History and future of domestic biogas plants in the developing world

The effect of manure-screening on the biogas yield of dairy manure was evaluated in batch digesters under mesophilic conditions (35 degrees C). Moreover, the study determined the biogas production potential of different mixtures of unscreened dairy manure and food waste and compared them with the yield from manure or food waste alone. A first-order kinetics model was developed to calculate the methane yield from different mixtures of food waste and unscreened dairy manure. The methane yields of fine and coarse fractions of screened manure and unscreened manure after 30days were 302, 228, and 241L/kgVS, respectively. Approximately 93%, 87%, and 90% of the biogas yields could be obtained, respectively, after 20days of digestion. Average methane content of the biogas was 69%, 57%, and 66%, respectively. Based on mass balance calculations, separation of the coarse fraction of manure would sacrifice about 32% of the energy potential. The methane yield of the food waste was 353L/kgVS after 30days of digestion. Two mixtures of unscreened manure and food waste, 68/32% and 52/48%, produced methane yields of 282 and 311L/kgVS, respectively after 30days of digestion. After 20days, approximately 90% and 95% of the final biogas yield could be obtained, respectively. Therefore, a hydraulic retention time (HRT) of 20days could be recommended for a continuous digester. The average methane content was 62% and 59% for the first and second mixtures, respectively. The predicted results from the model showed that adding the food waste into a manure digester at levels up to 60% of the initial volatile solids significantly increased the methane yield for 20days of digestion. Copyright 2010 Elsevier Ltd. All rights reserved.