Because the drainage system is over loaded by to many fixture units
Salt and water do make hydrochloric acid... Highly corrosive.
it is found in sewer pipes in the USA
Not normally even the connection to the public sewer is not covered
the toilet overflows if either the building sewer, branch line in your sewer system or trap in your toilet gets clogged.
3" - 6"
With a wire brush.
Absolutely not
You could install your own sewer cam and get a look however doing so would likely involve getting in to your sewer pipes which is best left to a professional. Call a plumber to look in to your pipes if he can't find anything install the cam while he's got the pipes open.
Probably not, but where does the water from the sink and shower go and what sewer problems are you having?
Pipes. It is used for alot of underground storm, sewer or freshwater pipes by cities.
yes
Yes it normally does.