This study aimed to evaluate the effect of high total solid concentrations on biogas production from cattle dung. The evaluation process was achieved using four laboratory biogas digesters fed with fresh cow dung at three concentrations; 10 %, 15% and 20% of total solids, three different retention times of 20, 25, and 30 days and three different stirrer speeds of 80, 100, and 120 rpm. The stirring period was 15 minutes every 4 hours. The digesters have been installed inside a water bath to keep

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... the digestion temperature at 40 ± 2ºC. The evaluation also, includes the estimation of energy production, energy consumption, and the net energy gained. The obtained results showed that increasing total solid concentration increase the biogas production rate (V/V/day) while the biogas productivity (V/kg TS add ) was decreased. The optimal conditions which gave the maximum biogas production and energy gained were; 15% of total solids (TS), 20 days of retention times, and 120 rpm of stirring speed. The maximum average biogas production rate and energy production at the optimal conditions were; 1.44 m 3 /m 3 /day and 28.881 kWh respectively. The energy consumption represents about 13.84% from the maximum energy production at stirring speed of 120 rpm and 28.66% from the minimum energy production at the non stirring digester, (control).