| Sludge thickening process is one of the important links of waste sludge treatment and disposal in the municipal wastewater treatment plant (WWTP). It includes mainly gravity thickening, mechanical thickening and air floatation thickening.The water content of excess sludge from secondary settling tank varies from 99.2% to 99.6%, but the water content of excess sludge, which is discharged from solid-liquid seperator for the Integrated Oxidation Ditch developed by Chongqing University, or discharged from aeration tank for some new wastewater treatment process is higher than 99.2～99.6%, Its water content is high and solid content is low, it is called low concentration excess activated sludge.Cavitation Air Floatation (CAF) technology has been widely applied industrial wastewater treatment, but hasn't been used in sludge thickening field.In this research, CAF technology was introduced into sludge thickening in WWTP, conditioner for excess activated sludge was selected. Factors such as flocculants, surfactant, solid loading, hydraulic loading, sludge skimming method, the aerator power and temperature were investigated. A Modified Cavitation Air Floatation (MCAF) equipment for low concentration activated sludge was developed and the factors such as flocculants, surfactant, solid loading, hydraulic loading, sludge skimming method, the aerator power and temperature were investigated, too. The dewatering characteristics of thickened sludge and effluent quality from CAF and MCAF were studied. The correlative mechanism of CAF and MCAF sludge thickening process were discussed. The features of CAF thickening process were summarized. Energy consumption, technology and economic analysis were illustrated. The results are as follows: 1,Single conditioners such as FO4440SH, FO4248, FO4190SH, PAC, FA920SH and dual conditioners such as PAC + FO4440SH, PAC + FO4190SH, PAC + FA920SH, PAC+AN926SHU, FO4440SH+FA920SH, FO4440SH+AN926SHU were used, given a comprehensive consideration of the sludge conditioning effect, operation management and cost, single conditioner FO4440SH was selected for the sludge conditioning from Chaoyangzhou WWTP. 2,As CAF-5 was employed to thicken low concentration sludge, flocculant FO4440SH was added with an optimal dosage of 1.0kg/tDS, 1227 was adopted as surfactant with an optimal dosage of 0.2kg/tDS,solid loading was 230kgMLSS/m3?d,and hydraulic loading was 90 m2/m3?d. The SS and dissolved COD in the effluent was 200～250mg/L and 30～40mg/L,respectively. And P in the sludge wouldn't release during the thickening process.3,MCAF-10, which is aimed to low concentration excess activated sludge, has been developed. When MCAF-10 was used to thicken low concentration activated sludge, with optimal dosage of FO4440SH and 1227 at 1.0kg/tDS and 0.2kg/tDS respectively, and with solid loading at 270kgMLSS/m3?d, hydraulic loading at 110 m2/m3?d, the SS and dissolved COD of the effluent was 100～150mg/L and 30mg/L or so,respectively. There was no P release of the sludge during the thickening process.4,MCAF exhibits excellent thickening efficiency with high solid loading and low SS effluent, therefore is more suitable for thickening low concentration excess sludge.5,A certain amount of surfactant enhances sludge thickening efficiency, while too much of it decreases the thickening efficiency due to the fact that the existing of micelle exhibits stronger hydrophilicity and decreases the adherence between sludge flocs and gas bubbles. With the addition of surfactant, the extra-cellular polymers in sludge surface is released to outside environment, which yields more compactable activated sludge flocs with less interstitial moisture and improved dewaterability.Since bubbles produced by CAF or MCAF are relatively large, the size of sludge flocs is not the key factor that affects sludge thickening for CAF or MCAF process. As long as the flocs' size reach 300~400μm, they can adhere to gas bubbles.6,Besides its simple unit, convenient operating and low cost, CAF thickening process possesses a short...
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