Optimization Of Coagulation Performance And Residual Aluminum Control For Yellow River Water With Low Temperature And Low Turbidity

Coagulation has been commonly used in water treatment process. Aluminum saltcoagulants, such as aluminum trichloride (AlCl3), aluminum sulfate (Al2(SO4)3),polyaluminum chloride (PACl) were widely applied in drinking water treatment.Particulate matter, colloidal and soluble substance could effectively removed bycoagulation and the subsequent processes. But studies showed that the use of aluminumsalt coagulants were the main reason for high aluminum concentration in drinking water.Aluminum in water is harmful to ecosystem and human body now has been arousedpeople’s attention.Low temperature and low turbidity water treatment has been the problem in watersupply treatment. The raw water of waterworks at the upper course of the Yellow Riveris seasonal changes. Typicalily, it will become low temperature and low turbidity duringwintertime. Small, fluffy and poor settling characteristics floc was generated in thecoagulation process from the water treatment plant when operated at the winter, thiscaused poor treatment effect and greatly increased the difficulty of the follow-uptreatment. Undesirable coagulation effect also lead to a high residual aluminumconcentration (more than0.5mg/L) in finished water, which posed a threat to the safetyof drinking water quality.The performance of four types of coagulants, high performance polyaluminumchloride (HPAC), polyaluminum chloride (PACl), aluminum sulfate Al2(SO4)3, and acomposite of PAC and FeCl3, were tested with low temperature and low turbidity rawwater from Yellow River for their relative coagulative effectiveness and residual aluminum in finished water by jar test in this paper.The results indicate that when the optimal dosage for turbidity removal by usingAl2(SO4)3or PACl, the residual aluminum concentration would exceed the nationalstandard (0.2mg/L).Both HPAC and composite coagulants of FeCl3and PACl couldreduce the residual aluminum concentration while being able to obtain a similarturbidity removal efficiency of Al2(SO4)3or PACl. When dosage of HPAC was at21mg/L, a turbidity of1.3NTU and a residual aluminum concentration of0.147mg/L inthe coagulated water after sedimentation could be obtained. Similarly, when thecomposite coagulant of PACl (12mg/L) and FeCl3(15mg/L) were used, a turbidity of1.18NTU and a residual aluminum concentration of0.074mg/L could be obtained. Theratio of PACl to FeCl3should be optimized when the composite coagulant wasemployed, otherwise both the residual aluminum and ferric iron in settled water wouldincrease. For HPAC, when the dose of addition was less than21mg/L, the residualaluminum concentration would be below the limits of National Drinking WaterStandards. Therefore, among the four types of coagulants, HPAC and compositecoagulant of PACl and FeCl3were a better coagulant for treating the source water withlow temperature and turbidity in terms of both coagulative effects and residualaluminum control in treated water.