Colonne d'eau

vignette|La colonne d'eau et ses différents étages d'espèces. La « colonne d'eau » est un concept utilisé en écologie aquatique pour désigner, représenter et étudier les « compartiments » et niches écologiques et espèces du volume d'eau compris entre le fond et la surface. En écologie, on désigne l'ensemble des êtres vivant dans la colonne d'eau par le terme « pélagos ». Le terme colonne d'eau est également employé en plongée sous-marine pour décrire la zone dans laquelle les plongeurs montent et descendent. Utilisation Ce concept est principalement utilisé en océanographie et pour les études et évaluations environnementales concernant les lacs, par exemple pour :

l'étude des phénomènes de stratification thermique ou saline (superposition de plusieurs couches d'eau ne se mélangeant pas) selon la profondeur, et l'étude des effets écologiques de cette stratification (zones mortes liées à des phénomènes d'anoxie notamment, effets microclimatiques en surface, etc ;
l'étu

Competition between cooling and shortwave radiation in shaping near-surface convection in pelagic waters of Lake Geneva

Hugo Cruz

When lake surface waters above the temperature of maximum density 4◦C cools, the verysurface fluid parcels become denser than their neighbours below. The latter process leads to a gravitationally unstable density distribution which may trigger ‘free convection’, i.e. the upper denser waters look for their equilibrium position somewhere down in the water column. This phenomenon usually occurs at night time, when the atmosphere is colder than surface waters. However, during daytime, it may happen that the lake surface cools—releasing heat to the atmosphere—while shortwave radiation penetrates through upper waters. In this scenario, the near surface waters of the photic zone can host an unstable layer due to the net surface cooling, whereas waters beneath it experience radiative heating and gravitational stabilization. Here, we investigate the competition between shortwave radiation and surface cooling when wind is low or negligible. Thus, depending on the relative intensity of surface cooling and shortwave radiation, the upper water column undergoes different gravitationally unstable density distributions and potentially convective regions. The objective of this Master Project is to investigate, describe, and characterise such convective regimes. To do so, the heat equation controlling temperature evolution of the upper water column subject to surface cooling and penetrative radiative heating is investigated. In parallel with in-situ observations, mathematical and numerical modelling allow to understand the phenomenon taking place. The results allow to infer how the relative importance of surface cooling and penetrating shortwave radiation may affect vertical heat and mass transfer within the lake and between the lake and the atmosphere.

2021

Performance and User Acceptance of GDM Filters for HWTS in a Rural Community of the Occupied Palestinian Territories

Alexis Baron

As everybody knows, access to safe water is vital for human beings. It is also considered as a human right to have access to safe drinking water, free from pathogens that can cause illness and death. According to the World Health Organization (WHO), unsafe water is still a part of the daily life of millions of people on Earth. As it is not always possible to build centralized treatment facilities, household water treatment and safe storage (HWTS) is a promising alternative for many remote areas in developing countries. It is in this perspective that the Italian NGO named Cesvi targets unserved communities of South Hebron in Palestine. It has the goal to provide HWTS units to households. These HWTS units are composed of a gravity-driven membrane (GDM) filter. The water is pushed through the membranes by the pressure of the water column. It can allow high bacterial removal and requires no energy and low maintenance. That’s why it is a very interesting technology for household scale. An valuation of the technical performance and the user acceptance of 9 HWTS units set up in the community of Khirbet Ghuwein was the main focus of the present study. The results have shown that most of the filters have a log-removal value (LRV) equal to 2 or more, while LRVs of at least 4 are obtained in the laboratory. However, this corresponds to a comprehensive protection according to the WHO international scheme to evaluate household water treatment technologies. The calculated removal efficiency of the filter can be influenced by the concentration of bacteria in the raw water, how the system is operated and the quality of the internal connections of the system. Nevertheless, it allows to observe the removal of total coliforms but also of E. coli, even if the latter were not very present in water. In contrast, GDM filters allow a high removal of suspended particles. Finally, the data do not allow to judge the safety of the water produced, as the HWTS unit seems to be sensitive and favorable to recontamination and regrowth of coliforms. The HWTS units are globally used by the households even if their use tends to decrease over time. Most of the users seem satisfied even if they are bored by cleaning the HWTS unit and think that it takes too many operational efforts. In addition, most of them would not be willing to buy one if it was not provided by the NGO. It seems that they used the filters without realizing the real purpose of them. It is very important in this kind of projects to accompany the delivery of filters with a workshop explaining the interest of such HWTS units and the proper way to use and maintain them. Despite the results that are not as conclusive as expected, the application of GDM filters in easy-to-use HWTS units is very promising but it still needs studies, applications, and improvements to allow a significant impact on the field.

2019

Performance and User Acceptance of GDM Filters for HWTS in a Rural Community of the Occupied Palestinian Territories

Alexis Baron

2019

Competition between cooling and shortwave radiation in shaping near-surface convection in pelagic waters of Lake Geneva

Performance and User Acceptance of GDM Filters for HWTS in a Rural Community of the Occupied Palestinian Territories

Photodegradation and fate of pharmaceuticals in the water column of a mid-sized lake

Competition between cooling and shortwave radiation in shaping near-surface convection in pelagic waters of Lake Geneva

Performance and User Acceptance of GDM Filters for HWTS in a Rural Community of the Occupied Palestinian Territories

Photodegradation and fate of pharmaceuticals in the water column of a mid-sized lake