Does absence of soil layer in gravel based constructed wetland affect the treatment efficiency? if so, how?
Gravel based CWs present higher macro porosity and allow better regulation of water flow. In addition they are less exposed to clogging
Horizontal subsurface flow constructed wetland with coarse sand and gravels shows good treatment efficiency for contaminants. you can also try plant specie Canna indica .
Soil layer is not used in the gravel based wetlands as I know from practice. Soil is sometimes used as the insulation layer on the top of the gravel, but there are lot of other types of mulch with better characteristics which can be used for insulation in cold climate application.
Soil was used as the filtration material in early days of this technology, but due the clogging problems, it's used only for free water surface wetlands nowadays and for other types of wetlands the washed sand and/or gravel is the main filter media.
Other thing is, that some special clay aggregates are used for P reduction, but I think it can't be considered as "soil" anymore.
The treatment efficiency is driven mainly by the available oxygen, but size of the media particals also play some role (sand is more efficient than coarse gravel..)
Dear Nitish
Its totally depends on the soil type and its grain size distribution . Literate base soil is good option instead of gravel and sand. The cost of the gravel and sand is high. High porosity increase the HRT and MLR. Silty and loam can used with less depth or mix with other. Soil Base Horizontal Flow Subsurface CWs can enhance the nutrients like N,P. It has good adsorption capacity for heavy metals. Regular maintenance need in both type of CWs .
Hence Soil Base Horizontal Flow Subsurface CWs is better.
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