Substrate Enhanced Constructed Wetlands For Nitrogen And Phosphorus Removal And Development&Application Of Alum Sludge-containing Lightweight Porous Substrate

Environmental pollution and resource shortage are the main problems facing human development at present.Excessive discharge of nitrogen（N）and phosphorus（P）in wastewater are the main causes of surface water pollution and eutrophication.Therefore,it is essential to develop alternative ways to strengthen N&P removal from wastewater.Different from the traditional wastewater treatment process,constructed wetland（CW）is an ecological wastewater purification process with considerable high efficiency,low energy consumption,simple operation and environmentally friendly feature.However,the removal efficiency of N&P in CW is unsatisfactory.This study aimed at enhancing N&P removal through substrate selection and optimization based on the in-depth analysis of the mechanism of N&P removal in CW.In terms of substrate selection,adhering to the concept of resource conservation and sustainable development,industrial waste woodchips was used as solid slow-release carbon source to improve denitrification performance,while alum sludge（AS）,a by-product of water treatment plant,was used as substrate to enhance P adsorption removal in CW.In addition,alum sludge and polyurethane were jointly employed as main components to develop a lightweight and porous appearance while size controllable substrate in terms of alum sludge-polyurethane（AS-PU）,which can resolve the problems existing in direct application of canonical AS substrate,e.g.high transportation and handling cost,risk of clogging and recycle difficulties etc.The enhancing effect of mixed substrates of alum sludge and woodchips on N&P removal in CW was studied by constructing a laboratory-scale substrate baffle flow constructed wetland（BSFCW）.The performance evaluation of AS-PU was carried out by characterizing its physical,chemical and adsorption properties.Thereafter,a vertical subsurface flow constructed wetland（VFCW）and a constructed wetland-microbial fuel cell（CW-MFC）were constructed to evaluate the performance of resultant AS-PU.As such,this study provides theoretical guidance and scientific basis for the recycling and optimization of woodchips and AS in strengthening N&P removal in CW.The main conclusions are as follows:（1）The average removal efficiencies of COD,NO₃^--N,NH₄⁺-N,TN and TP in BSFCW were 43.0%-81.9%,63.6%-96.1%,11.8%-75.3%,61.9%-74.4%and 75.0%-98.9%,respectively.The COD released by woodchips supplemented the lack of carbon source in the denitrification process and thus enhanced the N removal performance.There was a negtive correlation between the N removal efficiency and the distance of woodchips filling position and the inlet.（2）AS-PU₄ with the best physical and chemical properties was obtained after optimizing study based on he major components of AS and polyurethane with a few other aiding chemicals addition.The P-dsorption performance of AS-PU₄ was improved by lye reticulating.The specific surface area,apparent density,bulk density,porosity and 50%compressive strength of AS-PU_4w are 4.14m²/g,510kg/m³,270kg/m³,47.9%and 1236.9k Pa,respectively.（3）The adsorption capacity of P by AS and AS-PU₄ increases with the decrease of p H and the increase of initial P concentration,while the adsorption process follows the pseudo second order kinetic equation.The maximum theoretical P-adsorption capacity of AS and AS-PU₄ is,respectively,31.87 mg/g and 7.68mg/g.（4）The plants grew well in the VFCWs and CW-MFC with AS-PU_4was substrates,and the microbial biodiversity was higher in the substrate at different points of the system.The maximum removal efficiencies of COD,NH₄⁺-N,TN and TP obtained by VFCW and CW-MFC are 83.5%,91.8%,61.9%,88.4%and 80.7%,82.9%,71.3%,85.9%,respectively.The output voltage of CW-MFC ranges from 85m V to 717m V,while the maximum power density obtained is 85.10 m W/m³.Overall,the findings of this study show that the mixed substrate of woodchips and alum sludge effectively improves the N&P removal in the CW system.AS-PU_4w not only overcomes the disadvantages of high density and variable appearance of alum sludge,but also inherits the adsorption properties of alum sludge as well as the light and porous characteristics of the polyurethane.This study is of great significance for improving the N&P removal performance of CW and promoting the recycling of alum sludge.