NOVELTY - A flange is welded on a water outlet pipe of a third filter to be a connection port of an external pipe. An upper flange is connected with the upper end of an outer barrel. A flange cover is tightly fixed by a bolt to the upper flange. A decontamination pipe is connected at one end with one side of the lower part of the outer barrel, while the other end of the decontamination pipe is connected with a decontamination flange. A drain water pipe is connected at one end with the other side of the lower part of the outer barrel, and at the other end with a drain valve.    USE - Circulating water purifying device for straw gasification system.    ADVANTAGE - Venturi pipe causes sewage to swirl and settle after entering device, and dust in water to settle in device. Louver grid blades stop part of dust from moving upward to remain at bottom when water in lower part of device flows upward. Flow speed is slow when water enters settling chambers since volume of chamber is bigger than area of water outlet orifice, thus dust in water settles at bottom. Three filters and two settling chambers are connected in series such that dust and tar particles in water are filtered thoroughly and circulating water is purified completely. Parts which need to be maintained, disassembled and changed are detachably connected, thus achieving simple structure, convenient maintenance, safety and reliability.    DETAILED DESCRIPTION - The lower part of a filter is installed in a shell and is matched with the shell to form two settling chambers. The filter comprises an outer barrel, an upper flange, a flange cover, a water inlet pipe, a Venturi pipe, a louver grid, a filter membrane device, a supporting ring, a water outlet pipe, a drain pipe, a drain valve, a decontamination pipe, a decontamination flange and a flange cover. The water inlet pipe penetrates through the outer barrel and is connected with the upper part of the Venturi pipe along a tangent line direction, forming a 75-degree inclined angle with the axial line of the Venturi pipe. The lower end of the Venturi pipe is connected with the inner hole of the louver grid. The outer edge of the louver grid is connected with the outer barrel. The filter membrane device is installed above the Venturi pipe in the outer barrel and is connected by a bolt with the supporting ring on the inner wall of the outer barrel. The water outlet pipe is connected with the outer barrel above the filter membrane device. One end of the water outlet pipe is installed on two former filters, penetrates through the upper sealing plate and extends to the lower part of a settling chamber. The filter membrane device uses a macromolecular stereo glue filter membrane.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional schematic view of the circulating water purifying device.