In a covered wastewater reservoir, a gas passage is defined under the membrane cover and along the sides of the reservoir. An accumulation of bio-gases is held captive between the membrane cover and the liquid surface in a central region of the reservoir relative to the gas passage, by a flap-valve attachment condition extending between the accumulation of bio-gases and the gas passage. The new method for breaking this flap-valve attachment condition and for allowing bio-gases to flow from the accumulation of bio-gases toward the gas passage, consists of periodically and momentarily increasing a pressure in the gas passage to equal or to slightly exceed a pressure in the accumulation of bio-gases. In another aspect, the membrane cover is provided with a stiff and rough-surfaced bottom layer to facilitate the breaking of flap-valve attachment conditions when the pressure under the cover is made equal everywhere.