Synergistic Mechanisms For Enhancing The Dewaterability Of Sewage Sludge By Fenton Oxidation Combined With Penicillium Simplicissimum Or Tea Polyphenols

With the improvement of urbanization in China,wastewater treatment plants（WWTPs）are increasing dramatically.By December,2018,production of 80%moisture content sewage sludge is estimated to close to about 5.00?10⁷,and will exceed 6.00?10⁷ tons by 2020.The high water content of sludge seriously restricts the subsequent treatment and disposal of sludge.How to solve the problem of dewatering sludge with high water content has become a hot spot for scholars at home and abroad.Sludge extracellular polymeric substances（EPS）is highly hydrophilic and is a key factor in maintaining the structure of sludge flocs and restricting the dewatering of sludge.Therefore,degrading the extracellular polymeric substances of sludge and releasing the bound water in the microbial cells of sludge is the key to improving the dewatering performance of sludge.In recent years,Fenton advanced oxidation technology has been widely used in improving sludge dewaterability in view of their high efficiency,simplicity and environmental friendliness.Fenton system could generate hydroxyl radical via the decomposition of H₂O₂ catalyzed by Fe（II）.Hydroxyl radical is highly reactive and can oxidize most organic substances rapidly and nonselectively.Unfortunately,there are some disadvantages in the Fenton reaction in the sludge system,such as strict p H range limits（2.5-3.5）,high Fenton reagent,and the accumulation of ferric oxide sludge,which would block the Fe（III）/Fe（II）cycle and thus slow the Fenton reactions.It is reported that organic ligands（ethylenediaminetetraacetic acid（EDTA）,citric acid,and oxalic acid etc.）can effectively complex iron ions and inhibit Fe（III）precipitation in the system.However,the wide application of EDTA may cause some adverse environmental consequences because of its poor biodegradability.This dissertation mainly focused on the utilization of low weight molecule organic acids secreted by fungal strain and tea polyphenols（TP）to realize the effective Fe（III）/Fe（II）cycle and then improve the oxidation performance of Fenton system.However,the mechanism of fungal strain and TP enhanced Fenton oxidation on sludge dewatering is still unclear.Moreover,the mechanisms of dewatering during“Fenton-chelating agent”system were investigated,and then theoretical and technical support were proposed for efficient and low cost treatment.The main contents and results are as follows:（1）Fenton oxidation can significantly improve the dewaterability of activated sludge and thickened sludge.The specific resistance to filtration（SRF）of activated sludge and thickened sludge increased from 7.22×10¹² m/kg and 2.39×10¹³ m/kg to 7.67×10¹¹ m/kg and9.70×10¹¹ m/kg,a decrease of 89.4%and 95.9%,respectively.The mechanism could be illuminated as follows:Fenton oxidation led to the disintegration of sludge flocs（smaller particle size）,release of intracellular components（polysaccharides,proteins,nucleic acids,etc.）and bound water,while the degradation of highly hydrophilic sludge EPS into soluble organic matter,which may be the main reason for the improvement of sludge dewaterability.（2）Penicillium simplicissimum NJ12,a gluconic and oxalic acid-producing fungus isolated from sludge,was able to improve the dewaterability of sludge.The sludge SRF decreased by 97.2%from 1.98×10¹³ m/kg to 5.56×10¹¹ m/kg at 5%（v/v）inoculation dose（mycelium）and temperature of 28°C for 3 d of treatment time.Multiple stepwise regression analysis showed that the degradation of proteins in sludge Slime-EPS and the neutralization of Zeta potential were the main reasons for the improvement of sludge dewaterability by the fungal treatment.In addition,the treatment of P.simplicissimum NJ12 combined with Fenton oxidation showed that 50%less Fenton reagent and the final sludge SRF could be reduced to3.47×10¹¹ m/kg.The analysis by electron paramagnetic resonance（EPR）and confocal laser scanning microscope（CLMS）showed that the small molecule organic acids secreted by P.simplicissimum NJ12 were complexed with Fe（II）,which avoided the hydrolysis and precipitation of Fe（III）in Fenton system and greatly increased the generation of hydroxyl radicals.Sludge cell disintegration and bound water release may be the main reason for enhanced Fenton oxidation to improve sludge dewaterability.（3）Tea polyphenols（TP）are natural green organic compounds with chelating and reducing properties.TP-Fenton system was constructed to enhance Fenton oxidation.The efficiency of sludge dewatering during Fenton oxidation system can be greatly improved by adding small amounts of tea polyphenols（TP）.When the dosage of TP is 0.50 mmol/g DS（dry solids）,the dose of Fe（II）and H₂O₂ can be reduced by 50%compared to Fenton treatment alone,and the final SRF and CST of the TP-Fenton treatment system are 6.85×10¹¹m/kg and 22.7 s,respectively.Full wavelength scanning and isothermal titration calorimetry（ITC）showed that TP was strongly complexed with Fe（III）and Fe（II）under acidic conditions;electron spin resonance（EPR）showed that hydroxyl radical production was significantly higher in the sludge TP-Fenton treatment system than in the Fenton system alone.Therefore,it can be speculated that TP can reduce Fe（III）to Fe（II）in TP-Fenton system and accelerate the cycle of Fe in TP-Fenton system.（4）Fenton oxidation assisted by tea polyphenols also significantly improved the dewaterability of anaerobic digested sludge（ADS）.The CST of ADS decreased from the initial 258.9 s to 59.8 s with the addition of 0.80 mmol/g DS TP,50 mg/g DS Fe（II）and 75mg/g DS H₂O₂,respectively;organic phosphorus（OP）in sludge solid phase changes to inorganic phosphorus（IP）during sludge treatment by TP-Fenton.The final concentrations of organic phosphorus（OP）,apatite inorganic phosphorus（AP）and non-apatite inorganic phosphorus（NAIP）in the sludge were 1.10 mg/g,4.21 mg/g and 15.1 mg/g,respectively.After the solid-liquid separation,the SCOD,ammonia nitrogen and phosphorus contents of the final effluent filtrate were 621.6 mg/L,383.9 mg/L and 0.34 mg/L,respectively.In addition,cost estimation analysis showed that the total cost of the reagents in the TP-Fenton treatment process was 147.1 USD/ton DS,which is much lower than that of conventional Fenton conditioning,and therefore has a high economic feasibility.