Silt Density Index

The Silt Density Index (SDI) , also called fouling index or colloid index (KI), is a measure of the amount of particles in the water.

The time is measured in which 500 ml of water penetrates through a membrane at a pressure of 206.8 k Pa (30 psi about 2 bar) . After a certain period of time (normally 15 minutes) the test is repeated. The particles in the water reduce the amount of water that flows through the membrane over time and the membrane becomes clogged.

The SDI is the average percentage change per minute in the volume flow through a membrane over a period of 15 minutes (since the volume is set at 500 ml, the times required for membrane penetration can also be calculated directly instead of the volume flow) :

{\ displaystyle SDI = {\ frac {\ frac {t_ {x} -t_ {0}} {t_ {x}}} {T}} \ cdot 100 \, \%}

With

${\ displaystyle t_ {0}}$ : first measurement for 500 ml (in seconds)
${\ displaystyle t_ {x}}$ : second measurement for 500 ml (in seconds) after minutes ${\ displaystyle T}$
${\ displaystyle T}$ : Time between the start of the first and the start of the second measurement (in minutes)

The difference to a measurement of the filterable substances is the recorded particle size . With the SDI measurement, particles are recorded in the colloidal range. Membranes with a pore size of 0.45 micrometers are used.

These values are usually used in the inflow of reverse osmosis , as this allows an early prediction to be made about clogging of the membranes used. Values <5% / min are usual here.

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What is SDI , simplesdi.com.

Individual evidence

↑ Thomas Melin: Membrane Process - Basics of Module and System Design . 3. Edition. Springer, Berlin / Heidelberg 2007, ISBN 978-3-540-34327-1 , p. 260 .

[1] Thomas Melin: Membrane Process - Basics of Module and System Design . 3. Edition. Springer, Berlin / Heidelberg 2007, ISBN 978-3-540-34327-1 , p. 260 .