What is SDI?
 
Silt Density Index (SDI) 是一種用來定義水質污染指標的方法. 符合ASTM 4189-95, SDI 指數可用於預測 RO (Reverse Osmosis) 逆滲透膜被懸浮物或膠狀物質阻塞的情況. 

Description of the test 

The test described in ASTM4189-95 is performed using a 0.45 micron, 47mm diameter filter. The water to be tested is supplied to the filter at a constant pressure of 30 psi. The test involves measuring the time it takes to collect a 500 ml sample through the filter at the start of the test and comparing it with the time it takes to collect a 500ml sample after water has flowed through the filter (at 30psi) for 15 minutes. The sample times are applied to the formula below to obtain the SDI15 value.



The resulting value, SDI15, indicates the plugging of the membrane in percent-per-minute. Accordingly, the maximum SDI15 value is 6.7. (100÷15=6.7) An SDI15 value greater than 5 is generally considered too high to be meaningful (75% plugging).

On waters with high SDI, it's often useful to measure the SDI at 5 and 10 minute intervals. The resulting values, SDI5 and SDI10 can provide a better indication of the rate at which the membrane is plugging. For example, if you obtain an SDI 5 of 15, there's no point in taking the time it takes for an SDI15 test since you've already achieved 75% plugging (15 x 5=75). In this case, the water being tested is simply too high in colloidal or particulate matter to obtain a meaningful result.