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. In this study, several methods were used to prove that
the presence of micro-bubbles can interfere with SDI measurements.
The first 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. A fourth 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 was 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. Includes 2 references, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 340 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 11 |
| Published : | 03/01/2007 |
