Variation of pH during anaerobic processes of methane emission from drying and fermentation of cattle manure in central Mexico

In Mexico, two inventories of methane emissions from cattle manure have been conducted. In the first one (Gonzalez-Avalos 1994), Environmental Protection Agency (EPA) methodology was followed, while the second inventory (Gonzalez and Ruiz 1995) was supported on an experimental methodology from the Intergovernmental Panel on Climate Change (IPCC). Methane Emission Factors used in both methodologies were not appropriate to climate, cattle production, and handling of manure in Mexico. This work resulted from the necessity of estimating the Methane Emission Factors (MEF) from cattle manure, specialized for milk, meat, and dual-purpose, from several climates, breeds, feed, and manure management systems in order to obtain a representative inventory of methane emissions from cattle manure, in agreement to Mexican conditions. In this study we show the relation among pH values during the drying and fermentation processes of cattle manure, from different climates, cattle production systems and production functions. The quantity of formed biogas included methane contents as a function of the types of feeding, temperature, and the manure fermentation in an anaerobic lagoon at 35 degree C, are shown too. Although the variation of pH for the period of fermentation in the anaerobic lagoon was similar to that of the drying process, methane produced was a factor of 46 greater in the anaerobic lagoon than that of the drying and fermentation process. Also, for any initial values, if pH descended quickly close to 6 units, then methane production was inhibited drastically. On the contrary, maximum production of methane was linked to an initial pH value greater than 7 units, with a slow descent of pH during the fermentation process, but without reaching a minimum of 6 units. Finally, R super(2) = 0.9646 obtained among the exponential trends of pH and the samples humidity, is related to the bacteria population in the manure samples and, therefore, to the production of biogas and methane.