Exam Study Guide


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

A. Floatable and settleable

B. Floatable and dissolved

C. Dissolved and colloidal

D. Suspended and organic

Answer:  A. Primary clarifiers are very effective at removing floatable and settleable solids. Floatables include objects and debris that are buoyant. Fats, oil and grease (also called FOG) are included, 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. Settleable material is negatively buoyant (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 to 99 percent. Besides helping reduce the organic loading on downstream biological processes like trickling filters, rotating biological contactors, and activated sludge, primary clarification helps smooth out the hydraulic loading.

Primary clarifiers do not readily remove dissolved or colloidal solids, as these materials do not give in to gravity. Where additional removal of colloidal solids is needed to reduce loading downstream, chemical coagulants can be used to improve primary clarifier operation. Chemicals like ferric chloride, ferrous chloride, aluminum sulfate and polymers can be added to neutralize the charge of the colloidal solids and enhance settling in the clarifier — hence the term “chemically enhanced primary clarification.”


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

A. Slow microsand filtration

B. Ballasted flocculation

C. Weighted TDS capture

D. Enhanced sedimentation

Answer: B. Ballasted flocculation processes are sometimes referred to as high-rate clarifiers due to the higher hydraulic loading rates and shorter 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, increasing its negative buoyancy and 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. Several manufacturers offer ballasted flocculation units for drinking water and wastewater treatment. They are typically found where high amounts of water must be treated in a small footprint.

About the author

Ron Trygar, a certified environmental trainer, is the senior training specialist for water and wastewater programs at the University of Florida TREEO Center. He has worked in the wastewater industry for more than 30 years and holds Class A Wastewater Treatment Operator and Class B Drinking Water Operator licenses in Florida. 


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