Methane emissions from anaerobic sludge digesters in Mexico: On-site determination vs. IPCC Tier 1 method

Wastewater treatment is an important source of methane (CH₄) emissions. In most large-size aerobic treatment plants, the excess sludge is digested in anaerobic reactors (AD), with the concomitant CH₄ emissions. The guidelines of the Intergovernmental Panel on Climate Change (IPCC) have been adopted worldwide for quantifying the national emission inventories, which include wastewater treatment plants (WWTP) as a key category. The IPCC recommends using default emission factors (Tier 1) for countries with limited available data (such as Mexico and most developing countries). However, these estimates have a high degree of uncertainty, owing to the lack of reliable information about the operation process and local environmental conditions. In order to reduce uncertainty in the estimation of CH₄ emission from WWTP in Mexico, a country-specific emission factor was determined for AD associated with activated sludge process. This was accomplished with on-site data obtained from the AD of six activated sludge WWTP. In addition, the measured CH₄ emissions were compared to those resulting from the application of the IPCC Tier 1 method, using the recommended default methane correction factor (MCF: 0.8) as well as alternate values (0.32 and 0.26) recently proposed by the authors. Results show that the IPCC Tier 1 method, using the recommended MCF, highly overestimate CH₄ emissions compared with the values obtained on-site. In contrast, the alternate MCF achieved better estimations than the IPCC-recommended MCF, much closer to the observed emission values. The CH₄ emission factor proposed as country (Mexico) specific value is 0.49 kg CH₄/kg BOD_rem, which would allow the application of IPCC Tier 2 method. By doing so, the uncertainty associated with CH₄ emission from aerobic treatment plants with AD would be reduced. This, in turn, would provide important information for implementing appropriate CH₄ mitigation strategies for the water sector.