Do I have to continue paying for sewer service on a home after bankruptcy?

Are you still living in the property? If you, then you should pay for the services that were provided after your filing date, even if you surrendered the property. If the property is vacant, then you should be able to get the various utilities turned off, however, each locale may have different local ordinances.

No, you do not have to keep paying.

Assuming that you filed a Chapter 7 case which is a reasonable assumption since you got a discharge fast then you do owe the sewer services after you filed. The debts discharged were those you had before you filed for bankruptcy. Assuming that you did list the County Sewer Department as a creditor then you do not owe the charges before you filed the bankruptcy case. Since you probably had a "no asset" Chapter 7 case (as the vast majority of such cases) you might not owe the charges before you filed even if you forgot to name Sewer Department of the County as a creditor and gave the County notice of the filing. Most likely you did not give the County notice of the bankruptcy since the services were not turned off. However, you are stuck with the charges after the date of filing the bankruptcy case. In some cities or counties that sewer charges will become a lien on the property and bank will pay the sewer fees before or after the foreclosure since the bank wants the property without liens so that it can sell property. If the sewer charges are not a lien that is paid by the bank or the buyer of the property then the County can sue you for the outstanding charges after the date of the bankruptcy filing.

The bankruptcy law states that HOA's are still your responsibility during and after a Bankruptcy until the property is actually foreclosed.

I think you can stop paying, provided you scheduled that creditor, and listed them in your schedules as an executory contract. If you did not do that you can amend your schedules to do that.