Thank you for all the valuable contributions that have enriched this discussion so far.
Among the many useful information and ideas that you all shared, let me point out a very interesting and, I must say, unexpected observation raised by Gisèle Yasmeen.
In her report for the International Development Research Centre (IDRC) entitled “Urban Agriculture in India” (available at: http://doccentre.net/Besharp/resources-expertise/Urban_Agriculture_In_In...) she described the case of Dr RT Doshi – an internationally recognized leader in urban food production – who obtained biomass from sugar cane vendors for his own food production work.
This case shades a light on the fact (often neglected) that the relationship between street food vendors and farmers is not necessarily a one-way relationship, in which the farmer sells to the street food vendor the raw ingredient for cooking, and that is it. Rather, it can have a "reverse" direction, there where waste products from the sale of street food, for instance, can be put back into the production chain in the form of fertilizers or even as fuels (eg. exhausted oil can be turned into fuel for farming machines).
I would invite you all to send your thoughts about this!
Are you aware of cases or recycling projects that involve farmers and street food vendors?