Exam Study Guide: UV Disinfection; and Calcium Saturation

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 Exam Study Guide: Disinfection Byproducts; and Waste Brine Disposal. This time, you can test your knowledge about UV disinfection and calcium saturation.

Wastewater Treatment Sample Question:

When using ultraviolet light (UV) disinfection, which statement below regarding the effluent ultraviolet light transmittance (UVT) and turbidity would provide the most efficient inactivation of the pathogen?

A. The lower the turbidity and higher the UVT, the better the inactivation

B. The higher the turbidity and lower UVT, the better the inactivation

C. The UVT and turbidity must both be low for effective inactivation

D. The turbidity and UVT must both be high for effective inactivation

Answer: The answer is A, The lower the turbidity and higher the UVT, the better the inactivation. Effluent suspended solids tend to create a cloudiness to the water which we generally call turbidity. When the effluent water is turbid with suspended material, bacteria can live and grow within this particle, which can also shield the pathogenic organisms (some bacteria, cysts, spores and viruses) from the disinfectant.

The cleaner and clearer the effluent water is, the better the UV light can pass through the water directly into the pathogen, which either kills the organism, or prevents it from reproducing by mutating its RNA strand. A very clear effluent with low turbidity will then have a high UV light transmittance (UVT). Conversely, if the effluent contains many suspended particles and the turbidity is high, the UV light does not transmit well through the water, causing a low UVT.

Water Treatment Sample Question:

Which analysis listed below uses a small amount (about 0.3 g) of powdered calcium carbonate dissolved into a water sample to determine the amount of calcium saturation?

A. Langelier Saturation Index

B. Ryznar Index

C. Calcium hardness test

D. Marble test

Answer: The answer is D, the marble test. The marble test is a simple method to determine the saturation of calcium carbonate in a water sample and can be used to determine the potential corrosivity of the water being tested. Marble is a hardened form of calcium carbonate.

A sample of the water is collected and analyzed for calcium hardness, total alkalinity and pH. This sample and its results are known as the initial water sample.  A small amount, about 0.3 grams of calcium carbonate are added to the sample and stirred for 3 hours. Reagent grade calcium carbonate is essentially crushed chalk and is a white powder similar to lime. 

After stirring, the calcium is allowed to settle. The supernate above the settled calcium sludge is drawn off and filtered. The filtrate is then tested for calcium hardness, total alkalinity and pH. This sample is called the finished water sample.

If the initial water (before calcium saturation) was lacking the calcium, the water would allow the excess calcium to dissolve into it and become saturated. If the calcium hardness and total alkalinity of the finished water has increased, then it is assumed the initial water sample was not saturated and therefore corrosive.

If the analysis indicates the calcium hardness and total alkalinity of the finished water sample have decreased, the initial water was already saturated with calcium carbonate and has lost hardness. The initial water is then considered scale forming.

It is generally more acceptable to create a slightly scale forming finished water to decrease the likelihood that the water will be corrosive to the water distribution system piping and homeowners service lines and taps.


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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