After the question you uploaded about harmonized landfill tax within EU member states, I could put the following ideas for discussion:
a) Harmonized landfill tax would provide incentives for waste producers to find solutions for waste recovery, enhance recycling or re-use, and lead to a drop in the amount of waste produced.
b) Against a harmonized minimum tax might argue that national circumstances vary and that a harmonized tax at EU level will be thus not an adequate solution mainly on the basis of costs and competitiveness issues Especially for waste categories for which there is no alternative to landfill (e.g. construction material and mineral waste), and landfill taxes might argue to have no steering effect and could instead lead to fly-tipping. A flat landfill tax would hinder the mix of incentives and flexibility needed at national or local level to divert from landfill. The economic costs of waste landfills and population density differ among the Member States.
For example United Kingdom employs both tradable landfill allowances and a landfill tax. More generally, opponents might warn that introducing new minor taxes should be avoided as it is contrary to deregulation and reducing administrative costs.