Experiments were carried out during the cool season (20–30◦C) and the warm season (30–40◦C) and repeated three times. Three hundred kilograms dry weight of water hyacinth (300 kg), cut into small pieces of 5 to 10 cm pieces, and 1000 kg dry weight of fresh rumen residue (3/1 ratio) were diluted in 20 m3 of liquid and introduced into the digester. The metal cover that slides into the hydraulic joint hermetically seals the mixture. The gas is collected and stored in the gasometers. The volume of biogas is measured using an ELSTER HANDEL GMBH MAINZ volumetric meter, which has a limit of detection of less than 0.0010 m. Gas pressure is measured with a hand-crafted pressure gauge made of a 10 cm2 section of plastic tube attached (in a U-shape) to a graduated rectangular plank (Figs. 2 and 3). One extremity of the tube containing water is connected to the digester while the other is left open. Gas pressure causes the column of water to move and its displacement indicates the amount of pressure being created.