Frequently Asked Questions

Flow characteristics for a spiral element and flat sheet membrane are significantly different and are really only comparable on a qualitative basis. Variability in the membrane manufacturing process, differences in water composition, test procedures, and test equipment, used in a factory, laboratory, or elsewhere, will impact water flux results.

Reference: Understanding Variation of Experimental Flux and Rejection

Crossflow velocity limits for commercially available spiral-wound membrane elements depend on several factors, including element construction, maximum allowable pressure drop, and feed stream characteristics (such as viscosity and solids content). Recommended operating ranges are typically provided by the element manufacturer. For application-specific guidance, please contact Sterlitech for assistance in selecting appropriate crossflow velocities and operating conditions.

Sterlitech crossflow test cells (Sepa® CF, CF042, and CF016) operate in true crossflow filtration mode, meaning the feed flows tangentially across the membrane and produces both a permeate stream and a concentrate (retentate) stream. These systems allow continuous operation, with user-controlled pressure and crossflow rate, and enable ongoing sampling from both streams during testing.

The HP4750 Stirred Cell, by comparison, is a sealed batch filtration device (up to 300 mL feed volume) typically pressurized with compressed gas. It runs in normal-flow (dead-end) mode and does not have a concentrate stream. A stir bar helps reduce concentration polarization and simulates crossflow-like mixing at the membrane surface, but it is not true crossflow.