Treater

noun

1. One who treats, in any sense of the word.
2. One who treats; one who handles, or discourses on, a subject; also, one who entertains.
3. Anything that treats.
4. A vessel used to treat oil-water emulsions by any of several mechanisms so that the oil can be accepted by the pipeline or transport.
5. someone who negotiates (confers with others in order to reach a settlement)