Saving energy at a WWTP with membrane aerated biological reactors (MABRs): The results of a techno-economic analysis

Saving energy at a WWTP with membrane aerated biological reactors (MABRs): The results of a techno-economic analysis B. Ruffino*, G. Campo*, A. Cerutti* and M.C. Zanetti* * Department of Environment, Land and Infrastructure Engineering (DIATI), Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, IT (E-mail: [email protected]) Abstract Wastewater treatment plants (WWTPs) will need to make significant efforts to reduce and optimize their energy use in order to meet the climate neutrality goal under the European Green Deal. This study carried out a techno-economic analysis (TEA) comparing a conventional activated sludge (CAS) plant and a plant containing membrane biological reactors (MABRs), having both a treatment capacity of 25,000 equivalent inhabitants (e.i). The data used in TEA came from an experimentation, where, in order to reduce oxygen consumption and achieve high oxygen transfer efficiencies (OTEs), the air flow was controlled at the outlet of a pure, open-end MABR. OTE values of over 80% were recorded, which are not so common in the literature and are comparable to those obtained with a close-end configuration. Both COD and total nitrogen (TN) were successfully removed from samples of real wastewater with an efficiency of approx. 85%. The TEA showed that the MABR's energy requirement was only approx. one-fifth of that of the CAS. The CAPEX of the CAS system resulted of 123.7 k€. Compared to that, the MABR could become a sustainable investment, over a 5-year time horizon, if the overall cost of the MABR cassettes was less than 237 k€. Keywords Energy efficiency; oxygen transfer efficiency; pure MABR; simultaneous nitrification – denitrification; techno-economic analysis INTRODUCTION The recently (1 Jan 2025) entered-into-force revised Urban Wastewater Treatment Directive requires that municipal wastewater treatment plants (WWTPs), with a treatment capacity superior to 10,000 population equivalent (p.e.), target energy neutrality by 2045. WWTPs can accomplish this goal by using energy more efficiently and consuming less energy. The majority of the municipal WWTPs have a section of biological treatments, where organic substances and nutrients are removed with energy-intensive processes, such as conventional activated sludge (CAS) reactors. Typical CAS oxygen transfer efficiencies (OTEs) are still at very low values, 10–15%, which makes CAS an energetically unsustainable method for wastewater treatment. In this regard, upgrading current WWTPs with membrane aerated biological reactors (MABRs) may represent an opportunity. MABR is an advanced wastewater treatment technology that employs hydrophobic membranes to deliver oxygen directly to the biofilm grown on the membrane surface. According to literature, MABR technology offers three main advantages over CAS processes, namely: 1) high nitrification rates (up to 5 g N/m2·d), allowing process intensification, 2) high aeration efficiencies (4–8 kg O2/kWh), saving energy and 3) ability to achieve simultaneous nitrification and denitrification (Li et al., 2023; Ravishankar et al., 2022). This work carried out an assessment of the MABR economic viability, compared to a CAS, using values of OTEs and oxygen transfer rates (OTRs) obtained in a lab-scale experimentation carried out with real wastewater. OTE and OTR are the two most relevant parameters employed for a fullscale MABR design. Specifically, OTE impacts on the volume of air to be supplied to the biological process and, consequently, on the power of the air blower and energy expenditure (Elsayed et al., 2021). Conversely, OTR affects the extension of the surface area of the membrane to be installed and, consequently, purchase and installation costs (Castrillo et al., 2019). A recent literature search carried out in the Scopus database using the keywords “MABR” & “cost” revealed a substantial absence of recent studies dealing with the economic aspects related to the utilization of MABRs for new plants or for revamping of existing ones. MATERIALS AND METHODS The study was divided into three phases, with the specific aims and methods shortly described in Table 1. Phases 1 and 2 were experimental phases, carried out in a 3-L plexiglass reactor with a Zeelung lab-scale module (SUEZ Water Technologies and Solutions), with a surface to volume ratio of 83.3 m2/m3. The air was fed to the membrane using an oil-free compressor (AirCLEAN), wastewater was fed continuously with a peristaltic pump and a membrane pump was used for the recirculation of the wastewater inside the reactor. The air was supplied from one end of the membrane module at 30 kPa, in an open-end configuration. Table 1. Aims and methods of the three phases of the study. Aim Methods Phase 1 Test of the flow control at the outlet of the MABR as a strategy to obtain high OTEs. OTE assessment Reactor fed in a fed-batch mode with synthetic wastewater; variable outlet air flows (1 - 0.078 NL/h, decreasing trend) Phase 2 OTR assessment. Evaluation of the MABR performances in organic substances (COD) and TN removal Reactor fed in a continuous mode; HRT = 9.0 h (first period, days 1-20) and 7.4 h (second period, days 21-35); optimal air flow rate obtained in Phase 1; real wastewater collected from the Castiglione Torinese WWTP (2M p.e.) Phase 3 Techno-economic feasibility of using MABR cassettes, instead of traditional fine-bubble diffusers, for the aeration process in a WWTP of 25,000 e.i. OTE and OTR values obtained in Phases 1 and 2. Utilization of the discounted cash flow (DCF) method. Capital costs of the equipment obtained from the principal equipment’s providers Details on the analytical methods used in Phase 1 and 2 and on the economic assessment carried out in Phase 3 are provided in Campo et al. (2024). RESULTS AND DISCUSSION In Phase 1 the outlet airflow was made to progressively decrease from 1 NL/h to 0.078 NL/h, with a consequent increase in the OTE from about 25% to more than 80%. Such a value was of the same order of magnitude of those obtained with a dead-end configuration, but without the drawbacks related to such an operating mode. In Phase 2 the reactor was fed with a real wastewater, having a C/N ratio of approx. 6. The reactor reached a maximum OTE of 82% and an average OTR value of 6.2 ± 1.2 g O2/m2∙d (see Figure 1). Efficiencies in both COD and TN removal were over 80%, with denitrification efficiencies close to 100% (data not shown). With the support of the data obtained in Phases 1 and 2, a techno-economic analysis was conducted to evaluate the feasibility of renovating the biological section of a WWTP, with a treatment capacity of 25,000 e.i., using MABR cassettes rather than a traditional aeration method using fine-bubble diffusers. The oxygen demand of the carbon oxidation and nitrification processes were calculated to 550 and 606 kg/d, respectively, for a total of 1156 kg O2/d. Because of the lower OTE of the CAS reactor, the amount of air to be supplied was 5.7 times more than that of the MABR system. The supplement of approx. 67∙103 kg air/d, corresponding to 51.8∙103 Nm3/d, required 1773 diffusers working at an average of 70% of the maximum flow rate. The CAS CAPEX resulted of 123.7 k€, which included the cost of a spare blower, which was also considered for the MABR plant. The annual OPEX of the CAS plant was calculated equal to 55.0 k€, 84% of that for energy and the remaining amount for labor and maintenance. 3 4 5 6 7 8 9 0% 20% 40% 60% 80% 100% 0 5 10 15 20 25 30 35 OTR g O2/m2·d OTE % days OTE OTR Figure 1. Values of OTE and OTR recorded in Phase 2. Because of the better use of oxygen, the annual energy cost of the MABR plant was estimated to only 8.4 k€, approx. 5.5 times less than that of the CAS plant. Based on the OTR value of approx. 8 g O2/m2∙d, the membrane surface necessary for the wastewater treatment was of 340,000 m2. A total of 177 cassettes were calculated necessary, given that each cassette contains 48 modules with 40 m2 of membrane. As shown in Figure 2, which reports the cash flow for the two plants, a MABR plant could become a profitable investment, on a return time of 5 years, if the cost of the MABR cassettes was inferior to 237 k€, that is approx. 1340 € for each cassette. In that case, the savings in the energy cost due to the better utilization of oxygen could compensate the higher investment necessary for the MABR plant. It is evident that an increase in the OTR could reduce the number of cassettes necessary for the treatment and, consequently, the initial investment. Although very high OTE values were obtained by controlling the air flowrate downstream of the membrane, the OTRs stayed at relatively modest values. However, the increase in the OLR observed in the last period of the test (Figure 1) and the trend in the specific TN removal suggested that the achievement of higher oxidation efficiency per unit area of membrane is possible. 0 100 200 300 400 500 600 0 1 2 3 4 5 6 7 8 9 10 NPV k€ years MABR CAS Figure 2. Cash flow for a CAS and a MABR plant. A sensitivity analysis demonstrated that the cost of electric energy had the greatest influence on the maximum allowable value of the MABR investment, which was impacted by ±26% in comparison to the value determined in the reference scenario, according to a sensitivity analysis that imposed a variation of ±50% on the input parameters (cost of blower, diffusers, electric energy, and opportunity cost of capital). REFERENCES Campo, G., Cerutti, A., Zanetti, M.C, Ruffino, B. 2024 Membrane aerated biological reactors (MABRs) to enhance the biological treatment process at a WWTP. Journal of Environmental Management 371, 122921. Castrillo, M., Díez-Montero, R., Esteban-García, A.L., Tejero, I., 2019 Mass transfer enhancement and improved nitrification in MABR through specific membrane configuration. Water Research 152, 1-11 Elsayed, A., Hurdle, M., Kim, Y. 2021. Comprehensive model applications for better understanding of pilot-scale membrane-aerated biofilm reactor performance. Journal of Water Process Engineering, 40, 101894 Li, J., Feng, M., Zheng, S., Zhao, W., Xu, X., Yu, X., 2023 The membrane aerated biofilm reactor for nitrogen removal of wastewater treatment: Principles, performances, and nitrous oxide emissions. Chemical Engineering Journal, 460, 141693 Ravishankar, H., Nemeth, A., Massons, G., Puig, D., Zardoya, D., Carpi, N., Lens, P.N.L., Heffernan, B., 2022 Factors impacting simultaneous nitrification and denitrification in a membrane aerated biofilm reactor (MABR) system treating municipal wastewater. Journal of Environmental Chemical Engineering, 10, 108120