Water for Schools
Menu

Lesson plan Grade 6 Science

Marchfield Bend Septic Drainfield: Soil as a Treatment Step

A grade 6 lesson set at a fictional home with no sewer, where students trace how a septic tank and soil drainfield clean wastewater and find percent removal.

Objectives

  1. Students will explain how a septic tank and a soil drainfield each treat part of a home's wastewater.
  2. Students will calculate percent removal from before-and-after concentration data.
  3. Students will name a yearly well test that would warn a family that a drainfield is failing.

Vocabulary

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.
Groundwater
Groundwater is water that has soaked into the ground and fills the spaces between soil particles and cracks in rock underground. It is a large underground source of fresh water that wells and springs draw from.

Also: septic tank, drainfield, percent removal

Teacher overview

Many rural homes send their wastewater to a septic system rather than to a sewer, and the treatment that system provides happens mostly underground. This lesson uses the fictional community of Marchfield Bend, which has no sewer lines, to separate the two jobs a septic system does: the tank holds water still so grease floats and solids sink, while the soil of the drainfield does the actual cleaning as water trickles down through it. The common misconception this lesson corrects is that the tank cleans the water and the drainfield is only a place to get rid of it. Students work with fictional sample data in mg/L to see how much organic matter the soil removes, then decide what a family should test at the well each year.

Procedure

Hook (5 min): Pour muddy water into a jar half filled with sandy soil and let it sit while you talk. Ask students where the water goes at a house that has no sewer pipe. Collect two or three answers without correcting them, and write the word "drainfield" on the board.

Direct instruction (10 min): Sketch a septic system on the board: house, tank, then a set of buried trenches filled with gravel and perforated pipe. Explain that in the tank, grease floats to the top and solids sink to the bottom, so only the middle layer, called effluent, flows out to the trenches. Explain that the effluent then trickles down through several feet of soil, where bacteria living on the soil grains use the organic matter in it as food. Point out that the tank mostly separates while the soil does most of the treating, and that a system fails when the soil clogs or when water reaches groundwater before the soil has finished the job.

Model (7 min): Work one fictional example that is not on the handout. A home sends out effluent measuring 180 mg/L of organic matter, and a sampler set below its drainfield reads 36 mg/L. Subtract to find what the soil removed, 180 minus 36, which is 144 mg/L. Then divide by the starting amount and multiply by 100: 144 divided by 180 is 0.80, or 80 percent removal. Say the rule aloud as you write it, because students need it for the handout: removal percent equals the amount removed divided by the amount that went in, times 100.

Guided and independent practice (18 min): Students complete the handout on the fictional Tessmer home. Work items 1 through 3 together, then release students to finish items 4 through 10 in pairs. Circulate and check that students divide by the starting concentration rather than by the ending one, which is the most common error on item 3.

Closure and exit ticket (5 min): Each student writes one sentence on a card answering this question: "Which part of a septic system does most of the cleaning, and how do you know?" Collect the cards and count them. If more than a quarter of the class names the tank rather than the soil, reteach the two jobs with the jar demonstration before the next class.

Marchfield Bend Septic Drainfield: Soil as a Treatment Step — Grade 6 Science
Name: ____________________________ Date: ______________

Student Handout

The Tessmer home sits in Marchfield Bend, a fictional community with no sewer lines. Use these fictional numbers for every item.

  • The family uses 300 gallons of water per day.
  • Their septic tank holds 1,200 gallons.
  • The drainfield has 3 trenches, each 50 feet long.
  • Effluent leaving the tank measures 160 mg/L of organic matter.
  • A sampler 3 feet below the drainfield pipes measures 24 mg/L.
  • The county rule in this scenario: a drainfield needs 1 foot of trench for every 2 gallons of water used per day.
  1. The tank holds 1,200 gallons, and 300 gallons enter it each day. How many days does water stay in the tank?
  2. How many mg/L of organic matter did the soil remove between the tank outlet and the sampler 3 feet below the pipes?
  3. What percent of the organic matter did the soil remove? Use this rule: removal percent equals the amount removed divided by the amount that went in, times 100.
  4. A neighbor's tank sends out effluent measuring 200 mg/L, and that soil removes the same percent you found in item 3. How many mg/L are left below the neighbor's drainfield?
  5. What is the total trench length at the Tessmer home?
  6. Using the county rule of 1 foot of trench for every 2 gallons per day, how many feet of trench does a home that uses 300 gallons per day need?
  7. Compare your answers to items 5 and 6. Does the Tessmer drainfield meet the county rule?
  8. Explain why the soil under a drainfield counts as a treatment step and not just a place to send water.
  9. Name one test the family could run at their well each year to check that the drainfield is still doing its job.
  10. What would a rise in that test result tell the family about their drainfield?

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: 4 days.
    1,200 gallons divided by 300 gallons per day equals 4 days.
  2. Answer: 136 mg/L.
    The effluent measured 160 mg/L and the sampler below the field measured 24 mg/L, so 160 minus 24 equals 136 mg/L removed.
  3. Answer: 85 percent.
    136 mg/L removed divided by 160 mg/L that went in equals 0.85, which is 85 percent.
  4. Answer: 30 mg/L.
    Removing 85 percent leaves 15 percent, and 0.15 times 200 mg/L equals 30 mg/L. Students may instead find 85 percent of 200, which is 170 mg/L removed, then subtract: 200 minus 170 equals 30 mg/L.
  5. Answer: 150 feet.
    Three trenches at 50 feet each: 3 times 50 equals 150 feet.
  6. Answer: 150 feet.
    300 gallons per day divided by 2 gallons per foot equals 150 feet of trench.
  7. Answer: Yes.
    The home has 150 feet of trench and the rule calls for 150 feet, so it meets the rule exactly with nothing to spare.
  8. Answer: The soil is where the cleaning happens, because bacteria living on the soil grains use the organic matter in the effluent as food while the water trickles down through it. The tank mainly separates floating grease and settled solids, and bacteria in it break down some of the sludge, but most of the cleaning of the water happens in the soil. Accept any answer that credits the soil and its bacteria with removing material rather than only holding water.
  9. Answer: A yearly well test for bacteria, or a yearly well test for nitrate.
    Either test is correct. Accept any test that would show wastewater reaching the well.
  10. Answer: A rise would suggest that water from the drainfield is reaching the well before the soil has finished treating it, so the system needs to be inspected. Accept any answer that connects a higher reading to wastewater arriving at the well too soon.

Differentiation

Support

Give students a partly worked version of item 3, with the subtraction already done and the division set up as a labeled fraction, amount removed over amount that went in. Provide a calculator and a printed reminder that percent removal always divides by the starting number. Allow one-sentence answers on items 7, 9, and 10.

Extension

Ask students to find the largest daily water use, in gallons, that the fictional Tessmer drainfield could serve under the county rule of 1 foot of trench for every 2 gallons per day, and to explain what would likely happen in the soil if the family went well past that amount.

ELL supports

Pre-teach "septic tank," "drainfield," "effluent," and "percent removal" using a labeled cross section that shows the house, the tank with its floating and settled layers, and the buried trenches with arrows pointing down into the soil. Provide the sentence frame "The soil removed ___ of what went in, which is ___ percent." Allow items 8 and 10 to be answered with a labeled drawing plus one sentence.

Extensions

Have students compare the fictional Marchfield Bend system with a sewered town by listing which steps a treatment plant does with tanks and machinery that a septic system does with soil and time. A short home survey works well as a follow-up: students ask an adult whether their house is on a sewer or a septic system, and the class graphs the results by neighborhood.