Anaerobic Membrane Bioreactor (AnMBR) with degassing unit

Flow scheme of the technology

The AnMBR combines a bioreactor with either flocculant or granular biomass, such as an up-flow anaerobic sludge blanket reactor (UASB) with physical separation membranes, ultrafiltration (UF) for solid-liquid separation and membrane contactor for gas-liquid separation. As shown in Fig. 1, the UF membrane system is integrated with an up-flow anaerobic sludge blanket reactor (UASB) in a side-stream configuration.

   AnMBR

Fig 1. Example of the AnMBR of the case study Spernal in the project NextGen

In the MCU process, water passes through the outside (shell side) of the hollow fibres while a sweep gas (or vacuum) is applied to the inside (lumen side) of the fibres. Because the membrane is hydrophobic, it allows direct contact between gas and water without dispersion. Applying a higher pressure to the water stream relative to the gas stream creates the driving force for dissolved gas in the water to pass through the membrane pores. Then, the methane is transported using a vacuum pump or together with sweep gas to the biogas to generate electrical and thermal energy (Fig.1).

Pictures

Synergetic effects and motivation for the implementation of the technology

• Improvement of dewatering and reduction of dry matter

Excess sludge production in the AnMBR is very low, but still this need extracted from AnMBR from time to time, to avoid high solids concentration reaching the UF side-stream module. This excess sludge can be further stabilised in an anaerobic digester (AD) and dewatered, together with the primary sludge. This sludge still has a high CH₄ potential, and the organic matter is converted into biogas leading to a further reduction in the total dry matter content in the digested sludge. As a result, transport and disposal costs can be reduced. Furthermore, the treated sewage solids (i.e. biosolids) can be successfully used for fertilisation of land and for landscape purposes.

• High quality product by combining with other conventional treatment processes

MCU is applied as post-treatment to the AnMBR effluent. MCU applications include removing biogenic hydrogen sulphide, carbon dioxide, oxygen and ammonia. Coupling AnMBR with MCU for wastewater treatment allows for the recovery of the dissolved methane, clean water rich in valuable nutrients. Under UK winter, up to 100% of the methane is found dissolved in the liquid phase, as wastewater temperatures are in the range of 6-8°C. CH₄ has a high greenhouse gas potential, and its recovery and management are crucial to ensure the low environmental impact of the technology. The recovery of dissolved CH₄ has an energy demand. For its coverage, green energy should be used to contribute to the low environmental impact (Remy et al. 2022).

Technology requirements and operating conditions

The more organic matter is biodegraded in the AnMBR, the higher the biogas yield is. Several parameters affect biogas production and the methane yield in AnMBRs, including hydraulic retention time (HRT), solids retention time (SRT), temperature, pH and organic loading rate (OLR).

Operating conditions of the AnMBR and degassing unit in Spernal

The feasibility of anaerobic wastewater treatment in the UK has been demonstrated through pilot-scale trials that have taken place at Cranfield University since 2003. Until now, the work completed to date has shown that treating municipal low-strength wastewater (COD <400 mg/L) at temperatures between 6-22°C, with an average of 14°C, is feasible. Furthermore, it has the potential to replace traditional energy-consuming aerobic wastewater treatment processes. Hydrolysis is the limiting step, emphasising the need to provide long sludge retention times to ensure stable biogas production and solids can be maintained in the reactor by using a membrane filtration after the UASB. This combined system has been investigated at pilot-scale (Soares and Vale 2022). Membrane fouling can be controlled by intermittent gas-sparing practices using biogas, while maintaining the energy efficiency of the process. The membrane operation can be turned up-down, enabling design at average flow rather than full-flow to treatment.

Key performance indicators

During the start-up phase, total chemical oxygen demand (tCOD) removal rates of 60-70% were achieved in Spernal (Soares and Vale 2022). Coupling the UASB with a membrane enables the production an effluent with no TSS content, a tCOD below 20 mg/L and aBOD below 10 mg BOD/L. This effluent meets the requirements for water reuse, according to the EU Regulation 741/2020 (without considering the microbial removal performance). In addition, tCOD removal rates between 90-95% are expected (Soares and Vale 2022). Lew et al. (2022) observed a tCOD removal rate of 88% for municipal wastewater, while Chang et al. summarized for industrial wastewaters rates between 90% and 99%. Methane composition in the biogas is high (80%), facilitating its upgrading or other uses. Conversely, nutrient removal in the anaerobic reactor is negligible (5-10% phosphate removal and ammonia increase by 5-15% due to solids hydrolysis). Post-treatment for nutrient removal/recovery is necessary.

The sulphate content in the wastewater is a very important parameter because sulphate-reducing bacteria (SRB) being active under the anaerobic conditions reduce it to H₂S using hydrogen and acetate as substrate (Lovley et al. 1988). Competing for the same substrate as the methanogenic archaea, the SRBs can even outcompete methanogenic archaea, reducing the methane yield. Furthermore, H₂S can inhibit methanogenic archaea at a specific concentration. More information is provided in the factsheet about anaerobic digestion. Furthermore, the dissolved H₂S degasses together with the methane in the degassing unit and can lead to corrosion processes.

Links to related topics and similar reference projects

Case Studies applying the technology

Lleida, Spain

Tain, United Kingdom

Spernal, United Kingdom

Publications

• Chang, S., Anaerobic membrane bioreactor (AnMBR) for wastewater treatment, 2014
• Jiménez-Benítez, A., Ruiz-Martínez, A., Robles, Á., Serralta, J., Ribes, J., Rogalla, F., Seco, A., Ferrer, J., A semi-industrial AnMBR plant for urban wastewater treatment at ambient temperature: Analysis of the filtration process, energy balance and quantification of GHG emissions, 2023
• Kim, J., Hofman, J., Kenyeres, I., Poor-Pocsi, E., Kleyböcker, A., Heinze, J., Kraus, F., Soares, A., Paissoni, E., Vale, P., Palmer, M., Bloemendal, M., Beernink, S., Ros, S., Hartog, N., Frijns, J., D1.4 (2022) New approaches and best practices for closing the energy cycle in the water sector, 2022
• Lew, B., Tarre, S., Beliavski, M., Dosoretz, C., Green, M., Anaerobic membrane bioreactor (AnMBR) for domestic wastewater treatment, 2009
• Lovley, D., Phillips, E., Novel mode of microbial energy metabolism: organic carbon oxidation coupled to dissimilatory reduction of iron or manganese, 1988
• Plana Puig, Q., Nanu, C., Groza, I., Soares, A., Vale, P., Monokrousou, K., Makropoulos, C., Katsouras, G., Tsalas, N., Tazes, N., Tsimnadis, K., Lindeboom, R., Plata, C., Suters, R., Kenyeres, I., Plà, M., Filipsson, S., Hedman, F., Kim, J., Hofman, J.,, D1.3 New approaches and best practices for closing the water cycle, 2022
• Remy, C., Kraus, F., Conzelmann, L., Seis, W., Zamzow, M., D2.1 (2022) Environmental life cycle assessment and risk analysis of NextGen demo case solutions, 2022
• Soares, A., Paissoni, E., Vale Palmer, M., Decentralized energy recovery from anaerobic membrane bioreactor in Spernal (UK), 2022
• Soares, A., Vale, P., Anaerobic membrane bioreactor (AnMBR) in Spernal, 2022
• TETECH, https://www.te-tech.co.uk/case-studies/spernal-anmbr, 2023

Cross-links

• Ammonium sulphate production (air stripping & scrubbing)
• Low grade heat recovery from wastewater
• Nutrient removal and recovery via ion exchange and HFMC
• Struvite production