Exam Study Guide: Influent Wastewater Alkalinity; and Ion-Exchange Softening Plant Operation

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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 Centrifugal Pumps; and Cloudy Source Water. This time, you can test your knowledge about influent wastewater alkalinity; and ion-exchange treatment plant operation.

Wastewater Treatment Sample Question:

Why should the operator of a biological nitrogen-removal process be aware of the influent wastewater alkalinity?

A. To be able to calculate the alkalinity needed for complete denitrification
B. To ensure there will be enough alkalinity to completely denitrify
C. To ensure there will be enough alkalinity for nitrification
D. To ensure there will be sufficient alkalinity to aid in biological phosphorus removal

Answer: The answer is C, to ensure there will be enough alkalinity for nitrification. For complete nitrification, about 7.14 mg of alkalinity, as CaCO3 per liter are consumed per mg/L of ammonia-N oxidized to nitrate. If the influent waste stream contained 38 mg/L of ammonia-N, we could then assume we would need roughly 271 mg of alkalinity per liter to accomplish complete nitrification (38 mg/L NH3-N x 7.14 mg/L alkalinity for each mg of NH3-N = 271.35 mg of alkalinity).

Remember that other biological processes also require the presence of alkalinity to ensure a stable liquid pH, and downstream processes like disinfection with gaseous chlorine tends to consume alkalinity and lower the effluent pH. 

Nitrification bacteria require bicarbonate or carbon dioxide as their carbon source for metabolism, and will also produce some acid during nitrification. These two conditions consume available alkalinity from the liquid matrix, and could cause the pH to decrease well below 6.0. Process experts recommend about 80 mg/L of alkalinity remain in the effluent of biological treatment fluids to ensure the pH remains at or near a balanced condition of 7.0.

Water Treatment Sample Question:

Which of the following choices is the correct order of operation in an ion-exchange softening water treatment plant?

A. Service, brine, backwash, rinse
B. Service, backwash, rinse, brine
C. Service, backwash, brine, rinse
D. Rinse, brine, backwash, service

Answer: The answer is C, service, backwash, brine, rinse. Ion-exchange softeners can remove many different contaminants from a drinking water source, including calcium and magnesium. Various specialized ion-exchange resins can be used to remove nitrate, excess fluoride and TDS.

Service is when the actual work is being performed by the softener. This will occur until the media has removed the maximum amount of the target contaminant. Once the media is exhausted, the contaminants within the source water will begin to pass through the media. 

The unit is then taken out of service and backwashed. Backwashing expands the media bed, and loosens trapped particles and debris which flow out of the unit in the backwash water flow.

Brining the media is similar to regenerating the media. The brine contains the ions (in many cases sodium for salt softeners) that encourage the remaining contaminants to release from the media and be replaced with the ions in the brine.

Rinsing is the final step, where contaminant-free potable water is used to rinse away the released ions suspended in the brine and rinse away excess brine solution. Once rinsing is complete, the unit goes back into service.

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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