Study On Rural Domestic Wastewater Treatment In Subsurface Wastewater Infiltration System Enhanced By Ca-based Co-pyrolysis Biochar

In China,the discharge amount of rural domestic sewage is very huge.The treatment of centralized rural domestic sewage has received great attention and achieved remarkable results,but it is still difficult for decentralized sewage.Subsurface wastewater infiltration system（SWIS）is an eco-treatment technology,which has great potential for treating decentralized rural domestic sewage.However,traditional SWIS has a limited removal efficiency for nitrogen and phosphate（P）due to the lack of oxygen and carbon sources.Therefore,in this study,the co-pyrolysis biochar with excellent P adsorption capability was coupled with intermittent aeration to enhance SWIS.Co-pyrolysis biochars were prepared at three different temperatures（400°C,600,800°C）using rape straw and various Ca-rich materials（Ca CO₃ reagent,dolomite powder,marble powder and eggshell powder）.The P adsorption property were determined through batch adsorption experiments,physical-chemical characteristics and adsorption mechanisms were analyzed by material characterization techniques.Then the co-pyrolysis biochar with best P adsorption capability was selected as substrate fillers for SWIS.Four SWISs（control（A）,aeration（B）,biochar（C）,and biochar—aeration（D））were established to elucidate the implications of biochar and aeration in SWISs.The influence of hydraulic load,aeration rate,and influent C/N on removal effect were investigated to determine the optimal operating conditions.Finally,actual wastewater was treated by biochar+aeration system.The main conclusions of this study are as follows.（1）With the pyrolysis temperature increasing,P adsorption of co-pyrolysis biochars increased,and achieved best effect at 800℃.At this case,the P adsorption capacities of the biochars was Ca CO₃（CBC800）>marble powder（MBC800）>eggshell powder（EBC800）>dolomite powder（DBC800）>raw biochar（BC800）.Compared to BC800,co-pyrolysis biochars had higher p H_PZC,more Ca/Mg oxides and hydroxides,more functional groups,and bigger pore size.However,due to the filling effect of calcium-rich materials,the specific surface area of co-pyrolysis biochar was significantly reduced.（2）The P adsorption processes of co-pyrolysis biochars were dominated by chemisorption,and the adsorption mechanisms involved chemical precipitation,electrostatic attraction,and ligand exchange.The Q_max of MBC800 and EBC800 were 156.04 and 125.35mg/g,respectively,which were36.98 and 29.68 times higher than that of BC800.（3）The operation results of the SWISs showed that addition of biochar could short the start-up period of NH₄⁺-N,improve the TN removal rate and enhance the stability of the systems,but it increased the COD concentration slightly in effluent.Intermittent aeration improved the COD removal rate,but it inhibited the denitrification to affect the TN removal.Under all operating conditions,the biochar—aeration system（System D）had the best comprehensive decontamination effect,its optimal operating conditions were hydraulic load8 cm/d,intermittent aeration 6 h/d,and influent C/N 15:1,at which the COD,NH₄⁺-N,TN,and TP removal rates were 94.29%,98.83%,80.70%,and 100%,respectively.（4）System D was operated under optimal hydraulic load（8 cm/d）and intermittent aeration（6 h/d）conditions using the actual wastewater from the Farm Wastewater Treatment Plant of Sichuan Agricultural University as influent.The results showed that it maintained efficient and stable removal rate for wastewater with low nitrogen concentration,and the quality of effluent achieved the standard first grade of"Water Pollutants Discharge Standard for Rural Domestic Sewage Treatment Facilities"（DB51/2626-2019）of Sichuan Province.The research results provided theoretical and technical support for decentralized rural domestic wastewater treatment in southwest China and have high application value.