A bioreactor has been developed utilising floating plastic granules with a density close to water. The reactor can be either aerated or non-aerated and can be used for nitrification, denitrification or solids removal. Particular applications are in tertiary treatment to achieve suspended solids levels below 10 mg/l and ammonia levels below 5 mg/l, and in a pre-denitrification mode to reduce nitrate levels in effluent from bacteria bed works in order to comply with the UWWTD.