A study of twelve wastewater treatment plants in France confirms the effectiveness of the contact zone technique in controlling sludge bulking (with a reduction in the Sludge Volume Index in 91% of cases) and foaming (improvement of the situation in 75% of the cases). The study focused on low-F/M activated sludge units in which organisms such as Microthrix p. or type 0041 (characteristic of these plants) were identified. Contact zones do not entail any major limitations, a fact which advocates their use in this type of treatment facility. Rational operation of the plant (in terms of aeration and sludge production) and compliance with a few simple rules will ensure effective use of this technology.