In drinking water treatment, the inactivation of microorganisms increases with increasing disinfectant exposure (product of concentration and contact time. CT). Also, the formation of undesired (toxic) disinfection by-products increases with CT. The present study proposes a new concept that uses this undesired side effect of chemical water disinfection fur a Fast and reliable test of treatment efficiency. In laboratory systems. bromate formation during ozonation and the formation of trihalomethanes during chlorination were used to calculate the disinfectant exposure, which is a measure for the achieved degree of disinfection. (C) 2001 Elsevier Science Ltd. All rights reserved.