Why would a sewage treatment plant need security measures?

Protection from sabotage, protection from juvenile pranks.

They don’t want shit getting out of hand.

Vandalism of sewage treatment can result in untreated waste spills, especially into rivers, lakes, bays.

They don’t want the mob to drop dead bodies into the sewage tanks. Dead bodies clog the filter.

Absolutely. Close off those pipes and you can shut down a city.

Imagine if you flushed your toilet and everything backed up into your bathroom and you could no longer flush the toilet? If there were no security measures and the pipes were sabotaged, just imagine.

Marauding dung beetles, the shit they pull.

^Ha ha. XD

All good answers. Also to protect the dumbasses like me who would like to go explore those kinds of places. It’s probably pretty damn dangerous in and around there. fuckin dung beetles

If anything bad happened & there was no security…the shit would hit the fan.

Most simple answer to protect themselves against vandalism.

It’s not just for sabotage, which until recent years had never been much of a problem in most rural parts of the USA, but also for vandalism (including taggers, who would find the blank walls of large concrete tanks a great place to practice their art) and theft.

Theft at all kinds of industrial sites has been an issue for many, many years. There is a lot of money in scrap metal, piping and valves which abound at these types of places, not to mention the equipment yards and garages where earthmoving and excavating equipment, cranes and trucks are stored and serviced. (And the pieces and parts that are stored in those garages, and the fuel tanks that are often maintained at the site to operate all of that equipment.)

Aside from that, as @elbanditoroso said, you don’t want any shit going down at one of these places.

I recall my dad telling me once, after getting home from a business trip to New Jersey in the 1960s that the drinking water reservoirs there were fenced with chain-link fences to prevent access. (Where we lived in central Massachusetts there were a number of smaller surface water reservoirs that supplied the city of Worcester, and while there was never recreational access to them for swimming, boating of any kind or fishing, public roads ran right by them and they were “open” in that way. Those reservoirs haven’t been fenced yet, but the pumping stations are quite a bit more guarded than they ever were. I did laugh about the NJ reservoir fencing, but … it’s not so laughable now. I get it.)

Most work sites, interior and exterior have security- guards, cameras, etc.