A new helical membrane module for increasing permeate flux

Abstract

The membrane separation process such as MBR (membrane bioreactor) in wastewater treatment would be more competitive and environmental friendly, if membrane fouling was reduced and higher permeate flux was maintained without consuming extra material or extra energy. For this, a new helical membrane module with an inside helical spacer and outside cover membrane was designed which reduced filtration resistance. It increased mass transfer coefficients by 1.46–1.69 times that of flat membrane modules, when aeration level was at 0.2 m 3/h and the liquid cross flow rate was low (∼12 cm/h). Similarly increased were the ratios of pseudo-steady flux to initial flux ( F s /F o) for the helical membranes when the final helical angle was between 45° and 180° during the 1 or 2 h dynamic filtration of 500 mg/L kaolin suspension under constant trans-membrane pressure (2.8 or 3.2 kPa). The increased mass transfer is affected by liquid cross flow. When the cross flow rate was more than doubled (29 cm/h), the 90° helical membrane could maintain a relatively higher flux than the flat membrane at lower aeration intensity (0.1 m 3/h). The increase of aeration levels from 0.1 to 0.14 and to 0.2 m 3/h reduced cake layer deposition and fouling in the 135° helical membranes than in the flat membranes. More obvious decrease of filtration resistance in helical membranes than the flat membrane is attributed to the effectively enhanced turbulent mass transfer by the helical membrane surface distribution in the flow field, and the intensified interfacial turbulence/reduced boundary layer thickness by aeration bubbles and liquid flow. The maximal 48–69% flux increase obtained without extra energy consumption is a real advantage of the helical membranes.