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where Ce is effluent BOD5 (mg/L); Co is influent BOD5 (mg/L); A is fraction of BOD5 not removed as settleable solids near headworks of the system; Av is specific surface area for microbial activity (m2/m3); L is length of system (parallel to water flow) (m); W is width of

Free water surface system (FWS) Subsurface flow system (SFS)

Figure 11 Illustration of constructed wetland systems.

Free water surface system (FWS) Subsurface flow system (SFS)

Figure 11 Illustration of constructed wetland systems.

system (m); d is design depth of system (m); n is porosity of system; and Q is average hydraulic loading, m3/day.

In SFS, water level is below ground; water flows through a sand or gravel bed. It functions like a series of horizontal trickling filters. Emergent plants with extensive root systems are grown within the media. In the United States, flow path is normally horizontal, while some European applications use vertical flow paths. Because of its nature, SFS is suitable for waste that has less TSS and a low flow rate. It tolerates cold temperature, can be operated with low odor emission, and has a lower land requirement than FWS. However, both constructional and operational costs are higher than FWS. Based on the USEPA wetland design manual, the design equation for the SFS is [18]:

where Ce is effluent BOD5 (mg/L); Co is influent BOD5 (mg/L); KT is temperature-dependent first-order reaction rate constant (d-1); t is hydraulic residence time (day); Q is average flowrate through the system (m3/day); d is depth of submergence (m); n is porosity of the bed; and As is surface area of the system (m2).

The Gulf of Mexico Program (GMP) conducted a survey of treatment performances of 68 constructed wetland sites with a total of 135 pilot- and full-scale operations, and developed the Livestock Wastewater Treatment Wetland Database. It showed that the average concentration reduction efficiencies were 65, 53, 48, 42, and 42%, for BOD5, TSS, NH3-N, total nitrogen, and TP, respectively [21]. The reduction of fecal coliforms was 97%. Treatment performance of cattle feeding, dairy, poultry, and swine waste is illustrated in Table 5. As shown, good effluent can be produced for most cases. The survey showed that the hydraulic loading rate ranged from 3.8 to 5.5 cm/day.

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