The byproduct is a sort of like salt water, but a lot more concentrated. It’s mostly NaCl, but there’s also various other anions and cations such as Al, Ca, Mg, K, SO4 etc. Those metals came from the ocean, so you might be inclined to think that you can dump them back into the ocean. The problem comes when you dump a lot of that stuff and you get very high concentrations locally. When the concentration of those compounds is within the normal range, sea life can handle it. Once it’s above the limit, you can expect things to struggle or die. Eventually, it will get diluted in the ocean, but before that the concentrations will be high enough to cause damage to most living things.