Research On The Effect And Mechanism Of Biochar Coupling Fenton Oxidation On Enhancement Of Sludge Dewaterability And Aerobic Stability

With increasing urbanization level in China,the timely and effective treatment and disposal of excess activated sludge not only relates to the normal operation of wastewater treatment plants,but also significantly influences the sustainable development of economy and society.However,the high miosture content(MC)and excessive content of pollutants,such as heavy metals,etc.,in the shop inspected sludge cake lead to higher disposal cost,lower treatment and reuse efficiency,etc.,and restricts the implementation of low-cost disposal methods,such as aerobic fermentation and stabilization of sludge,etc.,and application of products.Thus,to enhance the sludge dewaterability,reduce heavy metal content in the cake and improve the aerobic fermentation and stabilization characteristics of sludge during pretreatment,the mechanism of synergistic effect of Fenton oxidation on sludge dewaterability and heavy metal removal was first clarified by Fenton oxidation experiment;then the effect and mechanism of the biochar coupling and Fenton oxidation on improvement of sludge dewaterability,heavy metals and nutrient element migration were explored by biochar(as supplementary skeleton)experiment;finally,the mechanisms of Fenton oxidation,biochar and biochar coupling for aerobic stability enhancement and effect on Cd stability and migration in soil were clarified by bio-drying and composting experiment in order to provide a feasible technical route to reduce and stabilize the excess activated sludge in situ.It is found in this study that:(1)In the study of the effect of Fenton oxidation on sludge dewaterability and heavy metal removal,the application of Fenton reagent and skeleton material CaO can significantly improve the sludge dewaterability and heavy metal content of the cake;the sludge dewaterability peaks at the Fe²⁺,H₂O₂ and CaO dosage of 90,30 and 90 mg/g DS,respectively,and 90.2%lower specific resistance to filtration(SRF)of sludge and 89.1%higher dewatering rate than CK group,respectively;the Cd,Cr,Ni,Pb and Cu removal,72.10%,46.23%,65.38%,63.16%and 80.80%,from the cake peak at the Fe²⁺,H₂O₂ and CaO dosage of 90,60 and 90 mg/g DS,and has the Zn,Cd,Ni,Pb,and Cu removal of more than 41.58%from the cake at the optimal centrifugal dewatering rate,respectively;the correlation analysis shows that the decrease of heavy metal content in the cake significantly correlates with the increase of sludge dewaterability.The BCR morphology of heavy metal shows that the Cd,Cr,Ni,Pb,Cu and Zn in the cake tends to shift to an active state;Zn and Cd are the most active among them,and acid extractable(F1)Zn and Cd increase by the factor of 3.46 and 26.16 at the optimal dewatering rate.(2)In the study on rice husk biochar as the supplementary framework for Fenton oxidation to condition the sludge,the sludge dewaterability is not significantly improved only by biochar conditioning,but the nitrogen and phosphorus is significantly enriched in the sludge cake;biochar coupling Fenton oxidation(FB)can significantly improve the sludge dewaterability;FB group has the best sludge dewaterability,20.0%and 10.5%lower MC and 98.5%and 76%lower SRF,and 15.6 and 0.66 times higher Y_N,respectively,than CK and F groups under the optimal Fenton conditions at the biochar dosage of 500 mg/g DS;medium and fine particle groups are more conducive to MC decline and TN and TP enrichment in the cake;the biochar added slightly enriches Cu and Zn,and helps Cd,Cr,Ni and Pb migration to the liquid phase;big particles are conducive to Pb transfer to the liquid phase,medium particles are conducive to Ni,Cr,Ni and TP enrichment,and fine particles are conducive to Cd and Cu transfer to the liquid phase;the BCR morphology of heavy metal tends towards transfer to active state;the results of three-dimensional fluorescence,infrared spectroscopy and UV254 analysis show that the coupling conditioning can improve the sludge dewaterability by T-EPS transfer to outer loose state and degradation,biochar skeleton and cationic flocculation;biochar adsorption enriches the nutrient elements released from the sludge,while the sludge floc weakened adsorption sites and functional groups,competitive adsorption of cations,such as Ca²⁺,Fe³⁺,etc.,and increase of solid content significantly decrease the heavy metal content in the cake.(3)In the study on coupling conditioning,bio-drying and composting,the FB group has higher heating rate,longer high temperature maintenance time,higher organic matter and TOC removal,lower moisture content,higher available component content,and higher humification level than CK and B groups at the addition of low conditioning material,showing that the coupling conditioning is more conducive to bio-drying and composting;FB50 group performs the best,and has maximum composting temperature of 58.5?,organic matters degradation rate of 26.4%and MC of 53.9%during composting,the organic matters content and MC decrease by 90.4%and 73.8%during bio-drying(the first eight days),respectively,48.9%and 24.2%higher than CK group,and TOC and C/N decrease by 32.3%and 34.6%,respectively;FB50 group has 26.5%and 74.7%higher available phosphorus and alkali-hydrolyzed nitrogen than CK group,and completely decomposes at HA/FA>1.4;heavy metals(Cu,Cd,Cr,Pb,Zn,Ni)reach the limits of Grade A sludge for agriculture,and are obviously passivated,and their bioavailability significantly reduces.Pot experiments show that the application to soil can promote the greengrocer biomass enrichment,reduce the Cd availability of plants in soil,and increase the soil pH,organic matter content and humus content;the biochar adsorbs the degradable organic matters,improves the spiracular structure of sludge,synergistically enriches the iron salt and accelerates aerobic fermentation and thorough decomposition of compost,while the thoroughly decomposed iron-rich compost effectively reduces the Cd bioavailability and migration characteristics in soil.