Exam Study Guide: Volatile Acid to Alkalinity Ratio; and Odor-Reducing Chemicals

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.
Exam Study Guide: Volatile Acid to Alkalinity Ratio; and Odor-Reducing Chemicals

Welcome back to TPO magazine's new and improved Exam Study Guide Series, which offers a pair of water/wastewater study questions each week with in-depth explanations of the answers. We covered a set of wastewater and drinking water treatment questions last week on the topics of Luxury Uptake of Phosphorus; and Chlorine Dosing Math. This week, you can test your knowledge about volatile acid test results, and odor-reducing chemicals. Take a look at the multiple-choice sample questions and answer explanations below.

Sample Question No. 1:

If the alkalinity test result of the anaerobic digester biosolids is 5,000 mg/L, what should the volatile acid test result be when the digester is running at an optimum volatile acid to alkalinity ratio?

A) 5 mg/L

B) 50 mg/L

C) 500 mg/L

D) 5,000 mg/L

Answer: The answer is C, 500 mg/L. A properly operating anaerobic digester will normally have a volatile acid to alkalinity (VA/Alk) ratio of 0.1 to 1.0 (0.1:1). This is equivalent to one part of volatile acid to 10 parts of alkalinity. If the alkalinity is 5,000 mg/L, the volatile acid value should be one tenth of this, or 500 mg/L. The organisms responsible for methane formation produce natural alkalinity and work best in a slightly alkaline environment while they consume the volatile fatty acids produced during the acid formation stage.

Since the successful operation of an anaerobic digester relies on the formation of methane to be used in boilers for heating the digesters, it is essential to maintain the best conditions for methane formers. If too much raw feed sludge is pumped into a primary digester, the amount of volatile acid formation could overcome the available alkalinity used as a buffer and the methane formers could slow or stop functioning. If the VA/Alk ratio rises to 0.5:1 or greater, care must be taken to allow the methane formers to recover and produce alkalinity and methane to keep the boilers fired and the digesters at optimum temperature.

Sample Question No. 2: 

The addition of what odor-reducing chemical to a surface water source must be followed by filtration to remove turbidity?

A) Granular activated carbon (GAC)

B) Powdered activated carbon (PAC)

C) Sodium hypochlorite

D) Magnesium hydroxide

Answer: The answer is B, powdered activated carbon (PAC). Powdered activated carbon is commonly added to the raw surface water source water feed to a treatment plant to adsorb taste and odor contaminants, natural organic compounds and synthetic organic chemicals from the water. PAC is usually mixed into a slurry first, then fed into the raw water feed or during the treatment process. Since PAC will turn the water a dark color, the turbidity created by the PAC must be filtered out before the treated water enters the clear well.

PAC must be handled with care as high concentrations of PAC dust in air can be explosive, therefore static spark and open flame must be prevented in and around PAC handling facilities. Wet PAC can also deplete oxygen from air creating an oxygen deficient environment.

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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