from end surfaces of end openings of the sleeve each of the produced water pipe 8 and the air inlet pipe 9 has a length of 150 mm the produced water pipe 8 is connected with a water outlet branch pipe 14 over the membrane module and the air inlet pipe 9 is connected with an aerating branch pipe 16 below the membrane module the vertical distance between the water outlet branch pipe 14 and the aerating branch pipe 15 is 1 3 m such that the whole membrane module may have a space of 120 mm in which it can suspendingly move upward and downward the water outlet branch pipe 14 is communicated with a suction pipe of a water pump capable of providing negative pressure water to be purified enters the interior of the membrane filament through the micro holes on the wall of the hollow fiber membrane filament 3 and converges into the produced water collecting chamber 7 then flows into the water outlet branch pipe 14 through the produced water pipe 8 and eventually is pumped out by the water pump the aerating branch pipe 15 is communicated with an air source compressed air provided by the.