def get_retained_mass(self, biomass_IDs):
    cmps = self.components
    mass = (cmps.i_mass * self._state[:(- 1)])
    return (self._V_max * mass[cmps.indices(biomass_IDs)].sum())
def get_retained_mass(self, biomass_IDs):
    cmps = self.components
    mass = (cmps.i_mass * self._state[:(- 1)])
    return (self._V_max * mass[cmps.indices(biomass_IDs)].sum())