Peculiar or unexpected behavior of Silt Density Index of pretreated seawater for RO desalination

Hiroshi IWAHORI, P.E.Jp Co-Authors: Satoru ISHIHARA, Masaaki ANDO, Naoki TADA

Membrane Division Nitto Denko Corporation 

gas_saturation_and_membranes.pdf

 

Introduction:

No turbidity, but a high SDI-values in UF/MF membrane treated seawater:

In a specific seawater condition, it is often observed to have a high SDI values unexpectedly in normal pretreatment conditions.

For example, UF/MF Pretreatment of seawater RO desalination:

We set up “Feed quality should be SDI < 4 , which is very easily achieved ”. However, it is unexpectedly observed that UF or MF filtrate having no particulate indicates sometime SDI = 4 or more.

We set up a hypothesis that a key factor of increasing SDI-value should be micro-air bubbles in the seawater. Because, pretreatment of highly pressurizing and degassing SDI- test solution gives a lower SDI-value than non-treatment.

We surveyed SDI-value increasing factors by using various types of MF with the same 0.45μm pore size. Further investigation indicated the unexpected phenomena is caused by adsorption of dissolved organic compounds and other substance inside SDI-filter-pore structure due to hydrophobic nature of the membrane material.

SDI measurements  ASTM:D4189-95

Seawater sample is passed through a 0.45micrometer membrane filter at a constant applied gage pressure of 207kPa(30psi), and the rate of plugging of the filter is measured.
The SDI is calculated from the rate of plugging;

 

SDIT= %P30/T=[1-ti/tf]x100/T

 

where: %P30 = plugging rate percent at 207kPa feed pressure T = total elapsed flow time, min(usually 15 min)
ti = initial time required to collect 500ml of sample, second

tf = time required to collect 500ml of sample after test time T (usually 15 min), second.

Therefore, normally the following SDI 15 is used for RO feedwater evaluation. SDI 15 = %P30 / 15 = [1 - ti / tf] x 100 / 15

SDI(5minutes) measurements

In case of raw seawater or %P30 (plugging rate percent) exceeding 75%, 5-min measurement is applied. The smaller volume of sample size, 100ml is used.

 

 

SDI5 =%P30 /5=[1-ti /tf]x100/5

Why not Measurement of Turbidity:

Drawback: SS analysis with optical-method used to be Poor Reliability in low concentration range in past days. Therefore, direct measurement of suspended matter by MF filtration was utilized.

First application: Quality control for Electronic Grade Water in 1969 Monitoring feed water QC for seawater RO in 1979 from DuPont technical operating manual. The SDI measurement has become a de facto standard for RO feed water QC.

In some SDI measurements with eutrophic seawater or sewerage water, even if UF or MF membrane pretreated it and no turbid matter existed, high levels of SDI-values which are not relating with particles in feed waters have been observed.

We found that pre-treatment of SDI test solution with high-pressurization at 6MPa decreased SDI-values and air bubbling increased SDI-values. Since degasification with membrane degasser results in reducing subsequent SDI values, we paid attention to micro- air-bubble. 

Hiroshi IWAHORI, P.E.Jp Co-Authors: Satoru ISHIHARA, Masaaki ANDO, Naoki TADA

Membrane Division Nitto Denko Corporation 

gas_saturation_and_membranes.pdf

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