VF should probably be called 'vertical downflow' because there are also systems where the wastewater is brought to the filter bottom and the wastewater passes the filter in upward direction and treated water is collected at the top of the bed. These systems have commonly been used in Brazil since 1980s. The beds are filled with crushed rock on the bottom, the next layer is coarse gravel, and the top layer is local soil planted with rice (Oryza sativa). This treatment system is called 'filtering soil' in Brazil (Figure 32). Other systems were also studied under experimental conditions in Australia, New Zealand, or Sweden. However, outside Brazil, the layer of soil is usually not used and beds are filled with gravel and usually planted with species com monly used for HSSF wetlands. At present, there is a very limited information on treatment effect of these systems with the exception of systems in Brazil where the results show good treatment performance.
BOD5-out = BOD5-E
Figure 29 The BOD5 submodel (UNET-IETC). DENI* 1.97, decomposition of organic matter by; ORMD, decomposition of organic matter by.
It is possible to consider all the factors that play a role for the efficiency of a CW by the use of models. Figures 29-31 show such a model that has been applied for design of subsurface wetlands. The model considers the oxidation of organic nitrogen to ammonium to nitrate, the uptake of ammonium and nitrate by plants, decomposition of organic matter measured as BOD in this case and the
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