Study On Enhanced Nitrogen Removal Characteristics In Biochar-based Constructed Wetland

The water pollution in of our country is serious.The ecological management of watershed water environment is a major project in China.As an ecological treatment technique,constructed wetlands(CWs)are of great significance for treating wastewater.However,the treatment performance of traditional CWs was limited by lack of oxygen,carbon sources,plant degradation and low temperature.To reuse degraded wetland plants,improve the nutrients removal efficiency and ensure the winter operation effect has become an urgent problem to be solved in the development of CWs.In this study,the decayed wetland plants were used to produce biochar via high temperature pyrolysis,which realized the resource utilization of decayed wetland plants and effectively avoided the problems of clogging and secondary pollution.In this study,the physicochemical characteristics of biochar with different pyrolysis temperatures were analyzed,and then the optimum biochar as substrate was introduced into the CWs.Nitrogen removal performance,the growth characteristics of plants and microbial population of different amounts of biochar under seasonal temperature variation was compared.The process of nitrogen retention,conversion and export in biochar-based CWs was also analyzed to study mechanism of nitrogen removal.The main conclusions obtained are as follows.(1)Pyrolysis temperature significantly affected biochar performance.BC300(pyrolysis temperature is 300 ?)has excellent pore structure.BC300 was incompletely charred,with a large specific surface area and a rough surface topography,which providing an attachment site for microbial colonization.The dissolved organic matter(DOM)concentration was 125.67 mg L-1,which was about 20 times that of BC600.DOM released from BC300 was bioavailable carbon source.In addition,the UVA+UVC humic acids-like available for bioavailability account for the highest proportion in BC300,up to 91.88%.Therefore,BC300 was selected as substrate in the present study.(2)Biochar addition strengthened nitrogen removal performance in CWs.Biochar could promote the growth of plants,increase plant biomass,and significantly increase the nitrogen uptake of plants.The rate of nitrogen removal via plant uptake in biochar-based CWs was up to 78.78%.Biochar could provide an attachment site for microbial colonization,which was attributed to the excellent pore structure and specific surface area.It could significantly increase microbial abundance.DOM released from biochar was supplied as a carbon source,providing an electron donor for the denitrification and promoting the growth of denitrifying bacteria.The proportion of Beta and gamma actinomycetes in the biochar added groups increased to 19.07%and 76.22%,respectively,which improved the nitrogen removal performance.(3)A variety of microbial pathways acted on nitrogen performance.In addition to the traditional nitrification-denitrification pathway,partial nitrification-denitrification and simultaneous nitrification-denitrification were also important pathways for nitrogen removal.In the present experiment,the nitritation rate is higher than the nitratation rate,and there was no accumulation of NO2--N in the effluent,and the proportion of simultaneous nitrification and denitrification via NO2--N is 58.20%,55.10%,50.89%,respectively.Therefore,multiple pathways in the biochar-based CWs worked together to remove nitrogen,which ensured nitrogen removal performance.(4)Optimal nitrogen removal performance in tidal flow biochar-based CWs.The removal efficiency of NH4-N in the four groups(tidal flow and intermittent aeration)was 85.83%,87.88%,96.19%,and 98.30%,respectively.Two different ways of oxygen supply could effectively improve nitrogen removal performance and increase the absolute abundance of microorganisms.Owing to the higher oxygen transportation efficiency,nitrogen removal performance was better in the biochar-based tidal flow CWs.