The analysis of residence time distribution functions originating from tracer studies is one of the main tools for the assessment of hydraulic performance in water and wastewater treatment units. In order to simplify the analysis, hydraulic indexes extracted from these functions are normally used. In general, these indexes are divided into two categories: short circuit and mixing indicators. However, some indexes may be related to more than one physical phenomenon ͑i.e., short circuit, mixing,

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... ecirculation, dead zones͒, leading to erroneous interpretation. In this work their capability to evaluate short-circuit and mixing levels in water and wastewater treatment units is assessed. Among the indexes analyzed, t 10 , which is the time necessary for 10% of the tracer to leave the unit, is recommended as a short-circuit indicator and the dispersion index ͑ 2 ͒ and the Morril index as mixing indicators, depending on the mixing level. JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / OCTOBER 2008 / 851 J. Environ. Eng. 2008.134:851-859. Downloaded from ascelibrary.org by UNIVERSIDADE FEDERAL DO on 07/15/13. Copyright ASCE. For personal use only; all rights reserved. Fig. 3 . Tendency of the short-circuit indexes with the increase in the short-circuit level: ͑a͒ ti and t 10 ; ͑b͒ t p , Th, and ICC; and ͑c͒ t 50 , HBP 1 and SEG 1 . JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / OCTOBER 2008 / 855 J. Environ. Eng. 2008.134:851-859. Downloaded from ascelibrary.org by UNIVERSIDADE FEDERAL DO on 07/15/13.