Bioenergy recovery from cattle wastewater in an UASB-AF hybrid reactor

New data on biogas production and treatment of cattle wastewater were registered using an upflow anaerobic sludge blanket-anaerobic filter (UASB-AF) hybrid reactor under mesophilic temperature conditions (37 °C). The reactor was operated in semi-continuous mode with hydraulic retention times of 6, 5, 3 and 2 days and organic loading rates of 3.8, 4.6, 7.0 and 10.8 kg COD_t m^-3 d^-1. Biogas volumes of 0.6-0.8 m³ m^-3 d^-1 (3.8-4.6 kg COD_t m^-3 d^-1) and 1.2-1.4 m³ m^-3 d^-1 (7.0-10.8 kg COD_t m^-3 d^-1), with methane concentrations between 69 and 75%, were attained. The removal of organic matter with values of 60-81% (COD_t) and 51-75% (COD_s) allowed methane yields of 0.155-0.183 m³ CH₄ kg^-1 COD_t and 0.401-0.513 m³ CH₄ kg^-1 COD_s to be obtained. Volatile solids were removed in 34 to 69%, with corresponding methane yields of 0.27 to 0.42 m³ CH₄ kg^-1 VS_removed. The good performance of the novel hybrid reactor was demonstrated by biogas outputs higher than reported previously in the literature, along with the quality of the gas obtained in the various experimental phases. The hybrid reactor investigated in this study presents comparative advantages, particularly in relation to conventional complete mixture units, considering economic factors such as energy consumption, reactor volume and installation area.