represents the mass balance of the fixed (inorganic)
solids. Where f is the volatile fraction of influent BG substrate obtained from measurement (0.808 in S1 and 0.816 in S2), F is the
mass flow of influent total solid (kg d
1
). a and b are the volatile
fraction (VS/TS) of biomass and undigested BG substrate, respectively (a ¼ w0.80 [43]). X and Y are the mass flow of biomass and
undigested BG (kg d
1
)