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Classroom activity Grade 9 Careers & CTE

Westerholt Digester Night Shift: An Operator Log Relay

A grade 9 careers relay where crews work a fictional digester's night log, computing solids reduction, gas yield, and engine output before each call-out decision.

Objectives

  1. Students will calculate the percent reduction in volatile solids across a digester.
  2. Students will convert a night of digester gas production into kilowatt-hours of engine output.
  3. Students will compare a computed indicator against a plant action level and state whether to call the supervisor.

Vocabulary

Anaerobic digester
An anaerobic digester is a sealed tank where bacteria break down sewage sludge without any oxygen, shrinking its volume and producing methane gas as a byproduct. Some treatment plants capture that gas and burn it to generate their own electricity or heat.
Alkalinity
Alkalinity is water's ability to resist becoming acidic, acting like a buffer that keeps pH from swinging wildly when acids are added. Water with good alkalinity helps protect fish and other aquatic life from sudden pH changes.
Action level
A trigger number in lead and copper testing. If too many homes test above this level, the utility must take steps like improving treatment and telling the public.

Also: volatile solids, biogas, volatile acids

Teacher overview

Overnight at a water reclamation facility, a single operator monitors the whole site, and the anaerobic digester is the equipment that asks the most of that operator. This relay puts crews of four through four rounds of a night log from the fictional Westerholt Water Reclamation Facility, passing one log page down the crew so that a reader, a calculator, a checker, and a caller each handle it before the round closes. Passing the page copies the way a shift actually works, since a number is read, worked, verified, and only then acted on. The common misconception this relay corrects is that a digester is a holding tank where sludge waits its turn; it is a heated, sealed biological reactor, and its operators judge it by how much of the volatile solids the bacteria destroy and by how the acid and alkalinity readings move against each other.

Activity Directions

  1. Before class, print four log pages for each crew, one Student Handout per student, and post the fictional Westerholt call-out rules where every crew can see them: call the supervisor when the volatile acids to alkalinity ratio rises above 0.10, and call when the digester temperature moves more than 2 degrees Fahrenheit during one shift.
  2. Open by naming the four crew jobs. The reader states the log entries aloud, the calculator works the numbers, the checker repeats the arithmetic independently, and the caller decides whether the supervisor gets a phone call. Explain that on a real night shift one operator does all four in sequence.
  3. Form crews of four and assign the first set of jobs. Every student keeps a handout and answers every question, though the crew works each log page together.
  4. Round 1 covers the solids report. Hand out log page one, set a twelve-minute timer, and have crews work Questions 1 through 3.
  5. Pass each job one seat to the left, then run Round 2 on the gas report with log page two, allowing ten minutes for Questions 4 and 5.
  6. Pass the jobs again and run Round 3 on the sludge feed pump with log page three, allowing six minutes for Question 6.
  7. Pass the jobs a final time and run Round 4 on the temperature record with log page four, allowing eight minutes for Questions 7 and 8.
  8. End the relay after the fourth round, when every crew has recorded two call-out decisions and every student has held all four jobs.
  9. Close with a ten-minute debrief on two questions. Ask why the fictional Westerholt plant sets an action level on a ratio rather than on the volatile acids reading by itself, and ask what the plant gains by burning its own digester gas in an engine instead of flaring it.
Westerholt Digester Night Shift: An Operator Log Relay — Grade 9 Careers & CTE
Name: ____________________________ Date: ______________

Student Handout

Every reading below comes from one night at the fictional Westerholt Water Reclamation Facility. Show the numbers you used, not just the result.

Round 1: Solids report

  1. The sludge fed to the fictional digester holds 42,000 mg/L of volatile solids, and the sludge withdrawn holds 21,840 mg/L. Percent reduction equals the feed value minus the withdrawn value, divided by the feed value, times 100. What is the percent reduction?
  2. The 2 a.m. sample shows volatile acids of 180 mg/L and alkalinity of 2,400 mg/L. The ratio equals volatile acids divided by alkalinity. What is the ratio, rounded to three decimal places?
  3. The fictional Westerholt rule is to call the supervisor when that ratio rises above 0.10. The ratio in Question 2 is 0.075. Should the crew call the supervisor?

Round 2: Gas report

  1. During the same 24 hours the fictional digester produced 63,000 cubic feet of gas while destroying 4,200 pounds of volatile solids. How many cubic feet of gas were produced for each pound of volatile solids destroyed?
  2. The plant's engine makes 1 kilowatt-hour of electricity for every 18 cubic feet of digester gas it burns. How many kWh could the 63,000 cubic feet from Question 4 produce?

Round 3: Sludge feed pump

  1. The crew feeds the fictional digester 12,000 gallons of thickened sludge in one night, pumping at 60 gpm. How many minutes of pumping does that take?

Round 4: Temperature record

  1. The fictional digester read 98 degrees Fahrenheit at 10 p.m. and 95 degrees Fahrenheit at 6 a.m. By how many degrees Fahrenheit did the temperature change across the shift?
  2. The fictional Westerholt rule is to call the supervisor when the digester temperature moves more than 2 degrees Fahrenheit in one shift. Using the change you found in Question 7, should the crew call the supervisor?

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: 48 percent.
    The feed of 42,000 mg/L minus the withdrawn 21,840 mg/L leaves 20,160 mg/L, and 20,160 divided by 42,000 is 0.48.
  2. Answer: 0.075.
    The volatile acids of 180 mg/L divided by the alkalinity of 2,400 mg/L gives 0.075.
  3. Answer: No.
    The ratio of 0.075 sits below the fictional action level of 0.10, so the night crew logs it and keeps working.
  4. Answer: 15 cubic feet per pound.
    The 63,000 cubic feet of gas divided by the 4,200 pounds of volatile solids destroyed gives 15 cubic feet for each pound.
  5. Answer: 3,500 kWh.
    The 63,000 cubic feet divided by 18 cubic feet per kilowatt-hour gives 3,500 kilowatt-hours.
  6. Answer: 200 minutes.
    The 12,000 gallons divided by a pumping rate of 60 gpm gives 200 minutes, which is 3 hours and 20 minutes.
  7. Answer: 3 degrees Fahrenheit.
    The reading fell from 98 degrees to 95 degrees, a change of 3 degrees across the shift.
  8. Answer: Yes.
    A change of 3 degrees is more than the 2 degrees the fictional rule allows, so the caller phones the supervisor.

Differentiation

Support

Give these crews a log page with the first substitution already written, so the fictional Westerholt solids question reads as 42,000 minus 21,840, divided by 42,000. Let the checker use a second calculator rather than reworking the arithmetic by hand.

Extension

Ask these students what the fictional Westerholt crew should watch over the following week if the volatile acids reading climbs to 380 mg/L while alkalinity stays at 2,400 mg/L, and what the new ratio would be.

ELL supports

Pre-teach digester, volatile solids, biogas, and action level with a simple cutaway sketch of a sealed heated tank with a gas line leaving the top. Provide a sentence frame for the caller: "I am calling or not calling because ___ is above or below the action level of ___."

Extensions

Arrange a call with an operator at a facility that generates power from digester gas, and ask what the engine covers in a typical day. Crews that finish early can write a fifth log page for the fictional Westerholt plant in which one reading crosses an action level and one does not, then trade pages with another crew to check the call-out decisions.