  We present results for the isospin-0 $\pi\pi$ s-wave scattering length
calculated in twisted mass lattice QCD. We use three $N_f = 2$ ensembles with
unitary pion mass at its physical value, 240~MeV and 330~MeV respectively. We
also use a large set of $N_f = 2 + 1 +1$ ensembles with unitary pion masses
varying in the range of 230~MeV - 510~MeV at three different values of the
lattice spacing. A mixed action approach with the Osterwalder-Seiler action in
the valence sector is adopted to circumvent the complications arising from
isospin symmetry breaking of the twisted mass quark action. Due to the
relatively large lattice artefacts in the $N_f = 2 + 1 +1$ ensembles, we do not
present the scattering lengths for these ensembles. Instead, taking the
advantage of the many different pion masses of these ensembles, we
qualitatively discuss the pion mass dependence of the scattering properties of
this channel based on the results from the $N_f = 2 + 1 +1$ ensembles. The
scattering length is computed for the $N_f = 2$ ensembles and the chiral
extrapolation is performed. At the physical pion mass, our result $M_\pi
a^\mathrm{I=0}_0 = 0.198(9)(6)$ agrees reasonably well with various
experimental measurements and theoretical predictions.
