Certification Action Line – Answers – November 2013

Nov. 1, 2013

1. d. Media filters use a filtration bed commonly consisting of one or more layers of sand, anthracite, and/or garnet media granules. Other granular materials, such …

1. d. Media filters use a filtration bed commonly consisting of one or more layers of sand, anthracite, and/or garnet media granules. Other granular materials, such as volcanic pumicite and glauconite greensand, also are used. They may use more than one size range of granule to capture different sized particulates. Different media materials settle after backwashing in the order of their respective specific weights. Filter media granules have the ability to remove suspended particulates through a number of different filtration mechanisms.

2. b. The effectiveness of gravity filters, commonly used in municipal water treatment systems, is dependent on achieving low water velocities through the media bed. At higher velocities, the shear forces of water flow against the suspended particles will tend to reduce the effectiveness of gravity filtration. Higher filtration flow rates will also result in high pressure drop through the filter.

3. a. At pressure drop increases of 15 or more psid, the captured suspended solids can begin to form a more compacted filter cake, which may become difficult to break up and remove during backwashing.

4. a. The multimedia filter is designed to remove a larger quantity of suspended solids between backwashing because larger filter media granules with a lower density are at the top of the filter bed with small media granules of higher density successively below it. The top of the bed thereby captures only the larger particles, leaving the smaller particles to be filtered by the finer media further within the entire bed depth.

5. c. Multimedia beds are sometimes effectively operated at flow velocities as high as 20 gpm/ft2, but their best efficiency is perhaps when they are operated in the range of 3 to 5 gpm/ft2

6. d. Both anthracite and volcanic pumicite are light materials and exhibit surface crevices and pocks that create swirling flow eddies. These centrifugal motions help to force suspended particles to settle and adhere onto the surfaces of the individual media granules.

7. b. A filter backwash rate of about 10 to 15 gpm/ft2is usually appropriate for anthracite and sand media. Single media filters of heavy materials such as garnet, pyrolusite, or KDF require higher backwash rates to lift these media. In cold water or with light-weight materials like activated carbon and/or with small size media like fine pumicite lesser backwash flow rates may suffice.

8. d.

9. b. Van der Waals forces are omnipresent electrostatic forces between all matter. For particles to stay attached to filter media surfaces however, the individual particles must roll along or migrate to a sufficiently quiescent harbor where the shear forces of flowing water are not able to overcome the van der Waals forces of molecular attraction. Smaller particles are typically better able to withstand the shear forces, because there is less surface area to drag against the flowing water.

10. False. With time the surface irregularities on granules will be lost due to abrasion against other granules. The media can become coated with organic material that cannot be removed by backwashing, and the media will also collect dense particles, over time, that will not be backwashed out. When this occurs the after-backwash pressure drop of the filter will increase substantially. Generally filter media will require replacement every 5 to 10 years, depending on water quality.

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