Selecting municipal wastewater treatment technologies for greenhouse gas emissions reduction: the case of Mexico for year 2030

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1 Selecting municipal wastewater treatment technologies for greenhouse gas emissions reduction: the case of Mexico for year 2030 Adalberto Noyola Instituto de Ingeniería, Universidad Nacional Autónoma de México, Coyoacán, Mexico City, Mexico Abstract The proper selection of treatment technologies may be an important opportunity for contributing to the reduction goals of greenhouse gases (GHG) emissions. Based on actual Mexican infrastructure data, the scenarios analyzed in this work show that the emission reduction from sewage treatment in Mexico could be as high as 34% if compared to the baseline scenario, depending on the treatment technologies. The results show that the anaerobic and aerobic scenario (AN+A) is a better option if compared to the full aerobic one (A), as it reduces in 4% the GHG emissions for year Moreover, if methane is used for in situ electricity cogeneration in larger facilities, the reduction reaches 27%. The results obtained may be helpful for policy and decision makers in evaluating cost effectiveness and feasibility of possible GHG mitigation strategies for wastewater treatment facilities, mainly for new infrastructure in developing countries. Introduction As many developing countries and emerging economies, Mexico needs to invest in wastewater treatment facilities, an aspect that has been neglected in the past. At present, 45% of municipal sewage enters a treatment plant, so a significant fraction is discharged untreated or partially treated into water bodies or used for crop irrigation, with significant impacts on public health and the environment. In order to face this infrastructure demand and compliance with environmental and water reclamation regulations, the selection of treatment technologies should take into account not only technological, economic and social aspects, but also environmental impacts, incorporating sustainability criteria and long-term planning (Muga and Mihecic, 2008). In this sense, emissions of greenhouse gases (GHG) are one of the most significant environmental impacts of wastewater management, which directly contributes to Climate Change (CC). Particularly, during wastewater treatment, carbon dioxide (CO 2), methane (CH 4) and nitrous oxide (N 2O) can be emitted to the atmosphere. With around 7% of CO 2 eq. emissions in Mexico, wastewater management can contribute with the national climate change policy. For that purpose, it is necessary to design and apply appropriate mitigation strategies in the current and future facilities for wastewater treatment. This may be accomplished by adopting improved processes, new tools and operational schemes avoiding or minimizing GHG emissions (Flores-Alsina et al., 2011). With 55% of untreated municipal sewage, the wastewater treatment facilities that will be built in Mexico should consider processes with a lower carbon footprint. Therefore, the aim of this study was to determine the baseline scenario of GHG emissions of municipal wastewater treatment in Mexico and to assess different technological scenarios in order to provide technical elements for the identification of effective mitigation strategies in this sector. The

2 work is based on real national infrastructure for sewage treatment and on water and climate change policies in Mexico. Methods In order to support decision making for more sustainable treatment systems, five technology scenarios were analyzed for a 100% treatment coverage at year 2030; also, a business as usual scenario was considered as a baseline. By this way, the currently applied technologies, as well as full aerobic and a combination of anaerobic treatment followed by aerobic post-treatment were considered. The current status of the applied wastewater treatment technologies in Mexico was taken from the 2010 National Inventory of Municipal Water and Wastewater Plants in Operation (CONAGUA; 2011). This official information was used as a basis for the classification of current treatment technologies in the country. From the 2186 municipal wastewater treatment plants (MWWTP) in Mexico, the two more adopted technologies are stabilization ponds (34.7%) and activated sludge (29.6%). These are followed by septic tank with different post-treatment processes (7.7 %) and the Up-flow Anaerobic Sludge Blanket Reactor (UASB) also with or without a post-treatment step (6.9 %). These four technologies correspond to 1724 facilities (78.9 % of total). The first two technologies are also the more representative in terms of treatment capacity, with a clear contribution of activated sludge with 52% of total, followed by stabilization ponds (14.5%). The methodology of the IPCC for national GHG inventories (IPCC, 2006) was used for determining CH 4 emissions, based on the proposed default factors (Tier 1). For each treatment facility in Table 1 (2186 treatment plants), organic matter removal (as biological oxygen demand, BOD) was estimated with the above-mentioned process simulator. In all cases, the influent BOD was fixed at 244 mg/l, based on a representative characterization of sewage in Latin America [15]. Scenarios: Water agenda scenario (WA).- Mexican Water Agenda 2030 states a long-term strategy in order to improve the water sector performance in Mexico and reach a 100% treatment coverage of collected sewage. In this case, the current diversity of treatment processes is applied. The difference between base line (BL) and WA scenarios is the treatment coverage (87 vs. 100% respectively). Aerobic-only processes scenario (A).- Based on the previous WA, this scenario considers that the new infrastructure for municipal wastewater treatment is exclusively aerobic. Anaerobic followed by aerobic post-treatment scenario (An+A).- In this case, the new municipal wastewater treatment plants will be combined anaerobic/aerobic systems, meeting the Water Agenda 2030 considerations. It takes into account the implementation of anaerobic processes, mainly UASB reactors, followed by other biological processes such as activated sludge, trickling filters, rotating biological contactors, pond systems, etc.). This combination is being increasingly used for municipal wastewater treatment in warm climate regions, particularly in developing countries such as Brazil, India, Colombia and Mexico (van Lier et al., 2010; Noyola et al., 2012). Zero methane emission scenario (ZME).- This scenario is based on the previous one (An+A) but considering that all the methane dissolved in the effluent is recovered and burned, using a hypothetical desorption operation with 100% efficiency. This results in an overall methane emission fraction of 5%, considering that the flare has a 95% burning efficiency. Biogas to energy scenario (BE).- In this case, the basis is again the An+A scenario, resulting in an overall capture and oxidation of methane of 83.75% as burning efficiency is kept at 95%.

3 However, biogas is used for electricity production in treatment facilities with capacities higher than 500 l/s, which results in lower indirect CO2 emissions in those cases. An electrical energy conversion of 2.5 kwh/m3 of biogas was used in this scenario. Results and Discussion The production of CO 2 eq. of the six scenarios is presented in Figure 1, for the time interval. Also, data from previous years is included ( ) as taken from official sources (Arvizu, 2008). The percentage reduction of GHG for each scenario is also shown for year GHG emission reduction from sewage treatment in Mexico could be as high as 34% if compared to the baseline scenario. This would be accomplished with the combined anaerobic-aerobic processes with 95% methane burning efficiency and electricity cogeneration in facilities with treatment capacity above 500 L/s. If production of electricity is not considered, the reduction of GHG emissions is limited to 14%. Clearly, the impact of biogas recovery for electricity production is highly significant for GHG emissions reduction. In addition, the anaerobic-aerobic scenario is a marginally better option if compared to the full aerobic one, as it reduces 4% the GHG emissions for year 2030 (14 and 10% respectively, if compared to baseline). This small difference increases to 27% if methane is used for in situ electricity cogeneration in larger facilities, as mentioned. In addition to the previous discussion, this data shows that a 100% treatment coverage using the present mix of technologies (WA scenario) would result in a slightly higher amount of GHG between years 1990 and 2030, which may be considered a satisfactory result in terms of GHG mitigation goals. However, significant improvements may be reached if different technological solutions are considered: in the current state of technologies, choosing investment policies following the AN+A scenario would result in slightly lower emissions in year 2030 if compared those produced 40 years back. More ambitious measures could be taken if BE scenario is considered by the selection of combined anaerobic and aerobic processes coupled to in situ electric energy production from biogas in the larger facilities, a task that may be accomplished with technology adaptation within reach. The anaerobic-aerobic scenario would result in slightly lower emissions in year 2030 (10,277 Gg CO 2 eq.) if compared to those produced 40 years back from this activity (10,500 Gg CO 2 eq.). However, if biogas is used for cogeneration in anaerobic-aerobic processes, the amount of CO 2 eq. in year 2030 would be 75% of the one produced in 1990.

4 Figure 1. Comparison of technological improvement scenarios for reducing GHG emissions by WWTP in Mexico (years ). WA: Water agenda; A: Aerobic-only processes; An + A: Anaerobic followed by aerobic post-treatment; ZME: Zero methane emission; BE: Biogas to energy (Noyola et al. submitted) The remarkable impact of biogas recovery for electricity production on GHG emissions has also been pointed out by Shahabadi et al. (2010). In fact, biogas production and recovery from wastewater constitute a renewable energy source and economically viable substitute for energy production [30] and would have significant environmental benefits in terms of GHG production [31]. The general approach to increase electricity (or thermal energy) production from biogas in municipal anaerobic treatment facilities should focus on two aspects: to maximize biogas production of anaerobic processes, with the concomitant recovery of the dissolved fraction, and to develop electricity generators with high conversion efficiency (Cao, 2011) for a wider application range, even for small treatment plants. These goals may be achieved if research efforts are linked with technology innovation and adaptation, both being pulled by the potential demand of this kind of equipment in a market that is looking for more sustainable options. Moreover, a rough economic estimation showed that adopting the anaerobic-aerobic scenario instead of the full aerobic one, would represent significant investment savings (at least 10%). More significant saving would result in operation and maintenance (around 40% of the annual expenditures on these items). Conclusions Municipal wastewater treatment represents an opportunity to reduce GHG emissions in developing countries. The scenarios analyzed in this work show that emissions from municipal sewage treatment in Mexico could be reduced in 34%, depending on the adopted treatment technologies (anaerobic and aerobic scenario (AN+A) with methane used for local electricity production in larger facilities.

5 The choice of treatment technologies is of major importance for accomplishing effective GHG emission reduction in the water sector. The adoption of treatment systems with lower electricity needs, such as the anaerobic processes, may be a low hanging fruit policy measure, considering the new facilities that will be built in warm climate developing countries with limited sewage treatment coverage. By making the right choices, two targets may be attained: the reduction of direct discharge of untreated sewage and the mitigation of GHG (methane) emissions from the water sector. References Arvizu J.L. (2008) Actualización del Inventario Nacional de Gases de Efecto Invernadero en la categoría de desechos. Instituto Nacional de Ecología (INE), México. Cao Y.S. (2011) Mass Flow and Energy Efficiency of Municipal Wastewater Treatment Plants, IWA. London, UK CONAGUA (2011) Inventario nacional de plantas municipales de potabilización y de tratamiento de aguas residuales en operación, Comisión Nacional del Agua, Mexico. Flores-Alsina X., Corominas L., Snip L., Vanrolleghem P. (2011) Including greenhouse gas emissions during benchmarking of wastewater treatment plant control strategies, Water Res., 45 (16), IPPC (2006), 2006 Guidelines for National Greenhouse Gas Inventories. Vol. 5 Waste. Prepared by the National Greenhouse Gas Inventories Programme. Eggleston H.S., Buendia L., Miwa K., Ngara T. and Tanabe K. (eds). IGES, Japan. Muga H., Mihecic J. (2008) Sustainability of wastewater treatment technologies, Environ. Manage, 88 (3), Noyola A., Padilla A., Morgan-Sagastume J.M., Güereca L.P., Hernández F. (2012) Typology of Municipal Wastewater Treatment Technologies in Latin America, CLEAN Soil, Air, Water, 40 (9), Noyola A., Paredes M.G., Morgan-Sagastume J.M., Güereca L.P. Submitted to CLEAN Soil, Air, Water. Shahabadi B.M., Yerushalmi L., Haghighat F. (2010) Estimation of greenhouse gas generation in wastewater treatment plants--model development and application, Chemosphere, 78 (9), Van Lier J.B, Vashi A., Van der Lubbe J., Heffernan B. (2010) Anaerobic sewage treatment using UASB reactors: Engineering and operational aspects, in Environmental Anaerobic Technology: applications and new developments, Fang, H.H.P. editor, Imperial College Press, London,