[tuatoru]

Yes it is. We do it with urban wastewater in thousands of locations around the world. This is not a hard problem, nor is it expensive.

[hnmullany]

Waste brine is substantially more dense than regular seawater, so it tends to form a coherent layer that doesn't mix easily: it's why waste brine sinks to the sea-bed and spreads out in a layer.

[tuatoru]

Diffusers are a thing. Fairly well understood.