IsUltrafiltrationBetterThanMicrofiltrationas Pretreatment for Reverse Osmosis

7/29/2019 IsUltrafiltrationBetterThanMicrofiltrationas Pretreatment for Reverse Osmosis

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Figure 3 - MFI test results for April 9, 2005

Fouling of High-pressure Membranes by UF vs. MF Pretreatment

After replacement of the UF membranes in early April 2005, all three high-pressure membrane

systems in both the MF and UF trains were chemically cleaned and adjusted to operate at a

constant flux of 12 gfd. Fouling of all high-pressure membranes was very rapid such that themaximum available feed pressure (250 psi) to the X-20 membranes had been reached in 167

hours (April 25th

). During this time, all high-pressure membranes were once again cleaned. Thenext event was replacement of the MF membranes. Rapid fouling of the RO membranes

continued such that once again the maximum available feed pressure to the X-20 membranes was

reached after about 350 hours (May 5th

). This event prompted another chemical cleaning of thehigh-pressure membranes. This operating experience suggested that operation at 12 gfd was not

practical, regardless of the choice of low-pressure membrane treatment. The decision was made

to lower the flux to 8 gfd to lower the fouling rate and allow a longer period during which tocompare the effects of MF vs. UF pretreatment on high-pressure performance. Following the

reduction in flux, operation was continuous through 1000 hours (June 3rd

).

The interruption of operation at 12 gfd by chemical cleaning of the high-pressure membranes and

subsequent decision to lower the flux to 8 gfd led to dividing the performance analysis into three

time periods as shown in Table 5.

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5.0, 9.0, and 15.8 percent and UF: 8.8, 21.8, and 25.9 percent). The highest percent removals for

MF and UF did not occur on the same sampling date. TOC rejection in all RO membranes wasuniformly high (> 97 percent) suggesting that almost all of the organic matter in both the MF and

UF filtrates comprises molecular weights that exceed 200 (see Table 2 for MWCO of high-

pressure membranes).

Table 10 - Rejection of TOC by low- and high-pressure membranes during April and May

2005

Feed

(mg/L)

Filtrate/

permeate

(mg/L)

Rejection

(percent)

MF 16.8 15 9.9

UF 17 13.6 18.8

MF-RO/NF

ESPA2 15.7 0.2 98.6

NF-90 15.7 0.2 98.6

X-20 15.7 0.3 98.3

UF-RO/NF

ESPA2 15.1 0.2 98.6

NF-90 15.1 0.4 97.5

X-20 15.1 0.5 96.7

CONCLUSIONS

When both the MF and UF systems were operating well, the UF system required less energy andgave a higher specific flux for the ESPA2 and NF-90 membrane systems. The difference inmembrane polymer chemistry for the X-20 system may have been responsible for far less

fouling, regardless of the pretreatment choice. Foulants still remain in the UF filtrate to produce

a decline in specific flux. Thus, components with molecular weight less than 150,000 Dacontribute to fouling. However, a large fraction of organic matter that passes through the MF

membrane but is retained by the UF membrane appears to contribute significantly to fouling.

While the UF system appears to offer an advantage for high-pressure membrane treatment, ferric

chloride addition is required and cleaning may be more frequent than for the MF system.

UF provided better removal of TOC, albeit removal was less than 20 percent as should be

expected. However, there was no significant difference in TOC rejection by high-pressuremembranes receiving pretreatment by MF vs. UF.

The quality and consistency of the secondary effluent and the type of screening provided before

the UF membranes had a large impact on the operability of the UF system. Inside-out hollow

fiber membranes are particularly susceptible to screening problems because they require finerscreens than pressurized MF and immersed MF and UF systems. Particulate components of

secondary effluent that are not easily identified (e.g. snail shells and algae) can foul fine screens

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very rapidly. Even filtered secondary effluent can challenge low-pressure membrane systems

when plant upsets result in elevated TSS concentrations.

All three high-pressure membranes generally performed better with UF pretreatment than MF

pretreatment. Significant differences were noted in the required net driving force and specific

flux decline. Additional engineering studies are required to determine if the measured differencesin pretreatment performance are significant in terms of capital and operating costs for full-scale

facilities.

REFERENCES

Eriksson, P. K. and Lien, L. A. (2002) Hydrophilic ultrafiltration and microfiltration membranes.

American Filtration & Separations Society's 15th Annual Technical Conference &Exposition, Galveston, TX.

Gagliardo, P., Merlo, R. P., Adham, S., Trussell, R. S., and Trussel, R. (2000) Application of

reverse osmosis membranes for water reclamation with various pretreatment processes.Proceedings of the Water Environment Federation 73rd Annual Conference &

Exposition, Anaheim, CA.Howe, K.J. and Clark, M.M. (2002) Fouling of microfiltration and ultrafiltration membranes by

natural waters.Environmental Science & Technology.36(16), 3571-3576.

Metcalf & Eddy, Inc. et al. (2003) Wastewater engineering - treatment and reuse. fourth edition.McGraw-Hill, Inc., New York, NY.

Reardon, R. D., DiGiano, F. A., Aitken, M. D., Paranjape, S. V., Foussereau, X. J., Kim, J. H. ,

Chang, S.-H., and Cramer, R. (In press) Membrane Treatment of Secondary Effluent forSubsequent Use. Water Environment Research Foundation, Alexandria, VA.

Schippers, J. C. and Verdouw, J. (1980) The modified fouling index, a method of determiningthe fouling characteristics of water.Desalination, 32, 137-148.

van der Graaf, J.H.J.M., Kramer, J.F., Pluim, J., de Koning, J., and Weijs, M. (1999)

Experiments on membrane filtration of effluent at wastewater treatment plants in TheNetherlands. Water Science & Technology.39(5), 129-136.

Van Houtte, E., Verbauwhede, J., Vanlerberghe, F., Beumer, M., and de Bruijn, F. (1998)

Completing the water cycle: reuse of WWTP effluent for drinking water, Koksijde,

Belgium. Water Reuse 98 Conference Proceedings, Lake Buena Vista, FL.

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IsUltrafiltrationBetterThanMicrofiltrationas Pretreatment for Reverse Osmosis