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Lesson plan Grade 8 Science

Brindley Crossing: Ultraviolet Dose Compared With Chlorine

A 55-minute grade 8 science lesson where students calculate ultraviolet dose from contact time and flow at a fictional plant, then compare it with chlorine.

Objectives

  1. Students will calculate ultraviolet contact time from a chamber volume and a flow rate.
  2. Students will calculate ultraviolet dose as intensity multiplied by contact time.
  3. Students will compare ultraviolet and chlorine disinfection against stated design criteria.

Vocabulary

Disinfection
Disinfection is the step that kills or disables germs in water so it is safe to drink. Water plants usually disinfect with chlorine, ultraviolet light, or ozone.
Chlorine residual
Chlorine residual is the small amount of chlorine still left in the water after it leaves the treatment plant. It keeps protecting the water from germs all the way to your faucet.
Pathogen
A pathogen is a germ, such as a bacterium, virus, or parasite, that can make people sick. Keeping pathogens out of drinking water is the most important job of water treatment.

Also: ultraviolet, contact time, dose

Teacher overview

Ultraviolet light disinfects water by damaging the genetic material inside bacteria, viruses, and parasites so they can no longer reproduce, and how well it works depends on the dose the water receives. Dose is intensity multiplied by contact time, so a plant that pushes water through the lamp chamber faster gives every drop less exposure. This lesson has students calculate contact time and dose for the fictional Brindley Crossing Water Treatment Plant at two different flows, then weigh ultraviolet light against chlorine using the plant's own criteria. Students commonly picture ultraviolet light as heating or boiling the water; it changes nothing about the temperature and leaves nothing behind in the pipes, which is exactly why plants that use it still add a small amount of chlorine afterward.

Procedure

Hook (5 min): Ask students how they would prove that water leaving a plant is free of living pathogens without tasting it or waiting days for a lab result. Collect two or three answers, then explain that operators cannot see disinfection happening, so they measure the conditions that guarantee it.

Direct instruction (13 min): Define contact time as the time one drop spends inside the ultraviolet chamber, found by dividing the chamber's volume by the flow rate. Define dose as intensity multiplied by contact time, measured in millijoules per square centimeter. Explain that the fictional Brindley Crossing engineers set a target dose of 40 millijoules per square centimeter for their design, and that ultraviolet light leaves no residual in the pipes. Write on the board the three criteria the plant judges a disinfectant by, since students will use them in item 7: delivering the target dose at every flow the plant sees, leaving protection in the distribution pipes, and forming no chemical byproducts.

Guided practice (12 min): Model the arithmetic on a different plant, not on the handout numbers. A chamber holding 30 cubic feet at a flow of 3.0 cubic feet per second gives a contact time of 10 seconds, and a lamp intensity of 3.0 milliwatts per square centimeter then gives a dose of 30 millijoules per square centimeter. Ask students what would happen to that dose if the flow doubled, and confirm the prediction before releasing them to the handout.

Independent practice (17 min): Students complete the handout in pairs, working through winter and summer flows at the fictional Brindley Crossing plant and finishing with the comparison item.

Closure and exit ticket (8 min): Students answer handout item 6 on an index card, explaining why the plant still adds chlorine after the ultraviolet chamber. Collect the cards and confirm that at least 80 percent of students name protection in the distribution pipes rather than extra killing power at the plant.

Brindley Crossing: Ultraviolet Dose Compared With Chlorine — Grade 8 Science
Name: ____________________________ Date: ______________

Student Handout

Use these two relationships for every calculation: contact time = chamber volume divided by flow rate, and dose = intensity multiplied by contact time. The fictional Brindley Crossing Water Treatment Plant has an ultraviolet chamber that holds 40 cubic feet, and its engineers set a target dose of 40 millijoules per square centimeter.

  1. In winter the plant runs 2.0 cubic feet per second through the 40-cubic-foot chamber. Find the contact time in seconds.
  2. The lamps run at an intensity of 2.0 milliwatts per square centimeter. Using your contact time from item 1, find the winter dose in millijoules per square centimeter.
  3. Summer demand pushes the flow to 4.0 cubic feet per second through the same 40-cubic-foot chamber. Find the summer contact time in seconds.
  4. With the lamps still at 2.0 milliwatts per square centimeter, find the summer dose in millijoules per square centimeter.
  5. At the summer contact time of 10 seconds, solve the equation I x 10 = 40 to find the lamp intensity in milliwatts per square centimeter that would reach the plant's target dose.
  6. Ultraviolet light leaves no disinfectant behind in the pipes, while chlorine keeps working all the way to a customer's tap. Explain why Brindley Crossing still adds a small chlorine residual after the ultraviolet chamber.
  7. Give one advantage of ultraviolet light over chlorine and one advantage of chlorine over ultraviolet light for this plant, using the criteria from the lesson.

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: 20 seconds.
    Contact time = 40 cubic feet divided by 2.0 cubic feet per second = 20 seconds inside the chamber.
  2. Answer: 40 millijoules per square centimeter.
    Dose = 2.0 milliwatts per square centimeter x 20 seconds = 40 millijoules per square centimeter, which meets the target exactly.
  3. Answer: 10 seconds.
    Contact time = 40 cubic feet divided by 4.0 cubic feet per second = 10 seconds. Doubling the flow cuts the contact time in half.
  4. Answer: 20 millijoules per square centimeter.
    Dose = 2.0 x 10 = 20 millijoules per square centimeter, half the winter dose and below the plant's target.
  5. Answer: 4.0 milliwatts per square centimeter.
    Solving I x 10 = 40 gives I = 40/10 = 4.0 milliwatts per square centimeter, so the lamps must run twice as bright to hold the dose when the flow doubles.
  6. Answer: Chlorine protects the water after it leaves the plant.
    Ultraviolet light only works inside the chamber, so water is unprotected once it enters miles of pipe where a break or a backflow could let contamination in. A small chlorine residual, commonly held between 0.2 and 2.0 mg/L in distribution, keeps working the whole way to the tap.
  7. Answer: Ultraviolet light forms no chemical byproducts, and chlorine leaves lasting protection.
    Accept any pair drawn from the lesson criteria. Ultraviolet light adds no taste, forms no disinfection byproducts, and handles parasites that resist chlorine. Chlorine is inexpensive, leaves a measurable residual operators can test anywhere in the system, and keeps working after the water leaves the plant.

Differentiation

Support

Give students a filled-in example showing contact time and dose for a third flow rate, such as 1.0 cubic foot per second through the same chamber, so they can see the pattern before starting item 1. Provide the two relationships on a card with each symbol labeled in words.

Extension

Ask students to find the flow rate at which the fictional Brindley Crossing plant would fall to half its target dose with the lamps at their normal intensity, then write a short recommendation about whether the plant should add a second chamber or install brighter lamps.

ELL supports

Pre-teach "dose," "intensity," and "contact time" using a flashlight and a strip of paper moved past the beam at two speeds, naming each term as students watch. Provide sentence frames such as "When the flow doubled, the contact time ___ because ___." Allow students to answer items 6 and 7 in a two-column chart before writing sentences.

Extensions

Have students find out which disinfectant their own utility reports in its yearly water quality report, then write a paragraph explaining what that choice means for how long the water stays protected on its way to their school.