eat the yucky stuff the water carries. Much like in the pumice planter setup, a layer of soil is planted and the vegetation feeds off the natural fertilizer in the gravel below. The somewhat purified effluent that comes out the other end of the wetland can be piped to other outdoor plants.
There are several innovative projects in the blackwater arena (which also work for graywater), based around similar principles to those above. Some of these as-yet-unique systems use solar-heated septic tanks or other cleaning systems to purify sewage to levels that would make utility companies jealous.