(23)In order to avoid possible internal market distortions, minimum requirements for the implementation of the extended producer responsibility system should be established in this Directive, while the practical organisation of that system should be decided at national level. In order to favour the substitution of substances and products generating micropollutants residues in urban wastewater, the contributions of the producers should be proportionate to the quantities of the substances contained in the products they place on the market and the hazardousness of those substances and their residues. The contributions should cover, but not exceed, the investment and operational costs for the monitoring activities for micropollutants, the collection, reporting and impartial verification of statistics on the quantities and hazardousness of products placed on the Member States market, and the application of the quaternary treatment to urban wastewater in an efficient manner. Those contributions should also cover operational costs of already established quaternary treatment at the date of entry into force of this Directive when it is necessary to meet the obligations of the extended producer responsibility system. It should also cover part of the investment costs of the established quaternary treatments, taking into account the depreciation of the investments and the deadlines of the financing obligations established in this Directive. Since urban wastewater is treated collectively, it is appropriate to introduce a requirement for producers to join a centralised organisation which can implement their obligations under the extended producer responsibility on their behalf.