The Role Of Tannic Acid In Fenton And Persulfate Oxidation In Improving The Dewatering Of Anaerobically Digested Sludge

Anaerobic digestion is a technical solution for stabilization,reduction and energy conversion of excess sludge that is vigorously promoted internationally.However,since the sludge after anaerobic digestion has smaller particle size and greater alkalinity,its dewatering is often difficult,which is very unfavorable for subsequent disposal and comprehensive utilization of sludge such as transportation and incineration.In recent years,the advanced oxidation technology of Fenton and persulfate based on the principle of·OH and SO₄^·-radical degradation of sludge cells and extracellular polymeric substances has been used in the field of sludge treatment due to its simple process,convenient operation,fast and high efficiency,etc.Especially the conditioning and dewatering of excess activated sludge gradually emerged and attracted wide attention from scholars at home and abroad.However,Fenton oxidation and persulfate oxidation generally have the problems of large dosage,low utilization rate and high processing cost in practical applications,which greatly restricts the pace of engineering application of such methods.Therefore,how to reduce the cost of chemical addition and energy consumption for Fenton oxidation and persulfate oxidation is a technical problem that needs to be solved urgently.In view of this,this article attempts to apply a wide-source,low-cost,and green-friendly plant extract-tannic acid（TA）to Fenton and thermal activation persulfate to oxidize and condition anaerobically digested sludge and promote its dewatering performance,explore the related mechanism,aiming to provide a technical reference for the efficient and low-consumption dewatering of anaerobically digested sludge.The main research results obtained are as follows:（1）Tannic acid can significantly promote Fenton oxidation and improve the dewatering performance of anaerobically digested sludge,reducing the dosage of Fenton reagent by half.Under the optimal conditions of p H=3.0,Fe²⁺dosage of 25 mg/g DS,H₂O₂ dosage of 31.25mg/g DS,TA dosage of 0.25 mmol/g VS,and reaction time of 30 min,compared with the original anaerobically digested sludge,CST decreased from 667.6 s to 25.5 s,and SRF decreased from 1.4×10¹⁴ m/kg to 9.0×10¹¹ m/kg.TA promoting Fenton strengthening effect sludge dewatering process,since the complex and reduction of the TA.Due to the complexation and reduction of TA,the Fe²⁺cycle is improved,and a large amount of·OH is generated to disintegrate the sludge flocs and greatly reduce the protein in TB-EPS,especially aromatic proteins and tryptophan proteins,floc structure becomes loose and porous and other factors may be the main reasons for the improvement of dewatering performance.（2）Tannic acid can greatly reduce the temperature in the process of thermally activated persulfate oxidation to promote anaerobically digested sludge dewatering.Under the condition of tannic acid dosage of 0.15 mmol/g DS,the temperature of thermally activated persulfate oxidation to improve anaerobically digested sludge dewatering can be reduced from 80℃to 55℃;the sludge CST decreased from 317 s to 39 s for 25 min,a decrease of88.0%,under the condition of 4 mmol/g DS sodium persulfate（PDS）and 55℃,and the bound water content decreased by 56.7%.Electron paramagnetic resonance detection showed that the large amount of·OH and SO₄^·-produced by thermally activated PDS coupled with tannic acid to disintegrate the sludge EPS is the key to improving the dewatering performance of anaerobically digested sludge.In addition,after treatment,the particle size of the sludge becomes smaller,the pores of the sludge floc structure increase,the absolute value of the zeta potential decreases,and the protein content in LB-EPS and TB-EPS is greatly reduced,and the content of small molecules of humic acid with strong hydrophobicity is increased and other factors also help to enhance the dewatering performance.（3）Tannic acid coupled with thermally activated persulfate promotes the dewatering performance of anaerobically digested sludge,and at the same time greatly promotes the release of phosphorus in the solid phase of the sludge.Under the conditions of PDS dosage4 mmol/g DS,TA dosage 0.15 mmol/g DS and 55℃,the release rate of total phosphorus（TP）in the sludge reached the maximum 67.9%.Further morphological classification showed that the phosphorus in the sludge mainly exists in the form of non-apatite inorganic phosphorus（NAIP）.After treatment,the form of phosphorus changes from pyrophosphate to polyphosphate.