LOW TURBIDITIES EFFECT ON THE COAGULATIONFLOCCULATION

Abstract

This investigation evaluated, in laboratory scale, the effect of low turbidity of the decanted water stored in the reservoir (80,000 m3) which thereafter supplies the units of the Puerto Mallarino- treatment plant in Cali, when the operation of the system is not possible with crude water. The control variables are: coagulant type and doses at different pH values, assessed with the jar test equipment. The coagulation diagrams developed allowed the identification of the maximum efficiency zones regarding turbidity removal and color. The study was done in three phases: Phase I: decanted water from the Puerto Mallarino plant was used utilizing the usual working coagulants: ferri chloride and aluminum sulfate; these two samples were stored separately, during 40 days, in 1,000 cm3 tanks. Phase II: decanted water was recollected in the Puerto Mallarino plant, regardless of the coagulant used in the moment of the collection. This water was stored in a 1,000 cm3 tank, performing tests during days 1, 3, 8 and 15 using ferric chloride as a coagulant in 3 – 15 mg L-1 dosage range. Phase III: the two working coagulants were used in order to find the highest color and turbidity removal percentage through the use of coagulation diagrams and to stablish the relationship between water quality and optimal coagulant doses. Finally, with the aid of coagulation diagrams, it is shown the turbidity removal with aluminum sulfate reaches 85% at 6.8 pH with a coagulant dose of 15 mg L-1, while a 90% efficiency of color removal is achieved at 6.0 and 6.5 pH with coagulant lower dosage (10 and 12 mg L-1).