Aluminium sulfate as coagulant for highly polluted cork processing wastewater: Evaluation of settleability parameters and design of a clarifier-thickener unit

Abstract

This is the second part of a master project on the chemistry of aluminium as coagulant in the treatment of highly polluted cork-process-wastewater. The main aim of this second part was to determine the influence of the operating conditions on the system's settleability parameters. It is well known that it is just as important to achieve good settleability parameters in the physico-chemical treatment of wastewaters as it is to attain a high level of decontamination. These parameters will determine the dimensions of the required equipment, and hence the costs of the installation. This part of the study therefore analyzes the influence of the different operating variables on the following settleability parameters: sediment volumetric percentage, settling velocity, sludge volume index and total suspended solids just after mixture with the coagulant. The ranges used for the experimental variables were: coagulant dose (83–166 mg L −1 of Al 3+), coagulation mixing time (5–30 min), stirring rate (60–300 rpm), contamination level of the wastewater (Wastewater II COD ≈ 2000 mg O 2 L −1, Wastewater III COD ≈ 3000 mg O 2 L −1), and pH (5–11). The optimal conditions found for the settling process were not the same as those that had been determined for the organic matter removal. In this case the optimal conditions were: coagulation mixing time (30 min), stirring rate (60 rpm), coagulant dose (83 mg L −1 of Al 3+) and pH (7–9). Finally, the Talmadge–Fitch method is used to apply the results to the design of a clarifier-thickener unit to treat 2 m 3 h −1 of wastewater. The required minimum area of the unit would be 4.11 m 2.