Particle Size Distribution, Count Rate, and Membrane Fouling in Hollow-Fiber Microfiltration

Abstract

Membrane processes are a widely used and a growing method to reclaim wastewater effluent.However, a major drawback is their energy intensity. Treating 1 million gallons of water requires in general 600-800 kW for microfiltration and 1,600-2,000 kW for reverse osmosis membranes.A better understanding of membrane fouling is key to reducing energy requirements, which in turn lower running costs. There are two fouling mechanisms: 1) the more known type is cake formation on the membrane caused by particles greater than 0.2 microns (backwashing and air sparging help mitigating this problem); 2) the lesser known is pore plugging caused by biogenic nanoparticles (< 0.2 microns), which is very difficult to mitigate. Nanoparticles have a higher potential to foul due to their smaller size. This phenomenon may account for up to 80% of flux reduction, causing irreversible fouling that necessitates costly chemical cleaning. Previously (Safarik and Phipps, 2006), studies at the Orange County Water District (OCWD) correlated fouling of microfiltration (MF) membranes with wastewater characteristics, using unfiltered, and pre-filtered wastewater (using 0.2 µm, 20K MWCO and 10K MWCO ultrafilters). This showed that removing nanoparticles larger than 200 nm only improved membrane performance by 20%. Nanoparticles larger than 3.5 nm (20KMWCO ultra filter) reduced membrane performance by 45%. But nanoparticles smaller than 2.5 nm (10K MWCO ultra filter) had little or no influence on membrane performance (Figure 1). In this study we wanted to further investigate the extent of nanoscale particles impact on membrane flux reduction and therefore on energy consumption.Samples were collected at the Santa Margarita Water District (SMWD), Irvine Ranch Water District (IRWD) and Orange County Sanitation District (OCSD) plants. At each plant samples were collected at several points (primary influent and effluent, secondary effluent) and a filtration series was conducted with 0.45, 0.2, 0.1, 0.08, 0.05, 0.03, and 0.01 micron membranes. At each step, filtrate was collected, COD was measured, and Zetasizer Nano analysis was performed to quantify particle size distribution and count rate (number of particles per second detected by the instrument, i.e. total amount of particles in the water). The photon count rate (expressed as kilocounts per second) is linearly correlated with the change in the filtration pore size (Figure 2). 518 Membrane Applications 2010

Membrane Processes

“…Better understanding of membrane fouling is one of the key in reducing the operational problems and operating costs in MBR plants. Smeraldi et al (2010) reported that membrane fouling in hollow fiber membrane can be attributed to pore plugging caused by biogenic nanoparticles (< 0.2 microns), which have a higher potential to foul due to their smaller size, and account for 80% of flux reduction, causing irreversible fouling that necessitates costly chemical cleaning.…”