Heavy metals are toxic for humans and the environment. Therefore it is important to limit the emission of these heavy metals in air, water and soil. An aqueous solution of sodium trimercapto-triazine, commercially supplied under the name TMT-15, is currently used for the removal of heavy metals occurring in the washing water from flue gas purification at the incineration plant of the Intercommunale Maatschappij voor Openbare Gezondheid (IMOG) located in Harelbeke (Belgium). This aqueous solution of sodium trimercapto-triazine belongs to a group of complexion agents which are expensive and a large dosage is necessary to obtain good effluent values. Therefore an optimization study for the consumption of TMT-15 and an examination for the alternative products was performed. Key performance indicators were effluent quality and chemical consumption. The optimization study was realized based on a central composite experimental design (CCD) approach performed with the Design Expert software. The amount of complexion, coagulation and flocculation agent was used as input variable in this design, while the effluent heavy metal concentrations and the corresponding metal discharge levy were used as responses that needed optimization. Preliminary jar tests were used to determine the experimental boundaries, while the Design Expert software was used to determine the optimal complexion, coagulation and flocculation agent concentration. Based on this approach a 19% reduction in chemical use could be achieved on lab scale and, because of safety precautions, a reduction of 10% was implemented in the actual plant. Further (cost) optimization of the installation was based on an alternative analysis which revealed that the same reagent supplied by a different supplier produced similar results, but was lower in cost price. (C) 2008 Elsevier B.V. All rights reserved.