where ISOinf.
is isotopic ratio of the influent sludge; ISObio.B.
is iso-
topic ratio of the biofilm in the BF; ISOeff.B.
is isotopic ratio of the
effluent sludge in the BF; Pinf./eff.B.
is contribution ratio of the influent
sludge to the effluent sludge of the BF; Pbio.B./eff.B.
is contribution
ratio of the biofilm to the effluent sludge of the BF.