Constructed-wetland systems attempt to replicate and utilise the water ‘purifying’ features of natural wetlands and apply them to the treatment of ‘dirty’ waters from municipal, industrial and agricultural sources. There are two factors that are primarily responsible for constructed wetland functioning:
- Macrophyte (plant) species
- Wetland design

Whilst it is bacteria that are responsible for the degradation of ‘polluting’ chemicals into less toxic forms (via their aerobic metabolic pathways), it is the plants which are responsible for supporting bacterial colonies on their roots and allowing the associated processes to take place.

The area of soil directly surrounding plant roots has been termed the ‘Rhizosphere’; here the concentration of bacterial communities is significantly higher than in soil only a few centimetres away; the plants actively encourage this association by producing exudates, which support the growth of the bacterial communities; bacteria ‘repay’ this debt by aerobically transforming complex, pollutant compounds into those which the plant can absorb and use itself. Some plants are specifically adapted to support the bacteria in flooded soils (as in wetland systems). Oxygen diffusion in aqueous solutions is 10-4 slower than in air, therefore anaerobic conditions would quickly arise in the rhizosphere (allowing anaerobic communities to proliferate and start producing methane (CH₄), ammonia (NH₃) and Hydrogen Sulphide (H₂S)); a number of plant species have made adaptations to their root systems which allow them to transport significant quantities of oxygen to the rhizosphere (through specialised gas spaces called aerenchyma), thus allowing them to survive in what would otherwise have been toxic environments.
Common reed (Phragmites australis) has probably been the most widely used species over the last thirty years, due mainly to its high oxygen transport rates and ability to grow in a variety of climes; research at the Wales Biomass Centre has looked at the potential for biomass crops (miscanthus, giant reed, reed canarygrass but specifically Willow) to be used in biofiltration systems.

The design of constructed wetland systems is an area of considerable research interest; determination of planting patterns, planting media, rate and orientation of water inflow are major considerations for designers as all have an influence on system efficiency. Planting patterns, and species used, are chosen to provide maximal root-water contact; the size and adsorption potential of the planting material (usually gravel-sand mixes) are important for flow conductivity and potential chemical adsorption/retention (particularly with regards to phosphorus). Water distribution throughout the treatment bed is the other major variable:

- SURFACE FLOW SYSTEMS – These are shallow basins where water flows above the level of the planting media, from the inlet at one end of the bed to the outlet at the other. Whilst they are relatively cheap to setup and maintain they consequently require a larger surface area for operation. They are frequently used to treat storm-water runoff and mine waters.

- SUBSURFACE FLOW SYSTEMS – can handle greater flow rates but tend to have higher costs. There are two types:

Horizontal flow systems (HSSF) – Water flows through the planting media, from inlet to outlet. The media becomes saturated with water and oxygen is only supplied by the plants; this is the general form of constructed treatment-wetlands. It is important that the correct media is used in order that water conductivity is not impeded. Suspended solids (SS) and organic materials (as measured by the Biochemical Oxygen Demand (BOD) test) are more effectively removed by this type of system than nitrogen and phosphorus.

Vertical flow systems (VSSF) – These systems have water applied to different parts of the bed and not just at one inlet end. An intermittent loading regime also introduces air into the system, effectively oxidising and reducing the bed at regular intervals; thus proving very effective in the removal of nitrogen and phosphorus from the out-flowing waters. However, this system requires a greater degree of investment to setup and maintain.

In general, constructed wetlands (primarily HSSF and VSSF) are used to treat secondary and tertiary wastewaters. Initially, wastewaters (which are usually from municipal and agricultural sources and therefore have a high percentage of solids, fats, proteins and organic materials) are introduced to a sludge tank/bed and allowed to settle, thus removing a high percentage of pollutants and solids; they are then passed on to wetlands to remove specific pollutant elements and for ‘polishing’. For systems serving high populations (~70+) it is usual for there to be a combination of beds for different stages of treatment, and also a number of beds can be used in just one stage; these beds may run sequentially or in parallel (recent research has shown positive results with regards to operation of the latter and quality of its effluent).

Work at the Wales Biomass Centre has suggested that there is a strong potential role for Willow in future biofiltration systems; not only are many species highly flood tolerant, biomass varieties have high growth rates (excellent for nutrient uptake), have a high biodiversity value and have a high degree of association with mycorrhizal fungi, something which may be exploited in the uptake of ‘waste’ nutrients. Biofiltration is still an emerging technology, and its potential marriage with biomass technologies has yet to be fully realised.