Enhancement Of Pollution Removal Of Swine Wastewater Using A Biochar-amended Aerated Vertical Flow Constructed Wetland

For the past few years,with the rapid growth of Chinese economy,the pig industry has been developed in a large-scale and intensive direction.As a result,pig wastewater has become a source of pollution that can not be ignored.Pig wastewater has the characteristics of high pollutant content,large emissions,and emerging contaminants.Meanwhile,the relatively concentrated discharge time makes the impact load strong and difficult to handle,which has great harm to the ecological environment and human health.As a new ecological sewage treatment technology,constructed wetlands?CWs?have been applied initially in the treatment of livestock and poultry wastewater,but CWs still exist the problems of poor reoxygenation capacity,insufficient carbon source and weak adsorption capacity,which caused poor nitrogen and organics removal.It seriously affects the application of CWs in the field of livestock wastewater treatment.In this study,the application of biochar combined with intermittent aeration technology in subsurface flow CWs was used to enhance the treatment efficiency of swine wastewater.First,the optimal operating conditions of intermittent aerated CW with biochar addition was determined by taking synthetic wastewater as the feedwater.Then,under the optimal operating conditions,the treatment efficiency of the intermittent aerated CW with biochar addition for different influent strengths of pig wastewater was investigated.Primarily,the removal efficiency of conventional pollutants and rules of nitrous oxide?N₂O?emissions from intermittent aerated CW with biochar addition for a long-term operating period were studied.In addition,the removal efficiency,migration and transformation characteristics of dissolved organic matter?DOM?in the intermittent aerated CW with biochar addition were explored by three-dimensional fluorescence excitation emission matrix spectroscopy?EEM?and parallel factor analysis?PARAFAC?.Finally,the quantitative reduction of antibiotics resistance genes?ARGs?and their distribution characteristics in the intermittent aerated CW with biochar addition were also evaluated.The main findings are as follows:?1?With the comparison of intermittent aerated CW without biochar addition,the optimal operating conditions of the intermittent aerated CW with biochar addition was determined.The results showed that the intermittent aerated CW with biochar addition could achieve the best removal of total nitrogen?TN?with the removal efficiency of 73.33%,when the aeration volume was 1.0 L min^-1 and the aeration time was 2 h d^-1.Meanwhile,the removal efficiency of chemical oxygen demand?COD?and ammonia nitrogen?NH₄⁺-N?reached 95.45%and 85.45%,respectively.In the treatment of synthetic wastewater,the N₂O emission fluxes(575.73?1877.7?g m^-2 h^-1)of intermittent aerated CW with biochar addition was significantly lower than that of intermittent aerated CW without biochar addition(745.85?2298.09?g m^-2 h^-1).?2?The treatment efficiency of the intermittent aerated CW with biochar addition for swine wastewater under different strengths was clarified.Under the condition of no aeration,the addition of biochar in the CW significantly improved the removal of COD,NH₄⁺-N,and TN,and reduced the N₂O emission fluxes.However,with the influent strength increased,the pollutant removal efficiencies decreased.In addition,the application of intermittent aeration has effectively enhanced the pollutants removal in CWs.Under intermittent aeration conditions,the addition of biochar in the CW also intensified the removal of COD and NH₄⁺-N.However,due to the lack of carbon sources,the denitrification process was inhibited.The removal efficiency of TN?30.92%?40.12%?in the CW with biochar addition was lower than that in the CW without biochar addotion?40.83%?48.70%?.Moreover,the addition of biochar failed to reduce the N₂O emission fluxes under the condition of strength 2 and strength 3.Simoultaneously,with the influent strength increased,the removal performance became worse,and the N₂O emission fluxes became larger.?3?The characteristics and rules of DOM removal from pig wastewater by the intermittent aerated CW with biochar addition were determined.Compared with the CW without biochar addition,the CW with biochar addition,the intermittent aerated CW without biochar addition,the intermittent aerated CW with biochar addition had the best removal of DOM with removal efficiency of 59.97%?72.20%.PARAFAC analysis showed that the DOM from effluent mainly dominated by UVA humic-like and UVC humic-like components.According to the the three-dimensional fluorescence indexes and ultraviolet-visible absorption spectrum characteristics,it was determined that DOM from intermittent aerated CW with biochar addition effluent was an endogenous input,and had high aromaticity,humification degree,and large molecular weight,which was not easily degraded by microorganism.It further illustrated that the lack of available carbon sources may be the possible reason for the poor TN removal of theintermittent aerated CW with biochar addition.?4?The reduction effect and distribution characteristics of ARGs from swine wastewater in the intermittent aerated CW with biochar addition were confirmed in this study.The intermittent aerated CW with biochar addition could remove target ARGs?tet A,tet M,tet O,tet W?with removal efficiency of 0.53?2.14 log.Meanwhile,biochar combined with intermittent aeration technology reduced the accumulation of target ARGs in substrates of CWs(1.29×10⁴?7.88×10⁵ copies g^-1).In addition,it was clear that the DOM removal had an effect on ARGs removal.Especially,the protein-like component,tryptophan-like component and the molecular weight?S_R?of DOM had significant correlations with the target ARGs removal.