Abstract: A solid organic waste is fermented in water in two phases, a hydrolysis/acidification phase and a methane fermentation phase. Solid organic waste is introduced in the first phase, of which waste at least 80% of the volume has a particle size larger than x mm, x being between 0.25 and 1.5 mm dependent on the introduced solid organic waste. The waste in the first phase is continuously or periodically stirred and is screened for the removal of particles smaller than x mm with liquid, whereafter the smaller particles are separated from the liquid and the so-separated liquid is at least mainly supplied to the methane fermentation space, while the separated smaller particles are recirculated to the first phase reactor and/or discharged.