Horizontal flow wetlands are shallow excavations with a synthetic or clay liner to prevent ingress or egress of water. They are filled with a media through which the liquid to be purified must flow. This can be anything from soil to light expanded clay aggregate, but 5-10 mm gravel is the most common. An inlet zone of larger media ensures the influent liquid is distributed effectively into the media. A similar outlet zone collects the treated liquid in drainage pipes which pass through the liner into a level control chamber where a simple plastic tube or swivel pipe allows the liquid level in the wetland to be controlled.

The media is planted with marsh plants, the common reed, Phragmites australis, being the most commonly used, hence the common name of reed beds applied to many sub-surface flow and surface flow wetlands. Other plants are often used however, especially when biodiversity is important.

Purification of the liquid occurs by a complex mixture of processes, some aerobic (using oxygen ) others anaerobic ( without oxygen ). The two most important mechanisms are filtration and bacterial removal. Dense populations of bacteria growing on the roots of the marsh plants contribute significantly to the treatment process. The roots fulfil a further function in slowing down the rate at which the media clogs with accumulated solids.   

Horizontal flow wetlands are the most common type in the UK and are used for secondary treatment (strongly polluting liquids such as industrial effluent or septic tank effluent), storm water treatment (run-off from roads and roofs either alone or in admixture with sewage), grey water (from washbasins and showers/baths) or tertiary treatment (polishing of effluents from other sewage treatment processes).

Vertical flow

Vertical flow wetlands are shallow excavations or above ground constructions either built of impermeable materials or lined with synthetic or clay materials to prevent ingress or egress of water. At the base of the excavation are drainage pipes which are usually turned up so they reach the surface at their ends. This allows air to move in and out of the wetland. The pipes are overlain by media or a number of different layers of media. These range from soil to light expanded clay aggregate, but gravels and coarse sands are most widely used.  

The liquid to be treated is applied as a pulse to slabs on the surface of the media, usually by pumping, but other devices which do not use power may be possible if their is sufficient fall on the site.  Marsh plants such as the common reed, Phragmites australis, are planted in the media. Other plants are often used however, especially when biodiversity is important.

The surface of the wetland floods to a depth of several centimetres then slowly percolates downwards through the media undergoing filtration and coming into contact with the dense microbial populations on the surface of the media particles and roots. Treatment is mainly aerobic (using oxygen), and is a series of cycles with pumping and draining phases.

Treated water leaves the wetland through the drainage pipes at its base, usually by gravity, and passes either to the watercourse or the next phase of treatment.

It is usual to have at least one vertical flow wetland not in use, thereby allowing a rotation between wetlands every few days. This prevents clogging of the surface and maintains treatment efficiency.

This type of reed bed has been adapted for the purpose of de watering sludge.

Surface Flow Wetlands

Surface flow wetlands are amongst the most simple to construct. They are very shallow excavations or shallow earth banked lagoons enclosing an area of land. Soil, or some other media such as gravel provides the growing media for the marsh plants. A liner may not be necessary. To avoid short-circuiting the surface should be virtually flat with a very gentle slope towards the outlet end.  Many species of plants can be used, a particular benefit if biodiversity is important.

A system of pipes or channels distributes the wastewater over the inlet end of the wetland, and a collection channel collects the treated liquid at the outlet end. The wastewater flows along the surface allowing settlement of solids and coming into contact with the bacterial populations on the surface of the media and plant stems.

Surface flow wetlands have  a lower efficiency per unit surface area than

sub-surface wetlands and therefore take up large areas of land.

Free water surface wetlands

Another simple type of wetland. They are basically shallow ponds filled with aquatic plants. Several may be provided, rather like a series of lakes on a river. This is actually a good analogy, because the treatment processes that purify water in natural lakes and rivers are exactly the same as those operating in free water surface wetlands.

Treatment mechanisms are complex but include settlement of solids and contact with bacteria suspended in the water and attached to surfaces. Algae can be seasonally important.

To avoid short-circuiting good distribution of wastewater at the inlet to the wetland is needed and similarly good collection facilities at the outlet end, although very big wetlands or multiple wetlands in series may not need these at all.

Free water surface wetlands have a lower efficiency per unit surface area than sub-surface flow wetlands and therefore take up large areas of land.