In the realm of pretreatment for reverse osmosis (RO) system design, multi-media filters (MMFs) and turbidity filters are two of the various technologies available for suspended solids removal, turbidity control, and silt density index (SDI) reduction. This article delves into the terminologies and fundamentals of these two filtration methods, providing insights into their functions, differences, and application selection criteria.
What is the function of an MMF or turbidity filter?
These pretreatment filtration technologies serve to reduce the level of suspended solids contained within an incoming feed water source. Reduction of suspended solids “upstream” of the RO increases the effectiveness of both the RO unit and the deionization (DI) technologies that are the foundation of high purity water systems.
Silt density index (SDI) estimates the quantity of suspended solids and colloids in water. SDI is measured following the ASTM D19.08 Standard Test Method for Silt Density Index of Water, using a 0.45-micron membrane. The SDI measurement of the incoming feed water informs which type of pretreatment filtration design is most appropriate. For example, choosing a technology consisting of finer media types may be preferable where greater total suspended solids (TSS) removal is required prior to water reaching the RO system.
What is a multi-media filter (MMF)?
An MMF contains a combination of several media types, offering distinct advantages over single-medium filters such as prolonged filtration runs and increased filtration rates.
Typically comprising gravel, garnet, sand, and anthracite, MMFs are strategically designed to address the varying micron sizes of particles present in feed water. Anthracite captures large particles, sand targets slightly smaller particles, and garnet captures the smallest particles. The presence of gravel at the tank’s bottom facilitates water flow through the tank’s screen.
What is the difference between MMFs and turbidity filters?
While MMFs can be considered a type of turbidity filter, it is helpful to distinguish between the two terms. Turbidity, denoting the quality of cloudiness or opaqueness due to suspended matter, accurately describes the function of an MMF. However, the term “turbidity filter” is typically reserved for systems using a single medium, such as AG Plus or Zeolite.
What are AG Plus and Zeolite?
AG Plus is a general service filter system employing a non-hydrous silicon dioxide media for suspended solids removal down to the 20-40 micron range. The fractured edges and irregular surface of the media create less pressure drop by allowing deeper sediment penetration without caking and blinding the bed surface. Filter-Ag has a lighter weight than traditional sand media, thereby requiring lower backwash rates and thus achieving better water conservation.
Zeolite is a finer material that can be used to lower the micron rating much farther. It’s an alternative, single media approach to MMFs that achieves a finer filtration to 3-5 microns with longer service run times.
How do I select the correct option for my specific high purity water application?
For optimal performance tailored to your high purity water application, consult our equipment experts and application engineers. Atlas High Purity Water offers various water sampling methods to assess feed water quality and determine the most suitable pretreatment filter design for your specific needs.
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About Atlas High Purity Water
Headquartered in Canton, Massachusetts, Atlas High Purity Water designs, installs, and services high purity water systems for critical applications at facilities throughout the Northeast.
Atlas High Purity Water is part of East Range Group, a team of four high purity water systems and service companies in the Northeast USA and Canada. East Range Group is dedicated to our core purpose of building phenomenal places to work for our incredible team of employees, and to building the best high purity water service company in North America.