A couple of vets in Milwaukee have their own cremation equipment, but the others contract with a pet cemetery that will pick-up, cremate, and return pets. If the owner doesn't want them back, I think the pet cemetery puts the ashes in a comunal grave. I think our humane society used to contract with them to dispose of euthanised animals, but since they built the new building in 2000, they have their own set up.
"I believe that most if not all shelters, vets, and other places that deal with animals cremate the remains. To send it to a local landfill would leech possible disease into the water supply or into the circle of life with rodents or other animals coming by to feed on the remains. '
There was a recent article in the paper about a number of hawks (eagles?) that were found poisened. It seemed that they had been dining on the repains of pets euthanised at an animal shelter up north and dumped in the local landfill.