The sewage tax (dutch: rioolheffing) is charged to residents of the Netherlands every year. The tax is charged in relation to the location you are in on the 1st of January.
The municipality spends money to properly maintain its sewer system. They repair holes, vacuum wells and replace parts of the sewer. The costs for such maintenances are paid with the collected money from the sewerage levy. In addition, users of the sewerage system also pay because they discharge water into the sewerage system.
The sewage tax is sent to the apartment owner and when the property is rented, the government states it is applicable for the tax to be charged to the tenant who is making use of the property. You can find details about this here and in article 8.1 from your contract. If you would also like to better understand the sewage tax you can always check out the official website of the Dutch government here.