Monitoring Membrane Fouling Potential: SDI Vs. Particle Detection

By John W. Clark, and Joseph R. Zimmerman

Membrane fouling can have a significant impact on the performance of reverse osmosis (RO) systems, as well as on the cost of operating the systems (downtime, membrane cleaning, membrane replacement, etc.). The Silt Density Index (SDI) test has become the standard measurement for determining the fouling potential of RO make-up water. However, SDI results only offer “snapshots” of the quality of the makeup water at any given time. To truly understand the water that is being treated, and to optimize the pre-RO filtration system performance, water plant operators should consider utilizing online particle detection analyzers.

SDI tests are run on grab samples, and only provide insight into the performance of the pretreatment system for the moment that the sample was taken. Where SDI falls short is in showing the operator what leads up to a high SDI reading, and what immediately follows. How can an operator go about optimizing a pretreatment system with only a few measurements each day? SDI testing doesn’t just fall short in the frequency of measurement, but there is also a lack of confidence amongst many water treatment professionals in the consistency of SDI results.