The Research On Secondary Dewatering Of Urban Sewage Dewatered Sludge Used Inorganic Conditioners

The secondary deep dewatering of urban dewatered sewage sludge is the very important link on the reduction, recycling and reducing the cost of sludge. But currently, the research on the secondary deep dewatering of urban dewatered sewage sludge at home and abroad is still very little, particularly on the research of dehydration mechanism of micromolecule inorganic conditioner.The purpose of this paper is to explore the dehydration mechanism of two kinds of micromolecule inorganic conditioners (AlCl3 and FeCl3) in the dewatering process, reveal the main factors which influence the dewaterability of sludge and lay a theoretical foundation for the research of micromolecule inorganic conditioners. In experiment, the moisture content of filter cake after dehydration of pressure filtration was as the evaluation index of the dewaterability of sludge. By studying the changes of moisture distribution, particle size, surface charge, EPS component and microstructure of dewatered sludge before and after conditioning, the dehydration mechanism of two kinds of conditioners (AlCl3 and FeCl3) was analyzed. Moreover, in order to further improve the secondary dewaterability of dewatered sludge, the compounds of silicate, FeCl3, AlCl3 and flyash were researched. The effects of dosage, adding ratio, concentration, mixed mode and adding order of conditioners on the secondary dewaterability of dewatered sludge were also investigated. The main results are as follows:(1)When AlCl3 was conditioned with dewatered sludge, the moisture content of sludge after dewatering decreased at first and then increased with the increase of AlCl3 dosage. The moisture content could reach the lowest 54.03% when the dosage of AlCl3 was 8 g/300 g dewatered sludge. The positive electricity of sludge particles was more and more strong and the particle size of sludge increased at first and then decreased. The particle size of sludge reached the maximum 299.56μm when the dosage of AlCl3 was 8 g/300 g dewatered sludge. The sludge flcos cohered more closely. Large amount of moistures which were not free water were turned into free water and removed from sludge under pressure. The dewaterability of dewatered sludge was improved finally.(2) When FeCl3 was conditioned with dewatered sludge, the moisture content of sludge after dewatering decreased at first and then increased with the increase of FeCl3 dosage. The moisture content could reach the lowest 55.7% when the dosage of FeCl3 was 10 g/300 g dewatered sludge. The surface charges of sludge particles were neutralized and the positive electricity of sludge particles was more and more strong. The particle size of sludge increased at first from 64.26μm to 97.18μm and then decreased. The particle size of sludge reached the maximum 299.56μm when the dosage of AICl3 was 8 g/300 g dewatered sludge. The contents of protein, polysaccharide and DNA of EPS in sludge increased with the increase of FeCl3 dosage. The result illustrated that the sludge floc and biological particles were disrupted and the moisture distribution in sludge was changed. A lot of bound water and free water were transformed into interstitial water. Under the action of outside pressure, the sludge floc was further flocculated and coagulated. The interstitial water inside of sludge floc was into free water and removed from sludge. Finally, the dewaterability of dewatered sludge was improved.(3) When silicate compounding with FeCl3, AICl3 and coal ash was used to condition with 300 g dewatered sludge, the secondary dewaterability of dewatered sludge of K2Si03 was better that Na2Si03. When the dosages of K2Si03, FeCl3 and flyash were 8 g,10 g and 10 g respectively, the moisture content of sludge could be decreased at the minimum of 39.9%. And when the dosages of K2SiO3, AICl3 and flyash were 8 g,8 g and 10 g respectively, the moisture content of sludge could be decreased to 41.5%. The dewaterability of dewatered sludge was basically not affected by the order of adding and solution concentration of agents. The optimum adding orders of conditioners were silicate, FeCl3, flyash and silicate, AICl3, flyash.