Optimization of management choices of clariflocculation process by means of the qualitative multi-criteria analysis

Abstract Every year ship traffic produces tons of liquid waste mainly consisting of bilge water and of washing water of tankers' tanks. The latter are called slop waters and are characterized by high salinity and by the presence of recalcitrant pollutants mainly of hydrocarbon origin: these characteristics promote the use of chemical-physical rather than biological treatment. In particular, in the present study the slop waters were subjected to a clariflocculation treatment by means of batch tests. This treatment involves the dosage of specific chemical reagents (coagulants and flocculants) added to water at different stages of the process. In order to establish the optimal reagents' type and dose, also considering the operating costs, the proposed study presents a frequency analysis belonging to the family of multi-criteria exploration. The application of this methodology to examine the validity of the different process alternatives has allowed to include in a single assessment both economic and extra-economic (measurable only in qualitative terms) procedures. Thanks to this qualitative and quantitative method, it was therefore possible to order the different treatment alternatives analyzed, identifying the one that allows optimizing the wastewater management, for a conscious choice of the most suitable solution to the problem.