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A stable anaerobic degradation of swine manure with ammonia concentration of 6 g-N/litre was obtained in continuously stirred tank reactors with a hydraulic retention time of 15 days, at four different temperatures. Methane yields of 188, 141, 67 and 22 ml-CH₄/g-VS were obtained at 37, 45, 55 and 60°C, respectively. The yields were significantly lower than the potential biogas yield of the swine manure used (300 ml-CH₄/g-VS). A free ammonia concentration of 1.1 g-N/litre or more was found to cause inhibition in batch cultures at pH 8.0 (reactor pH), and higher free ammonia concentrations resulted in a decreased apparent specific growth rate. Batch experiments with various mixtures of swine and cattle manure showed that the biogas process was inhibited when the swine-to-cattle manure ratio was higher than 25:75, corresponding to a free ammonia concentration of approximately 1.1 g-N/litre. Inhibition of the …