ECO-optimization of pre-treatment processes in metal finishing

Saving of bath chemicals and freshwater consumed within metal finishing systems and the reduction of wastewater produced can be important steps towards more sustainable processes. Furthermore, the application of regenerators is usually energy intensive. Accordingly, in an “eco–eco” (ECO) trade-off, importance should be given to both economic benefits as well as to several ecological measures. The approach presented here is the synthesis of an ECO-optimal reuse and recovery network (RRN) based on a simultaneous multi-objective system optimization model with dynamically chosen environmental objective. It includes a conventional cost objective and the potential environmental impact of the system. This second objective is represented by the maximal relative increase of indicator results. This, in turn, derives from a sensitivity analysis of life cycle inventory analysis (LCIA) integrated into mixed integer nonlinear programming (MINLP). This means, the LCA-rule, which claims that environmental indicators must not be aggregated, can be adhered strictly. In the case study, the synthesis procedure for eco-optimal design is applied to a large scale phosphating. Depending on the weighting ratio for the representative impact factor and the cost in the objective function, Pareto-optimal solution sets were calculated allowing to assess different eco-optimal RRN alternatives and to find a way out of the main dilemma between the contradicting behavior of energy demand and wastewater production. Up to 74%% improvement in terms of environmental indicators could be realized at low cost increase.