Certification Action Line: Filters & carbon

March 1, 2016

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

1. _______ is an example of a dual-function filter that provides chemical changes to a water supply and the mechanical process of separating out suspended particles.

a. Manganese greensand filter
b. String-wound fiber filter
c. Calcite filter
d. Activated carbon filter

2. True or False: A properly designed calcite filter of 36-inch bed depth and operating at 3 gallons per minute per square foot bed area can readily correct pH problems as low as 5.0. 

3. Where should calcite or calcite/magnesia neutralization be installed with respect to the water softener?

a. Before (upstream)
b. After (downstream)

4. What is the action by which activated carbon removes substances?

a. Absorption
b. Adsorption

5. True or False: The word "activated" in activated carbon means a catalyst coating has been placed on the media particles to affect strong molecular attraction forces.

6. True or False: The total pore volume of an activated carbon determines the types of contaminants the carbon will effectively reduce.

7. Which of the following constituent(s) is by and large insensitive to the adsorption characteristics of activated carbon?

a. Gasoline
b. Pesticides and herbicides
c. Chlorine
d. Earthy-musty odors

8. True or False: All activated carbon filter beds should be backwashed regularly.

9. True or False: Activated carbon will not remove bacteria.

10. True or False: An activated carbon filter that effectively removes chlorine may not effectively remove chloramines or trihalomethanes.


1. a, c and d. Manganese greensand oxidizes soluble contaminants such as iron to an insoluble precipitate; calcite neutralizes acidity; and activated carbon reduces or adsorbs contaminants such as chlorine and pesticides. All of these filters also physically separate suspended matter from water.

2. False. As a general rule, water supplies with a pH of 6.0 to 7.0 or having less than 50 milligrams/liter (mg/L) of free carbon dioxide can be handled with a calcite filter. Waters with pH in the range of 5.0 to 6.0 or those having more that 50 mg/L of free CO2 should be treated with a 5:1 mix of calcite (five parts) and magnesium oxide (magnesia) (one part). Higher CO2 concentration can sometimes be more effectively corrected with a chemical feed of soda ash (Na2CO3) or caustic soda (NaOH).

3. a. Both calcite and magnesia add hardness and bicarbonate alkalinity to the water, such as calcium bicarbonate and magnesium bicarbonate. This should be added before the water softener so that these ions are not distributed throughout the household water supply.

4. b. Activated carbon functions via an adsorption process in which matter adheres to the surface of the activated carbon. Absorption, on the other hand, is the process of taking matter in to the internal structure of the absorbing substance.

5. False. "Activated" refers to the fact that the activated carbon has been processed in a manner that creates large amounts of clean surface area in the carbon. The first step is to select a base material such as coal (bituminous or lignite); coconut shells; nutshells; peat; wood; fruit pits; or others with live, high pressure steam, carbon dioxide or acid that etches away the soft material in carbon and leaves behind a complex pore structure that has an affinity for organic molecules.

6. False. Total pore volume is an indication of the activated carbon’s adsorption capacity. The size distribution of the pore structure determines the types of contaminants the carbon will reduce effectively. Generally, activated carbons produced from lignite and peat tend to have larger pore diameters or macropores, which correspond to a higher molasses number. Shell activated carbons, on the other hand, tend to have a higher portion of small pore diameters that correspond to a higher iodine number.

7. c. The mechanism for removing chlorine with activated carbon is one of chemical reduction of chlorine to chloride rather than physical adsorption.

8. False. If activated carbon is being used to remove hazardous materials, it should not be backwashed because it will redistribute the media particles and cause some media that may be loaded with contamination to be redeposited at the effluent end of the filter. Any partial sloughing could then cause contaminants to prematurely appear in the treated water.

9. True. Activate carbon filters often offer an acceptable habitat for nonpathogenic or non-disease producing bacteria to grow. In fact, the level of heterotrophic bacteria in a water system is sometimes higher after a filter than before. If the water supply to the unit is potable, organisms pathogenic to man have not been demonstrated to grow in POU/POE treatment units.

10. True. The biggest problem with some activated carbon filters is a lack of contact time to remove anything other than chlorine taste and odor. Organic reduction and chloramine reduction need more contact time, requiring 5 to 30 minutes of empty bad contact time. On the other hand, chlorine can be effectively removed by activated carbon in a matter of seconds.

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