At present, the best technique for evaluating the possibility of colloidal pollution in the RO/NF system influent water is to measure the sedimentation density index (SDI, also known as the fouling index) of the influent water, which is an important parameter that must be determined before RO design.
During RO/NF operation, measurements must be made periodically (2-3 times a day for surface water), and ASTM D4189-82 specifies the standard for this test. The water inlet of the membrane system is stipulated that the SDI15 value must be less than or equal to 5.
Effective technologies for reducing SDI pretreatment are multi-media filters, ultrafiltration, microfiltration, etc. The addition of polydielectrics prior to filtration can sometimes enhance the above physical filtration and SDI reduction.
