Oxidation Reduction Of Excess Sludge And Sludge Utilization For Carbon Catalysts/Biodiesel Preparation

Wastewater treatment plant plays a significant role in the rapid urbanization,as an inevitable byproduct,excess sludge is generated and the yield continues to rise.Sludge is fetid and contains various pollutants such as organic pollutants and pathogens.Besides,abundant protein,lipid and other nutrients are found in sludge,owing to the luxuriant microbial community in sludge.Particularly,the water content of sludge reaches 93～99%.Therefore,sludge presents the properties of pollutional,recyclable,and high water content.Guided by the sludge disposal strategy of“reduction,utilization and harmlessness”,this study pays attention to water content of sludge,aiming to provide a new sludge reduction approach,synthesize functional sludge-derived carbon for environmental remediation,as well as produce biodiesel from sludge.This study would achieve the multi-win situation of reducing environmental pollution,avoiding resource waste and alleviating energy crisis.The main research contents and conclusions are as follows:（1）A sludge reduction method using manganese dioxide as oxidant was established.The sludge reduction efficiency can reach 73.30%with the water bath heating system.According to kinetics results,the reduction process was mainly controlled by surface chemical reaction.Two steps of floc destruction and microbial cell disruption occurred in sludge reduction,then the dissolved organic matters were oxidized by manganese dioxide,which promoted the further dissolution of sludge.Instead of electric heating,microwave radiation was introduced to construct the microwave-enhanced manganese dioxide oxidation system for sludge reduction.Microwave radiation can accelerate the disruption of microbial cell and the degradation of organic matter oxidized by manganese dioxide.The sludge reduction efficiency was67.97%in the microwave irradiation system,which was significantly higher than that of water bath under the same conditions（35.88%）.It shows that the introduction of microwave radiation could reduce acid consumption and reaction time,as well as improve sludge reduction efficiency.It indicates that microwave radiation coupled with manganese dioxide oxidation technology can reduce the acid consumption and reaction time,and improve the sludge reduction efficiency,which is an environmentally friendly and efficient sludge reduction method.（2）Nitrogen-doped sludge carbon of NC-700 was synthesized using excess sludge as precursor and urea as supplementary nitrogen source,which could act as a bifunctional material for methylene blue（MB）removal by the adsorption and catalytic oxidation through peroxymonosulfate（PMS）activation.Nitrogen doping regulated the elemental composition and microstructure of materials,since the N/C ratio and specific surface area increased,which improved the material performance for PMS activation and MB degradation.The adsorption process of MB on NC-700 followed the pseudo-second-order kinetics and Langmuir isotherm models,indicating that the adsorption process is a monolayer adsorption.98.70%of MB could be removed within 20 min in the NC-700/PMS system.Besides,NC-700 showed good reusability.Based on radical analysis results,N atom on the surface of NC-700 could combine with O atom from PMS,while C=O groups could provide lone-pair electrons to break O-O of PMS,then producing reactive oxygen species（ROS）.Particularly,singlet oxygen（¹O₂）was suggested to be the main ROS for MB degradation.（3）Sludge carbocatalyst with abundant oxygen functional groups using wet sludge as precursor was synthesized via hydrothermal coupled pyrolysis process.Results showed that the introduction of hydrothermal process and increasing the sludge water content could significantly enhance the oxygen content of sludge carbon,most of which are C=O groups,causing an enhanced performance of sludge carbon for bisphenol A（BPA）and sulfamethoxazole（SMX）degradation by activating PMS.Notably,the content of C=O group was found to possess a linear positive correlation with pollutant degradation rate of k value.Furthermore,¹O₂ was demonstrated to be the main ROS for pollutant degradation,since C=O groups were the main active sites for ¹O₂ generation.The BPA degradation process involves two routes of the scission of C-C and dehydroxylation as the first step,and the SMX degradation process involves four pathways of deamination,nitrification,hydroxylation and S-N bond break as the first step.Ecotoxicity evaluation indicated that the biotoxicity of BPA solution and SMX solution was significantly reduced after the oxidation treatment.（5）Hydrothermal process was introduced for biodiesel production by in situ transesterification using wet sludge as raw material.The biodiesel yield reached 8.63%with 2%（v/v）of acid concentration,which is the highest value compared with other reports.When the raw material was replaced by dried sludge with the same hydrothermal process,the biodiesel yield was 8.71%.The results demonstrated that hydrothermal process could break the distinction of biodiesel yield derived from wet or dried sludge.According to the gas chromatography-mass spectrometry results,methyl esters of palmitic acid and oleic acid were the main components of the obtained biodiesel.Then,the solid residue after biodiesel extraction was utilized as adsorbent,while the liquid residue was used as nutrient substrate.