Polyaluminium chloride and flocculation aids for colour removal from biologically treated textile mill effluent

Abstract

The objective of this work was to construct the polyaluminium chloride (PACl) diagram of coagulation for secondary effluent from a textile industry in order to determine the best coagulant dose and coagulation pH. PACl doses vary between 200 and 450 mg/L with pH values from 4.5 to 8.0. In addition, it was evaluated the effect of applying a copolymer of acrylamide and sodium-acrylate as a flocculation aid and the addition of a decolourant, composed by dicyandiamide, to increase the colour removal efficiency, at the best coagulant dose and pH obtained. Apparent colour removal of 70% was reached in the pH range and coagulant dose predicted for the enhanced coagulation process: high coagulant doses, between 200 and 450 mg/L; and low coagulation pH, between 4.5 and 5.0. The application of the copolymer and the decolourant resulted in an apparent colour removal between 10 and 30% less than the efficiency obtained with PACl application. These results indicate the need for further studies to optimise the application of these combined products with the PACl and improve the colour removal from textile mill effluent.