1 000 000 hectares is about 10,000 square miles or about 100x100 miles. That’s with a single layer of water, 1 ft deep.
If you were to put two layers, each one foot deep, one top of each other, you would reduce your square area to 5000 square miles, or 70x70 miles. Go for three layers and you have an area of about 55x55 miles.
I don’t know about the phosphorus part, but the areaa that is needed to use algae is not insurmountable.
Isn`t there potassium, nitrogen, and phosphate in sewage? Nasty, but, just an idea.