Wetland Cell

A wetland cell is a lined basin of gravel planted with reeds and cattails, built to treat water rather than hold it. Wastewater moves slowly through the gravel while the planting takes up the nutrients and organic matter it carries. It is a polishing stage, not a treatment system on its own: it always sits behind a septic tank, which settles the solids first, and what leaves is cleaner water rather than clean water.

The main choice is whether water runs over the gravel or through it. A subsurface-flow cell holds the water below the gravel surface, so the media stays clear of it and people, animals and insects have little contact; for a house that means a shallow lined excavation 30 to 45 cm (12 to 18 in) deep. A surface-flow cell leaves it open, which is simpler to build; the published sizing rests on it, at least twelve days from inlet to outlet. Open water near a house brings back the odour, contact and mosquito exposure. Retention time, liner and inlet and outlet levels are specialist design work on regulated wastewater, not an owner build.

Treatment is not steady. The biology slows in the cold, enough that some cells are roofed over, and the water leaving it is not pathogen-free for part of the year, so it goes to a buried dispersal field or a disinfection stage, never into the open. The planting is the maintenance that decides everything: plants must stay alive to take up nutrients, so neglected growth is lost treatment, not untidiness. Standing water breeds mosquitoes, and the cycle must be broken at least weekly: by draining or drawdown where possible, otherwise with a biological larvicide. Two failures are silent: gravel clogged by solids and biofilm, and uneven inlet distribution letting water short-circuit through part of the bed. Either way it runs on while treating less than it seems to. Digging into the media exposes anyone doing it to part-treated sewage.