Event though our station is surrounded by water we can not use it for most of our needs. Therefore for some demands we have to establish a sweet water supply. The easiest way is to bring sweet water from the shore and to fill corresponding tanks. Well, what kind of water do we need?
Drinking water for consumption
This water has to be absolutely food-safe. This water should be carried in tanks or bottles that are able to withstand the pressure difference between the surface and the operational depth. Alternatively the tanks might be equipped with a pressure valve similar to pressure cookers.
A human being needs about 2 litres of drinking water a day (see authoritynutrition.com or the same article on the CalamarPark Document Server). But there are some pinpoints to consider:
- A certain amount of it we take through our food
- We also need water for cooking
- What are the effects on water requirement under hyperbaric conditions?
- The amount of necessary water increases with physical work
- While making excursions with scuba equipment we breathe dehydrated air which increases the need of water
After all 4 litres of drinking water per person/day might be a good calculation base.
Tap water (Betriebswasser)
Water used for dish washing, showers or fine cleaning does not automatically have drinking water quality though it should be close to it. This sweet water can be transported and stored in corresponding tanks which makes the handling easier and cheaper. It should not be used for drinking or cooking. For details see the definition page of Wikipedia (english / deutsch). We have seen that drinking water will be stored in special bottles. But what about storing tap water:
Why not a rigid tank?
Let’s say we filled a rigid tank with sweet water and place it outside the station. If we try to take some water from it we have to replace the evolving space with air, otherwise the water would not even flow out. Bu remember that air is very precious in our station.
For regular refilling we have to remove that air again by compressing it into a tank (which requires energy). Additionally the growing amount of air in the tank would cause a growing buoyancy and the air pocket inside would be a perfect habitat for algae and fungi.
So a flexible tank
The easiest way to store the tap water is a certain balloon on the outside of the habitat. Decreasing amounts of purified water is replaced by seawater split from each other by the membrane of the balloon. The balloon might be exchanged regularly to clean the inside. Since sweet water is lighter than salt water the filled balloon would float upwards which makes it a windage for drifts. So the balloon should be hold in position with perforated shell that allows salt water to flow in and out. If this shell follows the streamline of the hull we have a full integrated system that is not vulnerable to any influence from the outside. To reach the balloon for exchanging a diver would have to open the perforated cover from the outside and close it again after the exchanging procedure.
For rough cleaning and toilet flushes we luckily can use sea water (lots of it if you like, don’t worry, it won’t finish).
Unfortunately there is no way to use the pressure drop (Falldruck) to let the water flow from the tap. Because…
- If the water inlet is located over the level of the corresponding faucet no water would flow! Air would flow out of the habitat into the balloon instead and water would rise from the moonpool until the ballon is filled and pumped up until reaching environmental pressure.
- If the water inlet is located below the level of the faucet no water would flow anyway (why should it?).
So we have to bit he bullet and install a little pump. The ballon and all inlets should be located below the moonpool level and the pump provides water whenever we need it.
See also the chapter ‘Sewage’ (not available yet)
Image by James Petts from London, England (Stream of water) [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons