Silt Density Index

Abstract

Abstract The most widely accepted and historically used predictor of the fouling potential of reverse osmosis feedwater is the plugging factor (PF), now commonly known as the Silt Density Index (SDI). The SDI procedure was standardized by ASTM International and is intended to be used as a measure of the fouling capacity of feedwater to reverse osmosis systems. The SDI is an index calculated from a test that measures the rate at which a 0.45-micrometer (μm) filter is plugged when subjected to a constant water pressure of 206.8 kPa (30 psi). The SDI gives the percent drop per minute in the flow rate of the water through the filter, averaged over a specified time-period, typically 15 minutes. Because the SDI is a relatively simple procedure and inexpensive to implement, it has been universally applied since the 1960s to assess the particulate fouling tendency of a feedwater intended for treatment by reverse osmosis (RO) membrane processes. Many facilities in the United States rely on automated SDI systems that are microprocessor controlled and fully automatic to allow operators to regularly monitor the feedwater. Care must be taken when employing the SDI with regards to accuracy and reproducibility, as the index is not based on any filtration mechanism and is not proportional with colloidal or particle concentration, in addition to the fact that there is no correlation factor for temperature nor for variations in membrane resistance. Even though there are legitimate concerns regarding the predictive value of the SDI, the index continues to be successfully used in the planning of RO facilities to guide the selection of pretreatment processes, and is often the basis of membrane guarantees and other plant performance contracts.