It is amazing to me that less than 100 years ago, people were studying the efficacy of disposing of Washington DC's garbage by feeding it to pigs.
Until very recently in Egypt, they used a system like this. It worked and kept people healthier than their current waste processing system, which does not include pigs.
I suspect that in the next decade or so, entrepreneurs will start feeding garbage to pigs again, because the cost of disposing of food garbage is getting to be very high. It will be sold to people as a "green" and "sustainable" solution.
One thing the study from 1919 points out is that pigs fed on city garbage brought a higher price than regular pigs - implying their pork quality - particularly fat quality - was better. This is contrary to what others have said about free-ranging pigs or slop fed pigs; basically, feeding garbage results in low-quality soft pork.
I recently got to eat some pig that was fed on some foul-smelling stuff*. The interesting thing: the meat tasted fine. Most of the fat of the pig (surprisingly not all) tasted nasty, smelling like the feed. Given my own extremely limited experience, it is hard to believe that pigs fed on garbage would taste better than the "regular pigs" of 1919.
Another detail from that study: pigs ate about 15-20# of high-quality garbage per day, and 30# during periods of low-quality garbage (e.g. summer, when there were too many watermelon rinds). That's a mind-blowing amount of garbage, if you consider that their smallest pigs were just 100#. You've got a pig eating more 10% of its weight in garbage per day - something I don't think a human could do day after day.
I remember being surprised to read that mast fed (e.g. free-ranging acorn-fed) pigs produced low-quality pork, considering how tasty and expensive the mast-fed stuff from Spain is.
It is very frustrating that there are so few certainties in pig fattening. It always seems there are exceptions.
That said, it is clear how to produce fantastic Mangalitsa pork - basically, grow them slowly and finish them on barley and/or wheat, killing them at 14-18 months. That method seems to consistently produce fantastic stuff.**
* As I ate the meat, I reflected that our ancestors ate whatever they could, and that some desperate people today eat nasty things on a regular basis. If you eat some really foul custom meat or game, it makes you appreciate the meat at the supermarket, which never tastes fantastic, but nor does it taste like vomit or fishmeal.
** We tend to kill ours around 10-12 months, for economic reasons. We may be shifting to heavier weights (and older pigs) in 2011.