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Lesson plan Grade 12 Careers & CTE

Threlwood Activated Sludge: Settling Data for a Lab Analyst

Students work as wastewater lab analysts, calculating sludge volume index for two aeration basins and comparing the center and spread of a week of readings.

Objectives

  1. Students will calculate sludge volume index from settled sludge volume and mixed liquor suspended solids.
  2. Students will compare the mean and range of daily index readings from two aeration basins.
  3. Students will calculate the pounds of suspended solids held in an aeration basin at a stated volume and concentration.

Vocabulary

Activated sludge
Activated sludge is a living brown mixture of helpful bacteria and other microbes that wastewater plants grow on purpose. The microbes clean the water by eating the waste in it.

Also: mixed liquor suspended solids, sludge volume index, settleometer, bulking, filamentous bacteria

Teacher overview

Activated sludge is a managed community of bacteria and protozoa, and the analyst who tracks it decides every day whether that community is settling the way the process needs it to. In this lesson students take the role of a laboratory analyst at the fictional Threlwood Water Resource Recovery Facility, calculating the sludge volume index for two aeration basins and then comparing a week of readings from each. The misconception the lesson corrects is the assumption that a taller sludge blanket in the settleometer means more biological treatment is happening, when a high settled volume paired with a low solids concentration is the classic signature of filamentous bulking and a plant about to lose solids over the clarifier weir. Students who hold that belief will read Basin B as the healthier basin.

Procedure

Hook (6 min). Before class, fill a settleometer or a 1-liter graduated cylinder with mixed liquor from a local treatment plant and stir it at the start of class so students can watch the sludge settle, or project a photograph of two settleometers side by side. Mixed liquor contains live wastewater bacteria, so wear gloves when handling a real sample, keep it at the front of the room, and wash hands afterward. Ask students which sample they would rather have in their plant and record the show of hands. Tell them they will revisit that vote at the end of the period.

Activated sludge briefing (10 min). Explain the process in the terms an analyst uses: mixed liquor is aerated so bacteria consume organic material, the mixture flows to a clarifier where the biological solids must settle and separate, and the settled sludge is returned to the head of the aeration basin. Introduce the sludge volume index as the volume in milliliters that one gram of those solids occupies after thirty minutes of settling, calculated as settled volume in milliliters per liter times 1,000, divided by mixed liquor suspended solids in milligrams per liter. Name the working rule the analysts at the fictional Threlwood plant use, that an index above 150 milliliters per gram indicates bulking sludge, and describe filamentous bacteria as the usual cause.

Modeled calculation (9 min). Work one index calculation aloud using a settled volume and a solids concentration the students will not reuse, so the model is instruction rather than a keyed answer. Write the units above each factor and show why the 1,000 in the numerator converts milligrams to grams. Ask two students to state what a rising index would mean for the clarifier before you move on.

Student handout (18 min). Distribute the handout and let students work in pairs. Circulate and watch for students who divide the solids concentration by the settled volume rather than the reverse, which produces an index near 0.009 and should be caught immediately.

Debrief and exit ticket (7 min). Return to the opening vote and have students defend or revise it using their answers to items 3 and 8. Collect item 8 as an exit ticket. If fewer than three quarters of the class identifies Basin A as the steadier basin, review the range calculations before the next class.

Threlwood Activated Sludge: Settling Data for a Lab Analyst — Grade 12 Careers & CTE
Name: ____________________________ Date: ______________

Student Handout

You are a laboratory analyst at the fictional Threlwood Water Resource Recovery Facility. Sludge volume index (mL/g) = settled sludge volume (mL/L) x 1,000 / mixed liquor suspended solids (mg/L). Show your work and label every unit.

  1. Today's settleometer test on Basin A gave a thirty-minute settled volume of 280 mL/L, and the laboratory reported mixed liquor suspended solids of 2,400 mg/L. Calculate the sludge volume index for Basin A. Round to the nearest whole number.
  2. Today's test on Basin B gave a settled volume of 420 mL/L with mixed liquor suspended solids of 2,100 mg/L. Calculate the sludge volume index for Basin B.
  3. Threlwood treats an index above 150 mL/g as a sign of bulking sludge. Which basin, A or B, shows bulking today?
  4. Basin A's index over the past five days was 112, 118, 121, 109, and 117 mL/g. Calculate the mean.
  5. Calculate the range of Basin A's five readings.
  6. Basin B's index over the same five days was 168, 182, 205, 196, and 214 mL/g. Calculate the mean.
  7. Calculate the range of Basin B's five readings.
  8. Using your means and ranges from items 4 through 7, which basin is holding a steadier settling condition? Name the basin.
  9. Basin A holds 0.42 million gallons of mixed liquor at 2,400 mg/L. Calculate the pounds of suspended solids in the basin, using pounds = concentration (mg/L) x volume (million gallons) x 8.34. Round to one decimal place.

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: 117 mL/g. Index = 280 mL/L x 1,000 / 2,400 mg/L. The numerator is 280,000, and 280,000 / 2,400 = 116.67, which rounds to 117 milliliters per gram.
  2. Answer: 200 mL/g. Index = 420 mL/L x 1,000 / 2,100 mg/L. The numerator is 420,000, and 420,000 / 2,100 = 200 milliliters per gram exactly.
  3. Answer: Basin B. At 200 milliliters per gram, Basin B sits well above the 150 threshold, while Basin A at 117 is comfortably below it. Basin B settled a larger volume from a lower solids concentration, which is the pattern filamentous bulking produces.
  4. Answer: 115.4 mL/g. The five readings sum to 112 + 118 + 121 + 109 + 117 = 577, and 577 / 5 = 115.4 milliliters per gram.
  5. Answer: 12 mL/g. The range = 121 highest minus 109 lowest = 12 milliliters per gram.
  6. Answer: 193 mL/g. The five readings sum to 168 + 182 + 205 + 196 + 214 = 965, and 965 / 5 = 193 milliliters per gram.
  7. Answer: 46 mL/g. The range = 214 highest minus 168 lowest = 46 milliliters per gram.
  8. Answer: Basin A. Basin A's mean of 115.4 sits below the bulking threshold and its readings span only 12 milliliters per gram, while Basin B averages 193 with a spread of 46. Basin B is both worse on average and less predictable, which is why an operator would begin adjusting the return sludge rate or checking for filaments there first.
  9. Answer: 8,406.7 pounds. Solids = 2,400 mg/L x 0.42 million gallons x 8.34 lb/gal. First, 2,400 x 0.42 = 1,008. Then 1,008 x 8.34 = 8,406.72, which rounds to 8,406.7 pounds of suspended solids held in the basin.

Differentiation

Support

Provide the index formula on a card with each variable labeled in words, and complete item 1 as a class before students continue. Give the five-day readings for each basin in a two-column table rather than in sentences, and supply the sums for items 4 and 6 so students perform only the division. Students who complete items 1 through 8 have met the first two objectives.

Extension

Ask students to calculate how much the return activated sludge rate would need to change to hold Basin B's solids inventory constant, then research two operational causes of filamentous bulking and what an operator does about each. Students can also plot both basins' five readings on the same axes and describe what a trend line would predict for the following week.

ELL supports

Pre-teach mixed liquor, settled volume, index, bulking, and inventory with a labeled process diagram tracing water from the aeration basin to the clarifier and back through the return line. Provide a worked example with the units written above each factor, and allow students to answer items 3 and 8 with a basin name plus one supporting number rather than a full sentence.

Extensions

Arrange for students to run a settleometer test on a real sample from a nearby facility, wearing gloves and eye protection and following the facility's handling guidance, or have a plant analyst bring a month of daily readings for students to summarize with the same statistics used here. Students can also examine microscope images of healthy floc and filamentous floc and write a short identification key an entry-level analyst could use at the bench.