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What should be the setting on the liquid alum chemical feeder in milliliters per minute when the flow is 2.5 MGD?
Liquid alum delivered to a water treatment plant contains 485 milligrams of alum per milliliter of liquid solution with specific gravity of 1.325. Jar testes indicate that the best alum dose is 9 mg/L. There is 3.785 in one gallon.



A. 74 mL/min
B. 81 mL/min
C. 92 mL/min
D. 103 mL/min
E. 118 mL/min

This question is part of CA Water Treatment Plant Operator Quiz Chapter 4
Asked by PaulMJr, Last updated: Feb 19, 2020

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4 Answers

RagweedFarmer

RagweedFarmer

Answered Feb 25, 2017

92mL/min, 92.0297
There are two difficult aspects to this problem, specific gravity, and the percentage ofalum in the solution.
Math Constant: Specific gravity of water = 1
Math Constant: 1 gram of water = 1mL
Math Constant: There are 1000 grams of water in one liter, and one milligram is one thousandth of a gram, so one milligram per liter (mg/L) is one millionth of a liter
Math Constant: 1,440 minutes in one day
Math Constant: 1 gallon = 3.785 liters
Step 1: 2.5mgd x 3.785 = 9.4625 million liters, 1 milligram is one millionth of a liter so 9.4625 liters is 1mg/L of 9.4625 million liters
Step 2: We need 9mg/L so, 9.4625 x 9 = 85.1625 liters of solution
Step 3: The liquid alum contains only 485 milligrams per milliliter. This is like saying it contains 485 parts alumout of 1000, or that it contains 48.5% alum. Because our solution is less than 100% we need to divide our needed amount by the percentage of the solution. So, 85.1625 / 48.5% = 175.59278 liters of a 48.5% solution needed.
Step 4: The liquid alum has a higher specific gravity (heavier per gallon) than water so we need less of it. 175.59278 / 1.325 = 132.52285 liters ofliquid alum per day needed
Step 5: 1,440 minutes in one day so 132.52285 / 1440 = 0.0920297 liters per minute needed
Step 6: 1,000 milliliters in 1 liter, so 0.0920297 x 1000 = 92.0297 mL/min
I hope that helped :)
 

irishdave

485 mg of alum per milliliter? Or per liter?

2. A dose of 9 mg/L would be 22.5 GPD, or 85.16 liters per day. Which would appear to lead to none of these answers.

3. There is [are] 3.785 what in one gallon?

Also, "testes"?
 

irishdave

485 mg of alum per milliliter? Or per liter?

2. A dose of 9 mg/L would be 22.5 GPD, or 85.16 liters per day. Which would appear to lead to none of these answers.

3. There is [are] 3.785 what in one gallon?

Also, "testes"?
 

John Smith

John Smith

Answered Oct 26, 2016

92 mL/min
 

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