The biodegradability of the feed mixture was determined from the intercept and was in the range of 47–56% of the influent volatile solids concentration shown in Table 2. The graphs used to determine the refractory fraction in the reactors are shown in Fig. 8. The biodegradability factor obtained from the graph indicates that the presence of resistant volatile matter is the major portion of volatile solids in the digester. This reasonably conforms to the experimentally determined VS destruction efficiency of 43–52%. The biodegradable fraction of volatile solids obtained from the graphical model and the experimental values are presented in Table 2.