Dr. Tom Ryan:

The definition of a larger city or town derives from the requirement in the directive to have secondary treatment in place. It is different depending on where the discharge goes. In short, however, if the discharge goes to an inland water, river or lake, a large town is defined as having a population of more than 2,000, while if it goes to a coastal area, it is 10,000. Anything above that requires, as a minimum, secondary treatment to be in place. It may require additional treatment but, as a minimum, it requires secondary treatment.