ReviewHorizontal sub-surface flow and hybrid constructed wetlands systems for wastewater treatment
Introduction
Constructed wetlands (CWs) are engineered systems that have been designed and constructed to utilize the natural processes involving wetland vegetation, soils, and the associated microbial assemblages to assist in treating wastewaters. 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. CWs for wastewater treatment may be classified according to the life form of the dominating macrophyte into systems with free-floating, rooted emergent and submerged macrophytes (Brix and Schierup, 1989). Most constructed wetlands for wastewater treatment are planted with emergent macrophytes but the design of the systems in terms of media as well as the flow regime varies. The most common systems are designed with horizontal sub-surface flow (HF CWs) but vertical flow (VF CWs) systems are getting more popular at present. Constructed wetlands with free water surface (FWS CWs) are not used as much as the HF or VF systems despite being one of the oldest designs in Europe (Brix, 1994, Vymazal et al., 1998a, Vymazal, 2001a). Constructed wetlands have been used for decades mostly for the treatment of domestic or municipal sewage. However, recently CWs have been used for many other types of wastewater including industrial and agricultural wastewaters, landfill leachate or stromwater runoff. As many of these wastewaters are difficult to treat in a single stage system, hybrid systems which consist of various types of constructed wetlands staged in series have been introduced.
The first experiments aimed at the possibility of wastewater treatment by wetland plants were undertaken by Käthe Seidel in Germany in early 1950s at the Max Planck Institute in Plön (Seidel, 1955). Between 1952 and 1956, Seidel carried out numerous experiments on the use of wetland plants for treatment of various types of wastewater, including phenol wastewaters (Seidel, 1955, Seidel, 1965a, Seidel, 1966), dairy wastewaters (Seidel, 1976) or livestock wastewater (Seidel, 1961). In early 1960s, Seidel intensified her trials to grow macrophytes in wastewater and sludge of different origin and she tried to improve the performance of rural and decentralized wastewater treatment which was either septic tanks or pond systems with inefficient treatment. She planted macrophytes into the shallow embankment of tray-like ditches and created artificial trays and ditches grown with macrophytes. Seidel named this early system the hydrobotanical method. Then she improved her hydrobotanical system by using sandy soils with high hydraulic conductivity in sealed module type basins planted with various macrophyte species. To overcome the anaerobic septic tank systems she integrated a stage of primary sludge filtration in vertically percolated sandy soils planted with Phragmites australis. So the system consisted of an infiltration bed through which the sewage flowed vertically and an elimination bed with a horizontal flow (Seidel, 1965b). This system was the basis for hybrid systems which were revived at the end of the 20th century.
Section snippets
Constructed wetlands with horizontal sub-surface flow
The most widely used concept of constructed wetlands in Europe is that with horizontal sub-surface flow (Fig. 1). The design typically consisted of a rectangular bed planted with the common reed (P. australis) and lined with an impermeable membrane. Mechanically pre-treated wastewater is fed in at the inlet and passes slowly through the filtration medium under the surface of the bed in a more or less horizontal path until it reached the outlet zone where it is collected before discharge via
Hybrid constructed wetlands
Various types of constructed wetlands may be combined in order to achieve higher treatment effect, especially for nitrogen. However, hybrid systems comprise most frequently VF and HF systems arranged in a staged manner (Fig. 3). There are now many fine examples of HF systems for secondary treatment and they proved very satisfactory where the standard required only BOD5 and SS removal. However, there has been a growing interest in achieving fully nitrified effluents. HF systems cannot do this
Conclusions
Constructed wetlands with horizontal sub-surface flow are a viable alternative for wastewater treatment for small sources of pollution especially when organics and suspended solids are the treatment target. Removal of organics (BOD5 and COD) and suspended solids is very high and steady over the years of operation. Removal of nutrients (nitrogen and phosphorus) is usually low and does not exceed 50% for municipal sewage when systems are dimensioned at about 5 m2 per population equivalent.
Acknowledgements
The study was supported by grant MSM 000020001 “Solar Energetics of Natural and Technological Systems” from the Ministry of Education and Youth of the Czech Republic and by grant No. 206/02/1036 “Processes Determining Mass Balance in Overloaded Wetlands” from the Grant Agency of the Czech Republic.
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