Dissolved oxygen (DO) is a key parameter in water quality. The DO concentration in a water body can be changed by interfacial phenomena such as reaeration and oxygen dissipation, which can be represented by the coefficients K2 and KD, respectively. Few studies have jointly correlated K2 and KD with physical and hydraulic parameters of the channel. The present work investigated the behavior of these coefficients over a range of hydraulic conditions, and developed semi-empirical equations capable of relating both coefficients. Reaeration and DO dissipation tests were conducted in a circular hydraulic channel with flow velocity ranging from 0.20 to 0.79 m.s-1 and depth ranging from 0.09 to 0.15 m. Estimates of K2 and KD were performed using the non-linear regression method. Semi-empirical equations were obtained based on classical dimensional analysis and multiple regression analysis. The comparison between measured and estimated coefficients yielded R2 for reaeration and dissipation of 0.940 and 0.844, respectively. KD was higher than K2 for all turbulence levels applied at the hydraulic channel. An estimate obtained by the relation between the semi-empirical equations indicates that the transfer of oxygen in the water-air direction (dissipation) is approximately twice as fast as the transfer in the air-water direction (reaeration).

RESUMOO objetivo deste estudo foi avaliar o perfil de oxigênio dissolvido (OD) em um trecho urbano da bacia do Rio Uberaba, identificar as principais fontes poluidoras e sugerir medidas de controle. Para tanto, o estudo foi dividido em cinco fases: (i) avaliação da qualidade da água durante um ano hidrológico; (ii) calibração e validação do modelo de qualidade da água; (iii) análise de sensibilidade dos coeficientes e dos dados de entrada do modelo; (iv) análise de incerteza; e (v) criação de cenários para a tomada de decisão. Os resultados mostraram que o Córrego das Lajes é a principal fonte poluidora do Rio Uberaba. O modelo mostrou-se confiável, uma vez que ele pôde ser calibrado e validado. Com a análise de incerteza foi possível mostrar que a probabilidade de o Rio Uberaba atender ao padrão ambiental exigido para a classe 2 (OD≥5,0 mg.L -1 ) é praticamente nula, mas que ela poderia ser elevada para 75,3% se a carga orgânica lançada no rio fosse removida com uma eficiência de 75%.Palavras-chave: modelagem de qualidade da água; análise de incerteza; Rio Uberaba. ABSTRACTThe objective in this study was to evaluate the Dissolved Oxygen (DO) profile along an urban stretch in Uberaba River basin and identify the main pollution sources. For this, the study was divided in five stages: (i) water quality evaluation during a hydrologic year; (ii) calibration and validation of the water quality model; (iii) sensibility analysis of the coefficient and entrance data for the model; (iv) uncertainty analysis; and (v) creation of decision-making scenarios. The results show that Lajes Stream is the main pollution source of Uberaba River. The model demonstrated to be reliable, once it could be calibrated and validated. Through the uncertainty analysis, it is possible to show that the probability of Uberaba River to attempt the environmental standard demanded for class 2 (DO≥5.0 mg.L -1 ) is practically zero, but it could be raised to 75.3% if the organic polluting load released into the river was decreased in 75%.

ABSTRACT.Longitudinal dispersion coefficient (D L ) is considered an essential physical parameter to water quality modeling in rivers. Therefore, the estimation of this parameter with high accuracy guarantees the reliability of the results of a water quality model. In this study, the observed values of longitudinal dispersion coefficient are determined for natural streams (with discharge less than 2.84 m ), a partir de ensaios de campo de estímulo-resposta com traçador salino (cloreto de sódio) e do método da propagação (routing procedure), corrigido para considerar a perda do traçador. Adicionalmente, uma equação semiempírica de previsão de D L foi desenvolvida a partir da análise dimensional e da técnica de regressão linear múltipla. A equação desenvolvida foi comparada com cinco equações empíricas de predição de D L existentes na literatura. O r 2 da equação gerada foi de 0,87, o que sugere que esta equação é adequada para a estimativa de D L para os cursos d'água estudados. A equação produzida gerou melhores resultados do que as cinco equações retiradas da literatura, apresentando uma acurácia de 71%.Palavras-chave: coeficiente de dispersão longitudinal, rios, traçador salino.

The aim of this study was to investigate the influence of humic substances (HS) on volumetric oxygen-transfer coefficient (K L a) under different turbulence intensities at the air-water interface. The experiments were carried out in an oscillating grid tank which provided three levels of turbulence intensities (Reynolds numbers of 5,116, 10,316, and 15,433). For each turbulence level, 15 different HS concentrations were tested and the K L a was estimated. The results showed that, regardless of the turbulence level in the tank, HS reduces K L a. The maximum reduction was 17%. The barrier effect, produced by the occupation of HS molecules at the interface, was the main phenomenon responsible for the reduction in the oxygen transfer. A relationship between the oxygen transfer velocity and turbulent kinetic energy showed that in the presence of HS, the air-water interface behaves like an immobile surface, reducing the oxygen diffusion and surface renewal.gas-water interface, grid-stirred tank, humic substances, oxygen mass transfer, reaeration

Melanoidin is a compound produced by food industries and distilleries, which has negative impacts on the water environment due to the high content of dissolved organic carbon and dark color. Consequently, this work aims to study the catalytic properties of multilayer graphene (MLG), molybdenum oxide (MoO 2 ), and manganese ferrite (MnFe 2 O 4 ) in the ozonation process to remove melanoidin from water solution. The results show that the reaction rate constant (k) of the melanoidin decolorization process using catalytic ozonation is 1.7 times higher than the non-catalytic ozonation process. The same results were observed for all catalytic materials with no significant difference among them. On the other hand, MLG was the most efficient catalyst in removing total organic carbon. The removal efficiency was 32% for the non-catalytic ozonation process and 63% for the catalytic ozonation using MLG. This increase in efficiency is attributed to a better production of hydroxyl radicals in the presence of MLG, which was confirmed using isopropanol as a radical scavenger. The efficiencies using MoO 2 and MnFe 2 O 4 were 46% and 51%, respectively. The results show that catalytic ozonation by MLG is a promising treatment for melanoidin removal.

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