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Worksheet Grade 12 Engineering & Design

Calverhoe Chloramine Conversion: Feed Rate and Ratio Worksheet

A grade 12 worksheet on a fictional district switching to chloramines: feed rates in pounds per day, the chlorine to ammonia ratio, and nitrification signs.

Objectives

  1. Students will calculate chlorine and ammonia feed rates in pounds per day using the pounds formula.
  2. Students will convert a percent hypochlorite strength into a daily volume of solution fed.
  3. Students will identify a monitoring pattern that indicates nitrification in a distribution system.

Vocabulary

Key words: chloramine, monochloramine, free ammonia, chlorine demand, hypochlorite, nitrification

Teacher overview

The fictional Calverhoe Water District is converting from free chlorine to chloramines to lower disinfection byproducts in a distribution system with long dead-end mains. Chloramine is formed by feeding chlorine and ammonia together, and the weight ratio between them decides whether any ammonia is left over. This worksheet walks students through the plant-side feed calculations, the volume of hypochlorite solution those pounds represent, and then the distribution-side monitoring that shows whether the conversion is holding. The misconception worth naming before students start is that chloramine is simply chlorine plus ammonia in any amount. Feeding too little chlorine for the ammonia present leaves free ammonia in the system, and free ammonia is food for nitrifying bacteria, which is exactly how a tank with long water age loses its residual.

Calverhoe Chloramine Conversion: Feed Rate and Ratio Worksheet — Grade 12 Engineering & Design
Name: ____________________________ Date: ______________

Student Worksheet

The fictional Calverhoe Water District treats 4.2 MGD. It targets a monochloramine residual of 2.6 mg/L leaving the plant, measured as chlorine. The district feeds chlorine and ammonia at a 5 to 1 weight ratio of chlorine to ammonia-nitrogen. Use the pounds formula for every loading item: pounds per day equals concentration in milligrams per liter times flow in million gallons per day times 8.34. Also use these relationships: a 1 percent solution equals 10,000 mg/L, and chlorine demand equals dose minus residual.

  1. The plant treats 4.2 MGD. Find the pounds of chlorine per day needed to deliver a dose of 2.6 mg/L.
  2. The district holds a 5 to 1 weight ratio of chlorine to ammonia-nitrogen. At a chlorine dose of 2.6 mg/L, find the ammonia-nitrogen dose in milligrams per liter.
  3. The plant treats 4.2 MGD. Find the pounds of ammonia-nitrogen per day needed to deliver a dose of 0.52 mg/L.
  4. The plant applies a chlorine dose of 3.1 mg/L and measures a residual of 2.6 mg/L. Find the chlorine demand.
  5. The plant treats 4.2 MGD and applies a chlorine dose of 3.1 mg/L. Find the pounds of chlorine per day actually fed.
  6. The district feeds 12.5 percent sodium hypochlorite, and a 1 percent solution equals 10,000 mg/L. Express 12.5 percent as milligrams per liter of available chlorine.
  7. The hypochlorite solution is 12.5 percent available chlorine by weight. Find the pounds of solution needed per day to deliver 108.59 pounds of chlorine.
  8. The hypochlorite solution weighs 10.0 pounds per gallon. Using 868.72 pounds of solution per day, find the gallons of solution fed per day.
  9. Water leaves the plant with a total chlorine residual of 2.6 mg/L. At the far end of a dead-end main the residual measures 1.1 mg/L. Find the percent of the residual that has been lost.
  10. A remote pump station feeds 95.0 pounds of chlorine per day and 22.0 pounds of ammonia-nitrogen per day. Find the weight ratio of chlorine to ammonia-nitrogen at that station, written as a number to 1.
  11. A ratio below 5 to 1 leaves free ammonia in the water. Explain in two sentences why free ammonia raises the risk of nitrification in a storage tank with long water age.
  12. Four sites in the fictional district report the values below. Name the site showing the clearest sign of nitrification and give the reason.
Site Total chlorine (mg/L) Nitrite (mg/L)
A 2.4 0.01
B 1.9 0.02
C 0.6 0.18
D 1.5 0.03

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 91.07 lb/day. 2.6 mg/L times 4.2 MGD times 8.34 equals 91.0728, which rounds to 91.07 pounds per day.
  2. Answer: 0.52 mg/L. Writing the ratio as 2.6 divided by the ammonia dose equals 5, the ammonia dose equals 2.6 divided by 5, which is 0.52 mg/L.
  3. Answer: 18.21 lb/day. 0.52 mg/L times 4.2 MGD times 8.34 equals 18.2146, which rounds to 18.21 pounds per day.
  4. Answer: 0.5 mg/L. Chlorine demand equals dose minus residual, so 3.1 minus 2.6 equals 0.5 mg/L of demand.
  5. Answer: 108.59 lb/day. 3.1 mg/L times 4.2 MGD times 8.34 equals 108.5868, which rounds to 108.59 pounds per day.
  6. Answer: 125,000 mg/L. Since 1 percent equals 10,000 mg/L, 12.5 percent equals 12.5 times 10,000, or 125,000 mg/L of available chlorine. This conversion treats the solution as if it weighed the same as water. The hypochlorite in item 8 is denser, at 10.0 pounds per gallon, so a solution that is 12.5 percent by weight actually holds about 150,000 mg/L.
  7. Answer: 868.72 lb/day. Writing 0.125 times the solution weight equals 108.59, the solution weight equals 108.59 divided by 0.125, which is 868.72 pounds per day.
  8. Answer: 86.87 gallons per day. 868.72 pounds per day divided by 10.0 pounds per gallon equals 86.872, which rounds to 86.87 gallons per day.
  9. Answer: 57.7 percent. The loss is 2.6 minus 1.1, or 1.5 mg/L. Then 1.5 divided by 2.6 equals 0.5769, which is 57.7 percent of the original residual.
  10. Answer: 4.32 to 1. 95.0 pounds of chlorine divided by 22.0 pounds of ammonia-nitrogen equals 4.318, which rounds to 4.32 to 1 and sits below the 5 to 1 target.
  11. Answer: Free ammonia feeds nitrifying bacteria. Accept any two-sentence response noting that leftover ammonia is a nitrogen source those bacteria oxidize, and that long water age in a tank gives them the contact time to grow, producing nitrite and consuming the chloramine residual as they do.
  12. Answer: Site C. Its nitrite has risen to 0.18 mg/L while its total chlorine has fallen to 0.6 mg/L, and that combination of rising nitrite with a collapsing residual is the signature of nitrification. The other three sites hold higher residuals with nitrite at or near the detection floor.

Differentiation

Support

Give these students items 1, 3, and 5 with the pounds formula already written out as concentration times 4.2 times 8.34, so the first calculations are substitutions rather than recalls. Provide a worked model on a different fictional plant, such as a 1.8 MGD system dosing 2.0 mg/L, and supply the table in item 12 with the nitrite column already highlighted.

Extension

Ask these students to work backward for the fictional district. Given that the remote pump station in item 10 feeds 22.0 pounds of ammonia-nitrogen per day, have them find the pounds of chlorine per day that station would need to reach exactly 5 to 1, then state how many additional pounds per day that represents.

ELL supports

Provide a labeled feed-point diagram showing where chlorine and ammonia enter and where the residual is sampled, so "dose," "demand," and "residual" mark three different points on one line. Offer sentence frames for items 11 and 12: "Free ammonia is a source of ___ for ___ bacteria," and "Site ___ shows nitrification because its nitrite is ___ while its chlorine is ___."

Extensions

Suppose the fictional Calverhoe district finds that the tank feeding the dead-end main turns over only once every nine days. Ask students to describe two operational changes that would shorten that water age, then explain which of the two would show up first in the nitrite readings and why.