Exam Study Guide: Luxury Uptake of Phosphorus; and Chlorine Dosing Math

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: Luxury Uptake of Phosphorus; and Chlorine Dosing Math

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 Aeration Tank Nitrification and Consumer Complaints. This week, you can test your knowledge the luxury uptake method of phosphorus removal with regards to phosphate accumulating organisms, and solve a math problem related to proper chlorine dosing in a water system. Take a look at the multiple-choice sample questions and answer explanations below.

Sample Question No. 1:

Under which conditions of the luxury uptake method of phosphorus removal do the phosphate accumulating organisms (PAOs) restore internal phosphate granules by taking in previously released phosphorus along with excess phosphorus found in the mixed liquor?

A) Aerobic conditions

B) Anaerobic conditions

C) Autotrophic conditions

D) Thermophilic conditions

Answer: The answer is A, with aerobic conditions in a biological phosphorus removal treatment facility using the luxury uptake method, organisms known as PAOs will have a selective advantage over non-PAOs to consume influent organic matter under anaerobic conditions in anaerobic, or fermentation basins. When they move into the aerated tanks with the MLSS flow and dissolved oxygen is available, they use the stored organic matter for metabolism and reproduction, but must first restore the phosphorus and other lost electrolytes. They recover the phosphorus they released plus excess phosphorus from the raw wastewater in the aeration tanks under aerobic conditions.

Sample Question No. 2: 

A water treatment plant uses 100 percent gaseous chlorine as its disinfectant from one-ton containers. How many pounds of chlorine would be used per day if the plant flow rate is 23.5 million gallons per day and the operator is meeting a chlorine dosage of 3.3 ppm?

A) 7.75 pounds/day

B) 77.55 pounds/day

C) 646.77 pounds/day

D) 6,467.7 pounds/day

Answer: The answer is C, 646.77 pounds/day. Below are the calculation steps using the following formula adapted from State of Florida Drinking Water Operator and Water Distribution System Operator Math Formula Sheets: feed rate in pounds/day = dosage in mg/L x flow in mgd x 8.34 pounds/gal over the purity of the chemical as a decimal.

To solve this math question, multiply as shown in the formula above. The pounds of chlorine per day = 3.3 mg/L dosage x 23.5 mgd x 8.34 pounds/gallon, which shows us 646.77 pounds per day of chlorine.

Since the question states that the chemical used is 100 percent chlorine, there is no need to divide this answer by 100, since it is already pure chemical. However, you might come across a question where the chemical is not 100 percent pure, as in the case of using a weaker version of chlorine like High-Test Hypochlorite (HTH) — a dry, granular chemical. HTH can be found in various strengths and 65 percent is a common trade strength used in water and wastewater treatment plants. If a math word problem states that the chemical used is 65 percent strength, you would divide that value as a decimal into the initial chemical pounds per day answer as 100 percent strength. See the example below:

Lbs/day of 65 percent available chlorine = pounds per day as 100 percent strength over trade strength as a decimal.

So, the pounds/day of 65 percent available chlorine = 646.77 pounds/day over 0.65, which gives us figure of 995 pounds/day of 65 percent HTH.

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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