This work evaluates the environmental impact of Wastewater Treatment Plants (WTP) based on data generated by the exergy analysis, calculating and applying environmental impact indexes for two WTP located in the Metropolitan Area of São Paulo. The environmental impact of the waste water treatment plants was done by means of evaluating two environmental impact exergy based indexes: the environmental exergy efficiency (η env,exerg ) and the total pollution rate (R pol,t ). The environmental exergy

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... efficiency is defined as the ratio of the exergy of the useful effect of the WTP to the total exergy consumed by human and natural resources, including all the exergy inputs. That relation is an indication of the theoretical potential of future improvements of the process. Besides the environmental exergy efficiency, it is also used the total pollution rate, based on the definition done by Makarytchev (1997) , as the ratio of the destroyed exergy associated to the process wastes to the exergy of the useful effect of the process. The analysis of the results shows that this method can be used to quantify and also optimise the environmental performance of Wastewater Treatment Plants. Keywords : exergy efficiency; environmental impact; wastewater treatment plants NOMENCLATURE B Deact. exergy of additional natural resources destroyed during waste deactivation [kW] B Disp. exergy related to waste disposal of the process [kW] B Nat.Res exergy of the natural resources consumed by the processes [kW] BOD Biochemical Oxygen Demand [mol/L] B Prep. exergy required for extraction and preparation of the natural resources [kW] B Product exergy of the useful effect of a process [kW] B Waste exergy of waste, which includes the exergies of solid wastes, rejected heat, and emissions [kW] COD Chemical Oxygen Demand [mol/L] η env,exerg environmental exergy efficiency R pol,t total pollution rate WTP Wastewater Treatment Plants