AP Environmental Science

## Learning Outcomes

- Understand what **secondary sewage treatment** is and how it differs from primary and tertiary treatment
- Explain how aerobic microorganisms reduce **biochemical oxygen demand (BOD)** and suspended solids
- Identify the main steps in the **activated sludge process**
- Compare **activated sludge** systems with **trickling filters**
- Explain why secondary treatment helps protect aquatic water bodiess from oxygen depletion
- Recognise what secondary treatment removes well and what it does not remove effectively
- Apply this knowledge to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

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

- Know that secondary sewage treatment is the **biological** stage that follows primary treatment
- Identify the role of **aerobic bacteria** in breaking down biodegradable organic waste
- Explain how secondary treatment lowers **biochemical oxygen demand (BOD)** and helps maintain dissolved oxygen in receiving waters
- Identify common secondary-treatment methods, especially **activated sludge** systems and **trickling filters**
- Understand the sequence of treatment: aeration or biofilm digestion, then settling of solids as sludge
- Recognise that secondary treatment removes most biodegradable organics and suspended solids, but does **not** remove many persistent pollutants well
- Understand that treated effluent is often disinfected after secondary treatment, commonly by chlorination
- Distinguish secondary treatment from **primary treatment** and **tertiary treatment**

## 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 secondary sewage treatment?
2. How do aerobic bacteria reduce biochemical oxygen demand in wastewater?
3. What is the difference between an activated sludge system and a trickling filter?
4. Why does lower BOD in treated wastewater help aquatic organisms in rivers and lakes?
5. Name one type of pollutant that secondary treatment does not remove effectively.

## Introduction

Secondary sewage treatment is one of the most tested parts of wastewater management because it links engineering, microbiology, and water pollution. AP Environmental Science often uses it to assess whether you can explain a process, connect that process to **dissolved oxygen** and **BOD**, and judge how well a treatment method solves an environmental problem.

At its core, secondary treatment is the stage where biology does the main cleanup work. After large debris and many settleable solids have already been removed, microorganisms are used to consume the remaining biodegradable organic matter. That matters because untreated sewage entering waterways raises oxygen demand, lowers dissolved oxygen, and can cause fish stress, fish kills, and dead zones.

> **Key Term: secondary sewage treatment**
> The biological stage of wastewater treatment in which microorganisms, usually aerobic bacteria, break down biodegradable organic matter remaining after primary treatment.

## Introduction to the process and why it matters

### What secondary treatment is designed to remove

Primary treatment mostly removes things that can be screened out or settled out. Secondary treatment goes after what is harder to remove physically: dissolved and fine suspended organic waste. This is why it is called **biological treatment**.

The central target is not just “dirt” in a general sense. It is organic matter that bacteria would otherwise decompose in a river, lake, or estuary. If that decomposition happens in the receiving water, the bacteria there consume dissolved oxygen. As oxygen levels fall, aquatic life suffers.

> **Key Term: biochemical oxygen demand (BOD)**
> A measure of how much dissolved oxygen microorganisms need to break down organic matter in water over time.

A high BOD means the water contains a lot of biodegradable organic pollution. A low BOD means much of that pollution has already been removed or decomposed.

This is a common exam trap: students often confuse **BOD** with **dissolved oxygen**. They are related, but they are not the same. BOD is the oxygen needed; dissolved oxygen is the oxygen actually present in the water.

> **Key Term: dissolved oxygen (DO)**
> The amount of oxygen gas dissolved in water and available for aquatic organisms.

The reason secondary treatment is environmentally valuable is straightforward:

1. Wastewater contains organic matter.
2. Bacteria break down that matter and use oxygen.
3. If this happens in natural waters, dissolved oxygen falls.
4. Fish and other aerobic organisms are stressed or die.
5. Secondary treatment shifts most of that decomposition into the treatment plant instead of the river.

That cause-and-effect chain is exactly the kind of explanation APES rewards.

> **Test Tip:** When an FRQ asks why sewage lowers water quality, build the chain clearly: organic waste increases bacterial activity -> BOD rises -> dissolved oxygen drops -> aquatic organisms are harmed.

### How secondary treatment fits into the full treatment sequence

A very common confusion is mixing up physical, primary, secondary, and tertiary treatment. Keep the order clear.

- **Physical treatment**: screens remove large debris like sticks, rags, and trash
- **Primary treatment**: settling removes many suspended solids and some organic matter
- **Secondary treatment**: aerobic microbes break down biodegradable waste
- **Tertiary treatment**: extra polishing, such as advanced filtration, nutrient removal, or further treatment before reuse or groundwater recharge

Secondary treatment is therefore not just “more settling.” Settling still happens, but the main treatment mechanism is biological decomposition.

In many APES sources, secondary treatment removes about **95-97% of organic waste** and **97% of suspended solids** by the end of the stage. It may also reduce some toxic metals, organic chemicals, and nitrogen, but much less completely than it removes biodegradable organics.

That last point matters. Do not overclaim what secondary treatment can do.

## How the main secondary-treatment systems work

### Activated sludge: suspended-growth treatment

The clearest example of secondary treatment is the **activated sludge process**, which is the most common system associated with this stage.

In this system, wastewater from primary treatment enters an **aeration tank**. Air or oxygen is pumped into the tank, and the wastewater is mixed with a dense population of aerobic microorganisms. These bacteria and other microbes feed on the dissolved and fine suspended organic matter.

> **Key Term: activated sludge process**
> A secondary-treatment method in which wastewater is mixed with aerobic microorganisms in an aeration tank and then sent to a settling tank so the biological solids can be removed.

Why is aeration necessary? Because the microbes doing the treatment are aerobic. They need oxygen to metabolise the organic material efficiently. Without enough oxygen, the process would not remove BOD effectively and could shift toward anaerobic conditions, which are less desirable in this stage.

As the microbes feed, they form clumps called flocs. The mixture then moves to a settling tank called a **secondary clarifier**, where the biological solids settle out.

> **Key Term: secondary clarifier**
> A settling tank in which biological solids formed during secondary treatment sink to the bottom and separate from the treated liquid.

Some of the settled biomass is recycled back to the aeration tank. This maintains a high microbial population and keeps treatment efficient. Excess sludge is removed for further processing.

A simple flow sequence looks like this:

- Primary-treated wastewater enters aeration tank
- Oxygen is added
- Aerobic microbes consume organic waste
- Mixture flows to secondary clarifier
- Solids settle as sludge
- Clearer effluent moves on to disinfection or further treatment
- Some sludge is returned to the aeration tank; excess sludge is removed

This explains why activated sludge is called a **suspended-growth** system: the microbes are suspended and mixed throughout the wastewater.

> **Key Term: suspended-growth process**
> A wastewater-treatment process in which microorganisms remain mixed within the water rather than attached to a surface.

A common APES extension is the fate of the sludge. Primary sludge and secondary sludge can be processed further with anaerobic bacteria. This can produce **methane**, which may be used as a fuel, and dried sludge cake, which may sometimes be processed for fertiliser use.

### Trickling filters: attached-growth treatment

The other major secondary-treatment method is the **trickling filter**. Despite the name, it is not just a simple filter like a screen. The main treatment mechanism is still biological.

In a trickling filter, wastewater is sprayed or allowed to flow over rocks or plastic media coated with microorganisms. These attached microbes form a **biofilm**, sometimes called a slime layer, on the surface of the material.

> **Key Term: trickling filter**
> A secondary-treatment system in which wastewater passes over media coated with microbial biofilm that breaks down organic matter.
>
> **Key Term: biofilm**
> A layer of attached microorganisms growing on a surface and feeding on organic matter in wastewater.

As wastewater trickles over the media, oxygen is available from the air spaces between the rocks or plastic surfaces. The microbes use that oxygen to break down the biodegradable organic waste. Bits of excess microbial growth can slough off and later be settled out.

This is called an **attached-growth** system because the microbes stay attached to a surface rather than being mixed throughout the water.

> **Key Term: attached-growth process**
> A wastewater-treatment process in which microorganisms grow on a surface or media while wastewater passes over them.

Students sometimes think trickling filters are anaerobic because they seem passive. That is incorrect. They are aerobic systems.

### Activated sludge and trickling filters compared

A useful way to revise is to compare the two side by side.

Activated sludge

- Microbes are suspended in wastewater
- Main location of treatment is an aeration tank
- Requires mechanical aeration
- Followed by a secondary clarifier
- Often gives strong control and strong effluent quality
- More energy-intensive

Trickling filter

- Microbes are attached to rock or plastic media
- Wastewater passes over biofilm-coated surfaces
- Oxygen comes from air circulating through the media
- Simpler in some cases
- Often lower energy demand than activated sludge
- Can have issues such as clogging or sloughing

A memory shortcut many students find useful is:

- **Activated sludge = microbes float**
- **Trickling filter = microbes stick**
  > **Test Tip:** If a question mentions an aeration tank and recycling sludge, think activated sludge. If it mentions wastewater passing over rocks or plastic media coated with bacteria, think trickling filter.

## Environmental effects, limits, and exam applications

### Why secondary treatment protects rivers and lakes

The most important environmental outcome is lower BOD in the effluent. If wastewater leaves the plant with much less biodegradable organic matter, bacteria in the receiving water have less to decompose. That means less oxygen is consumed there.

This helps prevent:

- oxygen sag in streams
- fish kills
- dominance by sludge worms and anaerobic organisms
- stress on sensitive species such as many insect larvae and game fish

In APES terms, secondary treatment is one of the main human responses to a classic water-pollution chain: **sewage -> high BOD -> low DO -> aquatic life damage**.

You may also see this linked to legislation. The **Clean Water Act (1972)** supported municipal wastewater treatment and aimed to improve the chemical, physical, and biological integrity of U.S. waters. That makes sewage treatment both a scientific and policy topic.

A useful enrichment detail is that U.S. secondary-treatment standards for publicly owned treatment works generally require strong BOD removal, often described as at least **85% removal**, with typical treated effluent around **30 mg/L BOD5** or less. Exact numbers are not always required for APES, but the idea of major BOD reduction is important.

### What secondary treatment does not remove well

A major exam distinction is knowing the limits of this stage.

Secondary treatment is very good at removing:

- suspended solids
- biodegradable organic matter
- some toxic metals and organic chemicals
- part of the nitrogen load

But it removes very little of:

- dissolved salts
- many persistent organic pollutants
- radioactive isotopes
- some pharmaceuticals and other micropollutants

This is why tertiary or advanced treatment may be needed when wastewater is intended for groundwater recharge or higher-quality reuse.

A frequent AP-style answer is that pollutants not effectively removed by ordinary secondary treatment include **persistent organic pollutants (POPs)**, **radioactive isotopes**, and **pharmaceuticals**.

> **Key Term: tertiary treatment**
> Additional wastewater treatment beyond secondary treatment, often involving advanced filtration, nutrient reduction, or further disinfection.

This also explains why chlorination often follows secondary treatment: although the biological stage greatly reduces waste, it does not guarantee that all living cells or pathogens are gone.

One more limit: chlorination can form **trihalomethanes** if chlorine reacts with remaining organic matter. These are undesirable because they are pollutants and possible carcinogens. Alternative disinfection methods such as UV radiation or ozonation can reduce that risk, but they are often more expensive or less persistent in effect.

### Common AP Environmental Science mistakes

Students often lose marks by making one of these errors:

- Saying secondary treatment is mainly physical settling
- Confusing BOD with dissolved oxygen
- Claiming secondary treatment removes all pollutants
- Forgetting that the key microbes are **aerobic**
- Calling trickling filters anaerobic
- Forgetting the settling step after biological treatment

A stronger answer names the sequence precisely:
aeration or trickling over biofilm -> microbial decomposition of organic matter -> settling of biological solids -> lower BOD and lower suspended solids in effluent

If asked to explain why sewage harms aquatic systems, include dissolved oxygen. If asked to explain how treatment works, include microbes, oxygen, and settling. If asked about limits, mention persistent pollutants or dissolved salts.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Secondary sewage treatment is the **biological** stage of wastewater treatment after primary treatment
- Its main purpose is to remove biodegradable organic matter and reduce **biochemical oxygen demand**
- High BOD means microorganisms need a lot of oxygen to decompose organic waste
- Lower BOD in effluent helps protect dissolved oxygen levels in rivers, lakes, and estuaries
- The **activated sludge process** uses aerobic microbes in an aeration tank followed by settling in a secondary clarifier
- A **trickling filter** is an attached-growth system in which wastewater passes over media coated with microbial biofilm
- Activated sludge is a **suspended-growth** process, while trickling filters are **attached-growth** processes
- Secondary treatment removes most suspended solids and biodegradable organic waste, but not many persistent pollutants well
- Persistent organic pollutants, radioactive isotopes, and many pharmaceuticals are not effectively removed by standard secondary treatment
- Sludge from treatment can be further processed, producing useful byproducts such as methane and dried sludge cake
- Secondary treatment is often followed by disinfection, commonly chlorination
- Tertiary treatment is needed when extra polishing or higher-quality reuse is required

## Key Terms and Concepts

- secondary sewage treatment
- biochemical oxygen demand (BOD)
- dissolved oxygen (DO)
- activated sludge process
- secondary clarifier
- suspended-growth process
- trickling filter
- biofilm
- attached-growth process
- tertiary treatment


Sewage Treatment - Secondary sewage treatment