This desk study deals with the mechanisms and parameters affecting particles separation from wastewater in mainly upflow anaerobic reactors. Despite the fact that the functioning of upflow anaerobic sludge blanket (UASB) systems depends on both physical parameters and biological processes, the physical parameters have been barely reported in the literature. The reason is that the underlying mechanisms are very complex and depend on various interrelated parameters. In addition, the lack of a serious attempt to gather the entire physical theme into one picture has resulted in just a superficial understanding of this field of science. Better understanding of the interaction and role of these parameters is essential for the development of anaerobic treatment technologies. In this study, the various parameters that might affect the solid liquid separation process by filtration through the sludge bed of a UASB have been elaborated. These parameters have been classified into (1) reactor operational conditions (temperature, organic loading rate, hydraulic retention time and upflow velocity), (2) influent characteristics (influent concentration, influent particle size and influent particle charge) and (3) sludge bed characteristics (particle size distribution, extracellular polymeric substances, and charge). The overall output of this study includes (1) a literature review, (2) structuring of this field of science, and (3) highlighting fields where research is needed.