淤塞密度指数（Silt Density Index，简称SDI）是测定在标准压力和标准时间间隔内，一定体积的水样通过一特定微孔膜滤器（例如用于逆渗透系统者）的阻塞率。它表征水中胶体物和悬浮物含量的多少。
淤塞密度指数测试有助预测滤膜表面何时会被微粒壅塞，从而采取预防措施。
英语百科
Silt density index 淤塞密度指数
The silt density index is a measure for the fouling capacity of water in reverse osmosis systems. The test measures the rate at which a 0.45-micrometre filter is plugged when subjected to a constant water pressure of 206.8kPa (30psi). The SDI gives the percent drop per minute in the flow rate of the water through the filter, averaged over a period of time such as 15 minutes.