Startup and initial operation of an MLE-MABR treating municipal wastewater

Membrane aerated biological reactors (MABRs) to enhance the biological treatment process at a WWTP

The goal of climate neutrality, under the provision of the European Green Deal, will require great efforts to wastewater treatment plants (WWTPs) to reduce and optimize their energy consumption. The utilization of membrane aerated biological reactors (MABRs) to renovate existing WWTPs could be an opportunity in this sense. In this study, the control of the flow at the outlet of a pure, open-end MABR was used as a strategy to minimize the oxygen consumption and obtain high oxygen transfer efficiencies (OTEs). OTE values of more than 80% were observed, which are not so common in the literature and are comparable to those obtained with a close-end configuration. High efficiencies (85%) were found for the removal of both COD and total nitrogen from samples of real wastewater. A techno-economic analysis, comparing a conventional activate sludge (CAS) plant with a MABR, both with a treatment capacity of 25,000 equivalent inhabitants (e.i.), found that the MABR only needed approx. 1/5 of the energy required by the CAS. A MABR plant could become a profitable investment, under a fixed return time of 5 years, compared to a CAS with a CAPEX of 123.7 k€, if the overall cost of the cassettes was inferior to 237 k€. A sensitivity analysis imposing a variation of ±50% on the input parameters (cost of blower, diffusers, electric energy, and opportunity cost of capital) demonstrated that the cost of electric energy had the highest impact on the maximum allowable value of the MABR investment, which was affected by ± 26% with respect to the value calculated in the reference scenario.

Simultaneous carbon, nitrogen and phosphorus removal in sequencing batch membrane aerated biofilm reactor with biofilm thickness control via air scouring aided by computational fluid dynamics

Membrane aerated biofilm reactor (MABR) is challenged by biofilm thickness control and phosphorus removal. Air scouring aided by computational fluid dynamics (CFD) was employed to detach outer biofilm in sequencing batch MABR treating low C/N wastewater. Biofilm with 177-285 µm thickness in cycle 5-15 achieved over 85 % chemical oxygen demand (COD) and total inorganic nitrogen (TIN) removals at loading rate of 13.2 gCOD/m2/d and 2.64 gNH4+-N/m2/d. Biofilm rheology measurements in cycle 10-25 showed yield stress against detachment of 2.8-7.4 Pa, which were equal to CFD calculated shear stresses under air scouring flowrate of 3-9 L/min. Air scouring reduced effluent NH4+-N by 10 % and biofilm thickness by 78 µm. Intermittent aeration (4h off, 19.5h on) and air scouring (3 L/min, 30 s before settling) in one cycle achieved COD removal over 90 %, TIN and PO43--P removals over 80 %, showing great potential for simultaneous carbon, nitrogen and phosphorus removals.

Looking deeper into the effects of scouring and aeration on membrane aerated biofilms: Analysis of nitrogen conversion, oxygen profiles and nitrous oxide emissions

This study investigated the effects of aeration and scouring strategies on the performance of Membrane Aerated Biofilm Reactors (MABRs) and the distribution of oxygen and nitrous oxide in the biofilm. Four flat sheet MABRs were operated with synthetic feed under different conditions: two with intermittent aeration (iMABR) and two with continuous aeration (cMABR). Scouring was induced by bubbling dinitrogen gas through the reactor bulk at low and high frequencies (LF and HF). In the iMABRs, a partial nitritation biofilm initially developed, but the biofilm adapted to the aeration strategy over time and became nitrifying. The cMABRs directly developed a nitrifying biofilm without a significant phase of partial nitritation. Limiting oxygen availability improved the overall performance with regards to total nitrogen (TN) removal by providing a better environment for anaerobic ammonium oxidation (Anammox) while limiting complete nitrification. Oxygen profiles were measured in the iMABR over time at different biofilms depths, showing that intermittent aeration led to various oxygen concentrations and temporal variations in the oxygen availabilities at different depths of the biofilm. Also, N2O emissions from the MABRs differed greatly between the different systems, but still remained lower compared to other reactor configurations for nitrogen removal, making the MABR technology a worthy alternative. The results showed large differences between the operating strategies of the MABRs and can help to gain more insight into the specific properties of MABRs for nitrogen removal.

Membrane aerated biofilm reactor system driven by pure oxygen for wastewater treatment

Pure oxygen is proposed for wastewater treatment due to its advantages over conventional air aeration. This study investigates a Pure Oxygen-based Membrane Aerated Biofilm Reactor (PO-MABR) for the first time under various operating conditions. The PO-MABR employs a gas-permeable membrane for direct diffusion of low-pressurized pure oxygen to the biofilm, ensuring exceptional carbon and nitrogen removal. The effectiveness of PO-MABR was investigated by varying operational conditions, including temperature, carbon-to-nitrogen ratio, gas pressure, and flow rate. Results indicate superior performance, with a 97% chemical oxygen demand removal and 19% higher total nitrogen removal than Air-Ventilated MABR (A-MABR) due to thicker biofilm and unique microbial structures in PO-MABR. Also, PO-MABR demonstrated resilience to low temperatures and effectively treated both high and low-strength wastewater. The findings emphasize the efficiency of PO-MABR in wastewater treatment, advocating for its adoption due to superior carbon and nitrogen removal across diverse operational conditions.

Nitrous oxide emissions from two full-scale membrane-aerated biofilm reactors

The upcoming change of legislation in some European countries where wastewater treatment facilities will start to be taxed based on direct greenhouse gas (GHG) emissions will force water utilities to take a closer look at nitrous oxide (N2O) production. In this study, we report for the first time N2O emissions from two full-scale size membrane aerated biofilm reactors (MABR) (R1, R2) from two different manufacturers treating municipal wastewater. N2O was monitored continuously for 12 months in both the MABR exhaust gas and liquid phase. Multivariate analysis was used to assess process performance. Results show that emission factors (EFN2O) for both R1 and R2 (0.88 ± 1.28 and 0.82 ± 0.86 %) were very similar to each other and below the standard value from the Intergovernmental Panel on Climate Change (IPCC) 2019 (1.6 %). More specifically, N2O was predominantly emitted in the MABR exhaust gas (NTRexh) and was strongly correlated to the ammonia/um load (NHx,load). Nevertheless, the implemented Oxidation Reduction Potential (ORP) control strategy increased the bulk contribution (NTRbulk), impacting the overall EFN2O. A thorough analysis of dynamic data reveals that the changes in the external aeration (EA)/loading rate patterns suggested by ORP control substantially impacted N2O mass transfer and biological production processes. It also suggests that NTRexh is mainly caused by ammonia-oxidizing organisms (AOO) activity, while ordinary heterotrophic organisms (OHO) are responsible for NTRbulk. Different methods for calculating EFN2O were compared, and results showed EFN2O would range from 0.6 to 5.5 depending on the assumptions made. Based on existing literature, a strong correlation between EFN2O and nitrogen loading rate (R2 = 0.73) was found for different technologies. Overall, an average EFN2O of 0.86 % N2O-N per N load was found with a nitrogen loading rate >200 g N m-3 d-1, which supports the hypothesis that MABR technology can achieve intensified biological nutrient removal without increasing N2O emissions.

The hybrid MABR process achieves intensified nitrogen removal while N2O emissions remain low

The hybrid membrane aerated biofilm reactor (MABR) process represents a full-scale solution for sustainable municipal wastewater treatment. However, most of the existing hybrid MABR processes retain large aerobic bioreactor volumes for nitrification, which is undesirable for energy and carbon savings. In this study, we used the plant-wide modeling approach with dynamic simulations to examine a novel hybrid MABR configuration with aeration controls that change the anoxic and aerobic fractions of the bioreactor volume. Result showed that the novel hybrid MABR showed "swinging" nitrification and denitrification capacities in response to diurnal loadings, achieving intensified nitrogen removal performance under both warm and cold temperature scenarios. N2O emissions from the hybrid MABR were only 1/5 of the emissions from the conventional activated sludge. The model predicted higher CH4 emissions from the hybrid MABR than the activated sludge process due to the methanogen growth in the oxygen-depleted MABR biofilm layer. Future measurements for CH4 emission are needed to obtain a holistic picture of the carbon footprint of the hybrid MABR process.

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.

Available online sensors can be used to create fingerprints for MABRs that characterize biofilm limiting conditions and serve as soft sensors

Membrane aerated biofilm reactors (MABRs) are a promising biological wastewater treatment technology, whose industrial applications have dramatically accelerated in the last five years. Increased popularity and fast industrial adaptation are coupled with increased needs to monitor, optimize, and control MABRs with available online sensors. Observations of commercial scale MABR installations have shown a distinctive and repetitive pattern relating oxygen purity in MABR exhaust gas to reactor ammonia concentrations. This provides an obvious opportunity for process monitoring and control which this paper investigates with the help of modeling. The relationship plots between the bulk ammonia concentration and the oxygen purity are defined as MABR fingerprint plots, which are described in the form of steady-state curves and dynamic trajectories. This study systematically investigated, analyzed, and explained the behaviors and connections of steady-state curves and dynamic trajectories with a MABR model in SUMO^®, and proposed a hypothesis about utilizing the MABR fingerprint plots to characterize MABR system performance, identify the limiting factor of biofilms, and possibly develop a soft senor for MABR biofilm thickness monitoring and control.