Research On Flocculation Characteristics Of Surface Water Withseasonal Turbidity Rapidly Varied In Cold Region Of Northwest China

The annual average temperature is low due to the high altitude in cold region of northwest China. The mainly surface water sources are snow melt water transported from long distance. It is difficult for water treatment plants(WTPs) to treat this kind of water because of its characteristics of perennial low temperature and turbidity, and seasonal turbidity rapidly varied, resulting in a decline of effluent water quality, water supply security and reliability. In this study, the Heiquan reservoir(Xining, China) was selected as the raw water to investigate the flocculation characteristics, flow field, and flocs breakage and re-growth using computer fluid dynamics(CFD) software combined with flocs morphology methods. Moreover, the mechanisms of flocs formation, breakage and re-growth were also discussed. In addition, the flocs strength quantitative evaluation model and broken rate coefficient model were established. The objectives were to solve the flocculation operation problems when treated similar actual water in cold region of northwest China; to provide theoretical and technical basis for snow melt water treatment in Xining and even in cold region of northwest China WTPs; and to guarantee the treatment efficiency in WTP and drinking water quality safety.The varying patterns of flow field in grid flocculation tank were investigated using CFD software basing on the analysis of raw water quality characteristics, and the grid flocculation tank were built combined with flow field analysis and flocculation experiment. The results showed that the surface water in cold region of northwest China was with characters of perennial low temperature and turbidity, seasonal turbidity rapidly varied, small particle size, high p H, organic matter concentration increased year by year. The flow velocity distribution was more nonuniform, the turbulence kinetic energy(k), turbulent dissipation rate(ε) and flocs fractal dimension were increased with the increase of flow velocity and lateral dimension of disturbed flow device. Increasing the eddy size and decay distance would weaken the disturbance in grid flocculation tank, reduce k, ε and flocs fractal dimension. Flocs size increased in initial stage and followed a decrease trend, and turbidity and particles in setted water showed opposite pattern. The velocity, eddy size, lateral dimension of disturbed flow device and decay distance displayed a good power function relationship with k and ε. The influence of longitudinal dimension of disturbed flow device was negligible. The optimal parameters of grid flocculation tank for raw water treatment were as follows: flow velocity was 0.10 m/s, eddy size was 40- 60 mm, lateral dimension of disturbed flow device was 10- 15 mm, longitudinal dimension of disturbed flow device was 1 mm, and decay distance was 350- 500 mm.The pilot scale flocculation tests of raw water were conducted using the optimized grid flocculation tank. The variation of flocs morphology and settling properties, using different coagulants and coagulant aids to treat low temprtature and turbidity raw water in cold region of northwest China, were investigated by flocs morphology method. The flocculation mechanisms were also considered. Further more, the effects of water quantity and turbidity on flocs morphology and settling properties were studied. Under the condition of low temperature and turbidity, flocs fractal dimension initially decreased, then increased, finally achieved a plateau with the increase of coagulation time. Increasing coagulant dosage could increase micro-flocs quantity, flocs size, growth rate, and fractal dimension; reduce turbidity and organic matters concentration of setted water. Polyaluminum chloride(PAC) flocs’ fractal dimension was the largest together with the best settling properties; the turbidity, oxygen consumption(OC) and total organic carbon(TOC) of setted water were lower than 0.5 NTU, 1.9 mg/L and 2.1 mg/L, respectively. Dosing cationic polyacrylamide(c PAM) could greatly improve the flocs properties and settleability. Polydiallyldimethylammonium chloride(PDADMAC) had a better promotion effect of flocculation at low dosage. The optimal dosage of c PAM and PDADMAC were 1.0 mg/L and 0.6 mg/L, respectively. Flocs morphology parameters increased firstly and then decreased, and water quality initially improved and then deteriorated with the increase of coagulant aid dosing time; and the optimal coagulant aid dosing time was after flocculation for 2 min. With the increase of water quantity, flocs size increased followed decrease, and the turbidity and organic matters of setted water increased in the initial stage and then decreased. When turbidity varied largely, the optimal coagulants dosage decreased firstly and then increased, and flocs size increased. Moreover, flocs would be broken which caused a decrease of flocs size and an increase of micro particles, and affect the effluent water quality..Jar tests and pilot scale tests were designed to investigate flocs breakage and re-growth characteristics with turbidity rapidly varied using PAC and PAC plus PAM. Flocs strength quantitative evaluation model and broken rate coefficient model were also established. The breakage process using low dosage of PAC was reversible, while breakage processes using high dosage of PAC, PAC plus PAM were irreversible. The size of re-growth flocs decreased, and the residual turbidity and particles were increased with the increase of breakage shear force and time. The size of re-growth flocs initially increased and then decreased when re-growth shear force increased. Additional dosage of PAC could significantly increase the flocs size of broken flocs, and improve water quality. Theoretically, flocs strength was a function of flocs diameter and fractal dimension, and the breaking force of fluid was proportional to the squared average velocity gradient and the fourth power of flocs diameter. The flocs strength order was: PAC>PAFC>PFS>AS. Moreover, dosing coagulant aid could obviously increase flocs strength. Broken rate coefficient was a function of shear force and fractal dimension.Through this study, grid flocculation tank was optimized based on raw water quality analysis, and the flocculation treatment methods of surface water with seasonal turbidity rapidly varied in cold region of northwest China were determined. In addition, flocs strength quantitative evaluation model and broken rate coefficient model were established. The results could not only to perfect the basic flocculation theory of seasonal turbidity rapidly varied water treatment, but also to solve the design and operation problems of flocculation process in cold region of northwest China WTPs. Consequently, drinking water safety could be guaranteed, and economic development choud be accelerated in cold region of northwest China.