Preliminary study on mini-modus device designed to oxygenate bottom anoxic waters without perturbing polluted sediments

The Tangential Guanabara Bay Aeration and Recovery (TAGUBAR) project derives its origins from a Brazilian government decision to tackle the planning and management challenges related to the restoration of some degraded aquatic ecosystems such as Guanabara Bay (state of Rio de Janeiro), Vitória Bay, and Espírito Santo Bay (state of Espírito Santo). This was performed by using the successful outcomes of a previous Ministry of Foreign Affairs and Directorate General for Cooperation and Development (i.e., Direttore Generale alla Cooperazione allo Sviluppo, MFA– DGCS) cooperation program. The general objective of the program was to contribute to the economic and social development of the population living around Guanabara, Vitória, and Espírito Santo Bays, while promoting the conservation of their natural resources. This objective was supposed to be achieved by investing money to consolidate the local authorities’ ability to plan and implement a reconditioning program within a systemic management framework in severely polluted ecosystems such as Guanabara Bay, where sediments are highly contaminated. Sediments normally represent the final fate for most contaminants. Therefore, it would be highly undesirable to perturb them, if one wishes to avoid contaminant recycling. In this context, we explored a bench-scale novel technology, called the module for the decontamination of units of sediment (MODUS), which produces an oxygenated water flow directed parallel to the sediment floor that is aimed to create “tangential aeration” of the bottom water column. The purpose of this is to avoid perturbing the top sediment layer, as a flow directed toward the bottom sediment would most probably resuspend this layer. Three kinds of tests were performed to characterize a bench-scale version of MODUS (referred to as “mini-MODUS”) behavior: turbulence–sediment resuspension tests, hydrodynamic tests, and oxygenation–aeration tests. In order to understand the functioning of the mini-MODUS, we needed to eliminate as many variables as possible. Therefore, we chose a static version of the module (i.e., no speed for the mini-MODUS as well as no water current with respect to the bottom sediment and no flume setting), leaving dynamic studies for a future paper. The turbulence tests showed that the water enters and exits the mini-MODUS mouths without resuspending the sediment surface at all, even if the sediment is very soft. Water flow was only localized very close to both mouth openings. Hydrodynamic tests showed an interesting behavior. An increase of low air flows produced a sharp linear increase of the water flow. However, a plateau was quickly reached and then no further increase of water flow was observed, implying that for a certain specific geometry of the equipment and for the given experimental conditions, an increase in the air flow does not produce any reduction of the residence time within the aeration reactor. Oxygenation–aeration tests explored three parameters that were deemed to be most important for our study: the oxygen global transfer coefficient, KLa; the oxygenation capacity, OC; and the oxygenation efficiency, OE%. An air flow increase causes an increase of both KLa and OC, while OE% decreases (no plateau was observed for KLa and OC). The better air flow would be a compromise between high KLa and OC, with no disadvantageous OE%, a compromise that will be the topic of the next paper.