Effects of several water quality parameters on arsenic removal by coagulation: laboratory experiments and a pilot-scale study

In laboratory experiments a traditional drinking water treatment method, coagulation/flocculation followed by solid/liquid phase separation has been applied in order to decrease arsenic concentration below 10 μg/L (which is the new Hungarian standard for arsenic). The goal of the research work was to examine the transition of the dissolved arsenic to solid form, to determine the factors which have significant effect on arsenic removal efficiency. The organic content of the water highly affected the arsenic removal process. The difference in the required coagulant dosage can be order of one magnitude depending on the organic content of the water. The phosphorous content also increases the required coagulant dose, since ferric phosphate precipitates are formed, decreasing the amount of coagulant available for arsenic removal. pH also proved to have significant influence when experiments were carried out at wide pH range. However, under more realistic conditions (pH is between 7.5 and 8), the effect of pH was not that significant. The inorganic carbon content at some extent favors the liquid/solid transition of arsenate, since it contributes to the buffering capacity of the water, therefore enhances the metal hydroxide formation process. However, the excess inorganic carbon has disadvantageous effect, since it competes with the arsenate ions for the free sites of metal hydroxides. Two oxidants (chlorine and potassium permanganate) and two coagulants (ferric chloride and aluminum sulfate) were studied in pilot scale experiments. The pilot plant was operated at Hajdúbagos, where the arsenic, iron and manganese concentration of the raw water is above the standard. In the experiments it was found the potassium permanganate + ferric chloride combination was the most efficient.