Exam Study Guide - April 2022

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By Rick Lallish

Which classification of anaerobic digester is the most efficient at pathogen reduction, increased volatile solids reduction and decreased detention times?

A. Heterophilic

B. Mesophilic

C. Thermophilic


Answer: C. Anaerobic digestion is done typically with either mesophilic (85-100 degrees F) or thermophilic (122-130 degrees F). The goal of thermophilic digestion is increased pathogen destruction, but also volatile solids reduction and decreased detention times when compared to mesophilic digestion. One drawback of thermophilic digestion is the increased energy requirement and cost. Because of this, mesophilic digestion is more common. More information may be found in the WEF textbook, Wastewater Treatment Fundamentals II – Solids Handling and Support Systems, Chapter 4.


By Drew Hoelscher


A. Will cause a water to smell like chlorine

B. Will cause a water to develop a strong metallic taste

C. Is regulated by the U.S. National Primary Drinking Water Regulations

D.  Will cause a water to develop an earthy-musty odor

Answer: D. 2-methylisoborneol (MIB) and geosmin are usually the primary cause of biological taste and odor complaints. Depending on the seasonal temperatures, source water nutrient level and other variables, summer and early fall are usually the most problematic. The most common approach in reducing the taste- and odor-causing compounds is feeding powder activated carbon as a slurry in the treatment process or installing granular activated carbon as a media in the filter bed. MIB is produced by bacteria (cyanobacteria and actinomycetes) and is released from the cells as the microorganisms die off. MIB and geosmin are problematic because their earthy or musty odor can be detected at very low levels (0.002 to 0.02 ug/L).

About the authors

Rick Lallish is water pollution control program director and Drew Hoelscher is program director of drinking water operations at the Environmental Resources Training Center of Southern Illinois University Edwardsville.  


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