The Effect Of Biochar On Bioretention Hydrologic Effect And Nitrogen Removal

Urbanization increases the area of impermeable surface and changes the underlying surface where stormwater runoff forms, resulting in a series of urban runoff issues, such as urban water cycle destruction, groundwater recharge reduction, urban rainstorm waterlogging, stream erosion, and water body pollution by contaminants from runoff. Increasing countries and regions have realized the seriousness of urban runoff issues, thus various stormwater management studies and practices have been conducted. Bioretention is a typical Low Impact Development practice, which can address both hydrologic and water quality issues, and has been widely used in developed countries. Bioretention can remove most of runoff pollutants, but its nitrogen removal is not satisfactory, NO3- concentration in effluent is often higher than that in influent. Some studies created an internal water storage layer at the bottom of bioretention to improve denitrification, but the results varied. Biochar is a thermal decomposition product from waste biomass pyrolysis with limited oxygen. Biochar has been reported to have high cation exchange capacity, large specific surface area, rich pores, and high organic carbon content, also its aromatic structure is very stable and hardly degradable. A novel application of incorporating biochar into bioretention filter media has been proposed in this thesis, to enhance the poor nitrogen removal in bioretention system.To evaluate the feasibility of the new technology, physicochemical properties of different biochars pyrolyzed from poultry litter and hardwood at 400? and 500? were analyzed, and successive batch extractions, batch NH⁴⁺ sorption and stormwater infiltration column experiments were conducted using these biochars. Based on these experiments, biochar suitable for stromwater treatment could be chosen. After that bioretention filter medium amened biochar was prepared to evaluate biochar impact on NHU+ sorption and water retention of filter medium. Two well-instrumented pilot-scale bioretention cells were built in parallel, a control ?Ctrl? and an experimental cell ?Exp? incorporated with biochar. Hydraulic conductivity test and infiltration test with a bromide tracer and nitrate pollutant were conducted on the two cells to testify the effect of biochar on bioretention hydrologic performance and nitrogen removal. Furthermore, runoff process of bioretention was stimulated and predicted using HYDRUS-1D.The results showed that poultry litter biochar ?PLC? had higher pH, ash content, cation exchange capacity ?CEC? that hardwood biochar ?HWC?, but lower C/N ratio and BET surface area. pH, ash conetent, C/N ratio and BET surface area increased with higher pyrolysis temperature, whereas CEC decreased. PLC leached more nutrients than HWC when successively rinsed in deionized water ?DI? and artificial stormwater ?SW? for 10 times, and most amount of nutrients was released in the first rinse. No measureable nitrogen leached from HWC except 0.07 ?mol·g-1 of org-N from HWC pyrolyzed at 400?. PLC produced at 500? leached 7.1-8.6 ?mol·g-1 of nitrogen, whereas 120-127 ?mol·g-1 of nitrogen were leached from PLC produced at 400?, mostly from its org-N release. NH⁴⁺ sorption was significant for all biochars. In stormwater, ion competition ?e.g. Ca2+? suppressed NH⁴⁺ sorption compared to DI water. NH⁴⁺ sorption was negatively correlated to the BET surface area of the tested biochars, but increased linearly with CEC. Ion exchange is the primary mechanism of NH⁴⁺ sorption by biochar. Stormwater infiltration column experiments in sand with 10% biochar removed over 90% of ammonium with influent NH⁴⁺ concentration of 2 mg·L-1, compared to only 1.7% removal in a sand-only column. HWC and PLC pyrolyzed at high temperature may be viable filter media for stormwater treatment.Biochar pyrolyzed from yellow pine wood at 550? were amended in a standard bioretention filter medium at a ratio of 4%?w/w?. Biochar amendment increased filter medium pH, decreased bulk density, increased total porosity and CEC. Biochar also enhanced NH⁴⁺ removal of filter medium by 31-114% Freundlich model fitted the NH⁴⁺ sorption isotherms ?R2>0.9883? better than Langmuir model. Biochar increased available water content of filter medium by 294% and saturated water content by 1.4 times, van Genuchten model fitted the water retention curves of the tested filter media well ?R2>0.9969?, hydraulic parameters were obtained using RETC.As to the pilot-scale bioretention cells, the saturated hydraulic conductivity of Exp cell with biochar was 1.5 times higher than that of Ctrl. Also the Exp cell increased mean residence time by 1.6h and water retention by 15%. Biochar enhanced denitrification in Exp cell via increasing water retention, pH, and carbon source, and decreasing dissolved oxygen and redox potential. Furthermore, biochar could donate electrons to drive denitrification via its reversible quinoid and hydroquinoid functional groups. The vadose zone of Exp cell with biochar reduced NO3-N concentration by 30.6-92.9%, while in Ctrl only-6-43.5%. NO3-N removal was enhanced by higher soil temperature.Bioretention runoff processes were simulated by HYDRUS-1D well with R2> 0.9783. Exp cell showed better hydrologic performance than Ctrl under various precipitation conditions. Specifically, Exp cell showed more water retention, less outflow; more treatment capacity, less overflow; longer delay of outflow and peak flow; more volume reduction of peak flow; and shorter ponding duration. Exp cell achieved satisfactory hydrologic performance with design area ratio of 7%.Overall, the technology of amending biochar into bioretention to improve nitrogen removal ?especially NO₃-? and hydrologic performance was testified to be promising. Biochar incorporation not only addresses the problem of poor nitrogen removal of traditional bioretenion, but could also optimize runoff process and relatively narrow design area. In addition, this technology expands the biochar application and provides a new way for organic waste reuse.