Design Of A New Constructed Wetland Using Shallow Geothermal Energy In Winter And Its Effect On Wastewater Removal

At present,there are two major problems in the world:the shortage of water resources and the serious pollution of water environment.The constructed wetland technology is a new type of sewage treatment project,which has the advantages of low operation cost,excellent treatment effect of pollutants,easy to maintain in the later stage,and is widely used.The wastewater treatment of constructed wetland mainly depends on the substrate,plants,microorganisms and other biochemical reactions of constructed wetland.However,in northern China,the winter temperature is often lower than 0℃,which will lead to the withering of plants,inactivation of microorganisms and greatly weakening of pollutant removal efficiency.Therefore,the development of constructed wetland is restricted.This paper proposes a new constructed wetland structure using geothermal energy to ensure the normal operation of constructed wetland in winter.The indoor experiment was established and the simulation results were verified by CW2D model.The main research contents and results are as follows:（1）Due to the influence of low temperature,the removal effect of constructed wetland is poor in winter.This paper proposes a new type of Constructed Wetland structure,which uses geothermal energy to improve the temperature of constructed wetland substrate,so as to ensure the normal operation of constructed wetland in winter.（2）Two indoor experimental constructed wetlands were established,and the experimental group laid water and heating pipes in layers in the wetlands.During operation,the hot water enters the wetland through the water inlet,and circulates through the water heating pipe to heat the wetland,and then flows out from the water outlet.The water from the heating pipe is collected,reheated and recycled.The control group was normal constructed wetland.The experiment shows that the experimental group constructed wetland can make the substrate temperature of constructed wetland reach 15℃,the wetland system can operate normally in winter,but the control group constructed wetland can not operate normally.（3）Through experiments,the removal of COD,NH₄⁺-N and TP in constructed wetland system was studied.The results showed that the removal rates of COD,NH₄⁺-N and TP in the experimental group were 69.41%,50.45%and 69.17%respectively,which were 17.50%,27.18%and 41.95%higher than those in the control group.（4）CW2D small-scale constructed wetland model was established to predict the operation effect of constructed wetland,and simulate the pollutant movement within 60 days.The results show that the COD removal efficiency reaches 72.5%.The removal efficiency of NH₄⁺-N reached 54.21%.The removal efficiency of TP reached 72.21%,and the simulation results were consistent with the actual constructed wetland results,which proved that CW2D constructed wetland model could be used to predict the operation effect of constructed wetland.（5）On the basis of indoor experiment and small-scale constructed wetland simulation research,CW2D model was used to simulate a medium and large-scale constructed wetland under planning and construction.The simulated operation lasted for 90 days,and the COD removal efficiency reached 67.8%.The removal efficiency of NH₄⁺-N reached 45.4%.The removal efficiency of TP reaches 68.1%.Therefore,the CW2D model can provide data and technical support for the actual constructed wetland construction and operation effect.