Treatment processes in wetland incorporate with several physical, chemical, and biological processes. The major physical process is the settling of suspended particulate matter which is a major cause of BOD reduction. The chemical processes involve adsorption, chelation, and precipitation, which are responsible for the major removal of phosphorus and heavy metals. In term of biological processes, the treatment is achieved by microorganisms (Gopal, 1999). Due to fixed film or free bacterial development, biological processes allow the degradation of organic matter, nitrification in aerobic zones and denitrification in anaerobic zones. The microbiological activity is the key parameter for their performance. The principle removal mechanisms in subsurface flow Constructed wetlands for some constituents in wastewater are summarized in table 1.

Table 1: Principle removal and transformation mechanisms in subsurface flow Constructed wetlands for the concerned constituents in wastewater (modified after Crites and Tchobanoglous, 1998)

For the role of plants in constructed wetland, they contribute to nutrient transformation, offer mechanical resistance to flow, increase the retention time, facilitate settling of suspended particulates, and improve conductance of water through the media as the roots grow. Particularly, the rhizomes of the reeds grow vertically and horizontally, opening up the soil to provide a hydraulic pathway through the media. Furthermore, they transport oxygen to the deeper layer of the media via the leaves and stems of the reeds down through the hollow rhizomes and out through the roots and hence help in oxidation and precipitation of heavy metals on the root surfaces (Gopal, 1999). However, Hiley and Hadjichristova (1998) stated that it is still debated whether the plants contribute any oxygen or not. In order to maximize the benefit in SF wetland, the full depth of the media should be compatible with the full plant root penetration so that potential contact points could be available throughout the profile (Reed et. al., 1995). The most frequently used plants species are Scirpus sp. (bulrush), Typha sp. (cattail), and Pragmites communis (reeds). Their typical characteristics are described below (Crites and Tchobanoglous (1998) and Reed et. al. (1995)).

Table 2: Typical characteristics of some plant species used in constructed wetland

Note: ppt = parts per thousand

Plant uptake of nitrogen and phosphorus is not a significant removal effect because they are taken up and usually released during decay. While uptake rates are potentially high, harvesting plant biomass can remove nitrogen and phosphorus but no research shows a significant removal performance due to harvesting. Harvesting plants is anyhow limited to both HSF and VSF systems.

The detailed schematic of HSF Constructed wetlands is shown in figure 4;

Figure 4: Detailed schematic of a horizontal flow system (HSF)

The VSF system illustrating more detail shown in figure 5 needs a well designed and constructed system to distribute the water equally over the whole area. The construction is therefore more expensive than for the horizontal flow systems. For VSF, filtrationis also an important removal mechanism. The bed media must be carefully chosen according to the wastewater constitution.

The water level is always at the bottom. Its best performance can be achieved by intermittent feeding when aerobic and anoxic phases alternate. Due to the higher effort in designing and constructing the VF properly, the performance of these systems in term of COD and nitrification is much higher than in the other constructed wetland systems.

Figure 5: Detailed schematic of an unsaturated vertical flow system (VSF)

Waterborne pathogens including helminth, protozoa, bacteria and viruses are of great concern in assessing water quality. pathogens in wastewater are usually associated with TSS and can be removed like TSS, mainly sedimentation. Thus removal of pathogens (measured by indicators) in wetlands appears to be correlated with TSS removal and hydraulic residence time (US EPA 2000). Analyses in Constructed wetlands show a significant reduction of pathogens about two to three logs, which mean more than 99% removal which is significant but usually not sufficient to meet standards for water reuse.