Study On Flocculation Characteristics Of The Low Temperature And Low Turbidity Raw Water Containing Anionic Surfactant (SDS)

Flocculation characteristics of raw water containing typical anionic surfactant-sodium dodecyl sulfate (SDS) under low temperature and low turbidity conditions have been studied. The removal efficiency of turbidity and SDS by polyaluminium chloride (PAC), polyferric sulfate (PFS), aluminum chloride (AlCl3), ferric chloride (FeCl3) and nano-silica was investigated. To futher characterization of the dynamic flocculation process of kaolin particles with SDS and the morphological properties of flocs fractal structures, optical online monitoring the flocculation process using PDA2000 and image analysis technology/the fractal theory were emploied. The major achievements in the thesis are as follows:(1) When the four inorganic coagulants and nano-silica used exclusively, FeCl3 and PAC exhibited the most significant removal efficiency of turbidity than the other two tested coagulants and showed a certain removal efficiency of SDS. After treated by silica nano-particles, the residual turbidity in solution was still high, but the removal efficiency of SDS was notably greater than the other inorganic coagulants did. SDS had a certain effect on the removal of turbidity by inorganic coagulants, and coagulants at the same dose had worse removal efficiency of turbidity but better the SDS removal efficiency when the solution contained higher concentration of SDS.(2) Using silica nano-particles as coagulant aid could dramatically enhance the PAC flocculation removal of turbidity and SDS. When ferric chloride, polyferric sulfate, and aluminum chloride served as main coagulants, there was a nano-silica critical dosage of 3mg/L for the removal of turbidity. When the nano-silica dosage was less than the critical dosage, the residual turbidity in supernatant decreased significantly with the increase of nano-silica dosage. However, when the nano-silica dosage exceeded the critical dosage, the residual turbidity in supernatant reversely increased with the increase of nano-silica dosage, while the removal of SDS only increased slightly.(3) When pH value was in the range of 6-9, PAC used with SiO2 can remove turbidity effectively, with the residual turbidity in supernatant is less than 1 NTU. The removal effect was obviously better than other combinations of coagulants and nano-silica. The pH value went too low (pH <6) or too high (pH> 9) being against the removal of turbidity. The best pH value for removal of SDS by PAC+ SiO2 was at 6. (4) SDS can enhance the surface electronegativity of the particles causing the increase of the absolute value ofζpotential on the particles surface, and affect the flocculation of coagulant. The coagulation mechanism of PAC is identified as the double-layer compression, the adsorption/charge neutrality and the bridging. The coagulation mechanism of nano-silica is suggested as the bridging, and mechanism for combination action of PAC and nano-silica is the combination of the absorption bridging and the charge neutralizationis with the former acting mainly and the later supplementally.(5) Nano-silica as flocculant enabled the particles in suspension to form large floc in a shorter period of coagulation, to reach the R-value peak earlier, and to increase the peak. Low concentration of SDS impeded the formation of floc at the beginning of flocculation, while high concentration of SDS promoted the flocculation process. When nano-silica served as flocculant aid, the strength factor and the recovery factor of the formed flocs increased compared with using PAC alone, the resistance against hydraulic shear force became stronger, and the re-coagulation of the broken flocs also enhanced.(6) Taking PAC as a main coagulant, both fractal dimension of flocs and the average equivalent particle size increased with the increase of the nano-silica dosage. Nano-silica promoted transition of the denser PAC floc, and the fractal dimension which characterize the filling degree of the internal floc increased, and the removal efficiency of turbidity and SDS significantly enhanced. During the whole flocculation process, the fractal structure of floc gradually evolved from a loose and porous, open-DLCA branch structure to a dense RLCA model structure. Thus, the sedimentation property improved and the fractal dimension value rised.