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 Pump Hydraulics; and Calculating Non-Carbonate Hardness. This time, you can test your knowledge about Imhoff tank maintenance, and water softening math.
Wastewater Treatment Sample Question
In a typical Imhoff tank, there is slot in the settling compartment. How is this slot cleaned in the event of a plugged condition or normal housekeeping?
A. Chaining
B. Reverse flow
C. Automatic backwash
D. High-pressure hose washdown
Answer: The answer to the question is A. Imhoff tanks are an older form of waste treatment — many operators may never see or operate one — but knowledge of how they work is beneficial to the wastewater operator. In some states, it's required for certification.
The Imhoff tank is a two-part process. The upper area is for sedimentation and the bottom is for digestion. The solids will settle and fall through the slot and begin to digest in the bottom. Occasionally, the slot will plug and the recommended course of action is to drag a chain along the slot. This process is known as chaining.
Water Treatment Sample Question
A 5-foot diameter ion-exchange softener contains forty two inches of exchange resin, which is rated to remove 20,000 grains of hardness per cubic foot. How may gallons of water can be softened before regeneration is necessary, if the raw water hardness is 400 mg/L?
A. 50 gallons
B. 3,434 gallons
C. 58,735 gallons
D. 116,465 gallons
Answer: The correct answer is C. Ion exchange softening — sometimes referred to as cation exchange softening — produces a zero hardness effluent until all of the attached sodium ions on the resin beads have been exchanged with calcium or magnesium ions. When the exchange resin only contains calcium and magnesium ions, a regeneration is necessary. The regeneration process is typically automated and operators only monitor softener effluent hardness for process control purposes.
About the authors: Rick Lallish is the Water Pollution Control program director at the Environmental Resources Training Center (ERTC) of Southern Illinois University Edwardsville. He provides training for entry-level operators in the wastewater field and operators throughout the state looking to further their education. Lallish was also named the 2017 Illinois Operator of the Year and 2018 president of the Illinois Association of Water Pollution Control Operators.
Drew Hoelscher is the program director of drinking water operations at the Environmental Resources Training Center in Edwardsville, Illinois.
