You can see an example in this video (I’ve set the timer so you’ll see it right when you click the link, no need to fast-forward). Is it the wastewater dump?
Ballast water? Ballast water discharge and the environment - Wikipedia
Anchor wash down. There are special hydrants in the fire main system to wash mud off the anchor chain and anchor as it is being weighed (ie reeled in). The ship in the video was presumably at anchor just before entering the lock and is still washing down. The fire main is fed directly from the sea (or whatever other body of water the ship is in).
Corry El, how does sea (salt) water affect those pump(s)?
It doesn’t affect the pumps in my aquarium.
The material would be selected for a reasonably long life in sea water, probably bronze perhaps stainless steel moving parts in contact with sea water for a fire pump depending on owner preference of first cost v maintenance cost. Systems which pump cargo and wash water in/out of the cargo tanks on a tanker like that (the ~50,000 dwt tonne oil product/chemical tanker ‘Ardmore Endeavour’ built in South Korea in 2013) are usually stainless, but that’s in part to prevent contamination of the cargo by rust particles. Sea water cooling piping in the engine room is usually copper-nickel. But the fire main piping might just be steel: eventually it corrodes and has to be replaced. But so is the hull of the ship just steel and, despite paint and anodic protection, will need major plating renewal eventually due to corrosion (though also stress-corrosion interaction). A ship like that is expected to be economical to keep running for 20-25 yrs. For ships that cost more to replace, the equation might shift to a longer life.
Now, THAT’S an explanation! Much obliged, C El.
Now, how about carnivorousplant’s aquarium pumps? 
Everything exposed to water is plastic.
I’m assuming your aquarium is the salt water type; how do you gauge salinity and balance it for your different fish (assuming more than one species)? What kind of salt is used?
This might require a separate thread.
I have a refractometer and a 1.025 reference fluid. The fish all live in the same salinity, 1.020 to 1.025. Coral likes a higher concentration of salt, 1.023-1.025.
The salt is a product made for marine aquariums. I think it is made from ocean water, but I’ve not found how it is made. Perhaps it comes from the guy who mops the floor at Morton’s factory.
The salt is a powder and is mixed with purified water.
Starting at 8:15 of this clip* (the episode of Victory at Sea which covers D-Day) you can see what it looks like from the deck.
*I tried using the OP’s technique for getting the clip to start at the desired point, but no matter how I parsed the URL it started at the beginning. Technology hates me.
Thanks! Informative and carnivorous!
What is your interest in aquariums?
^ Just curious. Decades ago, a girlfriend of mine’s father had a salt water tank in the club-basement*; this thread made me think of that and some questions popped into my head.
*Once, he put in this tiny black (with red tips, IIRC) shark, which got bigger and bigger until all the other fish were gone. He said he didn’t see that coming. :eek:
I spent 16 years in the Merchant Marines and it was not common for us to clean out the chain locker. (The place where all that anchor chain goes when it is not in the water.) We would wash the chain off as it came up.
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We mostly did it while hauling the anchor off muddy bottoms to clean it off, but also flooded the compartment(s) when in fresh water such as moving from salt water to fresh, such as going into a fresh water canal or a fresh water river… We did not want to to pick all that mud that stuck to the chain and put it into the locker, just to foul things up and have to hand clean it out later. When you did drop anchor you used an enormous amount of chain to hold town the anchor. Depend on the depth of the water, wind currents, water currents and length of the ship, our rule of thumb was that in a sheltered harbor about 50-70 feet deep we used 2X the length of the ship to hold down the anchor, minimum.
That said, you have lots of chain in the locker. It rusts and uses up all the oxygen quickly as it it is in an enclosed space. It is probably one of the most dangerous places to enter on as ship without a supplemental air tank.
So that is what all that water is coming out for. Preventive Maintenance.