Answers to these questions appear online at

Certification Action Line features questions and answers typical of those appearing in Water Quality Association (WQA) certification examinations. Some answers may not satisfy everyone or every condition.


1. Media filter beds commonly consist of granules of:

            a) sand

            b) anthracite

            c) garnet

            d) all of the above


2. Single media (e.g. sand) gravity filters use a slow filtration rate of about:

            a) 0.2 gallons per minute per square foot of filter bed surface area (gpm/ft2)

            b) 2 gpm/ft2

            c) 10 gpm/ft2

            d) 20 gpm/ft2


3. Media filters should be backwashed when their pressure drop increases by ________ pounds per square inch differential (psid).

            a) 10

            b) 15

            c) 20

            d) 30


4. & 5. Multimedia (e.g. 1.5 mm pumicite, 1.0 mm sand and 0.5 mm garnet) filters will hold (4) a or b particulates and are operated at (5) c or d flow velocities than single media (e.g. 0.5-1.5 mm sand) filters.

            a) more

            b) less

            c) higher

            d) lower


6. The irregular surface characteristic shape of pumicite and anthracite filter media help create _________, which assist in filtering suspended solids.

            a) locking interlinks

            b) tighter porosity

            c) particle diffusion

            d) flow eddies


7. To achieve at least 50 percent bed expansion, media filters should be backwashed at a rate of about ________.

            a) 1 to 5 gpm lft2

            b) 10 to 15 gpm lft2

            c) 20 to 25 gpm lft2

            d) 30 to 35 gpm lft2


8. Lower distributor/collector hubs or laterals are covered with a gravel support layer to:

            a) provide uniform distribution of backwash water flow

            b) prevent channeling of water flow

            c) prevent the blinding off by finer filter media of the distributor openings

            d) all of the above


9. Once suspended particles, especially very small particles, are touching the surface wall of a filter media granule, they will tend to be held there by:

            a) Zeta potential

            b) Van der Waals forces

            c) polysaccharide

            d) water shear forces


10. Properly backwash filter media will last indefinitely.

            a) True

            b) False