Large seawater desalination plants use reverse osmosis: the same principle as an under-sink system, at a different scale and a much higher pressure. Seawater carries far more dissolved salt than tap water, and the pressure needed to push water across the membrane rises with it.
The concentrate is not a fault
Every membrane process splits one stream into two: the water that crossed, and the water carrying what did not. At a plant that second stream is called brine, and it is saltier than the sea it came from. It exists for the same reason a household system has a drain line: the rejected load has to leave.
Why disposal is the hard part
Brine is denser than seawater, so returned carelessly it sinks and stays, forming a layer on the seabed where it was discharged. The engineering response is dilution and dispersion (outfalls designed to mix the stream quickly rather than pour it into one place), along with recovery of what is dissolved in it where that is economic. None of this is exotic. It is the same question a building services engineer answers on a much smaller scale when deciding where a concentrate line runs.
What it means for a small system
Two things carry over. First, a ratio of delivered water to concentrate is a real specification and worth asking for. Second, a supplier who claims a membrane system with no reject stream is describing something that would foul within days. Efficiency in this technology means a better ratio, never the absence of one.
