Abstract: A method of processing sewage. Non-biodegradable solids are first removed from the sewage for separate disposal. The sewage is then introduced to the bottom of a fermentation cell designed to optimize sedimentation and methane fermentation of settleable organic solids, most of which settle in the fermentation cell. Sulfate-reducing micro-organisms that release sulfides are growing in the fermentation cell. The sulfides released combine with multivalent metal particles in the sewage to form insoluble particles, a portion of which settles in the fermentation cell. The remaining metal sulfides, other suspended solids, microorganisms, nutrients, and pathogens in the sewage are then removed by natural means followed by Dissolved Air Flotation, Slow Sand Filtration and disinfection. Metal ions that escaped sedimentation in the fermentation cell are adsorbed by microorganisms that have a strong negative surface charge. Finally, reverse osmosis is carried out to produce purified water and a high-salinity concentrate.