Exam Study Guide: Sludge Blanket Depth; and Aluminum Sulfate as a Coagulant

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 Solids Retention Time; and Oxidizing Agents. This time, you can test your knowledge about sludge blanket depth, and aluminum sulfate as a coagulant.

Wastewater Treatment Sample Question:

It is  generally accepted that the sludge blanket depth in secondary clarifiers should not exceed 2 feet or 0.6 meters. By allowing a deeper sludge blanket, what undesirable outcome may occur?

A. Nitrification may occur in the secondary clarifier
B. Deeper blankets inhibit settleability of the sludge
C. Premature digestion will occur in the secondary clarifier
D. Solids may be washed out and into the effluent flow

Answer: The answer to the question is D. Monitoring the sludge blanket is a key process control parameter. The rule of thumb is never allow the sludge blanket to reach more than 25 percent of the clarifier sidewall. By allowing a deeper sludge blanket, the operator may encounter troubles with blanket washout due to excess flows (heavy rain event) or denitrification (sludge too long in the clarifier). Both of these conditions could affect your effluent solids readings and violate your NPDES limits.

According to the WEF OM-9 manual, “blanket depths should be kept below 0.6 m (2 ft.) whenever possible. Storing solids and maintaining deep blankets in the secondary clarifiers increases the likelihood that solids will be washed out of the clarifier and into the final clarifier.”

Water Treatment Sample Question

Feeding aluminum sulfate as a coagulant will cause the raw water alkalinity to:

A. Decrease
B. Increase
C. Stay the same
D. Dissolve

Answer: The correct answer is A. Primary coagulation is commonly achieved by dosing the water with metallic salts such as aluminum sulfate, ferric sulfate or ferric chloride. The chemical and physical reaction that takes place between the cationic metallic salts, alkalinity and suspended solids happens very quickly, which results in micro-sized sticky particles called floc. Floc particles gain mass and become heavy enough to settle out during sedimentation. 

If the source water is too low in alkalinity to produce aluminum hydroxide precipitation, an operator may choose to add lime to enhance the floc-forming ability of the aluminum sulfate. However, it is important to note that aluminum sulfate has specific pH ranges for optimal performance and lime will raise a water's pH.


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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