Environment

What happens to PET if it is dumped into landfills? 

It will stay there, inert, similar to glass.  It will not degrade biologically; one of the reasons it is such a good choice for packaging foods is its resistance to attack by microorganisms.  It will be crushed flat without fragmenting (no shards) and occupy less space that the more rigid glass.  It is resistant to the chemical species found in landfills and will not give rise to any harmful leachates.  Indeed, these very properties are utilized in stabilisation of landfills and processed baled PET bottles have been used for stabilisation of the foundations for road works.
 

PET will burn like paper, wood, and coal.  Since it is very difficult to ignite and usually melts away from any flame sources, accidental ignition is unlikely.  In this respect, it is similar to most natural organic materials.  It has the same calorific value as soft coal (22-23 MJ/kg) and this energy can be very efficiently recovered in up-to-date 'Waste to Energy' power generation facilities.  Residues (ash) are measured in parts per million and can be disposed of with normal fuel ash.  The complete combustion gases are carbon dioxide and water, the respiration gases of plants and animals.  In controlled incineration, air emissions are contained below regulatory limits.