The 1954 hit song "Mr. Sandman" was performed by The Chordettes, with words and music by Pat Ballard. There's no exact date to pinpoint, but some time after that, a man who worked for the Department of Sanitation would be called a "san man."

Another sanitation nickname is "strongest" (see "finest/bravest/strongest/boldest" above).

http://persweb.direct.ca/fstringe/oz/m496.html
Mr. Sandman, bring me a dream
Make him the cutest that I've ever seen
Give him two lips like roses and clover
Then tell him that his lonesome nights are over.
Sandman, I'm so alone
Don't have nobody to call my own
Please turn on your magic beam
Mr. Sandman, bring me a dream.

25 April 1971, New York Times, pg. SM34:
As part of his attempt to turn the department around, Kretchmer has gone to great lengths to win the confidence of the "san" men and their feisty little union leader, John DeLury.

2 June 1984, New York Times, pg. 27:
Other furnishings are of the "mungo" variety, a term used by sanitation workers for objects retrieved from the trash.
(...)
Part of the show will be "San-man's Place," an actual outdated sanitation office moved piece by piece to the gallery.

5 July 1986, New York Times, pg. 31:
"I swept the streets, whistled at the girls, did all the things san-men traditionally do," he said..