According to AG, there is no question of passing on costs in the case of a fixed amount of self-determination in the sewage treatment levy

X bv operates various forest parks, chalet parks and campsites. In addition to the rent for the chalets it receives, X bv also receives a contribution from park holidaymakers who own a chalet. X bv will charge both the tenants and the owners € 1.25 per m³ of water in connection with the purification levy charged to it. It is in dispute whether VAT should be calculated on the amount of sewage treatment levy charged on.

Source Taxlive