This video from Swiss start-up CortexiaExternal link gives a first-person view of how the Clean City Index programme works.

A video camera attached to a street sweeper, bike, or other slow-moving vehicle captures images of trash lying on the ground at a rate of two frames per second. 

The camera is connected to a computer, which receives the images and runs an algorithm to identify, count, and sort all the items of rubbish detected into some 40 highly specific categories.

Those 40 categories are then merged and funneled into 13 slightly broader waste types that city officials can more easily adapt to their sanitation planning needs. 

For example, what the computer programme identifies as a "takeaway burger box" and a "tissue" would both fall under the broader heading of "paper" in the programme's user app.