The Physi-chemical Characteristics Change Of Fine Sand Used As The Substrate Of Extremely Shallow Subsurface Constructed Wetland On Desertification Control

This research aims at the problem of desertification (fine sand) and proposes a newway of using constructed wetland for controlling desertification, and to make achievingthe desertification land restoration and sewage purification possible. The experimentsystem was built in a wastewater treatment plant of Xi’an City, and consisted of8unitshorizontal subsurface flow constructed wetland, which includes Festuca arundinaceaconstructed wetland (CW1), Lolium perenne L. constructed wetland (CW2),Cynodondactylon (Linn.)Pers. constructed wetland (CW3), Phragmites communisconstructed wetland (CW4), Typha orientalis constructed wetland (CW5), without plantconstructed wetland (CW6) Phragmites communis/Typha orientais constructed wetland(CW7), Festuca arundinacea/Lolium perenne L./Cynodondactylon(Linn.) Pers.constructed wetland (CW8) by depending on the different pants in constructed wetland,and includes the depth of0.1m (CW1~CW6) and0.6m (CW7, CW8) by depending onthe depth of filler in constructed wetland. The water of experimental system comes fromthe effluent of primary sedimentation tank, and each unit inflows and outflowsindependently, and HRT is6d. The research mainly studies the follow aspects:(1) Thedifferent depth of subsurface constructed wetland and the different cultived plant for theremoval efficiency of pollutants in water, and the physi-chemical characteristics changeof fine sand during the operation of the constructed wetland;(2)The plant growth status,the impact of the plant for the stability of systems and the selection of suitable plant forrestoring desertification land. The mainly results are shown as follows:1) The constructed wetland effluent of filler bed depth of0.1m without plants waspoor and unstable operation, and blocked up after running60d, which cannot achievethe intended purpose; While at the same HRT, The constructed wetland effluent of fillerbed depth of0.6m, COD achieved the A in first discharge standard, TN, NH4+-N and TPachieved the secondary discharge standards (GB18918-2002). Meanwhile, theaccumulation of nutrients in fine sand was better than that in the extremely shallowsubsurface constructed wetland, but after running240d, the constructed wetland wasalso blocked up; The COD, TN, NH4+-N and TP of effluent in extremely shallowsubsurface constructed wetland reached the A in first discharge standard (GB18918-2002), the units kept the high removal efficiency and stable operation during theexperiment, which can achieve the purpose of desertification restoration and sewagepurification;2) Compared with the original fine silt, the physi-chemical characteristics in theconstructed wetlands fine sand changed significantly（p＜0.05）; And fine sand porosity,organic matter, available nitrogen, available phosphorus increased, whereas the bulkdensity decreased, which improve the physi-chemical properties, and is beneficial forthe restoration of desertification land;3) Volatile solid content in each wetland fine silt decreased gradually from thesurface layer to the bottom layer, and the variation of surface layer was larger thanoriginal fine silt VS content0.67％, which bonds the particles of fine sand, and isbeneficial for crust in fine sand surface, produces effective resistance for wind erosion;4) As the cultived plant of constructed wetland and because of good adaptability,high quality root density, Festuca arundinacea and Lolium perenne L. grew better thanterrestrial group, and also showed strong growth ability in winter，and could form adense root net during constructed wetlands run, which produces effective resistance forwind erosion, so Festuca arundinacea and Lolium perenne L.can be as the preferredplants for restoration of desertification land.