Seven Bioreactor Landfill simulators (mixed gravel, gravel in layers, and controls without gravel with two levels of compaction, i.e. normal and lower density) were used to investigate the effect of different hydraulic conditions on the waste stabilisation process. The simulators with mixed gravel performed better by absorbing the temporal and spatial changes of the MSW particles, which allowed better contact of liquids and solids thus providing favourable conditions for micro-organisms to enhance the waste stabilisation process. Moreover, neutral pH levels seemed to be the “driving force” that enhanced physical, chemical and biological processes contributing to waste stabilisation in the simulators. After one year of operation, the residues of the different simulators were very close to achieving a Final Storage Quality status comparable to the Waste Acceptance Criteria for inert waste of the European Landfill Directive.