An up-flow anaerobic sludge blanket reactor (19) processes effluent water and converts dissolved organic substances into methane. The reactor has an inlet pipe (21) and an outlet pipe (12), a gas collector (4) hood (1) with a ridge trap discharging through an aperture (8). On one side the ridge trap is open near the ridge to a gas transfer box (7) that discharges the gas to the outlet (8). On the other the aperture (8) connects (7) to the gas collection chamber (4). The suction inlet is located outside the gas/water boundary layer (13). One gas transfer box (6) has one edge that is lower than the lower edge of a second gas transfer box (7).
