Simplify the following text for a Grade 9 native English-speaking person.
Original: Plumbing is usually distinguished from water and sewage systems , in that a plumbing system serves one building , while water and sewage systems serve a group of buildings or a city .
Simplified: Plumbing is different from water and sewage systems because plumbing system serves one building , while water and sewage systems serve a group of buildings or a city .