This research work aims to study the effect of mechanical stirring at different speeds and interval stirring periods on the biogas production yield and productivity and the energy balance in large scale biogas digesters. This investigation was conducted on two fixed dome digesters with 20 m3 total volume. The study includes three speeds of stirring (30, 45 and 60 rpm) and four stirring periods (15 min/hr, 15 min/2 hr, 15 min./3 hr and 15 min./4hr) ‗which equal to 6, 3, 2 and 1.5 hr/day, respectively. The obtained results showed that the stirring speed of 60 rpm was gave the high values of biogas production rate( 0.423 m 3 /m 3 /day), biogas productivity (0.423 m 3 /m 3 /day, 0.106 m 3 /kg TS add and 0.707 m 3 /kg VS consumed), energy production (9.379 MJ/m 3 /day), energy consumption in the stirring process (3.430 MJ/m 3 /day) and net energy gained (8.448 MJ/m 3 /day). However, the stirring period of 15 min./2 hr (3 hr/day) was gave the maximum biogas production rate and energy production at different stirring speeds and net energy gained at stirring speeds of 30 and 45 rpm. Meanwhile, the stirring period of 15 min./4 hr (1.5 hr/day) was gave the lowest energy consumption at different speeds and high net energy gained at stirring speeds of 60 rpm.

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 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).

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The main purpose of this study was to improve the biogas quality by keeping the methane content above 90%. To achieve this goal, it has been used a combined stage of slaked lime solution 50%and sodium hydroxide of 10 and 20% respectively at different stirring methods. The stirring methods were; without stirring, manual stirring, and mechanical stirring at speeds of 250, 200 and 150 rpm.The obtained results indicated that, a combined stage of slaked lime solution 50% and sodium hydroxide of 10%and 20%respectively with mechanical stirring at speed of 150 rpm showed a highly removal performance compared with the other different treatments. The methane content was above 99% through the first 32 hours compared with 19, 28 and 29 hours for manual stirring and mechanical stirring speeds of 250 and 200 rpm respectively. Moreover, the methane content was remained above 90% at removal time of 44 hours compared with 32, 40 and 41 hours for manual stirring and mechanical stirring at the same speeds respectively. In addition, the mechanical stirring at speed of 150 rpm gave the highest average removal efficiency of carbon dioxide 95.9% which keeping it less than 10% compared with 92.1, 94.6 and 94.7% for manual stirring and mechanical stirring at speeds of 250 and 200 rpm respectively. On the other hand, the highest average removal efficiency of hydrogen sulfide during purification process period off 44 hours was 79.2% compared with 78.5 and 78.9% for the two other mechanical stirring speeds of 250 and 200 rpm respectively.