Dumping grey water in large quantities (500 gallons).

You walked into a buzz saw with that question.

We have a cottage on a pristine lake.
We use to have a bit of leeway with outhouses etc.

A few years ago they caught an idiot who had setup a trailer on his lot. He would dump the black tank into a big beer cooler, drag it into the woods and dump it.
That's all it needed.
Now, no more outhouses. No more trailers. Full septic with mandatory pumpout every three years.

What you plan on doing ruins it for all.