Exam Study Guide: Soluble Organic Contaminant Removal; and the Langelier Index

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Exam Study Guide: Soluble Organic Contaminant Removal; and the Langelier Index

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. We covered a set of wastewater and drinking water treatment questions last time on the topics of Power Outage Procedure, and Ion Exchange Softening. This time, you can test your knowledge about removing soluble organic contaminants, and the Langelier Index.

Wastewater Treatment Sample Question:

The use of which chemical listed below can be a very effective method of removing soluble organic contaminants if used in conjunction with coagulation-sedimentation, filtration and disinfection?

A) Potassium permanganate

B) Anionic polymer

C) Sodium hydroxide

D) Activated carbon

Answer: The answer is D, activated carbon. If used along with other processes like activated sludge, enhanced primary treatment, filtration and final disinfection, activated carbon can be used to effectively remove soluble organic material from the wastewater being treated. Powdered activated carbon, also known as PAC, can be added to the wastewater influent where it adsorbs organic contaminants. As the PAC adsorbs the dissolved organic material, it settles in clarifiers along with other solids and is removed. The use of PAC requires a constant feed of the chemical as a slurry. Be careful though, powdered activated carbon dust can be explosive and must be handled with care and caution.

Granular activated carbon (GAC) can be used in the filtering process as a layer of the filter media or in a separate treatment contact unit where soluble organic material is adsorbed and removed from the water flowing through the unit. As the GAC continues to adsorb organics, it becomes exhausted and measurable COD begins to rise in the effluent. Once the service life of the GAC is reached, the GAC can be removed from the filter unit as a slurry, dried and regenerated by heating the GAC in a furnace. The organic material is released during the heating process and further destroyed in an afterburner unit. The GAC can now be reused in the treatment process.

Water Treatment Sample Question: 

The Langelier Index (LI), sometimes referred to as the Langelier Saturation Index (LSI), is a valuable tool to determine the corrosivity of potable water in a distribution system. The formula for Langelier Index is: Langelier Index = pH minus pHs. What factors are used to determine the pHs?

A) Temperature, alkalinity, dissolved oxygen, free chlorine residual

B) Temperature, calcium hardness, TDS, alkalinity

C) Chlorine demand, TDS, dissolved oxygen, total hardness

D) Temperature, alkalinity, velocity, total hardness

Answer: The answer is B, temperature, calcium hardness, TDS, alkalinity. The calculation of the pHs portion of Langelier Index is based off T.E. Larson’s method of determining calcium and bicarbonate concentrations. A table of values used in determining the pHs was developed and include the water temperature, the total dissolved solids (TDS) in milligrams per liter, the logarithmic value (Log10) of the calcium hardness and alkalinity, both as milligrams per liter of calcium carbonate (CaCO3).

These factors play a part in how saturated the water is with respect to its ability to form a protective scale on the water distribution mains. Once the pHs is determined, the calculation is performed to produce a number that is either negative, positive or neutral. A positive Langelier Index — for example +0.5 — indicates the water is slightly scale forming (non-corrosive). A negative Langelier Index — such as -0.5 — would indicate the water is corrosive, or aggressive. A neutral result (neither positive nor negative) would indicate stable water that will neither form a scale inside the water lines, nor be corrosive to the metal materials within. The pH portion of the LI calculation is just as it seems — the water’s pH value. 

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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