Study On The Performance And Evaluation Of Biochar Modified Bioretention Facilities For Pollution And Carbon Reduction

To cope with the severe impact of human activities on climate change,China has proposed national strategies for"carbon peaking"and"carbon neutrality".As an important type of infrastructure construction,there is an urgent need for sponge cities to realize the dual carbon strategy.Bioretention facilities,as typical sponge facilities,play a crucial role in pollution reduction and carbon reduction.They have the ability to purify pollutants and act as carbon sinks for runoff rainwater in small areas.However,the carbon sink function of traditional bioretention facilities is not apparent,and the synergistic mechanism between pollution control and carbon reduction remains unknown.In the context of achieving"double carbon"goals,improving bioretention facilities to enhance their efficiency in controlling runoff pollution and reducing greenhouse gas emissions is an important measure to achieve carbon neutrality in sponge cities.This study focuses on the advantages of biochar,which can adsorb and degrade various pollutants while improving the purification efficiency of bioretention facilities.The research aims to improve traditional bioretention facilities and investigate their synergistic efficacy and mechanism in reducing pollution and carbon emissions.The main results are as follows:（1）The pollution control effect of runoff pollution and carbon dioxide（CO₂）release pattern in biochar modified bioretention facilities were studied.The results showed that the modified facility had a strong chemical oxygen demand（COD）removal capacity and could maintain a relatively stable removal effect.The addition of 4%coconut shell biochar improved COD removal by 32.7%.The bioretention facility was stable in treating ammonia nitrogen（NH₄⁺-N）with a removal rate of more than 78.0%and good rainfall tolerance.Biochar addition was able to reduce CO2 emissions from the bioretention facility,with coconut shell activated carbon significantly reducing CO₂ emissions by 50.9-73.7%compared to corn stover.（2）The factors influencing runoff COD control and CO₂ emissions from the modified bioretention facility were investigated.The results showed that CO₂ emissions in the modified bioretention facility were linearly and negatively correlated with COD control effectiveness.At the same COD removal rate,high rainfall intensity resulted in higher CO₂ emissions of 1830.31mg/（m²?d）.The modified facility improved COD removal by 31.4%and NH₄⁺-N removal by6.3%at high organic matter concentrations.The increase in microbial metabolism and microbial mortality during the temperature increase from 18°C to 37°C counterbalanced each other and significantly improved COD removal without increasing CO₂ emissions.The CO₂emission during the rainy period was significantly lower than that during the dry period due to the decrease of oxygen content in the pores of the packing layer caused by the increase of water content of the packing layer and the inhibition of microbial metabolism.The addition of coconut shell biochar could reduce CO₂ emissions and increase the level of anaerobic respiration,promote methane（CH₄）emissions and reduce nitrous oxide（N₂O）emissions.（3）Evaluation of the pathways to achieve pollution and carbon reduction by improved bioretention facilities.A machine learning regression method was used to identify biochar improvement options targeting increased COD removal and reduced CO₂ emissions.Based on the experimental input data,a decision tree structure was output,and coconut shell biochar was selected as the improved biochar species with doping ratios between 3.0 and 5.0%,which can synergistically reduce COD and CO₂ emissions.It provides ideas and reference solutions for the improvement of pollution and carbon reduction effectiveness of biochar-improved bioretention facilities.The multivariate linear model of different influencing factors on COD removal rate,NH₄⁺-N removal rate and CO₂ emission was obtained by linear regression analysis to provide quantitative predictions for bioretention facility improvement measures and applicable scenarios.To evaluate the effect of improved bioretention facilities on pollutant control and CO₂ emission under rainfall conditions in Beijing,it was found that the improved measures were more likely to achieve pollution and carbon reduction efficacy under 4%coconut shell biochar conditions by GWP.