Effects of pH on coagulation behavior and floc properties in Yellow River water treatment using ferric based coagulants

Enhanced coagulation is one of the major methods to control disinfection by-products (DBPs) in water treatment process. Coagulation pH is an important factor that affects the enhanced coagulation. Recently, many studies focus on the coagulation effects and mechanisms, and few researchers studied the properties of flocs formed under different coagulation pH. Two inorganic polymer coagulants, polyferric silicate sulphate (PFSS) and polyferric sulphate (PFS), were used in Yellow River water treatment. The influence of pH on coagulation effect was investigated under the optimum dosage, and the results show that both coagulants gave excellent organism removal efficiency when pH was 5.50. According to the variation of zeta potential in coagulation process, coagulation mechanisms of the coagulants were analyzed. An on-line laser scatter instrument was used to record the development of floc sizes during the coagulation period. For PFSS, pH exerted great influence on floc growth rates but little influence on formed floc sizes. In PFS coagulation process, when pH was 4.00, PFS flocs did not reach the steady-state during the whole co-agulation period, while little difference was observed in floc formation when pH was 5.50 and above. The preformed flocs were exposed to strong shear force, and the variation of floc sizes was determined to evaluate the influence of pH on floc strength and re-growth capability. In comparison of the two coagulants, PFS flocs had higher floc strength and better recovery capability when pH was 4.00, while PFSS flocs had higher floc strength but weaker recovery capability when pH was 5.50 and above.

Coagulation effect and floc properties of polyferric silicate sulphate and polyferric sulphate in the Yellow River water treatment

Two inorganic polymer coagulants,polyferric silicate sulphate(PFSS) and polyferric sulphate(PFS),were comparatively evaluated in the Yellow River water treatment.The removal efficiency of turbidity and UV254 were investigated,and the zeta potential was measured.An online laser scatter instrument was used to determine the size distribution of flocs formed in the coagulation processes.Compared with PFS,PFSS forms flocs with higher growth rates and larger sizes.The formed flocs were exposed to a series of shear forces,and the floc strength was measured from the relationship between the applied shear speed and the resulting floc size.The flocs formed by PFSS had a higher strength.The floc breakage(expressed as breakage factor,Bf) and re-growth ability(expressed as breakage factor,Rf) under different shear forces and different shear periods were investigated,and it was found that larger Bf and Rf appeared after exposure to stronger shear force and longer shear period.Under the same shear condition,the flocs formed by PFSS showed a larger Bf while the flocs formed by PFS had better re-growth ability.