Introduction

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Natural wetlands have been used for wastewater treatment for centuries. In many cases, however, the reasoning behind this use was disposal, rather than treatment, and the wetland simply served as a convenient recipient that was closer than the nearest river or other waterway. Uncontrolled discharge ofwastewater led in many cases to an irreversible degradation of many wetland areas. Wetlands have been considered for a long time as 'wastelands', were scientifically neglected, and, therefore, the impact of wastewaters on different wetlands was not properly assessed.

However, there has been an explosive growth of knowledge about, and a radical change of attitude toward, wetlands during the last few decades. Wetlands have been recognized as providing many benefits including water supply and control (recharge of groundwater aquifers, drinking water, irrigation, flood control, water quality and wastewater treatment), mining (peat, sand, gravel), use of plants (staple food plants, grazing land, timber, paper production, roofing, agriculture, horticulture, fertilizers, fodder), wildlife (e.g., breeding grounds for waterfowl, preservation offlora and fauna), fish and invertebrates (shrimps, crabs, oysters, clams, mussels), integrated systems and aquaculture (e.g., fish cultivation combined with rice production), erosion control, gene pools and diversity, energy (hydroelectric, solar energy, heat pumps, gas, solid and liquid fuel), education and training, recreation, and reclamation.

Natural wetlands are characterized by extreme variability in functional components, making it virtually impossible to predict responses to wastewater application and to translate results from one geographical area to another. Although significant improvement in the quality of the wastewater is generally observed as a result of flow through natural wetlands, the extent of their treatment capability is largely unknown. While most of natural wetland systems were not designed for wastewater treatment, studies have led to both a greater understanding of the potential of natural wetland ecosystems for pollutant assimilation and the design of new natural water treatment systems. It has only been during the past few decades that the planned use of wetlands for meeting wastewater treatment and water quality objectives has been seriously studied and implemented in a controlled manner. The functional role of wetlands in improving water quality has been a compelling argument for the preservation of natural wetlands and, in recent years, the construction ofwetland systems for wastewater treatment. Constructed wetlands (CWs) can be built with a much greater degree of control, thus allowing the establishment of experimental treatment facilities with a well-defined composition of substrate, type of vegetation, and flow pattern. In addition, CWs offer several additional advantages compared to natural wetlands, include site selection, flexibility in sizing, and, most importantly, control over the hydraulic pathways and retention time. The pollutants in such systems are removed through a combination of physical, chemical, and biological processes including sedimentation, precipitation, adsorption to soil particles, assimilation by the plant tissue, and microbial transformations. Natural wetlands are still used for wastewater treatment but at present the use of CWs is becoming more popular and effective around the world.

CW treatment systems are engineered systems that have been designed and constructed to utilize the natural processes involving wetland vegetation, soils, and their associated microbial assemblages to assist in treating waste-water. They are designed to take advantage of many of the same processes that occur in natural wetlands, but do so within a more controlled environment. Some of these systems have been designed and operated with the sole purpose of treating wastewater, while others have been implemented with multiple-use objectives in mind, such as using treated wastewater effluent as a water source for the creation and restoration of wetland habitat for wildlife use and environmental enhancement. Synonymous terms to 'constructed' include man-made, engineered, and artificial wetlands.

The first experiments aimed at the possibility of waste-water treatment by wetland plants were undertaken in Germany in 1952 at the Max Planck Institute in Plon. From 1955 Seidel carried out numerous experiments on the use of wetland plants for treatment of various types of wastewater. Although Seidel's experiments were heavily criticized many researchers continued with her ideas. In the early 1960s, Seidel intensified her trials to grow macrophytes - she planted macrophytes into the shallow embankment of tray-like ditches and created artificial trays and ditches grown with macrophytes. However, Seidel's concept to apply macrophytes to sewage treatment was difficult to understand for sewage engineers and, therefore, it was no surprise that the first full-scale free water surface (FWS) CW was built outside Germany, in the Netherlands, in the late 1960s.

At present, there are many different types of CWs (Figure 1). CWs for wastewater treatment may be classified according to the flow regime into surface flow (SF or FWS) and subsurface flow (SSF) systems. The FWS CWs could be further categorized according to the life-form ofthe dominating macrophyte into systems with free-floating, floating-leaved, emergent, and submerged macrophytes. Within the SSF CWs it is possible to distinguish between systems with horizontal (subsurface) flow (HF or HSSF CWs) and vertical (subsurface) flow (VF or VSSF CWs). As many of these wastewaters are difficult to treat in a single-stage system, hybrid systems that consist of various types of CWs staged in series have been introduced. In the European sense, hybrid CWs are usually formed by a combination of HF and VF systems. However, any types ofCWs could be combined in order to achieve better treatment performance. This article will deal with CWs with FWS.

FWS treatment wetlands have some properties in common with facultative lagoons and also have some important structural and functional differences. Water column processes in deeper zones within treatment wetlands are nearly identical to ponds with surface autotrophic zones dominated by planktonic or filamentous algae. Deeper zones tend to be dominated by anaerobic microbial processes in the absence of light. However, shallow emergent macrophyte zones in treatment wetlands and aerobic lagoons can be quite dissimilar. Net carbon production in vegetated wetlands tends to be higher than that in facultative ponds because ofhigh gross primary production in the form ofstructural

Emergent plants

Surface flow

Submerged plants

Free-floating plants

Constructed wetlands

Floating-leaved plants

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