Designing a robust index for WWTP energy efficiency: The ENERWATER water treatment energy index.

The development and use of composite indexes has exploded in the last 15 years as a tool to summarise the large amount of information available nowadays. To ensure that composite indicators reflect faithfully the purpose of evaluation and are widely accepted and used, the users must understand the relationship between individual sub-indicators and the result (transparency) and the evaluation should not depend on weights, lack of information, etc. (robustness). It is proposed here for the first time a composite index to measure energy efficiency in a wastewater treatment plant, from the definition of the individual sub-indicators to the assignation of an energy label, easy to communicate to a broad public. Using as a demonstration real data from 44 wastewater treatment plants, the index robustness is tested and improved by uncertainty and sensitivity analysis results, finally achieving a robust algorithm which can be used by the large majority of wastewater treatment plants.