Biotreatment of wastewaters containing toxic compounds can be jeopardized by surges in substrate concentration. Preventative measures include tight monitoring and control to either dilute high concentration transients, or to divert them to surge tanks for later release and treatment. Using phenol as a typical toxic substrate, we have imposed 4 h phenol surges of 3,000, 6,000 and 8,000 mg/l from a steady-state feed of 500 mg/l, demonstrating such reduced treatment efficiency. In contrast, with the addition to the bioreactor of a small amount (3-10 % w/v) of inert polymer beads (Hytrel 8206) possessing a high affinity for phenol (partition coefficient of 39), significant detoxification was achieved, resulting in a 100 % increase in treatment efficiency, and good performance at phenol surges of up to 16,000 mg/l. The addition (passive operation) of polymers to a biotreatment system eliminated the need for intervention during toxic transients while, at the same time, demonstrating that the polymers perform this function automatically and at all times (aggressive operation).