AP Environmental Science

## Learning Outcomes

- Define **tertiary treatment** and explain how it differs from primary and secondary treatment
- Understand why tertiary treatment is used to protect aquatic waterwayss and support water reuse
- Explain how nitrogen and phosphorus are removed during advanced wastewater treatment
- Identify how filtration and disinfection improve effluent quality
- Compare chlorination, UV, and ozone as disinfection methods
- Apply sewage treatment knowledge to eutrophication, groundwater recharge, and APES exam questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand tertiary sewage treatment, with a focus on the following syllabus points:

- Know that **tertiary treatment** is treatment beyond conventional secondary treatment
- Identify the main goals of tertiary treatment: removal of nutrients, fine suspended solids, and pathogens
- Explain why tertiary treatment helps reduce **eutrophication** in receiving waters
- Understand the difference between **secondary treatment** and **tertiary treatment**
- Know that nitrogen removal may involve **nitrification** followed by **denitrification**
- Identify phosphorus removal methods such as chemical precipitation with lime, alum, or iron salts
- Understand that filtration is a polishing step and disinfection inactivates pathogens before discharge or reuse
- Recognise that chlorine, UV, and ozone are common disinfection methods, each with advantages and limits

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the main purpose of tertiary sewage treatment?
2. How is tertiary treatment different from secondary treatment?
3. Why does removing nitrogen and phosphorus matter for aquatic water bodiess?
4. What is the difference between nitrification and denitrification?
5. Why might a wastewater plant use UV or ozone instead of chlorine?

## Introduction

## Introduction

Municipal sewage treatment does not end when large solids settle out and aerobic bacteria digest organic waste. Even after secondary treatment, treated wastewater can still contain nutrients, tiny suspended particles, disease-causing organisms, and some chemicals that create environmental problems if the water is released into rivers, lakes, coastal waters, or groundwater.

That is where tertiary sewage treatment becomes important. In AP Environmental Science, tertiary treatment is the advanced polishing stage after secondary treatment. Its job is to improve effluent quality further, especially by reducing nitrogen, phosphorus, fine solids, and pathogens. This matters because untreated or partly treated wastewater can raise biological oxygen demand, lower dissolved oxygen, and drive eutrophication.

> **Key Term: tertiary treatment**
> Advanced wastewater treatment that occurs after secondary treatment and is used to remove remaining pollutants, especially nutrients, fine suspended solids, and pathogens.

### Why tertiary treatment is needed after secondary treatment

Secondary treatment already removes most biodegradable organic waste and suspended solids. That is why students sometimes assume tertiary treatment is optional extra cleaning with little environmental effect. That is a mistake. Secondary treatment is a strong improvement, but it still leaves enough pollutants to damage sensitive waters.

A useful APES comparison is the federal secondary-treatment baseline for municipal plants. Secondary treatment commonly aims for **BOD₅** and **TSS** levels of no more than **30 mg/L** as a 30-day average, with at least **85% removal**, and an effluent pH of **6.0-9.0**. Those standards show that secondary treatment is the minimum conventional level. Tertiary treatment goes beyond this baseline.

> **Key Term: secondary treatment**
> The biological stage of sewage treatment in which microorganisms, usually aerobic bacteria, remove most biodegradable organic matter and additional suspended solids.

Why is that extra step needed?

First, nutrients left in wastewater can still fertilize receiving waters. Nitrogen and phosphorus are often the most important nutrients in eutrophication. When wastewater rich in these nutrients enters a pond, lake, estuary, or slow-moving river, algae and aquatic plants can grow very quickly. At first, students sometimes think “more plant growth” sounds positive. The problem is what happens next: the bloom blocks light, deeper plants die, decomposers break down the dead material, and dissolved oxygen falls. Fish and invertebrates can then die.

Second, secondary-treated water may still contain pathogens. If that water is reused for irrigation, discharged near recreation areas, or allowed to recharge groundwater, disease risk becomes a major issue.

Third, very small suspended particles can remain in the effluent. These particles increase turbidity, reduce water clarity, and can shelter microbes from disinfection.

> **Key Term: eutrophication**
> The enrichment of water by nutrients, especially nitrogen and phosphorus, leading to excessive algal or plant growth and often reduced dissolved oxygen.

Cause-and-effect chain students should know:

- wastewater leaves secondary treatment with some nutrients and microbes still present
- effluent enters a sensitive water body or groundwater recharge system
- nutrients stimulate algal growth
- dead algae are decomposed by bacteria
- decomposition raises biological oxygen demand
- dissolved oxygen falls
- aquatic organisms are stressed or killed
  > **Test Tip:** If an FRQ asks why tertiary treatment reduces environmental impact, do not just say “it cleans the water more.” Link the process to outcomes such as lower nutrient loading, less eutrophication, lower pathogen risk, or safer groundwater recharge.

### Nutrient removal: how tertiary treatment controls nitrogen and phosphorus

The most tested part of tertiary treatment is nutrient removal. APES expects you to connect this directly to eutrophication control.

#### Nitrogen removal

Nitrogen in wastewater often enters the plant in forms such as ammonia or ammonium from urine and decomposing organic matter. Removing nitrogen is not one single step. It commonly involves two linked biological processes that students often confuse.

> **Key Term: nitrification**
> An aerobic biological process in which ammonia or ammonium is converted first to nitrite and then to nitrate.
>
> **Key Term: denitrification**
> A biological process, usually under low-oxygen or anoxic conditions, in which nitrate is converted to nitrogen gas and removed from the water.

These are not the same thing.

- **Nitrification** changes ammonia into nitrate
- **Denitrification** removes nitrogen from the wastewater by converting nitrate into nitrogen gas, which escapes to the atmosphere

That distinction matters. Nitrification alone does **not** remove nitrogen from water. It only changes nitrogen’s form.

A simple sequence is:

- ammonia/ammonium in wastewater
- aerobic bacteria convert it to nitrite, then nitrate
- under low-oxygen conditions, denitrifying bacteria convert nitrate to nitrogen gas
- nitrogen gas leaves the water

Some systems need an added carbon source, such as sugars or alcohols, to help denitrifying bacteria function efficiently. This is why some advanced plants add carbon-containing substances during treatment.

#### Phosphorus removal

Phosphorus can be removed biologically or chemically, but APES most often stresses chemical precipitation and nutrient control.

In chemical phosphorus removal, chemicals are added so dissolved phosphate forms insoluble solids that can then be removed. The source material specifically highlights **lime, Ca(OH)₂**, and web-based support also includes alum and iron salts as common options.

A basic idea students should remember:

- dissolved phosphate is hard to settle out by itself
- adding lime, alum, or iron salts causes phosphate to form solids
- those solids can then be filtered or settled out

This is called precipitation because the dissolved phosphorus becomes a solid precipitate.

> **Key Term: phosphorus removal**
> The removal of phosphate from wastewater, often by chemical precipitation or biological uptake, to reduce nutrient pollution.

A common misconception is that disinfection removes nutrients. It does not. Chlorine, UV, and ozone target pathogens, not nitrate or phosphate.

Another common misconception is that all plants remove the same amount of nitrogen and phosphorus. They do not. Tertiary treatment depends on plant design, permit limits, receiving waters, and reuse goals.

#### Biological nutrient removal

Some wastewater plants use microorganisms in carefully controlled stages to remove both nitrogen and phosphorus. This is often called biological nutrient removal.

> **Key Term: biological nutrient removal (BNR)**
> An advanced wastewater treatment approach that uses microorganisms under controlled conditions to remove nitrogen and phosphorus.

For APES, you do not need highly technical engineering details. What matters is the principle: different microbial conditions are created so bacteria can convert or store nutrients, allowing them to be removed from the water.

> **Test Tip:** On multiple-choice questions, watch for the trap “nitrification removes nitrogen.” The accurate statement is that nitrification converts ammonia to nitrate; denitrification is needed for actual nitrogen removal from the water.

### Filtration and disinfection: the final polishing steps

Tertiary treatment is not only about nutrients. It is also about polishing the effluent so the water is clearer and safer.

#### Filtration

After secondary treatment, small suspended particles can still remain. Tertiary filtration passes effluent through media that trap these particles.

Common examples include:

- sand filters
- carbon filters
- cloth or disk filters
- membrane filtration in more advanced systems

The APES framing often uses sand and carbon filtration as the classic tertiary example, especially for water that will be discharged to groundwater.

> **Key Term: effluent**
> Treated wastewater that leaves a treatment plant.
>
> **Key Term: effluent filtration**
> A tertiary treatment step that removes fine suspended particles from treated wastewater.

Why filtration matters:

- it lowers turbidity
- it improves water clarity
- it removes particles that may contain phosphorus
- it helps disinfection work better, because microbes are less likely to be shielded inside particles

This last point is easy to miss. UV light and chemical disinfectants work best when water is relatively clear.

#### Disinfection

The final barrier before discharge or reuse is often disinfection.

> **Key Term: disinfection**
> The process of killing or inactivating pathogens in treated wastewater before discharge or reuse.

The three main APES methods are chlorine, UV radiation, and ozone.

#### Chlorination

Chlorine is widely used because it is effective, relatively inexpensive, and leaves a residual disinfecting effect that can continue after treatment. But chlorine has drawbacks.

The source material highlights one of the most important: it can form **trihalomethanes** when it reacts with remaining organic matter. These compounds are undesirable because they are pollutants and possible carcinogens.

Chlorine residual can also harm aquatic life if discharged directly. That is why some systems add a follow-up step.

> **Key Term: dechlorination**
> The removal or neutralisation of residual chlorine from treated wastewater before discharge, to protect aquatic organisms.

#### UV disinfection

Ultraviolet radiation damages microbial DNA and prevents pathogens from reproducing. UV has the advantage of avoiding chemical residues and chlorinated byproducts. However, it usually works best when water is already clear and may require careful maintenance and energy input.

#### Ozone

Ozone is a strong oxidant that can destroy many pathogens and some other contaminants. It is effective but usually more expensive and more technically demanding than chlorine.

A clear comparison helps:

- **Chlorine:** effective, cheap, long-lasting residual, but can form harmful byproducts and may require dechlorination
- **UV:** no chemical residual, avoids chlorinated byproducts, but works best in clear water and may cost more
- **Ozone:** powerful disinfectant, can also oxidize some contaminants, but costly

The textbook notes that chlorine has often remained preferred because alternatives like UV and ozonation may be more expensive or less long-lasting.

### Why tertiary treatment matters for reuse, groundwater, and exam application

Tertiary treatment becomes especially important when treated wastewater is not simply discharged into a flowing river or the ocean, but reused or allowed to enter groundwater.

In arid and semi-arid regions, reclaimed water is valuable. Water that has undergone tertiary treatment may be used for:

- irrigation
- landscaping
- golf courses
- nurseries
- groundwater recharge

Groundwater recharge needs stricter quality because pollutants entering aquifers can remain there for a long time and are hard to remove. That is why tertiary treatment is often required before discharge into percolation basins or groundwater recharge systems.

This is also where APES connects sewage treatment to sustainability. Tertiary treatment is expensive, but it can reduce withdrawals from rivers and aquifers. In dry regions, that trade-off can make strong environmental sense.

Still, students should avoid overclaiming. Tertiary treatment does not remove everything. Persistent pollutants such as some pharmaceuticals, pesticides, or radioactive isotopes may remain unless even more advanced treatment is added. Sludge and biosolids management also creates separate environmental concerns.

A balanced judgement looks like this:

- tertiary treatment reduces nutrient pollution, fine solids, and pathogens
- it supports reuse and protects sensitive waters
- it is costly and energy-intensive
- some pollutants may still remain

If an FRQ asks for one tertiary process and its environmental benefit, a strong answer could be:

- **Process:** sand filtration after secondary treatment
- **Benefit:** removes fine suspended particles, lowers turbidity, improves disinfection, and reduces pollutant loading before groundwater recharge or discharge

Or:

- **Process:** denitrification
- **Benefit:** removes nitrate from wastewater, reducing eutrophication risk in receiving waters

Or:

- **Process:** UV disinfection
- **Benefit:** inactivates pathogens without leaving chlorine residual that could harm aquatic life
  > **Test Tip:** In FRQs, name the process first, then explain the mechanism, then link it to the environmental result. Structure it as: “This process works by... Therefore it reduces...”

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Tertiary sewage treatment is advanced treatment that occurs after secondary treatment
- Its main goals are to remove remaining nutrients, fine suspended solids, and pathogens
- Tertiary treatment helps reduce eutrophication by lowering nitrogen and phosphorus discharge
- Secondary treatment mainly removes biodegradable organic matter and much of the suspended solids
- Nitrification converts ammonia or ammonium into nitrate under aerobic conditions
- Denitrification removes nitrogen by converting nitrate into nitrogen gas under low-oxygen conditions
- Phosphorus can be removed by chemical precipitation using substances such as lime
- Filtration is a polishing step that removes fine particles and improves water clarity
- Disinfection targets pathogens and commonly uses chlorine, UV radiation, or ozone
- Chlorination can form trihalomethanes and may require dechlorination before discharge
- Tertiary treatment is especially important for groundwater recharge and water reuse
- Tertiary treatment improves water quality greatly, but it does not remove every possible pollutant

## Key Terms and Concepts

- tertiary treatment
- secondary treatment
- eutrophication
- nitrification
- denitrification
- phosphorus removal
- biological nutrient removal (BNR)
- effluent
- effluent filtration
- disinfection
- dechlorination


Sewage Treatment - Tertiary sewage treatment