Methane emissions by management and treatment of municipal wastewater in Mexico 2010

Management and treatment of wastewater has been identified as a major source of greenhouse gases (GHG). In wastewater treatment (WWT) systems without mechanical ventilation, methanogenic bacteria activate anaerobic decomposition of degradable organics, producing methane (CH4) as a byproduct. Methane produced during the management and treatment of wastewater is estimated to constitute between 8 and 11% to overall CH4 emissions, but a lack of quantitative methane emissions data from specific WWT processes prevents the design of effective mitigation strategies. We are developing a detailed WWT sector CH4 emissions inventory for Mexico. CH4 emissions have been estimated from the 2010 baseline year, using the IPCC Guidelines for National Greenhouse Gas Inventories. These estimates considered WWT plant infrastructures, volumes of waste treated, and the quality of input and output water and temperature. It is important to note that ambient temperature is one of the main factors influencing methane production, which in tropical and subtropical regions is close to optimal for mesophylic methanogenesis (35 °C), resulting in high biological activity and CH4 emissions from anaerobic systems compared to temperate latitudes. Mexico was divided into three regions: north, central and south in order to take into account these temperature variations. In order to evaluate methane emission inventories, direct measurements of WWT facility CH4 emission rates were performed during the SLCF-Mexico project in early 2013. The Aerodyne Mobile Laboratory performed emissions measurements at four wastewater treatment facilities in Mexico. Quantum cascade laser instruments were used to monitor CH4 and N2O, while a proton transfer reaction mass spectrometer monitored organics such as methanethiol. At three facilities, tracer release methods were used to quantify CH4 emission rates from anaerobic digestors. An Aerodyne aerosol mass spectrometer was also used to monitor particulate emissions from CH4 flares associated with the digestors. Measurement results will be presented. In 2010, the number of municipal wastewater treatment plants (MWWTP) in Mexico was 2186, with a total treated flow of 93.6 m3/s, representing just 45% of collected wastewater at the national level. The percentage of treated wastewater varies by region: in northern Mexico, 93% of the collected wastewater is treated, while the central and southern regions have treatment coverages of only 31% and 28%, respectively. As a result, the north has a 51% removal of biochemical oxygen demand (BOD) by WWT, while the central and southern regions remove only 17% and 16% of BOD, respectively. The management and treatment of municipal wastewater in Mexico for the year 2010 generated 600.4 Gigagrams (Gg) of CH4. By region: north 23.5% (141.1 Gg), central 53.4% (320.6 Gg) and south 23.1% (138.7 Gg). These values are directly related to the population of each region and the amount of BOD removed by different WWT systems.