Exam Study Guide: Treatment Ponds; and Lime-Softening Alkalinity

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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 Plastic-Media Trickling Filters; and Activated Alumina. This time, you can test your knowledge about treatment ponds, and lime-softening alkalinity.

Wastewater Treatment Sample Question:

A wastewater treatment pond has a total depth of 9 feet and has a surface area of 40,000 square feet. The lagoon is aerobic near the surface and anaerobic near the bottom. What type of treatment pond is this?

A) An aerobic pond

B) An anoxic pond

C) A septic pond

D) A facultative pond

Answer: The answer is D, a facultative pond. Waste treatment ponds that are designed and operate as aerobic/anaerobic are known as facultative ponds. Facultative, according to Merriam-Webster’s online dictionary, in our case means “exhibiting an indicated lifestyle under some environmental conditions but not under others.” For example, facultative anaerobes. We generally use the term facultative in wastewater treatment to mean bacteria that can live in either aerobic conditions or anaerobic conditions. The facultative pond in our question exhibits some of both conditions, and as many operators know, these facultative ponds can treat wastewater very well. Algae will grow near the surface, providing oxygen to the upper layers through photosynthesis. Anaerobic conditions prevail in the bottom sludge layers as bacteria continue breaking down solids that settle without the presence of dissolved oxygen.

Water Treatment Sample Question: 

As the pH rises toward 10.2 from pH 8.3 in the lime-softening process, what forms of alkalinity are converting and becoming new forms of alkalinity?

A) Carbon dioxide is becoming sodium hydroxide

B) Hydroxide alkalinity is becoming carbonate hardness

C) Bicarbonate alkalinity is becoming hydroxide alkalinity

D) Bicarbonate alkalinity is becoming carbonate alkalinity

Answer: The answer is D, bicarbonate alkalinity is becoming carbonate alkalinity. As lime is added to water that contains calcium and magnesium hardness, the pH begins to rise. Once the pH reaches 8.3, most all the carbon dioxide has been converted to carbonate forms. As pH continues to rise toward 10.2, bicarbonate alkalinity (and any calcium attached to it) becomes the carbonate form (CO3). If calcium was attached, it becomes calcium carbonate (Ca CO3) and will settle in the basin. If we continue to add lime slurry, the pH continues to rise. At pH 11.3, most all the carbonate forms of alkalinity become hydroxide alkalinity.


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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