Exam Study Guide - August 2021

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By Rick Lallish

(This is a corrected version of the question that appeared in this space in last month’s edition.)

The MPN method for fecal coliform testing has become on the most common methods for testing. What is a disadvantage for MPN fecal coliform analysis?

A. Sample toxins are diluted

B. Samples cannot be analyzed by membrane filtration

C. Difficult to interpret, either by observation or gas emission

D. Results are not very accurate and have a high probability of false positives

Answer: D. It is important for operators to understand the many types of laboratory procedures and analysis so that they can properly operate a wastewater treatment facility. Fecal coliform testing is required on most if not all NPDES permits. MPN has become the most common method. The advantages of MPN are ease of interpretation, sample toxins are diluted, and it is very effective on highly turbid samples, and samples that cannot be analyzed by membrane filtration. However, the method has distinct disadvantages that need to considered, such as the amount of time to get the results (up to 48 hours), the requirement for more glassware and media, probable false positives and sometimes inaccurate results. More information may be found in the OWP, CSU-Sacramento textbook: Operation of Wastewater Treatment Plants Vol. 2, (seventh ed.), Chapter 16 and Bacteriology, Microbiology for Beginners (Nisha Rijal). 


By Drew Hoelscher

Water is being pumped from a finished water storage basin at 275 gpm. The basin is 60 feet long, 30 feet wide and 18 feet deep. What is the flow rate into the basin, and how long will it take to fill, if the water level is currently at 11 feet and rising 18 inches every hour?

A. 336.6 gallons per minute, 12 hours

B. 275 gallons per minute, 12 hours

C. 611.6 gallons per minute, 4.7 hours

D. 275 gallons per minute, 4.7 hours

Answer: B. Accurately calculating rise and fall levels in water storage basins allows for a more controlled operation. For example, there is more opportunity for error to occur in water treatment when operators make multiple flow rate changes in short periods of time. Flow-paced chemical dosing equipment has helped eliminate some of these issues, but calculating flow rates to predict future operational changes is a part of basic water treatment operation.

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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