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Lesson plan Grade 12 Engineering & Design

Umberfield Sewer Inflow and Infiltration: A Wet-Weather Lesson

A grade 12 lesson where students compare dry- and wet-weather flows at a fictional wastewater plant, quantify inflow and infiltration, and price the extra treatment.

Objectives

  1. Students will compute inflow and infiltration as the difference between wet-weather and dry-weather plant flows.
  2. Students will express extra wet-weather flow as a percent of plant capacity and a treatment cost.
  3. Students will explain how sewer defects admit clean water and how utilities find them.

Vocabulary

Infiltration
Infiltration is when rain or melted snow soaks down into the soil instead of flowing away across the top of it. Water that infiltrates can feed plant roots or keep sinking to become groundwater.

Also: inflow, dry-weather flow, peak flow, collection system

Teacher overview

Sewers are designed to carry wastewater, yet in aging systems a rainy week can nearly double the flow arriving at the treatment plant, because stormwater pours in through defects and groundwater seeps in through cracked pipes. The ASCE 2025 Infrastructure Report Card graded US wastewater a D+, and inflow and infiltration is one of the problems behind that grade, because clean water that never needed treatment consumes plant capacity and budget. This lesson has students quantify that problem for the fictional town of Umberfield, computing the extra flow, its share of plant capacity, and its cost. Teaching note: some students assume the plant can simply refuse the extra water, so be ready to explain that the alternative to treating it is an untreated overflow into a street or a stream.

Procedure

Hook (6 min): Post two influent readings from the fictional Umberfield treatment plant: a sunny Tuesday at 2.0 MGD and a rainy Thursday at 3.6 MGD. Ask students where the extra water came from, given that people do not use more toilets and showers when it rains. Collect hypotheses on the board.

Direct instruction (14 min): Define the two components. Inflow is stormwater that enters the sanitary sewer through direct connections: roof downspouts piped to the sewer, cross-connected storm drains, and vented manhole lids in low spots. Infiltration is groundwater seeping through cracked pipes, failed joints, and deteriorated manholes, which continues for days after rain stops. Explain why both matter: every gallon is pumped and fully treated at real cost, and during storms the combined flow competes with actual sewage for the plant's fixed capacity. Sketch a simple two-line hydrograph, dry-day flow flat, wet-day flow spiking during rain and decaying slowly, and label which portion is inflow (the fast spike) and which is infiltration (the slow tail).

Guided example (10 min): Work one calculation as a class using different numbers than the handout: a plant with dry-weather flow of 1.5 MGD that reads 2.4 MGD in sustained wet weather. The inflow and infiltration is 2.4 − 1.5 = 0.9 MGD, which is 0.9 ÷ 1.5 = 60 percent added on top of the sanitary flow. Ask the check-for-understanding question and require a written answer on scratch paper before anyone responds aloud: why is the subtraction of dry-weather flow the essential first step? Look for the idea that dry-weather flow is the real sewage baseline.

Handout work (15 min): Students complete the six handout questions individually, showing units. Circulate for the common error of dividing by plant capacity when the question asks for a percent of dry-weather flow, and the reverse.

Discussion (6 min): Ask students how a utility would locate the defects behind Umberfield's numbers. Introduce smoke testing (blowing harmless smoke into the sewer and watching where it escapes), night flow isolation, and camera inspection, and connect each to whether it finds inflow or infiltration.

Exit ticket (4 min): Each student writes one sentence distinguishing inflow from infiltration and names the field method best suited to finding each. Scan responses for the fast-direct versus slow-seepage distinction before the next class.

Umberfield Sewer Inflow and Infiltration: A Wet-Weather Lesson — Grade 12 Engineering & Design
Name: ____________________________ Date: ______________

Student Handout

The fictional Umberfield wastewater plant has a permitted secondary treatment capacity of 4.0 MGD. Its average dry-weather flow is 2.0 MGD. During a sustained wet spell, metered influent averages 3.6 MGD. Treating water costs the utility $310 per million gallons. Answer with units shown.

  1. Compute the plant's inflow and infiltration during the wet spell, in MGD, as wet-weather flow minus dry-weather flow.
  2. Express that extra flow as a percent of the dry-weather flow.
  3. During the wet spell, what percent of the plant's 4.0 MGD capacity is being used?
  4. The wet spell lasts 12 days at 3.6 MGD. How many million gallons of extra water, beyond dry-weather flow, arrive over those 12 days?
  5. At $310 per million gallons, what does treating that extra water cost the utility?
  6. An inspection program finds and repairs defects that remove 0.5 MGD of the wet-weather extra flow. During a similar future wet spell, what percent of plant capacity would be used?

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: 1.6 MGD. 3.6 − 2.0 = 1.6 MGD of inflow and infiltration.
  2. Answer: 80 percent. 1.6 ÷ 2.0 = 0.80, so the wet spell adds 80 percent on top of the sanitary baseline.
  3. Answer: 90 percent. 3.6 ÷ 4.0 = 0.90, leaving only 0.4 MGD of unused capacity during the wet spell.
  4. Answer: 19.2 million gallons. 1.6 MGD × 12 days = 19.2 million gallons of extra water.
  5. Answer: $5,952. 19.2 million gallons × $310 per million gallons = $5,952 in added treatment cost.
  6. Answer: 77.5 percent. New wet-weather flow = 3.6 − 0.5 = 3.1 MGD, and 3.1 ÷ 4.0 = 0.775, or 77.5 percent of capacity.

Differentiation

Support

Provide the hydrograph sketch from direct instruction as a printed reference with "inflow" and "infiltration" already labeled, and pre-fill the subtraction in handout question 1 so students begin at the percent reasoning in question 2.

Extension

Have students estimate, for the fictional Umberfield system, the largest single wet-weather flow the plant could accept without exceeding capacity, and work backward to how much inflow and infiltration removal that would require if dry-weather flow grows to 2.2 MGD. (Capacity 4.0 − 2.2 = 1.8 MGD of room, versus 1.6 MGD of current extra flow.)

ELL supports

Pre-teach "inflow" and "infiltration" with a two-panel drawing: rain pouring into an open pipe connection, and groundwater seeping through a cracked pipe wall. Provide a sentence frame for the exit ticket: "Inflow enters ___, while infiltration enters ___."