Exam Study Guide: Hydraulic Loading Math; and RO Treatment 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: Hydraulic Loading Math; and RO Treatment Chemicals

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 last time on the topics of TF/AS Combination Treatment, and Acid Dilution Preparation. This time, you can test your knowledge about calculating hydraulic loading, and RO treatment chemicals.

Wastewater Treatment Sample Question:

A trickling filter plant has an influent flow rate of 7.5 mgd and uses a recirculation rate of 3,900 gpm. The trickling filter has a diameter of 176 feet and a media depth of 6.5 feet. The influent BOD is 235 mg/L. Calculate the hydraulic loading as gpd per square foot of the trickling filter.

A) 0.002 gpd per square foot

B) 83 gpd per square foot

C) 308.4 gpd per square foot

D) 539.4 gpd per square foot

Answer: The answer is D, 539.4 gpd per square foot. As I have mentioned previously in this forum, be careful when calculating equations in state exam settings. Be sure to use the formula sheets provided and follow the formula exactly. Be aware of hidden hazards designed to trip you up! In the question provided here, there are several pieces of useless data, and if you try to incorporate them into your calculations, you will get an incorrect answer. However, a close answer choice might be provided as one of the three distractors. Did you spot the extra information in the question? If you said the media depth and the influent BOD, you are correct!

The formula shown above indicates that the flow rate as gallons per day will be divided by the surface area of the filter as square feet. Please note that this is the total gallons per day, which includes the influent flow and the recirculation flow. You will need to add the two flows together to get the total flow applied per day. Since the formula requires the units of flow to be in gpd, convert all flow units first. 7.5 mgd x 1,000,000 = 7,500,000 gpd. 3,900 gpm x 1,440 minutes per day = 5,616,000 gpd. Add the two together to get 13,116,000 gpd. Next, figure the surface area of the filter: the area in square feet = 3.14 R2 or 0.785 D2. The area in square feet = 3.14 x 88 x 88 (remember to divide the diameter by 2 to get radius), or the area in square feet = 0.785 x 176 x 176. The area in square feet is 24,316.16 Now we can complete the calculation. Hydraulic loading in gpd per square foot = 13,116,000 gpd divided by 24,316.16 square feet. The hydraulic loading in gpd per square foot is 539.4. 

Water Treatment Sample Question: 

Which chemical can be added to the feedwater of a reverse osmosis (RO) treatment unit to reduce precipitation of calcium sulfate on the membrane?

A) Calcium carbonate

B) Potassium permanganate

C) Sodium chloride

D) Sodium hexametaphosphate

Answer: The answer is D, sodium hexametaphosphate. The addition of sodium hexametaphosphate (SHMP) is the most common chemical used to prevent calcium sulfate from becoming insoluble and precipitating onto the RO membrane. Once the SHMP is added, it inhibits the crystal formation of calcium carbonate and calcium sulfate, keeping them in a dissolved state. Since they cannot pass through the membrane they remain in the liquid being carried out of the membrane vessel as concentrate, or reject water. By keeping these inorganic minerals in solution, the RO unit can optimally produce permeate.

If the calcium sulfate were to precipitate onto the RO membrane, the scale that forms prevents the passage of water molecules through the membrane, called flux rate. As the flux rate declines, the production of permeate is reduced and element cleaning becomes necessary. To clean off inorganic impurities like the calcium sulfate scale, citric acid is commonly used as part of a clean-in-place procedure. This means that the RO unit is taken out of service and citric acid is pumped through the RO unit to dissolve the built up calcium sulfate scale. 

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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