Study On Mechanism Of Sludge-derived Iron-rich Biochar For Enhancing Sludge Deep Dewatering And Land Utilization Outside WWTP

Dewatering of sewage sludge is a worldwide challenge.Previous research found that Fenton-skeleton builder composite sludge conditioning technology can achieve efficient sludge deep dewatering.However,most iron element would remain in sludge cake during the sludge dewatering process.The subsequent disposal of the large amount of Fe-rich sludge became an urgent problem to be solved.In this thesis,Fe-rich sludge obtained from Fenton-alkaline slag(skeleton builder)sludge conditioning system was used for the preparation of Fe-rich biochar via pyrolysis process.A new mode of Fe-rich biochar recycling for sludge conditioning in wastewater treatment plant(WWTP)and application outside WWTP was proposed.The transformation of key elements(Fe,C,Al,Ca,Si,etc.)and their influencing on sludge dewatering and land application in the sludge recycling mode were discussed,and the environmental impacts of the new recycling mode were analyzed through life cycle assessment.The main findings are presented as follows:1.Study on the mechanism of Fe-rich sludge biochar reultilized as sludge conditioner for enhancing sludge deep dewateringFe-rich sludge biochar was prepared from sludge cake conditioned by Fenton-alkaline slag via pyrolysis process,and the biochar was used as conditioner to replace iron source as Fenton's reagent to activate H₂O₂ for hydroxyl radical generation for sludge deep dewatering.The research results showed that both homogeneous Fenton reactions initiated by the leached Fe²⁺from the biochar,and heterogeneous Fenton reactions initiated by the bonded Fe on the surface of the biochar,were revealed to contribute to the generation of·OH.Compared to raw sludge,CST(Capillary Suction Time)and SRF(Specific Resistance to Filtration)of the treated sludge could be reduced by 83.91%and 91.21%,respectively,under the optimized parameters of the pyrolysis temperature at 800 ^oC,the initial p H of sludge at 2.00,the dose of Fe-rich sludge biochar,alkaline slag and H₂O₂ at 0.792,0.072 and 0.650 g/g VS(Volatile Solid),respectively,and the reaction time of 40 min after the addition of H₂O₂.Based on the above research results,Fe rich biochar could be circulated in WWTP for sludge conditioning.In the process of 3 continuous cycles,the water content of sludge cake could be maintained around 46 wt%.2.Synergistic mechanism of Fe and Al for activation of H₂O₂ for the enhancement of sludge dewaterability with Fe-rich biocharDue to the high content of inorganic mineral elements in sludge,the effect of main inorganic mineral elements(Si,Al,P and Ca)in Fe-rich sludge biochar on the sludge conditioning process was discussed.It was found that Fe and Al compounds in Fe-rich biochar could form Fe Al₂O₄,when pyrolysis temperature was 800 ^oC,and the CST and SRF of sludge conditioned by Fe-rich biochar,obtained from sludge with adding 10 wt%Al₂O₃,could be decreased by 21.02%and 37.32%,respectively,compared to the system without adding Al₂O₃.Density functional theory calculations and sludge conditioning experiments showed that the strong chemical attraction of Fe,Al atoms in[Fe²⁺-Al³⁺]unit on the surface of Fe Al₂O₄ phase towards the two O atoms in H₂O₂ molecule made the H₂O₂ molecule to be more easily dissociated for the generation of·OH,which contributed to the inner sludge flocs of bound EPS(Extracellular Polymeric Substance)degradation,leading to the release of more bound water and the enhancement of sludge dewaterability.3.Molecular oxygen activation via the synergistic functioning of Fe and C in Fe-rich sludge biochar for the enhancement of dewaterabilityIn order to overcome the problems of high cost and potential risk of transporting H₂O₂,the feasibility and efficiency of in-situ H₂O₂ generation by Fe-rich sludge biochar under aeration were studied.It was found that when the pyrolysis temperature was 800 ^oC,the initial p H of sludge was 1.00,the dose of Fe-rich biochar was 400 mg/g DS(Dry Solid),the in-situ generation rate of H₂O₂ was the highest under aeration,and the concentration of H₂O₂ in liquid phase of sludge reached 0.83 mmol/L after 3 h of reaction.It was also found that the lower mass ratio of Fe:C,and the lower charge transfer impedance facilitated the electron transfer between Fe⁰ and Fe(II)in Fe-rich sludge biochar and O₂,improving the in-situ generation rate of H₂O₂.Sludge conditioning experiments showed that the in-situ generated H₂O₂ could cause obvious pre-oxidation effect for disintegrating sludge flocs,and combined with PDMDAAC(Poly dimethyl diallyl ammonium chloride)could greatly improve sludge dewaterability.CST and SRF of conditioned sludge could be reduced by 84.57%and 90.71%,respectively,compared with raw sludge,under the optimal sludge conditioning parameters of the initial p H of sludge at 3.70,and the dose of sludge biochar and PDMDAAC of 417.3and 10.73 mg/g DS,respectively.4.Silica dissolution characteristics of Fe-rich sludge biochar and its reutilization as silicon fertilizer to promote rice growthThe feasibility of using Fe-rich sludge biochar as silicon fertilizer for land application was explored.It was found that Ca O,which was the key component of alkaline slag used in Fenton-alkaline slag conditioning system,could react with quartz or aluminosilicate in sludge to form calcium aluminosilicate,which contributed to the increased the content of available silicon in Fe-rich sludge biochar.The available silicon content of Fe-rich biochar obtained from Fenton-Ca O conditioning system under the optimal pyrolysis temperature of 750 ^oC and the optimal dose of Ca O of 5.04 wt%could reach 77.25 mg/g and the accumulated dissolved Si from Fe-rich sludge biochar could reach 43.21 mg/g after repeated extraction for 17 days.In rice hydroponic experiment,the application of Fe-rich biochar prepared under the above optimized conditions increased the length of rice stem by 48.75%,and root by62.11%,and the biomass of rice stem by 76.85%,and root by 36.11%,compared with rice in blank group without adding any silicon in nutrient solution.The results indicated that Fe-rich sludge biochar can be reused as a slow-release silicon source in land application.5.Life cycle assessment of the novel recycling mode of Fe-rich biochar reused as conditioner for deep dewatering in WWTP—land application outside WWTPLife cycle assessments of the sludge treatment mode of“Fenton-alkaline slag sludge conditioning and deep dewatering—sanitary landfill”and the novel recycling mode of“Fe-rich biochar reused as conditioner for deep dewatering in WWTP—land application outside WWTP”were investigated.Compared with the former treatment mode,the contribution of the proposed new mode to global warming,acidification,eutrophication,respirable inorganics and ecotoxicity were all reduced,and the total environmental impact of the novel sludge recycling mode could be reduced by 21.71%.These results indicated that the proposed sludge recycling mode in this thesis could generate significant environmental benefits.In this thesis,a novel recycling mode of Fe-rich biochar reused as conditioner for deep dewatering in WWTP—land application outside WWTP was proposed,which would provide both new ideas and technical supports for the sustainable sludge treatment and disposal technology.