Study Of Free Nitrite Acid Combined With Electrochemistry To Improve Sludge Dewaterability

As municipal wastewater systems continue to expand their service areas,more and more municipal wastewater is discharged after being treated by municipal sewage plants.Municipal sludge,as a collection of pollutants in municipal wastewater,is also showing a rapid increase in production.The problem of sludge treatment and disposal has become a part of environmental problems that cannot be ignored.The high water content（WC）of municipal sludge creates difficulties for sludge transportation,storage,and reuse.Therefore,improving the dewatering performance of municipal sludge is the key point of sludge treatment,the purpose of this study is to find a new method of deep sludge dewatering to test its feasibility and investigate its dewatering mechanism.Many treatment methods are used for deep dewatering pretreatment of sludge,electro-coagulation（EC）is of interest to researchers because of its simplicity of operation,lack of secondary contamination,and improved sludge stability.In this project,the effects of electrolytic voltage,electrolysis time and electrode plate distance on sludge specific resistance to filterability（SRF）and sludge water content（WC）in electrochemical pretreatment were investigated.The SRF of municipal sludge was reduced from0.62×10¹³ m/kg to 0.13×10¹³m/kg the reduction rate was 78.9%,and the water content was reduced from 89%to 86.9%with a reduction rate of only 2.36%under the conditions of electrolysis voltage of 25 V,electrolysis time of 60 min,pole plate distance of 4 cm,and anode material of Fe pole plate.This is due to the fact that sludge cells cannot be lysed by EC.In the experiment,free nitrite（FNA）was found to be effective in lysing sludge cells to release intracellular material.An electro-coagulation（EC）process combined with added free nitrous acid（FNA）is described as a means of improving sludge dewaterability.The SRF and WC of the sludge were reduced by 89.57%and 18.90%,respectively,under the optimal working conditions of Fe plate as the anode,electrolysis voltage of 25 V,electrolysis time of 60 min,plate distance of 4 cm,FNA concentration of 1.13 mg/L,reaction time of 24 h,and p H value of 4.5.The EC process disrupted the sludge structure reducing the size（D50）of the sludge particles from 59.5 to 50.5μm.After the addition of FNA,the sludge cells lysed and the DNA concentrations and soluble chemical oxygen demand（SCOD）increased from 6.07μg/ml and 29 mg/L to364μg/ml and 588 mg/L,respectively,and the conversion of Fe（II）to Fe（III）was enhanced.Addition of FNA after EC therefore further improved the sludge dewaterability.Combined conditioning using EC and FNA can effectively destroy tightly bound extracellular polymeric substances（TB-EPS）and release bound water.In addition,the p H value is kept low which benefits sludge dewaterability and the removal of heavy metals.The concentrations of Zn and Mn in the sludge cake were reduced by 92.3%and69.0%,respectively.The Bureau of Reference（BCR）sequential extraction method showed increases in the percentages of the residual fractions of Zn and Mn,showing that EC combined with FNA is an efficient and versatile means of sludge conditioning.