Sulphur denitrators convert dissolved mineral matter (ammonium, nitrites and nitrates) into gas (nitrogen gas), which is one of the main components of air. The nitrogen gas is released into the atmosphere. Biofilters convert organic matter into dissolved mineral matter. The only gases emitted during this process are those produced by the breathing of aerobic bacteria (CO2). The hardness of sea water is around 14 French degrees. There is an empty space at the bottom of the filters, which allows the water to circulate through the filter candles (a sort of strainer that must not come into contact with the sand), as well as an empty space at the top, which allows the filters to be backwashed without losing sand. With the sand inside, not counting the weight of the water, each filter weighs about 17 metric tons. Every day, 1% of fresh seawater is added to the large tank. The fresh seawater is not sterilised (it contains very few micro-organisms). However, it is reoxygenated and its dissolved element content is rebalanced before being distributed to the various circuits. After being re-oxygenated, the raw seawater is passed over a specific substrate that retains most of the iron and manganese, but leaves enough for the needs of animals and plants. Some of it is collected as dry powder and processed by a specialised industry. Some of it goes back to the sea. It should also be noted that the water circuits of the two exhibitions are not connected. The "Mankind and Shores" exhibition contains 11 independent water treatment circuits (8 for sea water and 3 for fresh water); the "Journey on the High Seas" exhibition has 2 circuits, both for sea water. The power of the agitators can be adjusted independently, if this is found to be useful for the life cycle of the animals. The bio-balls are made of recycled plastic. Recycled glass can also be used, but it is much heavier and would have required much thicker concrete tanks. These bio-balls are self-cleaning due to the slow mixing effect obtained by the injected air bubbles. The water in the large tank is mechanically filtered, heated, aerated, sterilised (U.V.) and purified in biofilters (by micro-organisms), a skimmer and denitrators. The backwash water from the mechanical filters is recycled and can, depending on our needs, be fed into the large tank. The decanted sludge is recycled and reprocessed by an approved company. The water in the large tank is fully filtered 6 times a day and 1% of the volume is returned to the sea after purification.