Exam Study Guide: Primary Clarifiers; and Microsand Processes

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 Barscreen Maintenance, and Surface Water Characteristics. This time, you can test your knowledge about primary clarifiers, and microsand processes.

Wastewater Treatment Sample Question:

What are two types of solids effectively removed by primary clarifiers?

A) Settleable and floatable

B) Floatable and dissolved

C) Dissolved and colloidal

D) Suspended and organic

Answer: Primary clarifiers are very effective at removing floatable and settleable solids from the wastewater flow stream, so the answer is A, settleable and floatable. Floatable solids include objects and debris that are positively buoyant, meaning they float. Fats, oil and grease are included in this meaning, and primary clarifiers are well adapted for these substances. Also included in floatable solids are paper products; plastic wrappers and products; fruit and vegetable seeds; soaps; and detergents. Much of the floatable material removed by a primary clarifier is inorganic in nature.

Settleable material is negatively buoyant. In other words, it's denser than water. Grit, sand, heavy organic solids, coffee grounds and metal objects are examples of settleable solids. Primary clarifier settleable solids removal efficiency typically ranges from 95 percent to 99 percent removal.

Primary clarification not only helps reduce the organic loading on downstream biological treatment processes like trickling filters, rotating biological contactors and activated sludge processes, but also helps smooth out the hydraulic loading.

Primary clarifiers do not remove dissolved or colloidal solids very well as these solids do not readily give in to the physical force of gravity. In some cases where additional removal of colloidal solids are needed to reduce the loading on downstream processes, chemical coagulants can be used to improve the operation of a primary clarifier. Chemicals like ferric chloride, ferrous chloride, aluminum sulfate and/or polymers can be added to neutralize the charge of the colloidal solids and enhance the primary clarifier’s ability to remove them, thus the term Chemically Enhanced Primary Clarification.

Water Treatment Sample Question:

What is a process that uses microsand added to the flocculation stage to encourage floc settling called?

A) Slow microsand filtration

B) Ballasted flocculation

C) Weighted TDS capture

D) Enhanced sedimentation

Answer: The answer is B, ballasted flocculation. Ballasted flocculation processes are sometimes referred to as high-rate clarifiers due to the higher hydraulic loading rates and decreased detention times required for effective settling. Ballasted flocculation uses a microsand addition to the influent water of the flocculation stage to encourage faster settling of the flocs. Microsand essentially adds weight to the floc which increases its negative buoyancy causing it to settle more rapidly.

Inclined tube settlers are often used near the outlet weirs of the clarifying unit to encourage additional solids removal from the effluent. Several manufacturers offer ballasted flocculation units for both drinking water and wastewater treatment, and these might be found where high amounts of water must be treated in a small land mass. Basically they can treat a lot of water in a small footprint.


About the author: Ron Trygar is the senior training specialist for water and wastewater programs at the University of Florida's TREEO Center. Previously, he was the wastewater process control specialist at Hillsborough County Public Utilities in Tampa, Florida. He has worked in the wastewater industry for more than 30 years in a variety of locations and positions. Trygar became a Certified Environmental Trainer (CET) in 1998 and has since provided training for associations and regulatory agencies such as Florida Department of Environmental Protection (FDEP); Florida Water and Pollution Control Operators Association Short Schools; USABlueBook; Florida Water Environment Association sponsored training events; and local school environmental programs. Working alongside the FDEP Northeast District, Trygar helped begin the Florida Rural Water Association and FDEP joint operator certification review classes that are still given around the state today. He holds a Florida Class A wastewater treatment operator’s license and a Florida Class B drinking water operator’s license.



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