Why does our unit smell like sulfur after service?

A sulfur smell right after a service visit usually means hydrogen sulfide gas is forming in the holding tank. That rotten egg odor happens when bacteria that survive the pumping process start breaking down waste without enough oxygen.

The driver did not pump something else into the tank. A vacuum truck only removes waste. The issue is often the deodorizer. If the driver uses an old or weak batch of deodorizer, or does not add enough, it will not control the bacteria that produce the gas. Some formaldehyde-free deodorizers are less aggressive at killing those bacteria right away.

Another cause is leftover waste. If the tank has sludge or solids stuck to the bottom that the vacuum hose did not remove, that waste will start decomposing and smelling immediately. This is more common if the unit sees very heavy use between services. You should call the rental company and describe the smell. They can instruct the driver to use a fresh deodorizer and ensure a thorough cleanout on the next service visit.