Certification Quiz
Test Your Knowledge of Odor Control
True or False
1. T F Most concrete sewer corrosion is caused by the absorption of atmospheric carbon dioxide into the moisture on the sewer head space where it forms carbonic acid that reacts with the concrete.
2. T F Increasing wastewater pH by injection of a base chemical, such as sodium hydroxide (NaOH) or magnesium hydroxide (Mg(OH)2), will cause an increase in the emission of hydrogen sulfide (H2S) gas.
Multiple Choice
3. Which description below best defines the gas phase measurement of parts per million?
a. milligrams of gas pollutant per kilogram of gas mixture.
b. milligrams of gas pollutant per liter of gas mixture.
c. number of pollutant molecules for each million molecules in the gas mixture.
d. milliliter of gas pollutant per thousand liters of gas mixture.
4. Where is the most sulfide in a sewer produced?
a. In flowing wastewater with dissolved oxygen less than 1.0 mg/L.
b. In the biological slime layer on the sewer wall submerged in wastewater with dissolved oxygen less than 0.1 mg/L.
c. On the moist pipe wall above the wastewater.
d. Inside lateral connections.
5. What is the Dilution-to-Threshold (D/T) value of a 1.0-L odor sample if it requires 9.0 L to dilute the sample to the detection threshold as determined with a field olfactometer?
a. 100.
b. 10.
c. 20.
d. 9.
6. What is the control efficiency of an odor control scrubber that receives an airflow with a concentration of 15 ppm H2S and discharges 0.13 ppm H2S?
a. 96.5%.
b. 93.4%.
c. 99.1%.
d. 99.7%. 7. Estimate the useful life of a carbon bed air filter removing H2S from a foul airflow assuming the characteristics from the table below.
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Airflow at 70º F 500 ft3/min |
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Hydrogen sulfide concentration 5 ppm |
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Carbon in filter 1000 lb |
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Molecular weight of H2S 34 lb/mol |
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Volume of 1 lb•mol of any gas at 70ºF 387 ft3 |
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The carbon used can adsorb 30% of its weight in H2S | a. 31.6 months.
b. 8.3 months.
c. 12.0 months.
d. 11.3 months.
Short answer
8. What is olfactory fatigue and why is it particularly dangerous with respect to hydrogen sulfide?
9. What is meant by the term dilution-to-threshold (D/T) as determined with a field olfactometer?
Questions were developed by Dirk Apgar and the Water Environment Federation (Alexandria, Va.) Air Quality and Odors Control Committee and reviewed by the Association of Boards of Certification (Ames, Iowa) Validation and Examination Committee.
Answer Key
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1. False. While carbonic acid is formed when carbon dioxide is absorbed into the moisture on the exposed concrete, most concrete sewer corrosion is due to the absorption of hydrogen sulfide that is oxidized to sulfuric acid (H2SO4) by bacteria of the thiobacillus family. The H2SO4 reacts with calcium silicate (CaSi) and calcium carbonate (CaCO3) in the concrete matrix to form gypsum (CaSO4).
2. False. Increasing wastewater pH will decrease H2S emissions by increasing the dissociation of dissolved H2S gas into the ionic mes required to dilutsulfide (S-2) and hydrosulfide (HS-) species. The dissociated (S-2) and (HS-) ions remain in solution so there will be less dissolved gas available to be released.
3. Both c and d are correct. It is a fact of gas chemistry that the molecular fraction of the gases is equal to their volume fractions.
4. b.
5. d. D/T = (dilution volume)/(sample volume). So, D/T = (9.0 L)/(1.0 L) = 9
6. c. Percent efficiency = [(concentration at inlet – concentration at exhaust)/concentration at inlet] × 100. So, [(15 ppm – 0.13 ppm)/15 ppm] × 100%= 99.1%.
7. a. Step 1 is to calculate the carbon bed’s capacity to adsorb H2S. Pounds of carbon × (amount of H2S carbon can adsorb) = pounds of H2S the carbon can adsorb. So, 1000 lb carbon × (0.30 pounds of H2S per pound of carbon) = 300 lb H2S.
Step 2 is to calculate the pounds of H2S coming into the carbon bed. (Concentration of H2S entering filter) × (air flow ft3/min)/(volume of one mole of H2S) × (weight of one mole of H2S) = pounds of H2S entering filter per minute. So, (5 parts/1,000,000 parts) × (500 ft3/min) × (1 lb•mol/387 ft3) × (34 lb H2S / lb•mol) = 0.00022 lb/min H2S entering the filter.
Step 3 is to estimate carbon life using the results from steps 1 and 2. Pounds of H2S the filter can remove/pounds of H2S entering the filter per minute = minutes the filter can adsorb the H2S. So, 300 lb H2S/0.00022 lb/min = 1,363,636 minutes. Convert minutes to months: 1,363,636 minutes × (1 hour/60 minutes) × (1 day/ 24 hours) × (1 month/30 days) ≈ 31.6 months.
8. Olfactory fatigue is the reduction in a person’s ability to detect an odor after prolonged exposure to the odorous compound. With respect to H2S, this can result in a person unknowingly remaining or entering someplace where a dangerous level of the gas is present. When high concentrations are present the ability to detect H2S can be deadened almost immediately. In any case, it is important always to use personal H2S monitors to provide warning of dangerous gas levels when the nose may not.
9. Dilution-to-threshold (D/T) is the value of equivalent air volue the odor sample volume to the point that its concentration is just at the detection or recognition threshold. |
Reference
Water Environment Federation (2004). Control of Odors and Emissions from Wastewater Treatment Plants, Manual of Practice No. 25. Alexandria, Va.: Water Environment Federation.
©2007 Water Environment Federation. All rights reserved. |