron microscopic (SEM) images and component analysis of penetrating fluid revealed that extracellular polymeric substance (EPS) adhered to pore and that alkaline washing was an effective method to remove them. The study demonstrated that carbon membrane could be used as effective gas-permeable membrane in MABR for wastewater treatment.

Nitrogen removal characteristics and biofilm analysis of a membrane-aerated biofilm reactor applicable to high-strength nitrogenous wastewater treatment

A membrane-aerated biofilm reactor (MABR) capable of simultaneous nitrification and denitrification in a single reactor vessel was developed to investigate the characteristics of nitrogen removal from high-strength nitrogenous wastewater, and biofilm analysis using microelectrodes and the fluorescence in situ hybridization (FISH) technique was performed. Mean removal percentages of total organic carbon (TOC) and nitrogen were 96% and 83% at removal rates of 5.76 g-C m(-2) d(-1) and 4.48 g-N m(-2) d(-1), respectively. For stable removal efficiency, constant washing of the biofilm was needed. Dissolved oxygen microelectrode measurement revealed that the biofilm thickness was about 1600 microm, and that oxygen penetrated about 300 to 700 microm, from the outer surface of the membrane. Furthermore, FISH analysis revealed that ammonia-oxidizing bacteria (AOB) were located near the outer surface of the membrane, whereas other bacteria were located from the inner to the outer part of the biofilm. Combining these results demonstrated that simultaneous nitrification and denitrification occurred in the biofilm of the MABR system. In addition, stoichiometric analysis revealed that after 130 d(-1), the free ammonia (FA) concentration ranged within the concentration causing inhibition of the growth of nitrite oxidizing bacteria (NOB) and that AOB consumed 86% of the oxygen supplied through the intra-membrane. These results indicate that nitrogen removal not via nitrate but via nitrite was mainly achieved in the MABR system.