We have done a microscopic calculation of the s-wave optical potential for the pion-nucleus system at low energies, 0≤Tπ≤100 MeV, using a description based on hadronic degrees of freedom. We have obtained, separately, the real and imaginary parts of the optical potential coming from one-body and two-body processes. We have also separated the imaginary part of the potential associated to absorption and to quasielastic channels. We find that the imaginary part of the absorption channel is independent of the energy (within the range of energies considered here) in agreement with a recent empirical determination. We compare our results with phenomenological potentials obtained by fits to the empirical data. The comparison shows a good qualitative agreement. We discuss the dependence of the optical potential on the πN amplitudes, taken as an input in this model, and we find that a more repulsive isoscalar πN s-wave amplitude would provide better consistency between the π-nucleus theoretical results and the π-nucleus data. New experiments to provide more accurate πN phase shifts at low energies would be welcome.