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 Sampling Rules; and Risk and Resilience Assessments. This time you can test your knowledge about pumping heavy sludges; and slow sand gravity filters.

Wastewater Treatment Sample Question

Which of the following pumps would be the least affective for pumping heavy sludges?

A.  Progressing cavity
B. Rotary lobe
C. Centrifugal
D. Diaphragm

Answer: The answer to this question is C. Progressive cavity pumps are well suited for pumping sludges, especially for dewatering or thickening processes. Of the choices, a centrifugal pump is least suited for this operation.

Understanding the different types of pumps and what types of flows they can pump are important for the operator’s knowledge of plant operations. Pumps and pumping questions are included in most states certification examinations.

Water Treatment Sample Question

How are slow sand gravity filters backwashed?

A. Slow sand gravity filters are not backwashed.
B. Spraying high-pressure water over the entire surface area until all the sand appears clean.
C. Reversing the flow of water through the filter at a slow and steady rate until the sand appears clean.
D. Reversing the flow of water through the filter at a fast and steady rate until the sand appears clean.

Answer: The correct answer is A. Slow sand gravity filters were the first type of filters used in the production of potable water. Through engineering and technology advancements, slow sand gravity filters were replaced with conventional rapid sand filters and high-rate multimedia gravity filters. However, slow sand gravity filters are still sometimes used at water purification plants to filter backwash water before it is discharged to a receiving stream. 

As any filter, it eventually becomes exhausted and an operator will need to clean the filter by physically removing the top 1 to 2 inches of sand (schmutzdecke). After several cleaning cycles, the operator replenishes the sand to the original depth.

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.