Constructed wetlands are artificial wastewater treatment systems consisting of shallow ponds or channels which have been planted with aquatic plants and which rely upon natural microbial, biological, physical and chemical processes to treat wastewater. They have impervious clay or synthetic liners and engineered structures to control the flow direction, liquid detention time and water level. Depending on the type of system, they contain an inert porous media such as rock, gravel or sand [US EPA 2000].

Historically, Constructed wetlands were already used since centuries to treat a variety of wastewaters such as municipal wastewater, urban runoff, agricultural drainage, etc. However, this lesson focuses mainly on the treatment of municipal wastewater or its separated flows such as greywater. The Constructed wetlands according to this application are considered as a mayor treatment step, which usually need a pre-treatment and, depending on the reuse purpose, a post treatment.

This system can be divided into two types, on the one hand is free-water surface type (FWS) in which the water level is over the surface, and on the other hand is subsurface type (SF), in which the water level is maintained below the surface. The latter one can be further categorized into two types based on the pattern of flow, one with horizontal subsurface (HSF) and one with vertical subsurface flow (VSF) (Crites, et. al., 2000). The SF type can also be called “reed bed�. The illustration of each system can be seen in the figure below.

The free water surface Constructed wetlands (FWS) closely resemble natural wetlands because they look like ponds containing aquatic plants that are rooted in the soil layer on the bottom. The water flows through the leaves and stems of the plants. Their design and operation is very close to pond systems.

Figure 1: Schematic presenting each type of Constructed wetlands which A: FWS, B: HSF, and C: VSF (Brix, 1993)

The focus of this lesson is based on the Constructed wetlands with subsurface flow. This is due to several researches indicating that the pollutant removal efficiency is better than in FWS per unit of land, implying the area requirement is lower. These systems also pose no problem of mosquito or other insects breeding as well as the human, probably children, exposure to surface wastewater. Some disadvantages of this type are higher cost and have lower ecological value comparing to the FWS wetlands, which are of minor concerns.

The HSF and VSF systems do not resemble natural wetlands because they have no surface flow of water. They contain a bed of media which is typically gravel and sand, but also soil or crushed rocks can be also used. Within the media, emergent macrophytes are planted and the water is introduced beneath the surface of the media and is flowing through the roots and rhizomes of the plants. Conventionally, the flow in HSF systems is continuous, hence it creates a “saturated� condition within the wetland body whereas the flow in VSF systems is commonly intermittent, which results in an “unsaturated� and thus aerobic condition. Figure 2 depicts the photo of one VSF system in Hannover, Germany.

Figure 2: Unsaturated vertical flow Constructed wetlands in Hannover, Germany

It should be noted that FWS and SF Constructed wetlands work differently because the latter system does not support any aquatic wildlife. Some biological and chemical interactions only occur in an open water column and thus these will happen only in a FWS system. Moreover, Constructed wetlands should not be mixed with created or restored wetlands which are not designed for wastewater treatment but have the function of wildlife habitat.