What kind of garbage would I expect to see on the ground walking around a large Roman city, circa 50BC?

I am not an in-depth expert on Roman garbage. However it did some reading on Roman waste disposal-practises when preparing a tour of Monte Testaccio in Rome, which is one of the most prolific 'Garbage Dumps' of the Roman area.

J. Theodore Peña writes about Roman pottery in the archeological record. His work discusses practises of garbage disposal, recycling and re-use of Roman pottery.

Pottery was the 'plastic' of the Roman age in the way that almost everything foodstuff related or cookery related was done with it. Certainly where it pertained to fluids think wines or olive oil, condiments and preservatives were stored and transported in amphorae of different sizes. Also earthenweare vessels where used for most cooking uses.

Hence there was a lot of this material around and it is a very common archeological find of the period. However the amount of use it has seen it is not usually found in garbage dumps. Peña explains that ground up materials are very often used as building materials, either as make-shift tiles, of ground up into concrete or other building uses, mostly as filler material. Where possible vessels were cleaned out and re-coated with pitch, to make them 'sterile' (enough) for a new batch of wine or oil. This was done as fluids like these easily stick to the surface of the amphor, rotting or getting rancid, and therefore infecting a next payload. Broken vessels would be reformed with clamps and lead filling if possible.

So most pottery would find another use, but some of it would be discarded. Regular household trash, like bones and as from the frunace would most likely be used out on the fields, as fertilizer. Sometimes all these things were flung out into a seperate trash heap, but not nearly enough of it that one could say that people just threw their trash out.

Enter Monte Testaccio, a huge overgrown heap in the Ostia area of Rome. A mound, which has a roughly triangular shape, rises to a maximum height of ca. 36 m above its surroundings, with a perimeter of 1,490 m, a surface of 22,000 m^2 , and a volume on the order of 580,000 m^3. This mound is made up out of discarded pottery of almost exclusively olive oil containing amphorae from all over the Mediterrean. Builst from the first century bc till the 3rd century. Appearently this was the olive oil trade center of ancient Rome and somehow it wasn't considered viable to re-use these amphorae. in any way. Maybe because the cleaning-out and re-use of the vessels was too expensive and maybe the re-use of unclean olive oil in building was also not that feasible. Anyway it formed the most prominent Roman-era waste heap that survives to this day. There are none other known of this scale.

So Roman garbage disposals usually not as abundant as in our society. Usually there are small ones in domestic areas, which contain some litter that had no other particular use. But most of the stuff was reporposed in one way or another. With Monte Testaccio and a small amount of similar piles as an exeption. I'll edge in a bit of speculation in saying that most likely specific resuables would have been collected by door-to-doorsmen, but that I can't really verify.

Source: Peña, J. Theodore. Roman pottery in the archaeological record. Cambridge University Press, 2007.