Many of these systems are derived from the original system developed by Seidel at the Max Planck
Institute in Krefeld, Germany (Figure 35). The process is known as the Seidel system, the Krefeld system, or the Max Planck Institute Process (MPIP). The Seidel design consists of two stages of several parallel VF beds followed by two or three HF beds in series. The VF stages were usually planted with P. australis, whereas the HF stages contain a number of other emergent macrophytes, including Iris, Schoenoplectus, Sparganium, Carex, Typha, or Acorus. The VF beds were loaded with pretreated wastewater for 1-2 days, and were then allowed to dry out for 4-8 days. The thin crust of solids that forms on top of the VF beds is mineralized during the rest period and achieves an equilibrium thickness. The major idea behind this configuration is to achieve some removal of organics and suspended solids and nitrification of ammonia in the first VF stage while in the second HF stage further removal of organics and suspended solids and denitrification takes place.
In the early 1980s, several hybrid systems of Seidel's type were built in France with a system at Saint Bohaire, which was put in operation in 1982, being the best described. It consisted of four and two parallel VF beds in the first and second stages, respectively; the third, fourth, and fifth levels consisted of one HF bed in each level. A similar system was built in 1987 in UK at Oaklands Park. The first stage consisted of six vertical
beds (8 m2 each) intermittently fed and planted with P. australis. The second stage consisted of three vertical beds (5 m2 each) planted with P. australis, Schoenoplectus lacustris (bulrush), and Iris pseudacorus (yellow flag). The third stage was a HF bed (8 m2) planted with yellow flag and the fourth stage was 20 m2 bed planted with bulrush, Sparganium erectum (bur reed), and Acorus calamus (sweet flag).
In the 1990s and early 2000s, VF-HF systems were built in many European countries, for example, in Slovenia, Norway, Austria, France, and Ireland and now this type is getting more attention in more European countries (Figures 36 and 37). In Table 6, a typical treatment effect of the VF-HF system based on original Seidel's concept is presented. The Colecott system near Dublin consists of four VF beds (total of 64 m2) at the first stage, two VF beds
Vertical flow beds used in rotation
Figure 35 Layout of the hybrid VF-HF CW.
(60 m2) at the second stage, and one HF bed (60 m2) at the third stage. The system is designed for 60 PE.
VF-HF systems exhibit a high removal of organics (COD, BOD5) and suspended solids. As compared to
single HF systems, there is much higher removal of total nitrogen as a result of high nitrification in the VF section. Nitrate produced in the VF section is successfully reduced in the HF section. However, removal of phosphorus usually remains low unless filtration media with high sorption capacity are used.
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