Selection Of Enhancing Adsorption Substrates And Pollutant Removal Performances In Constructed Wetlands

Watershed pollution control has long become a problem,particularly for the effective treatment of the decentralized domestic wastewater discharged from vast rural areas and scattered residential areas lacks effective treatment,which always limits the country's economic and social development.As a natural treatment process with high efficiency,low consumption,eco-friendliness and simple operation and maintenance,constructed wetland has been widely used in processing various sewages including decentralized domestic sewage.However,the filler matrix in traditional constructed wetlands has relatively poor adsorption capacity for eutrophication elements such as nitrogen and phosphorus.The porosity and specific surface area of the wetland filler are important factors influencing the absorption of the filler matrix.The larger the specific surface area and porosity of the filler,the larger the amount of sewage contained,and the sewage can be adsorbed for a longer time through the wetland.Therefore,screening some strong adsorption type fillers applied into the constructed wetlands can significantly enhance the decontamination efficiency of wastewater.This paper selects one industrial tailing?coal gangue?,three kinds of natural ores?hematite,magnetite and manganese ore?and two different synthetic materials?new lightweight stone gravel and catalytic micro-electrolysis stone?.Each functional material was mixed with the gravel in a volume ratio of 1:1 as a filler of the subsurface flow constructed wetlands.In this study,the adsorption characteristics of different materials were firstly investigated in erlenmeyer flasks through isothermal adsorption and kinetic adsorption tests.Then the experiments on the removal and transformation of various contaminants including organic matter,nitrogen and phosphorus in constructed wetland with different fillers were carried out.Meanwhile,the heavy metals of the effluent was also analysed to evaluate the properties of these functional fillers.Finally,the suitable artificial wetland fillers and combinations could be screened based on the above exploration.The main results obtained in this paper are as follows:?1?Studies have shown that the theoretical adsorption of NH₄⁺-N and TP for the selected six materials is greater than that of gravel packing.Among them,isothermal adsorption and kinetic adsorption indicate that manganese ore has the largest theoretical adsorption capacity for NH₄⁺-N and TP,and the fastest rate.The absorbed amount of NH₄⁺-N by manganese ore is 0.5 mg/g,which is four times that of gravel adsorption.The amount of adsorption to TP was also six time?0.76 mg/g?,much as that of gravel.On the whole,manganese ore is suitable as a strong adsorption type artificial wetland filler added into the wetland,which can increase the adsorption amount of the matrix to the pollutants.This will provide a scientific reference for the water purification target and long-term operation prediction during the design of the constructed wetland.?2?Overall,the long-term decontamination performance of pollutants in each subsurface flow constructed wetland system with strong adsorption filler is basically better than that in control systems with gravels only.Except for the system with magnetite filler,the removal of TOC by other wetland systems is better than that of the control system,and the decontamination effect is stable.It indicates that these fillers had a certain adsorption effect on organic matter;the wetland system with manganese ore filler has the best long-term decontamination effect on NH₄⁺-N and TN,which is consistent with the results of static adsorption test.Additionally,the wetland system with bioceramics has better removal effect on NH₄⁺-N and TN than the wetland system with ore filler,which might be attributed to the larger pore size and is more suitable location for microbial membrane attachment.The long-term TP removal efficiency 45.19%⁸8.41%in the wetland system with ore filler is better than that in wetland system with bio-ceramic filler.From the whole experiment,the removal rate of phosphorus in each system gradually all decreased,indicating that the adsorption capacity of these fillers are limited and gradually began to reach saturation.?3?Long-term decontamination performance of each wetland system under different influent loads:With the increase of TN influent load,the removal rate of NH₄⁺-N decreases?37.49%⁷0.88%?but the removal rate of NO₃^--N increases?96.21%⁹8.23%?,which is largely due to the increasing the C/N ratio of the influent,providing more carbon source for microbial denitrification.The removal rates of TN and TP in each wetland system under different influent loads remained basically stable.With the extension of hydraulic retention time,the removal efficiency of TP in each wetland system gradually increases,but when the hydraulic retention time is more than 5 days,the TN effluent concentration in the wetland system decreases slowly,with a slight fluctuation sometimes.This indicates that the hydraulic retention time of the sewage in the subsurface wetland is not as long as possible.Considering comprehensively,the hydraulic retention time of the sewage in the wetland is suitable for 5 days.?4?Through long-term monitoring and comparison,we found that the leaching amount of heavy metals Fe,Mn,Cu,Zn and Cr in the wetland with strong adsorption type filler were greater than that in control system.However,the effluent concentration of heavy metals can meet the Class I water effluent of China's Surface Water Environmental Quality Standard?GB3838-2002?.Meanwhile,the amount of heavy metal ions leached in each wetland gradually reduced the running time of the wetland which was beneficial to the water purification.During the experiment,we also found that the growth of plants in the subsurface flow constructed wetland system was subjected to certain stress due to the addition of these functional materials.Among them,the wetland plants with magnetite added are most seriously stressed compared with other fillers.