Certification Action Line: Questions about filtration

Dec. 1, 2015

Certification Action Line features questions and answers typical of those appearing in Water Quality Association (WQA) certification examinations.

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. Answers for the questions on water filtration are listed at the bottom of the quiz.

1. "The removal of suspended solids by porous medium, largely by mechanical (straining and screening)" is a definition for which water treatment process?

a. Reverse osmosis
b. Softening
c. Filtration
d. All of the above

2. Home water treatment filters often combine filtration with which of the following chemical conditioning actions?

a. Oxidation
b. Adsorption
c. Neutralization
d. All of the above

3. What type filtration treatment is needed if green stains are on plumbing fixtures?

a. Activated carbon
b. Acid neutralization
c. Oxidation and filtration
d. All of the above

4. Sieves and screens are examples of _______.

a. Surface filters
b. Depth filters
c. String-wound fiber filters
d. All of the above

5. What advantage do multimedia depth filters have over single-media sand filters?

a. Require less backwash flow
b. Are better suited for open basin filters
c. Can operate for longer periods of time between back washing
d. All of the above

6. To obtain good cleaning during backwash, expand granular filter beds a minimum of _______ percent.

a. 50
b. 33
c. 25
d. 10

7. True or false: Particles smaller than the pore size of the filter cannot be removed in the filtration process. 

8. True or false: A rating of 20 microns (µm) on a filter cartridge means the filter will remove all particles greater than 0.02 millimeter. 

9. True or false: With submicron filtration, most cartridge filters can purify water. 

10. True or false: A cartridge filter can safeguard water against Giardia lamblia cyst contamination. 


1. c

2. d

3. b. Low pH reacting with copper and brass piping and fittings may cause green stains on plumbing fixtures.

4. a. Sieves, screens and strainers are surface filters, as opposed to depth filters, because only the surface layers do the actual filtering.

5. c. Multimedia depth filters can operate five or more times longer than single-media, sand filters before head loss requires backwashing. This is because turbidity is trapped and held throughout the entire bed depth rather than only the top 1 or 2 inches. Therefore, the multimedia bed can hold more sediment. In addition, multimedia construction lessens bed cracking, making it better suited for closed-pressure tanks. A 7- to 10-gallon-per-minute or gallon-per-minute/foot (gpm/gpf) service rate in multimedia construction functions is comparable to 2 to 4 gpm/gpf in single media filters, and the finer particles of garnet at the bottom trap finer turbidity particles in a multimedia filter.

6. a. A minimum bed expansion of 50 percent for at least 10 minutes helps ensure turbulent bumping and scouring of media particles against one another to better cleanse sticky deposits. In much the same way, washing your hands by rubbing them together is more effective than just rinsing them individually.

7. False. While straining particles larger than the filter’s pore size is an important mechanism, many other forces are at work in filters that help remove at least a portion of the smaller particles. For example, small particles might collide and stick together with other particles in the water or those already trapped in the filter, forming larger flocs that are effectively filtered. Also, water that flows close to the surface of a grain of medium sets up a hydrodynamic force that aids small particle removal. This layer of water flows slower than it would if it had been far away from a grain. A moving, suspended particle that comes close to the grain of medium, therefore, will tend to slow down on the side closest to the grain, while the other side will flow as usual. This imbalance of forces puts the particle in a spin, just like that of a hooking or slicing golf ball off a driver, which will take the particle on a curved path toward the medium grain.

8. False. Twenty microns is the same as 0.02 millimeters. However, a rating listed as 20 microns with no other qualification means the filter will stop a certain percentage of 20-micron particles. Usually this percentage is 75 to 85, and the rating is referred to as nominal. Only if the cartridge rating is listed as absolute will it stop all or at least 99.9 percent of the particles of the size range or larger.

9. False. Assuming the definition of a purifying filter as a "device that removes/disinfects all types of disease-causing microorganisms in raw water, including bacteria, viruses and protozoan cysts, so as to render the processed water safe for drinking" or potable, a cartridge filter with submicron filtration alone would likely not purify water. Minute mechanical imperfections, leaks around seals and grow-back can also occur. For a single filter cartridge to accomplish complete microbiological treatment, it would probably have to be certified to achieve 99.9999 percent reduction of bacteria, 99.99 percent  reduction of viruses and 99.9 percent reduction of cysts. Few mechanical cartridge filters have achieved this distinguished certification.

10. True. Giardia lamblia or protozoan cysts, are relatively large organisms of about 6 to 12 microns, and with the proper filter, they can be filtered out of water. Use a filter supported with performance test data across the full life or capacity of the filter.

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