Define Criteria and Constraints for Reducing an...

SECTION 1

Historical Context & Motivation

Humans have changed the environment for thousands of years. Early farmers cleared forests to plant crops. Factories during the Industrial Revolution filled the air with smoke. Over time, people began to notice that these changes harmed nature and human health.

As environmental problems grew bigger, scientists and engineers needed a way to organize their solutions. They could not just say "fix pollution" without a clear plan. They had to decide what success would look like and what limits they had to work within. This led to the practice of defining criteria and constraints.

1962

Silent Spring Published

Rachel Carson wrote about how pesticides were harming birds and ecosystems. Her book pushed people to demand clear environmental goals.

1970

First Earth Day & the EPA

The U.S. Environmental Protection Agency was created. For the first time, the government set specific pollution limits that companies had to follow.

1987

Montreal Protocol

Countries agreed to stop using chemicals that destroyed the ozone layer. They set clear criteria for how fast to reduce these chemicals and constraints on what substitutes could be used.

2015

Paris Climate Agreement

Nearly 200 nations set the criterion of limiting global warming to 1.5°C above pre-industrial levels. Budget, technology, and fairness became key constraints.

Each of these moments shows the same pattern. People identified an environmental problem, set goals for a solution, and figured out the limits they had to work within. Today we ask: How do engineers and scientists decide what counts as a good solution to an environmental problem?

SECTION 2

Core Principles & Definitions

When engineers work on environmental problems, they start by asking two questions. First, what must the solution achieve? Second, what limits do we need to work within? The answers to these questions are called criteria and constraints.

Criteria

Criteria are the standards a solution must meet to be considered successful. Think of them as your goals. For example, a water filter must remove at least 99% of bacteria.

Constraints

Constraints are the limits or restrictions on the design. These include budget, available materials, time, and laws. A school rain garden project might have a budget constraint of $500.

Environmental Impact

An environmental impact is any change to the natural world caused by human activity. Pollution, habitat loss, and climate change are all examples.

Trade-offs

A trade-off happens when improving one part of a solution makes another part worse. Choosing a cheaper material might mean it does not last as long.

✦ KEY TAKEAWAY

Think of criteria and constraints like planning a birthday party. Your criteria are your goals: fun music, enough food for everyone, and decorations. Your constraints are your limits: you only have $50, it must be indoors, and you have two hours to set up. You cannot plan a good party without knowing both your goals and your limits!

SECTION 3

Visual Explanation — The Design Process

Defining criteria and constraints is a key part of the engineering design process. The diagram below shows how this step fits into the bigger picture. Notice how criteria and constraints guide every decision that comes after.

The engineering design process begins with identifying a problem, then moves to defining criteria and constraints. This step (highlighted in purple) shapes every decision that follows. The dashed arrow shows that engineers return to criteria and constraints when they optimize.

Notice the dashed pink arrow going from step 5 back to step 2. This tells us something important. Engineering is not a straight line. When you test a solution and it does not meet your criteria, you go back and adjust. Sometimes you even change the criteria or constraints themselves based on what you learn.

SECTION 4

How It Works — Setting Up Criteria and Constraints

Anchoring Phenomenon: Stormwater Runoff at Riverside Middle School

Imagine your school parking lot sends dirty stormwater into a nearby creek every time it rains. The runoff carries oil, trash, and sediment. Fish populations in the creek have dropped 40% in five years. Your class is asked to design a solution. How do you start?

Step 1 — Identify the Environmental Impact

First, name the specific problem. "Pollution" is too vague. A better statement is: "Stormwater runoff from the school parking lot carries sediment and oil into Cedar Creek, harming aquatic life." Being specific helps you set measurable goals.

Step 2 — Write Criteria (Your Goals)

The solution must reduce sediment in runoff by at least 80%.
The solution must filter out oil and grease before water reaches the creek.
The solution must handle a rainfall of up to 2 inches per hour without flooding.

Step 3 — Write Constraints (Your Limits)

Budget: The school has only $2,000 for the project.
Space: The solution must fit in a 10 m × 3 m area next to the parking lot.
Time: Construction must be completed during spring break (one week).
Safety: No materials that are toxic to students or wildlife may be used.

🔬 NGSS Connection

This lesson connects to the Science and Engineering Practice of "Asking Questions and Defining Problems" and the Crosscutting Concept of Cause and Effect. Human activities (cause) lead to environmental impacts (effect). Defining criteria and constraints is how engineers move from understanding a cause-and-effect relationship to designing a solution.

SECTION 5

Types of Criteria and Constraints

Not all criteria and constraints are the same. Engineers sort them into categories so they can compare solutions fairly. The diagram below shows the main types you should know.

This diagram organizes criteria (goals) on the left in cyan and constraints (limits) on the right in purple. Social/legal constraints and trade-offs appear at the bottom. The crosscutting concept of Systems and System Models reminds us that all these parts work together.

Examples of criteria and constraints across different environmental issues

Category	Example Criterion	Example Constraint
Water Quality	Reduce sediment by 80%	Cannot use chemical treatments
Air Quality	Cut CO₂ emissions by 30%	Budget of $10,000 per year
Habitat	Restore 2 acres of wetland	Must not block a road
Waste	Divert 50% of trash from landfill	Only 3 collection bins allowed

SECTION 6

Worked Example — School Recycling Program

Let's walk through a full example. Lincoln Middle School sends about 800 pounds of waste to the landfill each week. The principal wants students to design a recycling program to reduce this impact. Here is how to define the criteria and constraints.

Defining Criteria and Constraints for a School Recycling Program

Step 1 — State the Environmental Impact ClearlyBe specific about the problem. Lincoln Middle School sends about 800 lb of waste per week to the local landfill. This contributes to land pollution and produces greenhouse gases as trash decomposes.

Impact: 800 lb/week of school waste sent to the landfill

Step 2 — List Criteria (Goals)Ask: "What does success look like?" Make each criterion measurable. Criterion 1: Reduce landfill waste by at least 50% (from 800 lb to 400 lb per week). Criterion 2: At least 75% of students participate by the end of the semester. Criterion 3: Sort recyclables into paper, plastic, and aluminum categories.

Three measurable criteria identified

Step 3 — List Constraints (Limits)Ask: "What limits do we face?" Budget: The school can spend no more than $300 on bins and signs. Space: Only 4 extra recycling stations can fit in the hallways. Time: The program must launch within 3 weeks. Rules: The custodial staff can only empty bins once per day.

Four constraints identified: budget, space, time, rules

Step 4 — Identify Trade-offsMore sorting categories (paper, plastic, aluminum, glass) would improve recycling quality but would need more bins. This conflicts with the space constraint of only 4 stations. A trade-off is to combine glass and aluminum into one "metals and glass" bin. This slightly lowers recycling quality but stays within the space limit.

Trade-off: combine categories to fit space constraint

Step 5 — Organize into a Summary TableWrite everything in a table so you can compare solutions fairly. Each proposed design can be checked against these criteria and constraints. A design that meets all criteria and stays within all constraints is a strong candidate.

Criteria and constraints are ready to guide the design

🔗 PATTERN: CAUSE AND EFFECT

Notice the cause and effect pattern. The cause (school waste) leads to an effect (landfill pollution). Criteria describe how much we want to reduce the effect. Constraints describe the real-world limits on what we can do about the cause. Recognizing patterns like this is a key part of thinking like a scientist!

SECTION 7

Strengths and Limitations of Different Approaches

There are different ways to tackle an environmental problem. Each approach has strengths and limitations. The table below compares three common strategies for reducing stormwater runoff at a school. Notice how the same set of criteria and constraints can lead to very different designs.

Comparison of three stormwater solutions against our criteria and constraints

Feature	Rain Garden	Permeable Pavement	Bioswale Channel
Sediment removal	High (85–95%)	Moderate (60–80%)	High (80–90%)
Cost	$800–$1,500	$5,000+	$1,200–$2,000
Space needed	10 m × 3 m	Entire parking lot	15 m × 1 m channel
Build time	3–5 days	2–4 weeks	4–7 days
Meets $2,000 budget?	Yes ✓	No ✗	Borderline
Fits 10 m × 3 m space?	Yes ✓	No ✗	No ✗

✦ KEY TAKEAWAY

Permeable pavement works great for sediment removal, but it fails two constraints (budget and space). This shows why you must check every solution against both criteria AND constraints. A solution that meets all criteria but breaks a constraint is not usable. It is like building the perfect treehouse but discovering it does not fit in your backyard.

SECTION 8

Connecting to Advanced Topics — Sustainability and Scale

In middle school, you define criteria and constraints for local projects. In high school and beyond, scientists and engineers apply these same ideas to much larger problems like climate change, ocean pollution, and biodiversity loss. The crosscutting concept of Scale, Proportion, and Quantity helps us understand how the approach changes as the problem gets bigger.

How defining criteria and constraints scales from classroom to global problems

Feature	Middle School Level	Advanced Level
Problem scope	School or neighborhood	City, nation, or global
Criteria	Simple, measurable goals	Complex goals with multiple stakeholders
Constraints	Budget, space, time, safety	International law, politics, economics, technology limits
Data used	Classroom measurements	Satellite data, computer models, long-term studies
Trade-offs	Simple (e.g., cost vs. space)	Complex (e.g., economic growth vs. emissions reduction)

The exciting part is that the thinking skills you practice now are the exact same skills used by environmental engineers and policymakers. When you define criteria and constraints for a school project, you are practicing the same process that shaped the Paris Climate Agreement. The scale changes, but the method stays the same.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

A student says, "Our criterion is to stop all pollution." What is wrong with this criterion? A) It is too specific. B) It is not measurable or realistic. C) It is actually a constraint, not a criterion. D) Nothing is wrong — it is a good criterion.

PROBLEM 2 — BASIC

A town wants to reduce plastic waste in the river by 60%. They have $5,000 and 6 months. Which of the following is a constraint in this situation? A) Reduce plastic waste by 60%. B) The river has plastic waste in it. C) The budget is $5,000. D) Plastic harms fish.

PROBLEM 3 — INTERMEDIATE

An engineering team designs a rain garden to filter stormwater. The garden removes 90% of sediment (criterion: at least 80%), but it costs $2,500 (constraint: budget is $2,000). What should the team do? A) Accept the design because it exceeds the criterion. B) Reject the design because it violates a constraint. C) Ignore the budget because the environment is more important. D) Change the criterion to 90% so the design matches.

PROBLEM 4 — APPLIED

Your neighborhood wants to reduce light pollution that disturbs migrating birds. The community has $1,200, can only change 20 streetlights, and must follow city electrical codes. A neighbor proposes replacing all 20 lights with bird-safe amber LEDs that cost $50 each (total: $1,000) and reduce harmful wavelengths by 70%. Which statement best evaluates this proposal? A) It fails because 70% reduction is not good enough. B) It meets the budget and quantity constraints, and reduces harmful wavelengths significantly. Further review of electrical codes is still needed. C) It should be rejected because LEDs are not natural light. D) It is perfect and no further review is needed.

PROBLEM 5 — CRITICAL THINKING

Two student teams are tasked with reducing cafeteria food waste. Team A sets the criterion of "reduce food waste by 40%" with constraints of $200 budget and 2 weeks. Team B sets the criterion of "reduce food waste by 90%" with the same constraints. After testing, Team A achieves a 45% reduction and Team B achieves a 60% reduction. Which team's approach was better, and why? A) Team B, because they reduced more total waste. B) Team A, because they met their criterion and stayed within constraints. C) Both teams failed because neither reached 90%. D) Team B, because aiming higher always leads to better results.

SUMMARY

Summary

When engineers tackle an environmental problem, they begin by clearly naming the environmental impact — the specific change that human activity is causing. Next, they define criteria, which are the measurable goals a solution must achieve, like reducing sediment by 80%. They also identify constraints, which are the limits on the design such as budget, space, time, and laws. A good solution must meet all criteria AND stay within all constraints.

When criteria and constraints conflict, engineers evaluate trade-offs — giving up a little in one area to gain in another. The crosscutting concepts of Cause and Effect and Systems and System Models help us understand that every part of the problem is connected. The same process used for a school rain garden is used for global agreements like the Paris Climate Agreement. By learning to define criteria and constraints now, you are building the skills of a real engineer!