Applications And Mechanisms Of Fenton-like Advanced Oxidation Processes In Sludge Dewatering And Pollutant Degradation

Disposal of sewage sludge with a high water content accounts for a major portion of the cost in wastewater treatment plants.Dewatering is a crucial process in the sludge treatment and faces significant technical challenges.Reducing water content of sludge cake is proven to be an efficient method to decrease the transportation and disposal costs.The highly hydrated and negatively charged extracellular polymeric substances(EPS)in sludge colloids play important roles in sludge dewatering.The degradation of EPS can promote the release of bound water and the reduction in water content of sludge.On the other hand,industrial wastewaters usually contain a mixed pool of pollutants in a wide range of concentrations,which can cause great harm to aquatic organism and human body.The development of cost-effective technical solutions is needed to successfully deal with the increasingly complex problems in wastewaters.Fenton process is a widely studied and used catalytic method based on the generation of hydroxyl radicals(HO)from hydrogen peroxide with iron ions acting as homogeneous catalyst at acidic pH and ambient conditions.The Fenton system is attractive because of its fast reaction rates,low toxicity,and simplicity to control.Nevertheless,the Fenton system has some intrinsic drawbacks that limit its widespread application,such as strict pH range limits,high H2O2 dosage,and the accumulation of ferric oxide sludge,which causes the decline of oxidation rates and requires a separation step.Therefore,in this work,we focused on the optimization of the source of iron in Fenton system to promote the degradation of EPS and the agglomeration of fine sludge colloids,which could accelerate the filtration rate and reduce the water content.Another effort was to introduce some reducing agents with low reaction rates into Fenton system to accelerate the circulation of Fe3+/Fe2+and the oxidation reactivity.Moreover,the mechanisms of dewatering and Fenton-reducing agent system were investigated.The main contents and results are listed below:(1)A Fenton-like dewatering process,i.e.,a combination of sulfuric acid,hydrogen peroxide and ferric sulfate.was developed,which can generate HO· to promote the degradation of EPS and the release of bound water in sludge.On the other hand,Fe3+ can agglomerate fine sludge colloids to form large flocs,which can accelerate the filtration rate.Uniform design was used to optimize the dosage of Fe3+,H2O2 and pH in the composite conditioner.Differential scanning calorimetry(DSC)and coupled plasma-atomic emission spectroscopy(ICP-AES)were employed to quantify the bound water content and typical inorganic elements(Zn,Cu,Ca,Cd)in the treated sludge samples,respectively.The results show that the bound water was released and converted into free water and the typical heavy metals were released from the sludge after the Fenton-like treatment.Meanwhile,the extended DLVO theory shows that the higher hydrophobicity and the less stable sludge flocs also contributed to the decrease of moisture content.(2)Fe@Fe2O3 nanomaterial,combined with polydiallyldimethylammonium chloride(PDMDAAC)and H2SO4,was used to enhance the sludge dewatering.This composite conditioner exhibited an excellent dewatering capability with low water content and high filtration rate.A higher content of proteins,polysaccharides,nucleic acids and total organic carbon(TOC)was observed in the samples after treated by the composite conditioner.These results demonstrate that more EPS could be decomposed into small dissolved organics and subsequently released into the liquid phase,which is beneficial to the decrease of water content.The presence of HO· was verified using a DMPO spin-trapping ESR technique,and stronger ESR signals of HO· were observed after complexing with DMPO in the mixed solution of Fe@Fe2O3 and H2SO4.The ESR results clearly demonstrate the existence of HO· for enhancing sludge dewatering and EPS decomposition with the dose of the composite conditioner.(3)The changing profiles of bound water content and distribution in the activated sludge treatment by NaCl addition and pH modification were investigated.Both the salt addition and acid treatment altered the flocculated matrix and increased the contents of LB-EPS,resulting in the release of trapped water and reduction in sludge volume.The sludge conditioned by H2SO4 yielded the lowest moisture content(70.8%)and the least capillary suction time(CST,11.1s).In addition,the chemical treatments also affected the distribution of bound water.The internal water content increased upon the dose of NaCl,leading to a decreased water content trapped in EPS.Hence,salt addition caused a slower filtration rate,but did not effectively decrease the total water content,although the EPS were decomposed.Under acidic conditions,cell lysis helped to release the intracellular water and further enhance the sludge dewatering performance.In contrast,conditioning with low-dosage of alkali increased the bound water content and deteriorated the sludge dewaterability.(4)The comparison of Fenton oxidation enhanced by three different kinds of reducing agents was investigated.Hydroxylamine resulted in the highest reaction rates under the optimum condition compared with cysteine and ascorbic acid.The range of the effective pH was expanded after the usage of three reducing agents.The highest reactivity of Fenton system was obtained at pH 4.0.Furthermore,the batch addition of reducing agents yielded much better Fenton reactivity.