The project compiled data on disinfection by-products (DBPs) as well as biocidal active substances from product types (PTs) 1-5, 11 and 12. DBP formation potential of each active ingredient was evaluated. In a modeling approach, distribution of selected DBPs and biocidal active substances for applications in solution and on surfaces was estimated. The formation of approximately 60 DBPs was investigated in laboratory simulations in solution and on surfaces. In addition, samples from real disinfection applications were analyzed. Based on the results, the current procedure for environmental risk assessment of the DBPs and proposals for modifications were discussed, which aim to simplify and harmonize the environmental risk assessment of the DBPs.