Performance And Mechanism Of Nitrogen And Phosphorus Removal In A Pyrite-based Modified Bioretention System

Climate change and urbanisation have led to several severe environmental problems such as increased runoff volume and non-point source pollution.Bioretention have been widely used in stormwater treatment to treat those problems due to their convenience and flexibility.However,since the conventional amended media function is single and unreasonable design,conventional bioretention facilities generally can not achieve simultaneous removal of dissolved nitrogen and phosphorus nutrients,some of them even face pollutant leaching problems,especially under various stormwater conditions.To solve the low and unstable nitrogen and phosphorus removal problem,this study proposed a new type of biochar-pyrite bi-layer bioretention system,and evaluate its nitrogen and phosphorus treatment performance and by-production leaching under different operating periods and various stormwater conditions.Besides,microorganisms and material-related nitrogen and phosphorus removal mechanisms were explored,and several implications for engineering applications and future researches were also proposed.The specific results are:（1）The pollutants leaching during the initial operation period was evaluated.Phosphorus and organic matter are the main leaching pollutants for the bioretention system.Biochar-pyrite modified bioretention system has very low initial leaching problems,with total dissolved phosphorus（TDP）less than 0.4 mg/L,and chemical oxygen demand less than（15 mg/L）.those pollutant leaching all become stable after 5rainfall events.Meanwhile,the total iron leaching was also very low,only about 0.1 mg/L after 10 rainfall events.（2）The variation of hydraulic performance,nitrogen and phosphorus removal performance was tested under both short-term（three months）and long-term operation（14 months）.Biochar-pyrite modified bioretention system can maintain very good hydraulic performance with the operation prolong,which only decreased from 292.2 to272.0 mm/h.Ammonium nitrogen（NH₄⁺-N）removal and dissolved organic nitrogen（DON）increased from 70.4±6.4%and 87.5±5.2%to 98.1±0.1%and 88.7±5.1%respectively.although nitrate nitrogen（NO₃^--N）removal decreased from 42.5±6.8%to17.2±5.3%,total dissolved nitrogen（TDN）removal remain relatively unchanged（65.6±3.6%and 64.1±2.5%）.TDP removal did not show a significant difference between short-term and long-term operation,which were 80.3±4.1%and79.0±5.1%.Total iron and sulfate by-product shows lower concentration under long-term operation,which could meet the demand of the second class and first class of GB/T 14848-2017（total iron≤0.2 mg/L,sulfate≤50mg/L）.Biochar-pyrite modified bioretention system shown stable dissolved nutrients removal as well as low by-product generation in both short-term and long-term operation.Despite higher denitrification performance,woodchip systems show more severe NH₄⁺-N,DON,TDP leaching problems.Therefore,it does not show superior denitrification and dephosphorization efficiency than pyrite improved facilities.In comparison,the sand bioretention system has relatively low pollutant removal performance in both long-term and short-term operation.（3）The effects of various stormwater conditions on nitrogen and phosphorus removal and by-product generation were studied.The biochar-pyrite modified bioretention system can achieve stable and efficient dissolved nutrients removal during most stormwater conditions.DON、NH₄⁺-N and TDP show little variation between various stormwater conditions,with their highest removal efficiency reached 96.0%、98.2%and90.2%.The removal performance of NO₃^--N and TDN increased with the increase of antecedent drying duration,with the highest removal efficiency of them can reach 49.2and 79.2%during 10 d antecedent drying duration.Different stormwater conditions did not influence sulfate and total iron production.In contrast,woodchip-modified also faced more serious leaching of NH₄⁺-N,TDP,DON,and OM with the increase of the antecedent drying duration.The sand-based bioretention system shows a similar nitrogen and phosphorus removal pattern with the biochar-pyrite modified bioretention system,while the whole removal performance was lower.（4）The mechanism of nitrogen and phosphorus removal and by-product reduction was proposed.Vadose zone and submerged zone cooperate for nitrogen and phosphorus removal as well as by-product reduction during wet and drying alternation in biochar-pyrite modified bioretention system.Biochar addition greatly improves the adsorption and transformation rate of NH₄⁺-N and DON in the vadose zone.It also increased the volumetric water content of the vadose zone,which effectively limited the dissolved oxygen diffusion,promoting in-situ denitrification in the vadose zone.besides,the lower dissolved oxygen also protect the anoxic environment in the submerged zone,which,in turn,reduces the aerobic oxidation of pyrite as well as the leaching of iron.In the meantime,pyrite and the leached organic matter were used in the submerged zone for mix mixotrophic denitrification.Sulfate reduction can be achieved using the organic matter washed out from the biochar layer by stormwater and die microorganisms after denitrification,which decreased the sulfate concentration in the effluent.The pyrite layer,in return,can further remove the phosphorus from both influent and biochar layer leaching by both amorphous iron oxides and hydroxides adsorption and ferric iron precipitation firmly,which avoid potential second leaching.