Nanofouling of Polypropylene Microfiltration Membranes by Clarified Secondary Effluent

Featuring Jana Safarik
Orange County Water District

Location:  Engineering Hall 2430
Free and open to the public
 
Abstract:
The Orange County Water District is utilizing an MF/RO/UV process in a 70-mgd advanced water reclamation facility to purify secondary clarified effluent (SCE).  This water possesses a significant load of biological detritus such as microparticulates, nanoparticulates and dissolved organic matter (DOM) which contributes to rapid fouling of the MF membranes with consequent loss of performance.  The primary objective of this project was to elucidate the fouling mechanism of 0.2 µm pore size hollow fiber polypropylene (PP) MF membranes.  Laboratory bench scale studies indicated there are two mechanisms of MF fouling 1) pore blocking via surface cake formation by particulates greater that the average membrane pore size, which may be alleviated by regular backwashing and air scouring  and 2) pore plugging due to intercolation of dissolved organic matter (DOM) and nanoparticulates smaller than 0.2 µm into the membrane matrix, which is difficult to mitigate with regular backwashing and is responsible for 80% of flux loss necessitating frequent CIP’s (clean in place).  The size of nanoparticles in SCE responsible for MF fouling was determined by differential filtration using 10kDa and 20kDa molecular cut off (MWCO) ultrafilters.  Filtrate MF fouling potential was assessed using a bench scale PP MF membrane test cell.  The 10kDa MWCO filtrate had no effect on membrane flux, but the 20kDa MWCO filtrate did cause a slight decrease in MF flux, indicating fouling particle size started between 2.5 and 3.5 nm.  To prevent the nanoparticulates from entering the membrane matrix a commercially available coagulant, Sumaclear 700 (Summit Research Labs, Flemington, NJ) was used to aggregate the nanoparticles into microparticles larger than the average pore size.  The efficacy of this process was assessed using two Siemans Water Technology AS10V continuous filtration-submerged (CMF-S) pilot systems.  One unit received coagulant pretreatment as the other unit (control) did not received coagulant pretreatment.  Each unit was equipped with 2 polypropylene MF membranes and operated at 9 GPM.  Membranes were backwashed and air soured every 22 minutes.  Coagulant concentrations of 10ppm, 5ppm and 2.5ppm were tested for their ability to improve membrane performance and to increase times between cleaning intervals.  The membrane cleaning interval was increased from 21 days to 69, 48 and 35 days at 10, 5 and 2.5ppm Sumaclear 700, respectively.  Test membranes were autopsied and examined by scanning electron microscopy (SEM).  The SEM revealed deep penetration of nanoparticles into the membrane matrix further supporting the pore plugging hypothesis.  The coagulant pretreated membrane cross sections also showed evidence of nanoparticulate penetration but the foulant material mainly sorbed close to and onto the membrane surface.  Results from this study demonstrate that preventing nanoparticles from entering the membrane matrix will improve membrane performance, increase cleaning intervals and decrease O & M costs.