This European Standard specifies the design principles and performance requirements for secondary treatment by biological fixed-film reactors at wastewater treatment plants for more than 50 PT. The primary application is for wastewater treatment plants designed for the treatment of domestic and municipal wastewater. Biological fixed film reactors include biological trickling filters, rotating biological contactors, submerged bed reactors and biofilters. Differences in wastewater treatment throughout Europe have led to a variety of systems being developed. This standard gives fundamental informations about the systems; this standard has not attempted to specify all available systems. Detailed information additional to that contained in this standard may be obtained by referring to the Bibliography.