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The determination of the residence time distribution function and the test with tracers can be used to describe the hydrodynamic behavior of a vessel. It is possible to assess whether the water is well distributed within a tank, or whether there are short-circuits and dead zones. This technique was applied to evaluate if the hydrodynamics of a mollusk purification tank, built outside the recommended specifications, provides a good distribution and renewal of water within its volume. Salt water was used as a tracer in a step tracer experiment. The salt concentration was measured continuously at different points inside tank. The result for the average residence time was 33 minutes, close to the theoretical value and the volumetric efficiency of the tank is practically equal to 1. The results indicate that the entire volume of the tank was used and that the water is well distributed with low occurrence of short-circuits or dead zones. The system also showed the ability to maintain an adequate dissolved oxygen concentration and temperature. The technique used in this study can be used to evaluate tanks built outside the international recommendations or presenting performance. A determinação da função da distribuição do tempo de residência e o ensaio com traçadores podem ser empregados para descrever o comportamento hidrodinâmico de um volume. É possível avaliar se a água está bem distribuída dentro de um tanque, ou se há caminhos preferenciais e zonas mortas. Esta técnica foi aplicada para avaliar se a hidrodinâmica de um tanque de depuração de moluscos, construído fora das especificações recomendadas, proporciona uma boa distribuição e renovação da água dentro do seu volume. A água salgada foi usada como traçador em um ensaio com estímulo em degrau. A concentração de sal foi medida continuamente em pontos distintos do tanque. O resultado para o tempo médio de residência foi de 33 minutos, próximo ao valor teórico e eficiência volumétrica do tanque é, praticamente, igual a 1. Os resultados indicam que todo o volume do tanque está em uso e que a água é bem distribuída com baixa ocorrência de fluxos preferenciais ou zonas mortas. O sistema também apresentou capacidade de manter a concentração de oxigênio dissolvido e temperatura adequadas. A técnica utilizada nesse estudo pode ser empregada para avaliar tanques construídos fora das recomendações internacionais ou que apresentem desempenho ruim.

Mestrado em Engenharia Alimentar - Instituto Superior de Agronomia Manila clam (Ruditapes philippinarum), mussel (Mytilus sp.) and peppery furrow shell clam (Scrobicularia plana) are species with high abundance in the Tagus estuary. However, these species may have high levels of contaminant metals. This study aimed to determine the effect of depuration of manila clam, mussel and peppery furrow shell clam on the content of contaminant metals (Cd, Pb and Hg) and on macro, micro and trace elements (S, Ca, K, Cl, Fe, Cu, Zn, As, Se, Br, Rb and Sr). The results revealed that the depuration was effective in reducing the levels of Pb in peppery furrow shell clam and Manila clam. In the mussel no decrease was registered in this contaminant. During the depuration no decreases were registered in Hg and Cd. Mussel and peppery furrow shell clam condition decreased through the depuration, as indicated the glycogen content in the adductor muscle of these bivalves. In peppery furrow shell clam the mortality rate reached 11 %. Manila clam proved to be the most adaptive to depuration conditions. The depuration also decreased the excess of Fe in Manila clam and peppery furrow shell clam, whose levels were above the tolerable upper limit (UL). In the mussel this decrease resulted in a disadvantageous loss of this mineral