Atenolol is one of the most commonly prescribed cardioselective b-adrenergic blockers, used in antihypertensive, antianginal and antiarrhythmic treatment. After human consumption, it is excreted via urine mainly as an unchanged compound (90%), with a small percentage of atenolol glucuronide (0.8–4.4%) and hydroxyatenolol (1.1–4.4%, hydroxylation of the benzylic position) (Radjenovic et al., 2008). Atenolol has been found to be partially removed in wastewater treatment plants, which has been shown to be mainly due to biotransformation. In laboratory studies with sludge from both the aerobic stages of full-scale conventional activated sludge treatment plants and sludge from a laboratory-scale membrane bioreactor, it was shown to be quantitatively transformed to the carboxylic acid product of primary amide hydrolysis, atenolol acid (Radjenovic et al., 2008; Helbling et al., 2010). Atenolol acid has been observed to be further degraded, but so far no products of further transformation have been identified.