I'm sorry, but we seem to be at cross purposes here. Obviously, when I pump out the grey tank, I pump it into the sea. I can choose when and where. It is quite a big tank, and could take several day's water, but as I said, I normally pump it out every second day. As I am never in harbours or marinas, I have never seen it as a problem. What point are you trying to make?
We have a sump for our shower. Shower water runs by gravity to the sump. In the sump is a Rule 2000gph bilge pump and float switch. The sump holds about 8 litres. It all works automatically when you have a shower and simply pumps the grey water over the side. The sump is really just an isolated section of bige by the shower that is easily accesed for cleaning. Works fine for us and the large bilge pump has never succumbed to hair or crud. It just keeps working