Performance Assessment of Hydraulic Efficiency Indexes Edmilson Costa Teixeira, Ph.D. 1 and Renato do Nascimento Siqueira, Ph.D. 2 Abstract: 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, recirculation, dead zones, 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. DOI: 10.1061/ASCE0733-93722008134:10851 CE Database subject headings: Hydrodynamics; Water treatment plants; Wastewater management; Tracers. Introduction The concern about water quality and quantity is increasing more and more around the world. This scenario has led to the search for more efficient treatment processes and improvement of design criteria. Water and wastewater treatment unit projects are usually based on hypotheses of ideal flow regimes plug flow and complete mixing. For plug flow, the fluid entering the unit is equally dis- tributed on the cross section and moves with the same velocity toward its outlet. The complete mixing condition is characterized by the homogeneity of the fluid properties inside the unit at any location and time. However, the flow pattern inside the unit deviates from the ideal and the performance of the water/wastewater treatment pro- cess is usually lower than that expected during its design stage. Therefore, it is important to evaluate the hydraulic performance of the unit to understand its effects over the treatment process. Evaluation of the hydraulic performance should be done by direct comparison of the flow pattern inside the unit and that expected for ideal flow conditions, such as in Teixeira 1993, who used laser Doppler anemometry for velocity measurements. However, this practice is not always possible and alternative methods are usually required. The most frequently used method is based on the interpretation of residence time distributions RTD. The interpretation of RTD functions is based on the compari- son of some hydraulic indexes extracted from these functions with the expected values for the ideal flow condition. Neverthe- less, there are many hydraulic indexes proposed in the literature to evaluate the performance of water/wastewater treatment units and their interpretation may lead to different conclusions. The lack of criteria to evaluate the hydraulic efficiency of treatment units was the main motivation for this work. A compi- lation of the majority of the hydraulic indexes used in the litera- ture and their suitability to evaluate the hydraulic efficiency of water and wastewater treatment units was analyzed. The analysis is based on the physical concept and statistical variability of each index. Residence Time Distributions RTD can be obtained by a tracer technique, which consists of an instantaneous injection of a known quantity of a conservative substance e.g., salts, fluorescent dyes, etc. at the inlet section of the unit and the subsequent monitoring of the tracer concentration with time at its outlet section Hart 1979. In order to permit the comparison of different works and fa- cilitate their interpretation, RTD functions are usually normalized. The normalization is done by dividing the measured concentra- tion C by the initial average concentration Co, which is the ratio between the mass of tracer that was injected into the flow and the effective volume of the unit, and the time t by the theoretical retention time T, which is the ratio between the ef- fective volume of the unit V and the flow rate Q. To correct the RTD functions so that the mass balance is sat- isfied, the normalized concentration, C = C / Co, where  = normalized time  = t / T, is usually divided by the recovery rate Rec, which is defined as Rec =  0  Cd 1 Experiments that display a value of Rec substantially different from 1.0 must be considered with caution Stamou and Adams 1988. 1 Senior Lecturer, Dept. de Engenharia Ambiental, Univ. Federal do Espírito Santo, Cx. Postal 01-9011, Vitória, ES 29060-970, Brazil corre- sponding author. E-mail: edmilson@npd.ufes.br 2 Senior Lecturer, Coordenadoria de Mecânica, Centro Federal de Edu- cação Tecnológica do Espírito Santo—CEFETES—UnED São Mateus, Rua Duque de Caxias, 194A, Carapina, São Mateus, ES 29.930-000, Brazil. E-mail: renatons@cefetes.br Note. Discussion open until March 1, 2009. Separate discussions must be submitted for individual papers. The manuscript for this paper was submitted for review and possible publication on March 1, 2005; ap- proved on January 30, 2007. This paper is part of the Journal of Envi- ronmental Engineering, Vol. 134, No. 10, October 1, 2008. ©ASCE, ISSN 0733-9372/2008/10-851–859/$25.00. 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.