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Worksheet Grade 11 Science

Bewley Reclamation Plant: BOD and TSS Removal Efficiency

A grade 11 worksheet on a fictional wastewater plant, computing BOD and TSS removal efficiency and daily pollutant loading with the pounds formula.

Objectives

  1. Students will calculate BOD and TSS removal efficiency from influent and effluent concentrations.
  2. Students will use the pounds formula to find daily pollutant loading and the mass removed.
  3. Students will judge whether a plant's effluent meets the secondary treatment standard.

Vocabulary

Biochemical oxygen demand (BOD)
Biochemical oxygen demand, or BOD, measures how much oxygen tiny microbes will use up as they eat the waste in water. Dirtier water has a higher BOD.
Total suspended solids (TSS)
Total suspended solids is the amount of solid material floating or suspended in water, measured by filtering a sample and weighing what gets left behind. High TSS makes water look cloudy and can smother fish habitat and clog treatment equipment.
Secondary treatment
Secondary treatment uses billions of helpful microbes that eat the dissolved waste in the water. The plant keeps the microbes supplied with air, then settles them out, leaving much cleaner water.
Effluent
Effluent is the cleaned water that leaves a treatment plant at the end of the process. It is tested carefully before being released to a river, lake, or the ocean.

Also: removal efficiency

Teacher overview

Secondary wastewater treatment is judged by two numbers: how clean the discharged water is, and how much of the incoming pollution the plant removed. This worksheet uses the fictional Bewley Water Reclamation Facility to practice both. Students compute removal efficiency for biochemical oxygen demand (BOD) and total suspended solids (TSS), then use the pounds formula to convert concentrations into the daily mass of pollutant the plant handles. A common student error is to assume that a high removal percentage automatically means the plant meets its permit; the permit also caps the effluent concentration, so a plant with high removal can still fail if it started with very dirty water, and item 7 forces that distinction. The federal secondary standard of 30 mg/L for BOD and TSS is used as the compliance benchmark.

Bewley Reclamation Plant: BOD and TSS Removal Efficiency — Grade 11 Science
Name: ____________________________ Date: ______________

Student Worksheet

The Bewley Water Reclamation Facility is a fictional plant that treats 3.2 MGD of municipal wastewater. On a given day its raw influent carried a BOD of 240 mg/L and a TSS of 260 mg/L. After primary and secondary treatment, the effluent BOD was 18 mg/L and the effluent TSS was 20 mg/L. Use the pounds formula where needed: loading (lbs/day) = concentration (mg/L) x flow (MGD) x 8.34.

  1. Calculate the plant's BOD removal efficiency as a percent, using removal efficiency = (influent minus effluent) divided by influent, times 100.
  2. Calculate the plant's TSS removal efficiency as a percent.
  3. The federal secondary treatment standard limits monthly-average effluent BOD and TSS to 30 mg/L. Does Bewley's effluent meet the standard for both, based on this day's readings?
  4. Find the BOD loading entering the plant, in pounds per day, using the influent BOD of 240 mg/L and the flow of 3.2 MGD.
  5. Find the BOD loading leaving the plant, in pounds per day, using the effluent BOD of 18 mg/L and the same flow.
  6. Using your answers to items 4 and 5, how many pounds of BOD does the plant remove per day?
  7. A plant upstream reports 95 percent BOD removal but is cited for a permit violation, while Bewley is in compliance at a lower removal percentage. Explain how that is possible, referring to the effluent concentration.
  8. Suppose a wet-weather event raises the flow to 4.0 MGD while the influent and effluent concentrations stay the same. Does the BOD removal efficiency change? Explain why or why not.
  9. At the 4.0 MGD flow, calculate the pounds of TSS removed per day, using the influent TSS of 260 mg/L and the effluent TSS of 20 mg/L.
  10. Secondary treatment relies on microorganisms in an aeration tank. In one or two sentences, explain what those microorganisms do to the BOD in the wastewater.

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: 92.5%.
    Removal = (240 - 18) / 240 x 100 = 222 / 240 x 100 = 92.5 percent.
  2. Answer: 92.3%.
    Removal = (260 - 20) / 260 x 100 = 240 / 260 x 100 = 92.3 percent.
  3. Answer: Yes, both meet the standard.
    Effluent BOD is 18 mg/L and effluent TSS is 20 mg/L, both at or below the 30 mg/L secondary limit.
  4. Answer: 6,405 lbs/day.
    240 mg/L x 3.2 MGD x 8.34 = 768 x 8.34 = 6,405 lbs/day (6,405.12 before rounding).
  5. Answer: 480 lbs/day.
    18 mg/L x 3.2 MGD x 8.34 = 57.6 x 8.34 = 480 lbs/day (480.38 before rounding).
  6. Answer: 5,925 lbs/day.
    6,405 - 480 = 5,925 lbs/day of BOD removed. Equivalently, (240 - 18) x 3.2 x 8.34 = 5,925.
  7. Answer: The permit caps the effluent concentration at 30 mg/L as well as requiring at least 85 percent removal, so a high removal percentage alone does not guarantee compliance.
    A plant with very high-strength influent can remove 95 percent and still leave more than 30 mg/L in the effluent, which is a violation, while Bewley's lower-strength influent lets a smaller removal percentage still finish below 30 mg/L.
  8. Answer: No, it does not change.
    Removal efficiency compares influent and effluent concentrations, which are unchanged, so the percentage stays the same; only the pounds-per-day loading rises with flow.
  9. Answer: 8,006 lbs/day.
    (260 - 20) mg/L x 4.0 MGD x 8.34 = 240 x 4.0 x 8.34 = 960 x 8.34 = 8,006 lbs/day (8,006.4 before rounding).
  10. Answer: They consume the dissolved and suspended organic matter.
    The microorganisms use the organic material as food, breaking it down through aerobic respiration, which lowers the BOD before the water is clarified and discharged.

Differentiation

Support

Provide the pounds formula and the removal-efficiency formula on a printed reference strip, and pre-fill the concentration-times-flow product in items 4 and 5 (768 for item 4) so students focus on carrying units through the multiplication. Allow a calculator for every computation.

Extension

Suppose a new industrial customer raises the influent BOD to 320 mg/L at the original 3.2 MGD flow. Have students calculate the plant's new daily loading and determine what effluent BOD would keep removal at 92.5 percent.

ELL supports

Pre-teach the terms "influent," "effluent," "loading," and "removal efficiency" with a simple labeled flow diagram of a plant showing water entering dirty and leaving clean. Allow items 7, 8, and 10 to be answered with short phrases or a labeled sketch rather than full sentences.