Lagoons and Wetlands

Lagoons and wetlands may be used for leachate treatment when low investment and operational cost, and simple and robust installation are required. This technique also does not demand a highly qualified staff [27]. In the United Kingdom lagoons and wetlands are very popular for leachate treatment. They operate as polishing stages after other biological processes [28].

The first automated plant in the United Kingdom designed to treat large quantities of strong leachate was built at Bryn Posteg Landfill Site in central Wales in the early 1980s. The plant consisted of a large aerated lagoon as a main element and was capable of treating up to 150 m3 of leachate per day. Effluent was discharged to local wastewater treatment works. The long retention time of the leachate and high efficiency of floating aerators allowed high standards of effluent to be achieved. In the winter, at a mean temperature 4°C and mean hydraulic loading rate of 84 m3/day, COD removal reached 98%. Both BOD5 and ammonia nitrogen had a 99% removal efficiency. High effectiveness (above 90%) in Fe and Zn removal was also achieved in suspended solids [27].

Normally, leachate is treated in a complex of lagoons and wetlands where flexible anaerobic and aerobic conditions are assured. The leachate retention time reaches 10-20 days and hydraulic loading rate varies from several tens to several hundreds of m3 per day [4,29,30]. Plants applied in constructed wetlands are terrestrial or aquatic species. Wetlands with reed (e.g., Phragmites ausralis) and willow (e.g., species Salix iminalis,

Salix dasyclados, Salix cinerea, and a cultivar Salix delamere) beds are the most popular [29,30]. However, experiments with other macrophytes such as Stenotaphrum secundatum (tropical to subtropical terrestrial species), Lemna minor, Eichhornia crassipes, and Myriophyllum verticellatum (free-floating aquatic species) have also been performed [31].

Figure 4 presents the process scheme of a leachate treatment plant near Oslo, Norway, that treated leachate from an old landfill receiving domestic and commercial wastes as well as sludge. The plant consists of an anaerobic lagoon, aerated lagoon, two parallel horizontal subsurface flow constructed wetlands, and one free water surface constructed wetland. The mean retention time of leachate is approximately 40 days at a mean hydraulic loading rate of 120 m3/ day. After 6 months of plant operation, the overall removal for COD, BOD, N, P, Fe, and pathogens (E.coli) varied from 65 to 95% (without data from wintertime) [30].

Satisfactory results are achieved in lagoons and wetlands, due to their ability to take large volumes of significantly diluted, treated leachate. Thus, the fluctuation of loads and hydraulic loading rates do not significantly influence treatment results. Also, the influence of toxicity and heavy metals on microorganisms and plants is limited. Disadvantages of lagoons and wetlands include the large area required for their construction and problems with maintaining effluent high standards during wintertime [4,30].

Figure 4 Landfill leachate treatment plant with anaerobic lagoon (1), aerobic lagoon (2), two parallel horizontal subsurface flow constructed wetlands (3), and one free water surface constructed wetland (4) (from Ref. 30).

Figure 4 Landfill leachate treatment plant with anaerobic lagoon (1), aerobic lagoon (2), two parallel horizontal subsurface flow constructed wetlands (3), and one free water surface constructed wetland (4) (from Ref. 30).

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