Abstract: An improved sewage treatment system including a horizontally elongated tank divided transversely into a primary, aeration compartment, a secondary, settling compartment and a tertiary, filtering compartment. In a first aspect of the invention, overflow of treated effluent from the tank enters a further treatment section, which may constitute a longitudinal extension of the tank, wherein the effluent is constrained by baffles to flow in an elongated path while having chlorine or ozone mixed therewith to provide further purification. Another aspect of the invention resides in the novel structure of the filtering section wherein filter screens of different guage or opening size are used to achieve a greater degree of purification.