Why aren't hog factory farming installations required to have on-sight waste water treatment facilities?

There are single factory hog farms reportedly producing 2,000,000 6-month old hogs a for slaughter every year. In terms of bodily waste, that’s roughly equivalent the city of San Diego California. Imagine if San Diego stored all the human waste in a lagoon and periodically shot it out of a manure cannon. Then, of course, you tile the land and let the run-off flow into the nearest body of water.