A method for treating process water of a flotation arrangement is disclosed. The process comprising the steps of a) dewatering overflow of a mineral flotation circuit in a gravitational solid-liquid separator to separate a sediment from a supernatant comprising water, silica-containing particles and soluble SiO2, fine particles, microbes, and residual flotation chemicals; b) subjecting the supernatant to cleaning flotation, in which at least 90% of the flotation gas bubbles have a size from 0.2 to 250 m, in a cleaning flotation unit for collecting at least silica-containing particles, for separating at least silica-containing particles from the supernatant into cleaning flotation overflow, and for forming purified process water as cleaning flotation underflow; c) removing cleaning flotation overflow as tailings; and d) recirculating purified process water into the mineral flotation circuit. A process water treatment arrangement is also disclosed.