The amount of gas produced depends on the slurry in the digester volume (Fulford, 1988). The digester volume is also related to the retention time measured in days and the loading rate, in terms of manure solids per unit liquid volume (San Thy et al., 2003). According to experiences in China, 97% of the total yield of gas from fermenting cattle manure will be produced in a period of 50 days at 350C. The hydraulic retention time (HTR) in anaerobic digesters is determined by calculating the number of days required for displacement of the fluid volume of the culture. At a given organic loading rate, the HTR is lower when using high water — content feeds than when using those containing less water (Fannin and Biljetina, 1987). The detention time is dependent on all the factors discussed above. Generally a retention time of between 30 and 45 days and in some cases 60 days is enough for substantial gas production (Clanton et al., 1985; Carcelon and Clark, 2002). A study by Hill (1982) found that detention times for digesters designed to produce maximum daily methane volume varied from 7.9 days for dairy waste to 14.8 days for poultry, and similar wide variations in loading rates existed between the two wastes.