Described are a tertiary disinfection system and process for treating a liquid effluent from a domestic secondary treatment unit. The liquid effluent has pathogenic micro-organisms, remaining suspended solids and dissolved organic pollutants. The system has an inlet, a subterranean initial treatment zone filled with a filtering material for removing some suspended solids, dissolved organic pollutants and pathogenic micro-organisms. The initial treatment zone allows the liquid effluent to percolate downward. The system also has a subterranean polishing zone extending horizontally outward from the initial treatment zone, and filled with a filtering material having pore sizes allowing upward capillary dispersion of the liquid effluent. The system also has a subterranean aeration zone having an interface with the initial treatment zone and the polishing zone, to provide aeration enabling aerobic accommodation of microflora for disinfection of the effluent.