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.
