The Basics

Filtration Classification

Filtration is a process that is widely used for removing particulate matter from water. Nearly all water treatment plants employ some form of filtration. Water filtration can be classified a number of different ways, the most common one we shall discuss here. The first classification holds three different items:

• Filtration that is slow enough to include a biological cleaning component (0.1 m/h). This means that bacteria are allowed to grow in the filter media and these bacteria improve the quality of the water leaving the filter. Slow Sand Filtration (SSF) used to be widely employed but has fallen out of use in recent years because of the high labor requirements.
• Filtration as a polishing step, normally using pressure or head to force the water through the filter. This type of filtration is normally used after a flocculation and flotation/settling stages and has much larger filtration rates (15m/h) than SSF. Although there are many designs and variations on this theme the most common mode of use is the Rapid Gravity Filter (RGF)
• Filtration using Membranes. This is a technology which has rapidly come of age in the last 20 years. Although simple membranes (cellulose based filter paper for instance) have been available for many years, they were commonly used by simply forcing the water through, which is nowadays called "dead end" filtration. A big advance was the discovery of "cross-flow" filtration, where water flows, under pressure, at high velocities, perpendicular to the membrane. This keeps the membrane clean and prevents "clogging". Membrane manufacturing has also come of age, with pore sizes down to a few angstrom. It is now possible to produce ultra-clean water (18MOhm or better) with many impurities down to a few Parts per Billion (ppb) or even Parts per Trillion (ppt).

As mentioned above, there are many interesting aspects of filtration technology. We will discuss Rapid Gravity filtration, Granular Activated Carbon Filtration, Filtration theory in granular filtration, Filtration theory in membrane filtration and Membrane Fouling.