Exam Study Guide: Influent Organic Matter Removal; and Chemical Feed Rates

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 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 UV Disinfection; and Calcium Saturation. This time, you can test your knowledge about influent organic matter removal, and chemical feed rates.

Wastewater Treatment Sample Question:

How much influent organic matter is expected to be removed in primary treatment using a primary clarifier?

A. 10 to 20 percent

B. 20 to 35 percent

C. 35 to 50 percent

D. 60 to 75 percent

Answer: The answer to this question is B. Primary clarifiers are designed primarily for the removal of solids. You can expect 60 to 75 percent removal of the influent solids content. An operator should be aware that the removal of some organic matter is a coincidental benefit. The primary clarifier also removes approximately 20 to 35 percent of the BOD. This is important to remember when factoring the BOD load on the secondary treatment process.

According to the 2018 WEF OM-9 manual, “A primary clarifier can be expected to remove 20 and 35 percent of the influent organic matter and as much as 60 to 75 percent of the influent suspended solids.

Water Treatment Sample Question:

An operator is feeding 12 percent sodium hypochlorite with a specific gravity of 1.20 to a 1 mgd rate of flow. How many milliliters per minute should the chemical feed pump deliver for a 6 milligram per liter dosage?

A. 41 milliliters per minute

B. 110 milliliters per minute

C. 50 milliliters per minute

D. 131 milliliters per minute

Answer: The correct answer is B. It is very common for water treatment facilities to operate by using sophisticated computer programs that can adjust chemical feed rates based on flow changes and/or dosage changes. However, it is still an important practice to be able to calculate chemical dosages and chemical feed rates manually. This particular problem gives you the concentration, specific gravity, rate of flow and the desired dosage to calculate the feed rate in milliliters per minute.

First, convert the concentration of sodium hypochlorite in percent to milligrams per liter:

Then, calculate how many gallons of 12 percent sodium hypochlorite are required per day to provide a 6 milligram per liter dosage:

Finally, convert the feed rate gallons per day to milliliters per minute:

(41.67 gpm)(3785 mL/gal) / 1440 minutes per day = 109.53 milliliters per minute

About the authors: Rick Lallish is the Water Pollution Control program director at the Environmental Resources Training Center 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 also was 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 (ERTC) in Edwardsville, Illinois.


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