With the entry into force of the new Water Protection Act in 2016, wastewater treatment plants (WWTPs) must install an additional treatment stage (elimination of micro-pollutants) within the next 20 years. As an immediate consequence, municipalities have to decide whether an upgrade is worthwhile or whether they should join a larger WWTP. In the latter case, considerable dismantling costs are to be expected. A concept has been developed and tested which, instead of dismantling the plants, provides for their conversion into modern recirculating aquaculture systems for the breeding of freshwater fish. Infrastructure interfaces and basins are available and land acquisition is favourable. Overall, it was expected that this would reduce the prime costs to such an extent that production could be more competitive. In this feasibility study, the conversion of two sites was examined in detail. For this purpose, facilities were planned at the sites and at the same time the costs and, in particular, the added value of reusing or co-using the WWTP infrastructure were calculated.