This application relates to an apparatus and method for treating effluent. The effluent may be derived, for example, from an upstream secondary treatment plant. The apparatus employs a plenum positionable within a fluid-impervious container. The plenum includes a fluid filter portion containing a hydrophilic material, such as peat, capable of transporting effluent by capillary action and a fluid collection portion for receiving effluent from the upstream effluent source. The apparatus also includes a fluid wicking conduit containing hydrophilic material for wicking effluent from the fluid collection portion. The fluid wicking conduit may be configured so that any appreciable amount of effluent introduced into the apparatus will be wicked even if the amount is substantially less than the fluid holding capacity of the fluid collection portion of the plenum. This enables the apparatus to operate substantially continuously and passively so long as some effluent is resident in the plenum. At the same time, the fluid holding capacity of the fluid collection portion can be sized to accommodate periodic surges in effluent flow. The apparatus may further include a dispersal mound of soil and vascular aquatic plants covering or adjacent to the plenum. The invention functions as a vegetative tertiary filter.