Exam Study Guide: Effluent Coliform Count; and Enteric Virus Removal

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Exam Study Guide: Effluent Coliform Count; and Enteric Virus Removal

Welcome back to TPO magazine's Exam Study Guide Series, which offers a pair of water/wastewater study questions with in-depth explanations of the answers. Last time, we covered a set of wastewater and drinking water treatment questions on the topics of Fusible Plugs; and Sodium Thiosulfate. This time, you can test your knowledge about plant effluent coliform count, and enteric virus removal.

Wastewater Treatment Sample Question:

The plant effluent coliform count fails to meet required standards for disinfection. The chlorine residual is normal. Inspection of the disinfectant supplies and equipment reveals no problems. What is the most probable cause of the elevated effluent coliform count?

A) Capacity of chlorinator too low

B) Too little solids in effluent

C) Chlorine demand too high

D) Contact time too short

Answer: The answer is D, the contact time is too short. To effectively kill the coliform bacteria, an adequate dosage of the disinfecting chemical and long enough contact time with that disinfectant are necessary. In the case shown above, the chlorine residual is normal and the chlorine supply and equipment are working okay. Had the chlorine demand been too high (answer choice C), the chlorine residual would not be normal, it would be very low, which would affect the ability of the chlorine to disinfect.

Some state regulatory agencies require specific minimum hydraulic retention times in the chlorine contact tanks. For example, Florida Administrative Code requires no less than 15 minutes of contact time at peak hourly flow to ensure effective disinfection of coliform group bacteria.

Water Treatment Sample Question: 

According to the EPA Enhanced Surface Water Treatment Rule (ESWTR), the removal or inactivation of cryptosporidium oocysts, giardia cysts and enteric (intestinal) viruses shall be:

A) 99 percent removal of cryptosporidium oocysts, 99.9 percent removal of giardia cysts and enteric viruses

B) 99 percent removal of cryptosporidium oocysts, 99.9 percent removal of giardia cysts and 99.99 percent inactivation of enteric viruses

C) Four-log removal of cryptosporidium oocysts, five-log removal of giardia cysts and six-log removal of enteric viruses

D) Two-log removal of giardia cysts, three-log removal of enteric viruses and four-log removal of cryptosporidium oocysts

Answer: The answer is B, 99 percent removal of cryptosporidium oocysts, 99.9 percent removal of giardia cysts and 99.99 percent inactivation of enteric viruses. Many water treatment facilities use calculations to show compliance with EPA’s enhanced surface water treatment rule (ESWTR), including the calculation for disinfectant residual concentration and contact time (CT value). CT values ensure there is sufficient disinfectant residual and contact time to kill or otherwise inactivate organisms like enteric viruses and giardia protozoa.

The term ‘log removal’ essentially means the effectiveness of the treatment system to remove or inactive these pathogens. Logarithmic removal (log removal) is equated to a percentage of removal. Answer B uses the actual percentage value as a number in its statement: 99 percent, 99.9 percent and 99.99 percent.  If answer B was rewritten to reflect the logarithmic values, we would state it as: two-log removal of cryptosporidium oocysts, three-log removal of giardia cysts and four-log removal of enteric viruses. When a facility is required four-log removal, they are essentially required to ensure 99.99 percent of enteric viruses are killed.

About the author: Ron Trygar is the senior training specialist for water and wastewater programs at the University of Florida’s TREEO Center. Previously, he was the wastewater process control specialist at Hillsborough County Public Utilities in Tampa, Florida. He has worked in the wastewater industry for more than 30 years in a variety of locations and positions. Trygar became a Certified Environmental Trainer (CET) in 1998 and has since provided training for associations and regulatory agencies such as Florida Department of Environmental Protection (FDEP); Florida Water and Pollution Control Operators Association Short Schools; USABlueBook; Florida Water Environment Association sponsored training events; and local school environmental programs. Working alongside the FDEP Northeast District, Trygar helped begin the Florida Rural Water Association and FDEP joint operator certification review classes that are still given around the state today. He holds a Florida Class A wastewater treatment operator’s license and a Florida Class B drinking water operator’s license.


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