Domestic sewage is the waste water discharged in the daily life of residents. It mainly comes from residential buildings and public buildings, such as houses, offices, schools, hospitals, shops, public places and industrial toilets. The pollutants contained in domestic sewage are mainly organic matter (such as protein, carbohydrate, fat, urea, ammonia nitrogen, etc.) and a large number of pathogenic microorganisms (such as parasite eggs and intestinal infectious viruses, etc.). The organic matter present in domestic sewage is extremely unstable and easily decomposes to produce foul smell. Bacteria and pathogens multiply by taking organic matter in domestic sewage as nutrients, which can lead to the spread of infectious diseases. Therefore, domestic sewage must be treated before it is discharged.

Because of reduction of membrane cost in recent years, the membrane separation technology has been a practical option to apply to the treatment of sewage and domestic wastewater. In 1990's, MF membranes, which are represented by polyethylene hollow fiber membranes as well as polyolefin flat sheet membranes, for the Membrane Bioreactor (MBR) method were developed, leading to establishment of an epoch-making method of permeating in the high density activated sludge to filter.