Wastewater Treatment in Constructed Wetlands with Horizontal Sub-Surface Flow

Wetlands have played a crucial role in human history. Major stages of the evolution of the life itself probably took place in nutrient-rich coastal waters. Some of the first prehistoric cultures, such as those of the early mesolithic settlements around the post-glacial lake margins and coasts of Europe and those of the coastal Indian communities in North America, depended on wetlands for food and materials for building, shelter and clothing (Maltby, 1991). Boulé (1994) in his excellent overview on an early history of wetland ecology pointed out that the early Sumerians knew the names of plants and animals that occupied the marshes of the Tigris and Euphrates Rivers, as evidenced by clay tablets on which those names were inscribed (Kramer, 1981). The Babylonians, who followed the Sumerians in Mezopotamia, not only had names for wetland plant species, but also established municipal reed beds and reeds harvested from these beds were used to make rugs, coarse mats to strengthen walls of clay brick, and very fine mats to serve as a foundation for dikes made from material dredged from the rivers (the original filter fabric).

The value of a wetland is a measure of its importance to society. Wetland functions are valued to various degrees by society, but there is no precise, general relationship between wetland functions and the value of wetlands to society, and values can be difficult to determine objectively. A wetland’s value can be weighed directly or relative to other uses that could be made of the site; thus, the location of a wetland affects its value to society (Lewis 1995).

Wetlands are transitional environments. In a spatial context, they lie between dry land and open water – at the coast, around inland lakes and rivers, or as mires draped across the landscape. In an ecological context, wetlands are intermediate between terrestrial and aquatic ecosystems. In a temporal context, most wetlands are destined either to evolve into dry land as a result of lowered water tables, sedimentation and plant succession, or to be submerged by rising water tables associated with relative sea-level rise or climatic change. Wetlands often form part of a large continuum of community type, and therefore it is difficult to set boundaries. Consequently, few definitions adequately describe wetlands with the problem of definition usually arising on the edges of wetland, toward either wetter or drier conditions (Vymazal, 1995a).

The three most important physicochemical properties of the soil that are affected by flooding are pH value, ionic strength, and oxidation-reduction potential (Eh or redox potential) (Patrick et al., 1985).

Flooding the soil causes an increase in the concentration of ions in the soil solution, although the increase may not persist throughout the growing season. In slightly acid and acid soils, the reduction of insoluble Fe, and possible Mn compounds, to more soluble forms accounts for much of the increase in cations. In neutral to slightly alkaline soils, Ca²⁺, and Mg²⁺ in the soil solution make significant contributions to the ionic strength. Ferrous and manganous ions produced through reduction reactions displace other cations from the exchange complex to the soil solution (Patrick et al., 1985).

Numerous lines of evidence indicate that aquatic angiosperms originated on the land. Adaptation and specialization to the aquatic habitat have been achieved by only a few angiosperms (< 1%) and pteridophytes (< 2%). Consequently, the richness of plant species in aquatic and wetland habitats is relatively low compared with most terrestrial communities (Richardson and Vymazal, 2001). Most are rooted, but a few species float freely in the water (Wetzel, 2001).

Tiner (1999) pointed out that plants growing in wetlands and water are technically called hydrophytes. However, today’s usage of the term hydrophyte is different than its original use. In the 1800s and early 1900s, it was used to define aquatic plants that were plants growing in water (Schouw, 1822, as reported in Warming, 1909) or plants with perennating buds beneath the water (Raunikaer, 1905, 1934). Warming and Raunkiaer were among the earliest of the plant ecologists to use the term hydrophyte. Hydrophytes were distinguished from helophytes, which included various wetland plants depending on whose definition was used (Tiner, 1999). Raunkier’s life-form were based on a plant’s adaptation to the critical season (e.g., winter) mainly the degrees of protection possessed by the dormant buds (Smith, 1913). According to this system, hydrophytes (plants with perennating rhizomes or winter buds) and helophytes (plants with buds at the bottom of the water or in the underlying soil) were the two types of cryptophytes (plants with dormant parts below ground), while other wetland plants were included in other life-forms, such as phanerophytes (trees and shrubs) (Smith, 1913). Raunkiaer’s helophytes did not include all typical marsh plants. Tinner (1999) reported that Warming (1909) was probably the first ecologist to arrange plant communities according to the degree of soil wetness. He recognized aquatic plants (water-plants) that spend their entire life submerged or with leaves floating at the surface and terrestrial plants that are mostly exposed to air, including marsh plants. Vegetation was then separated into numerous “oecological classes” based principally on soil properties. The first of the groupings was for soil that was very wet: 1) hydrophytes (formation in water) and 2) helophytes (formation in marsh). Clements (1920) might have been the first ecologist to expand definition of hydrophytes in include helophytes as a type of hydrophyte.

Constructed wetland 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 wastewater. They are designed to take advantage of many of the processes that occur in natural wetlands, but do so within a more controlled environment. Synonymous terms to constructed include manmade, engineered or artificial wetlands.

Constructed wetlands can be built with a much greater degree of control than natural systems, thus allowing the establishment of experimental treatment facilities with well-defined composition of substrate, type of vegetation, and flow pattern. In addition, constructed wetlands offer several additional advantages compared to natural wetlands including site selection, flexibility in sizing, and most importantly, control over the hydraulic pathways and retention time (Brix, 1993a).

The technology of wastewater treatment by means of constructed wetlands with horizontal sub-surface flow was started in Germany based on research by Käthe Seidel commencing in the 1960s (e.g., Seidel, 1961, 1964, 1965 a,b, 1966) and by Reinhold Kickuth in the 1970s (e.g., Kickuth, 1977, 1978, 1981).

Seidel intensified her trials to grow helophytes and hydrophytes in wastewater and sludge of different origin and she tried to improve the performance of rural and decentralized wastewater treatment which was either septic tank or pond system with low purification effect. She planted helophytes into the shallow embankment of tray-like ditches and created artificial trays and ditches grown with helophytes. The system worked effectively and Seidel named this early system the “hydrobotanical method” (Börner et al., 1998). She improved further this method by using sandy soils with high hydraulic conductivity in sealed module type basins planted with different species of helophytes. To overcome the anaerobic septic tank system she integrated a stage of primary sludge filtration in vertically percolated sandy soils grown with Phragmites australis (Seidel, 1965b). The first vertical stage was so called “filter or filtration bed”. The second horizontal flow stage was called “elimination bed” (Fig. 5-1) and was usually planted with Scirpus lacustris. However, too much significance was attracted to the absorption of nutrients by plants and this has been a target for this fundamental concept have been developed over the years. The system itself was revived later and now it is called a “hybrid system”. Both vertical and horizontal flow stages have been used separately as well. criticism (e.g., Nümann, er, 1970). However, numerous variants based on this fundamental concept have been developed over the years. The system itself was revived later and now it is called a “hybrid system”. Both vertical and horizontal flow stages have been used separately as well.

Constructed wetlands have long been used primarily for treatment of municipal or domestic wastewaters. However, at present, constructed wetlands are used for wide variety of pollution, including agricultural and industrial wastewaters, various runoff waters and landfill leachate (Kadlec and Knight, 1996; Vymazal et al., 1998a; Kadlec et al., 2000; Vymazal, 2006b; see also Table 1-1).

In the following section we try to summarize the major types of wastewaters which have been treated in HF constructed wetlands. Wastewaters and polluted waters are divided into five major groups: municipal/domestic, industrial and agricultural wastewaters, runoff waters and landfill leachate. Agroindustrial wastewaters such as those from wineries, distilleries, food and meat processing are included in industrial wastewaters despite these wastewaters are produced during processing agricultural products. Separate section deals with Endocrine Disrupting Chemicals which may occur in many various types of wastewater. The text does not deal with constructed wetlands for sludge dewatering.

Haberl et al. (1998) reviewed the situation concerning the use of constructed wetlands in Austria. They pointed out that due to lack of proper wastewater treatment plants in rural and mountain area CWs had been in discussion as an appropriate option for about 15 years in Austria. Several people promoted CW as on-site plants for these areas but at the same time regulatory authorities did not approve the CW technology. The major objections were the lack of long-term experience, winter operation, hygienic problems and clogging of the substrate. After emotional discussions several experimental CWs were built. One of the most extensively studied experimental plant of all times was CW at Mannersdorf built in 1982 (Fig. 7-1) - it had been monitored for 7 years (e.g., Haberl and Perfler, 1989, 1990). At the same year, HF system in Lainzer Tiergarten was put in operation.

The HF constructed wetlands turned out to be a very appropriate technology providing high stability in its efficiency with low levels of operation and maintenance. Removal of organics and suspended solids was efficient, however, HF CWs often proved insufficient as far as the removal of nutrients was concerned.