Micro-bubble interferences with Silt Density Index measurements on ultrafiltration permeate

Fraser Kent, Rami Messalem, Pierre Côte, Steve Siverns, Roy Arviv, O. Silner

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review


The formation of micro-bubbles in immersed membrane processes was examined to evaluate Silt Density Index (SDI) interferences. Water produced by ultrafiltration (UF) membrane processes is well suited to feed reverse osmosis (RO) systems and generally exhibits silt density indices of approximately 2. In some instances, the formation of micro-bubbles in immersed membrane systems can lead to higher SDI values as a result of bubbles blocking pores within the 0.45μm SDI filter paper. These higher SDI values do not correspond to higher fouling rates for downstream RO processes since micro-bubbles do not adversely affect RO filtration. Several methods were used to prove that the presence of micro-bubbles can interfere with SDI measurements: One method involved filtering UF permeate through an SDI filter paper and subsequently vacuum drying the filter paper to examine the permeability that was recovered. The permeability recovery should give an indication of the amount of original permeability reduction that was associated with the presence of micro-bubbles. The second method involved collecting SDI values measured on two types of UF permeate. The same feed water was sent to both an immersed UF system (where a vacuum is used to drive the process) and a pressurized UF system. The results using the same feed water and same membrane showed average SDI values that were >50% higher for the vacuum driven system where it is expected that micro-bubbles are present. A third method repeated the previous test, but with the raw feed being continuously aerated. Again the results showed significantly higher SDI values for the permeate associated with the vacuum system. The SDI values were much higher for both systems compared with the non-aerated tests. Another method involved conducting multiple SDI tests on RO permeate and comparing the results with SDI values found on the same RO permeate that was pressurized while in contact with air and then depressurized to form micro-bubbles. The RO permeate that was loaded with micro-bubbles showed SDI values that were significantly higher. A final demonstration is presented where a high pressure pump is used to re-dissolve micro-bubbles present in UF permeate. SDI values obtained from UF permeate were compared with the SDI values from the same UF permeate where the micro-bubble concentrations were reduced by sampling after the UF permeate was pressurized to ∼6000 kPa (900 psi). The average SDI values of the UF permeate were significantly higher than those obtained after the water was pressurized. The data that is shown in this paper presents irrefutable evidence that the presence of micro-bubbles in a solution causes elevated SDI values. It also points to a practical solution for reducing SDI values by taking SDI samples downstream of a high pressure pump. In most cases SDI values are only of interest when an RO system is present downstream of a UF system, therefore, a high pressure pump is usually available to provide this pressure.

Original languageEnglish
Title of host publication2007 Membrane Technology Conference and Exposition Proceedings
StatePublished - 1 Dec 2007
Event2007 Membrane Technology Conference and Exposition - Tampa, FL, United States
Duration: 18 Mar 200721 Mar 2007

Publication series

Name2007 Membrane Technology Conference and Exposition Proceedings


Conference2007 Membrane Technology Conference and Exposition
Country/TerritoryUnited States
CityTampa, FL


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