With reference to Mr. Mullinix's question: Why not include all 40,000 citizens in charging them the sewer usage rates? Simply put, you cannot charge a person for a service that they do not receive. The sewer service does not benefit those who are not connected to the system.
As for paying for the schools to which you have no children going to, this is a tax, not a direct service to which a person does benefit.
A possible reason as to why the other 31,000 citizens are not connected to the sewer system may be because they do not want to be at the mercy of city billing going up at the whim of those in power. Those living outside the city pay almost double for water verses those living inside city limits, or so I am told. Why then would they want to incur extravagant sewer bills?