Exam Study Guide: RBC Unit Operation; and Water Hardness Testing

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.

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 Wastewater Filters and Calcium Hydroxide Reactions. This week, you can test your knowledge about RBC units and water hardness testing.

Wastewater Treatment Sample Question:

Which of the following choices represents normal, proper operating conditions of a rotating biological contactor (RBC) unit?

A) A uniform, shaggy looking brown-to-gray growth with no algae or bare spots.

B) A uniform white growth with a distinct sulfur odor.

C) A heavy, shaggy dark brown-to-black growth with slight septic odors.

D) A thick green appearance, mostly on one half of the RBC disc.

Answer: The answer is A, a uniform, shaggy appearing brown-to-gray growth with no algae or bare spots. The biomass found on the RBC discs is made of aerobic and facultative bacteria that form sticky slimes that attach to the discs as they rotate through the applied waste stream. The growth tends to take on a shaggy, brownish/gray appearance with a non-offensive, musty odor. Since most RBC units are covered, algae growth is limited with the lack of sunlight.

The attached growth found on an RBC should not be white in color with sulfur odors, as this indicates a septic or high organic loading condition. Black appearance also is a possible organic overload condition, or could indicate an influent solids overload from other unit process such as digester supernatant or solids washing out of primary clarifiers.

The biomass covering the RBC unit should be uniform and not heavier on one half. If this occurs, the RBC drum will rotate unevenly, causing poor BOD removal efficiency and excessive wear on bearings and shafts.

Water Treatment Sample Question: 

Which is a true statement regarding hardness testing?

A) Hardness is defined as the total concentration of the iron and manganese ions.

B) Bromcresol green is added until a pale straw color is reached.

C) DPD indicator is added until a pink color appears.

D) EDTA is used as a titrant to an endpoint that is a pure blue color.

Answer: The answer is D, EDTA is used as a titrant to an endpoint that is a pure blue color. Ethylenediaminetetraacetic acid, otherwise known as EDTA, is used in the hardness test as a titrant that causes a sample to change color from a red/pink color to a pure blue color. The hardness test requires the sample to be pre-treated with other reagents including potassium hydroxide and sulfuric acid solutions. Once the pure blue color is reached, the analyst stops titrating with EDTA and measures the amount of EDTA in milliliters to reach the endpoint. A calculation using the amount of EDTA, the sample size and the normality of the EDTA is used to determine the hardness of the water as calcium carbonate, CaCO3.

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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