Exam Study Guide

WASTEWATER By Rick Lallish

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:  B. Primary clarifiers are designed primarily for solids removal. You can expect 60 to 75 percent removal of the influent solids content. Operators 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 Water Environment Federation OM-9 manual, “a primary clarifier can be expected to remove 20 to 35 percent of the influent organic matter and as much as 60 to 75 percent of the influent suspended solids.”

DRINKING WATER By Drew Hoelscher

A common chemical compound fed to oxidize iron and/or manganese:

A. Ca(OH)2

B. Al2(SO4)3

C. KMnO4


Answer: C. The chemical symbol KMnO4 represents potassium permanganate, a strong oxidizing agent that can oxidize soluble iron and manganese into insoluble iron oxide and manganese dioxide. To ensure proper feed rates, perform a series of jar tests to determine the proper dose. Dosing the water with too much KMnO4 can cause the water to turn pink, and dosing with too little KMnO4 will allow soluble iron and manganese to pass through the treatment plant, causing discolored water complaints from customers. 

About the authors

Rick Lallish is water pollution control program director and Drew Hoelscher is program director of drinking water operations at the Environmental Resources Training Center of Southern Illinois University Edwardsville. 


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