The refractory fraction can be determined graphically from the intercept of a
plot of (S/S0) and (S0*t)−1 , where S = substrate concentration (g/l), S0 = initial VS concentration (g/l) and
t = retention time (d). 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