Using the quantum transport equations for interacting electrons and phonons we study the phonon effects in the thermoelectric transport in impure metals. The contributions of both equilibrium phonons (the diffusive part) and non-equilibrium phonons (the drag effect) are investigated. We show that the drag effect which dominates in the thermopower of pure samples is strongly suppressed even by a small impurity concentration owing to the inelastic electron-impurity scattering processes. As a result we find the form of the phonon drag thermopower of an impure metal taking into account both normal and Umklapp processes of the electron-phonon scattering.