Well, the old way to do it is to use sea water flushing which leads to its own problems over the life of the ship with pipes corroding. New(ish) environmental regulations dictate that you can't just flush everything overboard anymore, it needs to be processed through a sewage plant. Despite probably being able to ignore most environmental laws, the Navy normally at least tries to abide by the current laws to be a good citizen and not pollute the oceans as well as having an easier time when visiting foreign ports.

The process plants run better on fresh water but fresh water is expensive and heavy on a ship so a vacuum system allows for less water per flush and really works pretty well if people don't flush stupid things.

Stupid things that my co-workers have fished from sewage plant besides the things mentioned in the article: Towels Screwdrivers Glasses Rags baby wipes/ flushable wipes Pens

As long as only body waste and a small amount of toilet paper are used, the systems are actually fairly reliable. Teaching people that that is ALL that can be flushed is an on going problem. The chemicals seem to be another issue, possibly a design issue but there are ways around that too. Urine crystals build up on the walls of the pipes and reduce the diameter for the waste water to flow. The vaccum systems typically have smaller diameter pipes then the old style "gravity" flush systems. There are companies that make continuous dosing systems that go in the end of the branch lines to help keep them clear.

A lot of issues with maintenance can be solved by design of branches in the piping system with isolation valves so the whole system doesn't have to be shut down to fix something. Even replacing a toilet means shutting off the branch or system since it opens a hole in the vacuum system and leads to loss of vacuum.

Some more information: https://www.boatus.org/clean-boating/sewage/msd-types/

Manufacturers: https://jetsgroup.com/jets-group/the-highest-standards/the-h... https://evac.com/