This study evaluates the residence time (RT) and total phosphorus (TP) in a small hypereutrophic lake in the city of Fortaleza, Brazil. The results indicate that RT predicted by a complete-mix model is very similar (R² = 0.83) to that simulated with a 2-D hydrodynamic model (CE-QUAL-W2). Simple power-laws were fitted to describe RT and TP concentration at the lake inlet as functions of lake inflow, yielding correlations of R² = 0.84 and 0.70, respectively. The combination of these correlations with a complete-mix approach provided a comprehensive model that predicted TP values measured at the lake outlet reasonably well (R² = 0.60). In addition, a direct empirical correlation between simulated TP concentration and precipitation was also obtained. The simulations indicate a nearly periodic behaviour of RT and TP, with the seasonal variations being much higher than the interannual ones. Finally, an application of the model showed that a reduction of 99% of the input TP load was required to reach 100% compliance with the required water quality standards; this could be achieved by connecting the residences to the sewage network. The methodology proposed in this research can be easily applied to other lakes in the Brazilian northeast and extended to other tropical regions around the globe.

Abstract This study investigated experimentally the air entrainment and pressure drop in low-cost ejectors composed of two pieces shaped from PVC bars inserted in a 25 mm T-junction of the same material. The hydraulic behavior was very similar for the different ejector designs, and linear relationships between the water and air flow rates were fitted. However, when a rotameter was installed at the air line, the head losses resulted in a pronounced decrease (3-fold) in the air entrainment rate. The maximum air-water entrainment ratios reached by the low-cost ejectors was 1.7, while the pressure drop was about 80% of the upstream pressure. The results suggest that these ejectors have a better benefit-cost ratio than conventional ones for applications such as aeration and mixing in reactors, tanks and water bodies. Comparing our results with those obtained previously by using water both as primary and suction fluids, it was shown that under gas-liquid flow conditions the entrainment ratio was about 2.5 times larger than that for the single-phase case, while the pressure drop was about 15% higher. This was attributed to the lower density of the air and the higher dissipation of turbulent kinetic energy due to bubble-liquid interactions in the two-phase flow case.

Abstract: Field surveys and integral modelling were carried out to study the effect of bubble plumes on the flow hydrodynamics and the time-evolution of water temperature profiles in a shallow lake with maximum depth of about 4 m. Then, model simulations were performed to verify the feasibility of destratification of ten water-supply reservoirs with capacities of 10-2,000 hm3. Finally, hydrological modelling was conducted to assess the impact of destratification on evaporation suppression and its effects on reservoir yield. The results indicate that net temperature reductions of the order of 1.0°C can be obtained, which corresponds to evaporation suppressions of approximately 10%. In turn, this resulted in increases in the regulated flow of 2-12% that could be described by a general correlation. Lastly, a simplified feasibility analysis provided an operational cost of about 1.0 US$ per cubic meter of saved water, which suggests that artificial destratification cannot be discarded as an alternative for supplementing water supply in rural water-scarce regions.

Abstract: This paper investigates phosphorus dynamics in a highly polluted system composed of a drainage channel followed by a shallow reservoir in the city of Fortaleza, Brazilian semiarid. During the dry season, significant non-point source loads of total phosphorus originated from residences unconnected to sewers were identified along the channel, resulting in an increasing flow rate of untreated sewage towards the reservoir. During the rainy season, as a consequence of dilution, phosphorus concentrations decreased by about 5-fold. The results also revealed that the reservoir was acting similarly to a waste stabilization pond, with phosphorus removal efficiency of about 33%. Assuming one-dimensional flow for the channel and complete mixing for the reservoir, it was possible to adjust phosphorus decay coefficients of kc = 2.2 day-1 and kr = 22.8 year-1, respectively, which were higher than their corresponding values available in the literature. This was attributed mainly to the relatively high temperatures and shallow water depths. The results also revealed an inverse dependence of kc and kr on water and wind velocity, with potential thresholds of respectively 0.3 and 5.0 m/s for resuspension of bed sediments and internal phosphorus loading.

Field surveys were carried out to investigate the surface jet flows and the resulting circulation patterns generated by diffused aeration in a shallow lake. In conrast to previous studies, the experimental conditions included point-source bubble plumes with very high air flow rates (100-400 L/min) relative to the shallow water depth (1.5 m). The results indicate that the surface jet velocity can be described by linear profiles. The decay of this velocity with distance displayed a similar behaviour to that reported in the literature, but the surface jet spread much faster under the present conditions. Combining our results with published data allowed for the generation of dimensionless correlations for the velocity and depth of the surface jet. A good agreement between the total circulation flow rates predicted by using the proposed correlations and integral modelling was also obtained. Lastly, a simple returning flow model was proposed to describe the circulation flow patterns induced by the bubble plumes. The results were also applied to assess the impact of circulation on vertical algae migration, which is important for water quality management.

Este estudo investiga as relações entre vazão e evaporação em função da morfologia de lagos em uma região semiárida do Brasil. Inicialmente, foi proposta uma nova metodologia para classificar a morfologia de 40 reservatórios no Estado do Ceará, com capacidades de armazenamento variando entre aproximadamente 5 e 4500 hm3. Em seguida, foram realizadas simulações do tipo Monte Carlo para estudar o efeito da morfologia do reservatório (incluindo suas formas reais e cônicas simplificadas) sobre o processo de armazenamento de água em diferentes níveis de garantia. Os reservatórios foram classificados como convexos (60,0%), ligeira mente convexos (27,5%) ou lineares (12,5%). Quando a aproximação cônica foi usada em vez de a forma real do lago, ocorreu uma compensação entre vazão e perdas por evaporação com diferentes tendências para os reservatórios convexos, ligeiramente convexos e lineares. Usando a aproximação cônica, os erros na previsão da vazão da água foram de aproximadamente 5% da média da vazão anual afluente, o que pode ser considerado insignificante para grandes reservatórios. No entanto, para reservatórios menores, este erro se tornou relevante. Portanto, este trabalho apresenta um novo procedimento para corrigir as relações entre vazão-evaporação que foram obtidas assumindo uma aproximação cônica, em vez de a morfologia real do reservatório. A combinação desta correção com o Diagrama Triangular de Regularização é útil para predizer rápida e objetivamente as perdas de vazão e evaporação do reservatório em ambientes semiáridos.

This study investigates the relationships between yield and evaporation as a function of lake morphology in semi-arid Brazil. First, a new methodology was proposed to classify the morphology of 40 reservoirs in the Ceará State, with storage capacities ranging from approximately 5 to 4500 hm3. Then, Monte Carlo simulations were conducted to study the effect of reservoir morphology (including real and simplified conical forms) on the water storage process at different reliability levels. The reservoirs were categorized as convex (60.0%), slightly convex (27.5%) or linear (12.5%). When the conical approximation was used instead of the real lake form, a trade-off occurred between reservoir yield and evaporation losses, with different trends for the convex, slightly convex and linear reservoirs. Using the conical approximation, the water yield prediction errors reached approximately 5% of the mean annual inflow, which is negligible for large reservoirs. However, for smaller reservoirs, this error became important. Therefore, this paper presents a new procedure for correcting the yield-evaporation relationships that were obtained by assuming a conical approximation rather than the real reservoir morphology. The combination of this correction with the Regulation Triangle Diagram is useful for rapidly and objectively predicting reservoir yield and evaporation losses in semi-arid environments.

RESUMO Este artigo apresenta um modelo integral para avaliar o impacto da transferência de gás na hidrodinâmica de plumas de bolhas. O modelo é baseado no princípio de similaridade do tipo Gaussiana e em relações funcionais para o coeficiente de entrada turbulenta e o fator de amplificação da quantidade de movimento devido à turbulência. O efeito da transferência de massa na hidrodinâmica de plumas de bolhas é investigado considerando diferentes tamanhos de bolhas, vazões gasosas e profundidades de água. Os resultados revelaram um impacto relevante quando bolhas finas são consideradas, mesmo para moderadas profundidades da água. Além disso, as simulações do modelo indicaram que para plumas de bolhas fracas (ou seja, com vazões gasosas relativamente baixas e profundidades de água e velocidades relativas elevadas), tanto a dissolução como a turbulência podem afetar a hidrodinâmica das plumas, o que demonstra a importância de se levar em conta o fator de amplificação da quantidade de movimento. Para condições de águas mais profundas, simulações dos processos de dissolução e descompressão das bolhas utilizando o modelo atual e modelos clássicos disponíveis na literatura forneceram resultados semelhantes tanto para sistemas de aeração como de oxigenação. Análise de sensibilidade mostrou que a profundidade da água, seguida do tamanho das bolhas e da vazão gasosa são os parâmetros que mais afetam a hidrodinâmica das plumas. Finalmente, correlações adimensionais são propostas para avaliar o impacto da transferência de massa na hidrodinâmica das plumas, incluindo tanto sistemas de aeração como de oxigenação.

ABSTRACT This paper presents an integral model to evaluate the impact of gas transfer on the hydrodynamics of bubble plumes. The model is based on the Gaussian type self-similarity and functional relationships for the entrainment coefficient and factor of momentum amplification due to turbulence. The impact of mass transfer on bubble plume hydrodynamics is investigated considering different bubble sizes, gas flow rates and water depths. The results revealed a relevant impact when fine bubbles are considered, even for moderate water depths. Additionally, model simulations indicate that for weak bubble plumes (i.e., with relatively low flow rates and large depths and slip velocities), both dissolution and turbulence can affect plume hydrodynamics, which demonstrates the importance of taking the momentum amplification factor relationship into account. For deeper water conditions, simulations of bubble dissolution/decompression using the present model and classical models available in the literature resulted in a very good agreement for both aeration and oxygenation processes. Sensitivity analysis showed that the water depth, followed by the bubble size and the flow rate are the most important parameters that affect plume hydrodynamics. Lastly, dimensionless correlations are proposed to assess the impact of mass transfer on plume hydrodynamics, including both the aeration and oxygenation modes.

Nesta pesquisa, desenvolveu-se uma solução analítica para previsão e análise do desempenho de injetores de fertilizantes. As equações de conservação de massa, quantidade de movimento e energia, foram aplicadas ao processo de mistura dos fluidos de sucção e motriz através de uma formulação adimensional. Injetores de alto desempenho foram avaliados experimentalmente, utilizando-se água, tanto como fluido motriz como de sucção, de forma que os coeficientes de perda de carga em cada componente desses equipamentos, fossem obtidos através de calibração. A formulação proposta apresentou estreita concordância com os dados experimentais e forneceu valores adequados para os coeficientes de perda de carga implícitos nas equações. Através de curvas teóricas de rendimento, geradas a partir dos coeficientes de perda de carga calibrados, constatou-se que, quanto maior é a densidade do fluido de sucção menores são as faixas de vazão de operação e os rendimentos alcançados.

An analytical solution was developed in this research to predict and evaluate the performance of fertilizer injectors. The conservation equations for mass, momentum and energy were applied to the mixing process between the driving and suction fluids through a dimensionless formulation. High performance injectors were evaluated experimentally using water both as driving and suction fluids in order to determine the head-loss coefficients for each component of these accessories through calibration. The proposed solution presented excellent agreement with the experimental data and achieved adequate values for the implicit head-loss coefficients in the equations. Theoretical efficiency curves generated on the basis of these coefficients showed that the higher the density of the suction fluid, the lower the ranges of flow ratio and efficiencies reached.

Desenvolveu-se, nessa pesquisa, um programa computacional para previsão e análise do rendimento de ejetores para bombeamento de misturas de água com sólidos em suspensão, nas atividades de mineração. A estrutura do programa consiste na entrada de dados de curvas características obtidas com o ejetor operando com água, tanto como fluido motriz, como de sucção, fornecidas pelo fabricante, para calibração dos coeficientes de perda de carga e simulação do rendimento de tal equipamento para diferentes vazões, massas específicas e concentrações de sólidos no fluido de sucção, baseando-se nas equações de conservação de massa, quantidade de movimento e energia. Os resultados obtidos com o programa apresentaram adequada concordância com dados experimentais disponíveis na literatura.

A computer program was developed in this research to predict and evaluate the performance of ejectors for pumping of mixtures of water and solids in suspension in the mining activities. The structure of the program consists in inputting data points from characteristic curves obtained with the ejector operating with water both as driving and suction fluid, supplied by the manufacturer, for calibration of the head loss coefficients and simulation of the ejector performance for different flowrates, specific mass and concentration by weight of the solids in the suction fluid, based on the conservation equations for mass, momentum and energy. The results obtained with the program presented good agreement with experimental data available in the literature.