septic paying for sewer

A landlord can pass on whatever charges the property may be imposed. It is extremely common for landlords to pass on exessive cost of increased taxes onto their tenants. With that said, your township probably voted for a bond to have a public sewer system. The cost of such sewer is probably calculated per property and passed onto the property owners as a increased tax or fee. The landlord has right to charge you for those increases under normal cercumstances, unless he has specifically excluded them from your rental contract.
So yes, you can be charged the sewer fee even though your property is not connected to the sewer.

The landlord can charge city impact fees if it is explained in the contract. If it isn’t you may be able to dispute the new charge. You may want to get the city involved if the landlord is required to change over the septic system over to the new sewer and refused not to. They may be able to force him to make the change. I doubt it is a requirement. Its probably an option the everyone voted on and now the costs are passed on down to you.