Generally, I’d think this comes under the Church’s principle of “Stewardship of the Environment”. We do have a responsibility, within reason, to care for our natural surroundings and appreciate God’s creation. Also, it’s basic respect to maintain the man-made surroundings out of consideration for the others walking on the street, sidewalk, etc. Littering surely is a venial sin.
In your case, where it is commonplace, there is also the risk of disease when the trash and garbage become excessive. Exposing one’s neighbor to injury or disease would possibly be a mortal sin, if it’s done with complete disregard and full knowledge of the possible harm.
But if you are saying there is absolutely nowhere to throw out the trash, no collecting service, no bins, etc, and you MUST leave it out, then I’d say the obligation would be something like containing all your trash in a bag and sealing it, try to be as neat as possible.
Are there rules in your city about what people should do? Is there some service that cleans the streets weekly, etc? If so, then your actions should be guided by the municipality’s laws.
