Disclosed is a system and method for treating municipal and sanitary wastewater that uses only mechanical devices and processes, which eliminates biological processes and settling tanks. The system includes a three-output Richter-type separator that separates wastewater into three fluid streams according to the specific gravity of the solids within the fluid streams. The lighter-than-water and heavier-than-water solids streams are combined and the resultant sludge is mechanically dewatered without intermediary biological-process systems or sedimentation. The partially-clarified water component can be directly filtered by a membrane filter and optionally optically or chemically disinfected for reuse or disposal. The system advantageously simplifies municipal and sanitary wastewater treatment eliminating traditional primary and secondary treatment stages, and significantly reducing the system's operational footprint. The system and method can be scaled to very large municipal systems.