Biogas generation by anaerobic digestion from prickly pear cactus (Opuntia ficus-indica) and nejayot

Abstract

In Mexico, the marketing of prickly pear (Opuntia ficus-indica) and the production of tortillas generate agricultural waste and by-products as a result of their widespread consumption. The residues from the prickly pear harvest and nejayote, a by-product of the tortilla-making process, present an opportunity to be utilized as substrates in anaerobic digestion (AD) for biogas generation. This study focuses on analyzing the percentage and energy expenditure of the biogas obtained through this sustainable practice and its impact on waste management and renewable energy production. A pilot-scale Upflow Anaerobic Sludge Blanket (UASB) reactor was used to perform AD. It was fed weekly with pre-treated waste from prickly pear pads and nejayote. Various physicochemical parameters were monitored during the study to assess process performance, including pH, conductivity, alkalinity, alpha ratio, total solids, and volatile solids. Using a BUCK Scientific® gas chromatograph, it was determined that the generated biogas contains 62% methane (CH₄) and 38% CO₂, with a yield of 195 NmL/gVS and 300 NmL/gVS. As the adaptation process progressed, it became evident as a source of renewable energy, highlighting its potential in the renewable energy sector. However, the analysis of the energy expenditure required for reactor maintenance and feeding, compared to the net energy generated by biogas, indicated that prickly pear and nejayote-based biogas production is a sustainable process. Weekly, 0.0581 kW to 0.06279 kW of energy was produced, utilizing 16.652 kW. It is relevant to consider that the reactor was in an adaptation stage, during which the reactor scale, feeding frequency, and the variability of monitored parameters can influence the quantity of biogas produced