Question: What is a sewage system?
Answer:  A sewage system is a facility where there are multiple 'sewer systems' for carrying liquid and other objects away from a building or facility. 
Why: There are three types of sewage systems. First there is the Separate System... the Combined System.. and a Partially Separate System. Sewage facilities most often are used for a specific type of sewage, they won't use more then one brand of sewage systems. 
Hope this help's!!!
 ~Kenzie~