Grades: 9-12 (Science, Chemistry and Biology)
Time: 60 minutes
Space Requirement: Classroom
Methodology: Independent Learning, Group Discussion and/or Class Discussion
No extra materials needed
Objectives: Students will learn about the sources of ammonia in ground water, the consequences of high ammonia concentrations in water supplies, and the manner in which these problems are usually dealt with. Students will learn about biological filtering as a way to use bacteria to remove ammonia from water. Students will learn about the chemical reaction between chlorine and ammonia which results in the creation of chloramine and will learn about chloramine. Students will be able to demonstrate calculations involving concentrations, volumes and dilution factors.
Directions/Procedure:
- Distribute printed copies of the Lesson Three Handout to the class.
- Read through the information on the handout with the class.
- There are many different ways to approach the questions on the second page of the handout. You could ask each student to work on them individually, allow them to work in groups or you could do it as a whole class with you marking their responses on the board.
- The nature of these questions requires the students to understand each question and get a correct answer before being able to answer the next so students may need some guidance when working through them. You can provide additional information or hints from the answers in the “For the Teacher” section below but give them lots of time to try working it out before giving them the answers.
Evaluation: Students can be evaluated on participating in working on the calculations if they were done in class. If the students are working on the calculations in their groups or individually they can be evaluated on the work they submit. Students should demonstrate the proper use of units and an understanding of how concentration measurements work.
For the Teacher:
Answer Key for the questions on the Lesson Three Handout:
1. Chloramine concentrations can be found by subtracting the free chlorine concentration from the total chlorine concentration.
Chloramine concentration of first beaker is 10mg/L - 10mg/L = 0mg/L
Chloramine concentration of second beaker is 0mg/L - 0mg/L = 0mg/L
2. In the salt and sugar example: The mass of salt in the first beaker is 0.1L * 20mg/L = 2mg and the mass of sugar in the second beaker is 0.1L * 4mg/L = 0.4mg. When they are mixed, the final solution will contain that very same 2mg of salt and 0.4mg of sugar but the volume increases to 200mL. If the final concentrations of salt and sugar are calculated the results are 10mg/L salt and 2mg/L sugar. Students should realize that by doubling the volume the concentrations are diluted to 50% even when no chemical reaction takes place. This indicates that when mixing the chlorine and ammonia samples the concentrations will reduce to 10mg/L free chlorine and 2mg/L ammonia due to dilution before the chemical reaction begins.
3. The 10mg/L free chlorine available will all be used to react with 1mg/L of the ammonia. This will leave 1mg/L ammonia and 0mg/L free chlorine remaining.
4. The 10mg/L free chlorine used in the reaction will turn into 10mg/L chloramine. Since the final free chlorine concentration is 0mg/L, the final total chlorine concentration is 10mg/L + 0mg/L = 10mg/L.
Related Links:
Nitrogen Cycle https://www.thoughtco.com/what-is-the-nitrogen-cycle-607607
IBROM treatment system http://www.safedrinkingwaterteam.org/ibrom.html
Disinfection byproducts http://www.lenntech.com/processes/disinfection/byproducts/disinfection-byproducts.htm
Operation Water Biology
Lesson Three
Ammonia and Chloramine Part One
One of the most important chemicals to the water treatment process is ammonia. The chemical formula for ammonia is NH3.
More information on the nitrogen cycle can be found at
https://www.thoughtco.com/what-is-the-nitrogen-cycle-607607
Ammonia is something that many water treatment facilities deal with in one way or another. It is common for ground and surface water sources to contain ammonia because ammonia can come from so many sources. Ammonia can be added to soil by nitrogen-fixing bacteria as part of the nitrogen cycle, decay of plants and animals or agricultural and industrial processes. Ammonia is highly soluble so it gets dissolved and transported by surrounding ground water.
In the areas that do have ammonia in their raw water it is a very problematic source of chlorine demand. For each milligram of ammonia in the water it takes 10-15 mg of chlorine to react with it and get rid of it. The reaction between ammonia and chlorine is much faster than the rate that chlorine kills bacteria so you cannot use chlorine to disinfect water that contains ammonia. Unfortunately the most widely used method of removing ammonia is to add chlorine. In a process called "break-point chlorination" chlorine is continuously added to water until all of the ammonia and bacteria have been removed, or in other words, until the chlorine demand has been met.
More information on disinfection byproducts can be found at
http://www.lenntech.com/processes/disinfection/byproducts/disinfection-byproducts.htm
This works if there is only a little bit of ammonia but if there is more than 0.3 mg/L ammonia in the raw water then so much chlorine would have to be added to get rid of it that it would result in dangerous levels of chlorination byproducts. You can see that there are cases where the only options seem to be, a) not using break-point chlorination and thus leaving bacteria in the water, or b) disinfecting the water at the risk of adding harmful amounts of chemicals to it. This means that some treatment facilities have to use very complicated and expensive methods, which often still involve the use of other chemicals, to take ammonia out of the water before they add chlorine.
More information on biological filtration facilities can be found at
http://www.safedrinkingwaterteam.org/ibrom.html
One new option that communities with ammonia problems have is biological filtration. This is a safe, chemical free, method of removing ammonia. In a biological filtration facility one of the stages of filtration is to pass the water through a special filter that is full of nitrifying bacteria. These bacteria take in the ammonia and some oxygen and perform a bio-oxidation reaction.
They oxidize the ammonia into nitrite NH3 + O2 -> NO2- + 3H+ Then further oxidize that into nitrate, NO2- + H2O -> NO3- + 2H+.
The bacteria gain energy from these reactions and are specialized to do them very efficiently. This process is part of the natural nitrogen cycle and does not produce any harmful byproducts. The nitrate that is produced by this process can easily be removed from the water by the reverse osmosis membrane in the final stage of the filtration process.
The reaction between chlorine and ammonia can be written as NH3 +HOCl -> NH2Cl + H2O. In this chemical equation NH3 is ammonia nd HOCl is hypochlorous acid which is formed when the chlorine first dissolved in the water. The primary result of this chemical reaction is NH2Cl, a chemical know as chloramine.
Chloramine is a disinfectant like chlorine, it is a weaker disinfectant than chlorine but it lasts much longer in water. The chlorine concentration in water can gradually decrease as the chlorine evaporates out but chloramine does not do this. This makes it useful for making sure water stays disinfected throughout drinking water distribution systems. In areas where there is no, or very little, ammonia in the raw water treatment facilities might still want to use chloramine for this purpose. After chlorinating (disinfecting) the water, as the last step in the treatment process they add ammonia and more chlorine to the water so that they react and create chloramine.
Example
If you test a water sample for both free and total chlorine and get values of 2.0 mg/L free chlorine and 2.5 mg/L total chlorine then you know that the concentration of chloramine in that sample must be 0.5 mg/L.
With this information, you may be wondering if there is chloramine in your own tap water and how you might be able to measure it. The fact of the matter is, part of the process of finding chloramine concentration is the total chlorine concentration test that you have already done. The other part is a second kind of test which is called a free chlorine test. Understanding the difference between free and total chlorine is very important. The free chlorine test finds the concentration of regular unreacted chlorine like the kind added to water during the chlorination process or the kind found in chlorine bleach. The total chlorine concentration test finds the combined concentrations of the regular unreacted chlorine and the chlorine that has been in a reaction and is now chloramine. Since total chlorine is free chlorine plus chloramine, the total chlorine concentration must always be greater than or equal to the free chlorine concentration. This also means that the total and free chlorine tests can be used together to find the chloramine concentration of a water sample. Subtracting the free chlorine concentration from the total chlorine concentration will give you the chloramine concentration.
Consider an experiment that could put this information to use. Imagine two beakers, each with exactly 100mL of water. The first has a free chlorine concentration of 20mg/L, a total chlorine concentration of 20mg/L and an ammonia concentration of 0mg/L. The second beaker has both free and total chlorine concentrations of 0mg/L and an ammonia concentration of 4mg/L. What do you think would happen if the contents of these two beakers were poured together into a larger beaker?
This problem can be broken into smaller pieces that should be considered one at a time.
1. What is the chloramine concentration in each of the two original beakers?
2. Pouring the beakers together will result in a total volume of 200mL; this volume change could dilute the chemicals. Might this have an effect on the concentrations of chlorine and ammonia before any chemical reaction even occurs? Consider the result of two 100mL beakers being poured together if one had 20mg/L of salt and the other had 4mg/L of sugar.
3. Assuming that a free chlorine concentration of 10mg/L is exactly enough to react with 1mg/L ammonia, what should the final free chlorine and ammonia concentrations be?
4. If all of the free chlorine used in the reaction becomes chloramine what should the final chloramine and total chlorine concentrations be?