A Pilot Study On Desertification Soil Restoration Using Municipal Wastewater As The Water And Fertilizer Sources

In order to simultaneously realize municipal wastewater purification and desertification soil restoration, sandy soil was used as the substrate of constructed wetlands. And the effects of constructed wetlands under different operational conditions were analyzed. The feasibility of constructed wetlands using wastewater as water and fertilizer source, simultaneously treating municipal wastewater and amending of desertification soil was discussed. The constructed wetlands were built in a wastewater treatment plant in Xi’an City, and consisted of 8 units. The depth of constructed wetlands were 0.1 and 0.6m, and Festuca arundinacea, Lolium perenne, Cynodon dactylon, Phragmites australis and Typha orientalis Presl were cultivated, respectively. The water of experimental system came from the effluent of primary sedimentation tank, and the inflow and outflow of each unit were independent. The experimental time was about two years.The main research contents include:(1) The analyzing of the changes of physical and chemical properties of desertification soil in constructed wetlands running under different conditions of various kinds of plants, hydraulic loadings, depths of substrate, seasonal changes;(2) Analysis of soil nutrient accumulation rate in desertification soil;(3) Removal efficiencies of organic matter, total nitrogen and total phosphorus in the effluent of constructed wetlands.The main conclusions are as follows:1) In the two-years operation,the COD Cr in the effluent of constructed wetlands could meet the first class mostly, while the removal efficiency of COD Cr in surface flow constructed wetlands was much better under the condition of longer hydraulic retention time. During the first year of operation of 0.1m-depth constructed wetland, the concentration of TN and TP in effluent could meet the standard A of the first class mostly,and both of their removal efficiencies were better than that of 0.6m-depth surface flow constructed wetlands. The removal efficiencies of COD Cr, TN and TP were no significant differences(P>0.05) among 0.1m-depth subsurface constructed wetlands, which showed that the species of plants had no significant effect on the effluent quality.2) After about two years of restoration, compared to the original sandy soil, soil organic matter, total nitrogen, available nitrogen, total phosphorus and available phosphorus content in soil increased significantly(P<0.01). It indicated that the fertilizer in wastewater could be quickly enriched in desertification soil.3) The structure of desertification soil has been improved obviously, along with significant decreasing of bulk density(P<0.01) and significant increasing of porosity of soil(P<0.01). Finally, soil bulk density and porosity tend to be stable. It was more favorable to the growth of plants, and the increasing of porosity could reduce the blockage of constructed wetlands and keep the system stable.4) During two years of restoration, the soil was considered to be slightly alkaline soil with the pH of soil ranging from 7.5 to 8.5. The pH change of sandy soil was insignificantly(P>0.05) than the original sandy soil, while the electrical conductivity increased significantly(P<0.01). However, the sandy soil not yet produced salinization.5) After the first year restoration, the nutrient content of sandy soil had no significant differences(P>0.05) between 0.6m-depth subsurface flow constructed wetlands and 0.1m-depth subsurface flow constructed wetlands. It could be known that the depth of the substrate had no effect on the nutrient enrichment of the desertification soil. However, in the second year of restoration, nutrient content of the substrate of 0.6m-depth surface flow constructed wetlands was significantly higher than that of 0.1m-depth subsurface flow constructed wetlands(P<0.05) except for available phosphorus. Therefore, the surface flow constructed wetlands, compared to the subsurface flow constructed wetlands, were more conducive to restore the desertification soil.6) The nutrient enrichment in 0.1m-depth subsurface constructed wetlands substrate had no significant differences(P>0.05), which showed the plant species had no effect on nutrient enrichment of desertification soil in the first year of restoration. In the second year of restoration, the nutrient index of 0.1m-depth subsurface constructed wetlands under different hydraulic loadings, had no significant differences(P>0.05) yet, which indicated that the hydraulic loading had no significant effect on the nutrient enrichment of 0.1m-depth subsurface constructed wetlands.7) During the study period, there were no significant differences in pH, bulk density and porosity(P>0.05), which showed the depth of substrate, plant type, hydraulic load and operation mode had no effect on these three indexes.8) The soil organic matter, total nitrogen, available nitrogen and total phosphorus of desertification soil had the similar change rule in spring. Available phosphorus decreased significantly in summer and autumn and increased in spring. The pH in the spring, summer and autumn showed a downward trend, while the winter has increased9) After two years of restoration, the nutrient indexes of the constructed wetlands substrate could better meet the linear equation and was an increasing function. Seen by the linear equation of surface flow constructed wetlands nutrient enrichment rate was higher than subsurface constructed wetlands except for available phosphorus.