Exam Study Guide: Wastewater Filters; and Calcium Hydroxide Reactions

Maintaining your education is important, especially in a career that demands licensing exams. Prove you’re an expert operator by answering these questions and others from our Exam Study Guide Series.
Exam Study Guide: Wastewater Filters; and Calcium Hydroxide Reactions

Welcome back to TPO magazine's new and improved Exam Study Guide Series, which offers a pair of water/wastewater study questions each week with in-depth explanations of the answers. We covered a set of wastewater and drinking water treatment questions last week on the topics of Trickling Filters and Lime Treatments. This week, you can test your knowledge about wastewater filters, and calcium hydroxide chemical reactions in the softening process.

Wastewater Treatment Sample Question:

Which type of filter is used to reduce the organic loading on downstream biological processes?

A) Tertiary filter

B) Vacuum filter

C) Traveling bridge rapid sand filter

D) Roughing filter

Answer: The answer is D, roughing filter. A roughing filter may be found in situations where very high organic strength waste must be biologically treated. The roughing filter is essentially a high-rate trickling filter placed ahead of other biological treatment processes such as rotating biological contactors (RBCs), other trickling filters or activated sludge processes. The roughing filter is placed in the flow pattern just after the primary clarifiers, but before the downstream biological processes.

Roughing filters could see organic loading rates of 300 pounds or more of BOD per day per 1,000 cubic feet of media, whereas a standard-rate trickling filter might see up to 25 pounds of BOD per day per 1,000 cubic feet of media.

Water Treatment Sample Question: 

In the lime softening process, when calcium hydroxide is added to water to be softened and the pH is increasing, which chemical reaction will occur?

A) Alkalinity converts from the bicarbonate form to the carbonate form, and then calcium can be precipitated as calcium carbonate.

B) Hardness is converted to alkalinity becoming permanent hardness.

C) Hardness will convert to bicarbonate hardness, and non-carbonate hardness will precipitate.

D) Alkalinity converts from the bicarbonate form to carbon dioxide, and magnesium precipitates as magnesium carbonate.

Answer: The answer is A, alkalinity converts from the bicarbonate form to the carbonate form, and then calcium can be precipitated as calcium carbonate. As the pH of the raw water increases with the addition of lime (calcium hydroxide), the alkalinity begins to convert from bicarbonate to carbonate form. At around a pH value of 10.2, the calcium found in the raw water that caused the initial hardness bonds with the excess carbonate to become a calcium carbonate precipitate (CaCO3-). The heavier-than-water precipitate will settle to the bottom of the lime softening unit for removal by sludge scrapers or other removal mechanisms.

Alkalinity and hardness are two distinctly different parameters and tests, but have some things in common with each other. Calcium and magnesium that make up hardness also contribute to elevated alkalinity of the water. Alkalinity is sometimes described as the ability of water to resist a change in pH, or as a buffer against acids that cause a decrease in pH. Calcium and magnesium found in the earth’s crust in the form of calcium and/or magnesium bicarbonate readily dissolve in water, giving the groundwater naturally occurring alkalinity and hardness.

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.


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.