AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/8/8-11)
Unit 8 · Topic 8.11
8.11 Sewage Treatment
Sewage treatment plants clean wastewater in stages: primary treatment removes solids physically, secondary treatment uses bacteria to break down organic matter, and tertiary treatment removes leftover nutrients and chemicals. The water is disinfected before release, and the leftover sludge must be dealt with.
Key terms
- primary treatment
- secondary treatment
- tertiary treatment
- sludge
- disinfection
Why treat sewage?
Raw sewage contains human waste, food, soaps and chemicals. It carries pathogens (disease-causing organisms) that spread diseases such as cholera and dysentery. It is full of organic matter that bacteria decompose, using up oxygen in rivers. Its high biological oxygen demand (BOD), the amount of oxygen microbes need to break down the organic matter, is the main measure of that problem. And it adds nitrogen and phosphorus that cause eutrophication.
The stages
| Stage | Type of process | What happens | What it removes |
|---|---|---|---|
| Primary | Physical | Screens catch large objects; grit settles out; in settling tanks, solids sink as sludge and grease floats off | Large debris, sand, much of the suspended solids |
| Secondary | Biological | In aeration tanks, air is bubbled in and aerobic bacteria eat the dissolved organic matter; the bacteria then settle out in a second tank | Most of the remaining organic matter (lowers BOD) |
| Tertiary | Chemical or ecological | Chemicals, filters or constructed wetlands remove what's left | Nitrogen, phosphorus, and some other chemicals |
| Disinfection | Chemical or physical | Chlorine, ultraviolet (UV) light or ozone kill pathogens before release | Bacteria and viruses |
Measuring how well it works
Treatment plants test their water before and after each stage. BOD shows how much organic matter is left; counts of fecal coliform bacteria (bacteria from the intestines of people and animals) show whether disease-causing organisms could be present; and nitrogen and phosphorus levels show the risk of eutrophication downstream.
In the United States, the Clean Water Act requires city sewage plants to provide at least secondary treatment. Tertiary treatment is added where the receiving water is especially sensitive, such as a lake or bay already suffering from algal blooms.
Sludge and other challenges
The solids removed during treatment form sludge. It can be broken down in anaerobic digesters, which produce methane that the plant can burn for energy. The remaining solids can be landfilled, incinerated, or treated and spread on farmland as fertilizer, called biosolids. Spreading biosolids recycles nutrients but raises concerns about heavy metals, pathogens and chemicals they may contain.
Many older cities have combined sewer systems that carry both sewage and storm runoff in the same pipes. Heavy rain can overwhelm the treatment plant, sending untreated sewage straight into rivers, an event called a combined sewer overflow.
Standard treatment is not designed to remove many pharmaceuticals, personal care products and microplastics, so some pass through into rivers (topic 8.3). Homes not connected to sewers use septic systems, where solids settle in an underground tank and liquid seeps into the soil through a drain field.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
BOD removal through two stages
Wastewater entering a plant has a BOD of 250 mg/L. Primary treatment removes 30% of the BOD. Secondary treatment removes 85% of the BOD that remains. (a) What is the BOD after secondary treatment? (b) What is the overall percent removed?
Show the solutionHide the solution
- Step 1: After primary: 30% removed, so 70% remains. 250 × 0.70 = 175 mg/L.
- Step 2: After secondary: 85% of the remaining 175 is removed, so 15% of 175 remains. 175 × 0.15 = 26.25 mg/L.
- Step 3: (b) Overall removed = (250 − 26.25) ÷ 250 × 100 = 223.75 ÷ 250 × 100 = 89.5%.
- Step 4: Note: you can't just add 30% + 85% = 115%. The second percentage applies only to what was left after the first stage.
Answer: (a) About 26 mg/L (26.25 mg/L). (b) 89.5% removed overall.
Common mistakes
- Mixing up primary and secondary treatment. Primary is physical settling; secondary is biological breakdown by bacteria.
- Saying primary or secondary treatment removes nitrogen and phosphorus well. Removing nutrients is the job of tertiary treatment.
- Adding percentages from successive stages. Each stage's percentage applies to what remains after the previous stage.
On the exam
- Expect to identify which stage does what. Keep the pattern straight: physical, biological, chemical or ecological, then disinfection.
- Link tertiary treatment to preventing eutrophication; that connection often earns points.
Connected topics
Videos
Check yourself
4 questions on 8.11 Sewage Treatment. Pick an answer to see if you got it, and why.
In which stage of wastewater treatment do bacteria in aerated tanks break down dissolved organic matter?
Which sequence lists the steps of modern wastewater treatment in the correct order?
A wastewater plant switches its final disinfection step from chlorine to ultraviolet (UV) light. Which is the main environmental benefit?
A treatment plant discharges into a bay that suffers from algal blooms every summer. Which upgrade would most directly reduce the plant's contribution to the blooms?
0 of 4 answered