Wen Po Cheng

A study of coagulation mechanisms of polyferric sulfate reacting with humic acid using a fluorescence-quenching method.

A fluorescence-quenching method is developed to assess the effect of pH on the coagulation mechanism of humic acids (HA) reacting with metal ions. A polyferric sulfate (PFS) synthesized in our laboratory is adopted as the coagulant to simplify the hydrolysis process and increase the experimental precision. The following results are discovered. When the concentration of PFS increases from 2 to 10 mg/L, the effective pH range of HA removal changes from 4.0-5.0 to 4.0-7.5. At increased coagulation pH, the ferric ions may still react with HA but unable to neutralize the surface charge completely. The residual concentrations of HA measured by fluorescence spectrophotometer are lower than those by TOC, as a consequence of the fluorescence-quenching effect. This demonstrates that the coagulation of HA by PFS at low pH is mainly due to charge-neutralization. The adsorption of the HA on the pre-formed iron hydroxide flocs is accompanied by the dissociation of Fe ions from the floc structure until the equilibrium has been reached, which is evidenced by the presence of the Fe-HA complexes in the solution during adsorption experiment. This is quite different from the characteristics of flocs formed by PFS associated with HA in the coagulation. Within the pH range investigated, the complex-formation and the hydrolysis are the two competitive reactions happened between the hydroxide ions in solution and the functional groups of HA. Therefore, the removal of HA is not caused by adsorbing onto the iron hydroxide resulted from PFS hydrolysis, but through the complex-formation between the PFS and the HA.

Influence of eutrophication on the coagulation efficiency in reservoir water

Water from the three reservoirs, Min-ter, Li-yu-ten and Yun-ho-shen, was examined for concentration of chlorophyll a, ultraviolet absorption (UV(254)), fluorescence intensity (FI), concentration of dissolved organic carbon (DOC), and fractionation of dissolved molecules by molecular weight. The water samples were collected over the change from spring to summer (May to July but before the typhoon season) when the water temperature and extent of eutrophication increase. Analytical results indicate that the concentration of DOC is proportional to the concentration of chlorophyll a, but not to the values of UV(254) and FI. Therefore, eutrophication, extraneous contaminants of small molecules, and the extracellular products of algae cause an increase in DOC, but a decrease in the proportion of large organic molecules such as of humic substances. The fraction of DOC with a molecular weight of less than 5000 Da increases with the concentration of chlorophyll a. All these data suggest that changes in the quality of water after eutrophication make the treatment of drinking water more difficult. The method of enhanced coagulation was recently developed for removing DOC. However, the results of this paper demonstrate that the efficiency of DOC removal falls as the degree of eutrophication increases. When the percentage of DOC with small molecules excreted by algae increased by 1%, the efficiency of DOC removal decreased by approximately 1%, implying that enhanced coagulation are not able to remove the DOC excreted by the algae during eutrophication, and resulting an increased concentration of trihalomethanes formation in water disinfections process.

Evaluating the Coagulants of Polyaluminum Silicate Chlorides on Turbidity Removal

Abstract Polyaluminum silicate chloride (PASiC) is a new inorganic coagulant and has different characters from other cationic inorganic products used in water treatment. The optimal formulas and conditions to produce and use PASiC are not thoroughly well‐known. PASiC can be formed by treating AlCl3 solution with silicate compounds and the insoluble aluminum silicates are produced thereafter. It has been found that partially decreasing alum acidity with silicate would form a more stable solution than polyaluminum chloride (PAC). PASiC may enhance aggregation by the silicate in it to remove turbidity especially for the water of low turbidity and alkalinity. In this research, a series of PASiC coagulants were produced from the different basicities (B) and Al/Si molar ratios at the different aging temperatures and aging times. Accordingly, PASiCs prepared by the different basicities (B) and Si/Al molar ratios were added in a synthetic water with low turbidity and alkalinity to evaluate the efficiency of PASiC...

Study of the coagulation property of polyaluminum silicate chloride coagulants prepared with ultrasonic assisted NaOH dosing

Abstract Polyaluminum Silicate Chloride (PASiC) is a new water treatment coagulant. It contains silica that can be hydrolyzed to produce SiO2 as coagulant aid. Hence, this coagulant consumes little alkalinity and is effective at low dosages. It is, therefore, especially suitable for treating water of low alkalinity and low turbidity. When manufacturing PASiC, the alkalinity solution involved in the polymeric reaction must be added slowly to the aluminum salt solution to avoid local over‐saturation which enhances the production of high‐valence medium polymer species (Alb) (e.g. Al12AlO4(OH)24 7+, Al13). In this research, the use of an ultrasonic vibrating system to assist the alkalinity dosing during the formation of PASiC was studied. The ultrasonic vibrating system breaks up the NaOH solution into fine mists so that the latter can be more evenly dispersed into the aluminum silica solution to avoid local over‐saturation. This ultrasonic‐assisted NaOH dosing method leads to a more stable polymerization of aluminum changing ratios of Ala, Alb, and Alc in the final product and raising the Alb portion in the coagulant. Long‐term observations also show that PASiC prepared with the ultrasonic‐aided NaOH dosing at 0.25 mL/min is more effective in removing turbidity.