AP Environmental Science

## Learning Outcomes

- Understand what **primary sewage treatment** is and where it fits in wastewater treatment
- Identify the main steps involved, including screening, grit removal, settling, and skimming
- Explain how a **primary clarifier** separates solids by density
- Distinguish clearly between **preliminary treatment**, primary treatment, and secondary treatment
- Recall typical removal percentages for **TSS** and **BOD**
- Apply this knowledge to AP Environmental Science free-response and multiple-choice questions

## AP Environmental Science Syllabus

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

- Know that primary sewage treatment is an early, mainly physical stage of wastewater treatment
- Identify the role of screening, grit removal, sedimentation, sludge collection, and scum skimming
- Explain how a primary clarifier works and why reduced flow allows solids to separate
- Distinguish between materials removed in primary treatment and pollutants that remain in the water
- Understand that primary treatment removes about **50–70% of total suspended solids (TSS)** and about **25–40% of biochemical oxygen demand (BOD)**
- Explain why primary treatment reduces the load on secondary biological treatment
- Know that primary treatment does not disinfect wastewater and does not remove many dissolved pollutants
- Recognise the difference between **sludge** and **scum**

## 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 primary sewage treatment?
2. How does a primary clarifier separate waste materials from wastewater?
3. What is the difference between **sludge** and **scum**?
4. Why are screening and grit removal important before or during early sewage treatment?
5. Why is secondary treatment still needed after primary treatment?

## Introduction

Primary sewage treatment is the first major cleaning stage that wastewater passes through after it arrives at a treatment plant. Its job is not to make sewage fully clean. Instead, it removes the easiest materials to separate: large debris, heavy settleable solids, and light floatable matter. This matters because if these materials stay in the wastewater, they clog equipment, raise treatment costs, and make later biological treatment less effective.

For AP Environmental Science, students often lose marks by describing sewage treatment too vaguely. You need to be precise about what primary treatment actually does: it is mainly **physical separation**. Wastewater slows down, heavier solids sink, lighter materials float, and the partially clarified liquid moves on for more treatment.

> **Key Term: wastewater**
> Water that has been used by humans and contains dissolved or suspended wastes.

## Introduction to the process

Before focusing on the clarifier itself, it helps to see the big picture. Wastewater from homes, businesses, and some industries enters a treatment plant carrying many different types of pollutants: toilet paper, food particles, grease, sand, organic matter, pathogens, nutrients, and sometimes toxic chemicals. No single step can remove all of these.

Primary treatment works because not all pollutants behave the same way in water. Some are large and easy to catch. Some are dense enough to settle. Some are light enough to float. Others are dissolved and stay mixed in the water. Primary treatment targets the first three groups only.

> **Key Term: primary sewage treatment**
> The early, mainly physical stage of wastewater treatment that removes large debris, settleable solids, and floatable materials before secondary treatment.

That final phrase, **before secondary treatment**, is critical. Primary treatment is a preparation stage as much as a removal stage. By taking out much of the suspended material first, it makes the biological stage more efficient and protects pumps, pipes, and tanks from damage.

### Screening and grit removal: the first protections

In precise wastewater terminology, **screening** and **grit removal** are often classed as preliminary treatment rather than primary treatment proper. However, AP Environmental Science questions sometimes group them with the early physical stage, so you should understand them as part of the overall first phase of sewage processing.

**Screening** removes large debris such as rags, sticks, plastics, paper, and trash. This is less about water chemistry and more about plant operation. If these items are not removed, they jam pumps, block channels, and interfere with settling tanks.

**Grit removal** targets dense inorganic particles such as sand, gravel, coffee grounds, and eggshell fragments. These do not decompose easily. Instead, they settle in pipes and abrade machinery. Grit is dangerous because it wears down moving parts and fills tanks that should be used for other solids.

> **Key Term: screening**
> The removal of large debris from wastewater using bars or screens.
>
> **Key Term: grit removal**
> The removal of heavy inorganic particles such as sand and gravel from wastewater.

Students often confuse these two. Screening removes **large** items, regardless of whether they are light or heavy. Grit removal removes **dense** particles, even if they are quite small.

This distinction matters in FRQs. If a question asks why a treatment plant uses screens, the best answer is not “to clean the water” in general. A stronger answer is: screens remove large objects that would clog or damage downstream equipment. If the question asks about grit chambers, explain that they remove abrasive inorganic particles that would otherwise settle in pipes and tanks.

> **Test Tip:** If an APES question asks about the “first” wastewater step, check the wording carefully. In strict process terms, screening and grit removal are preliminary treatment, while primary treatment is settling in a clarifier.

### Sedimentation in the primary clarifier

The core of primary sewage treatment happens in a **primary clarifier**, also called a settling tank or sedimentation tank. Here, wastewater flow is slowed down enough that gravity can do the work.

When water moves quickly, particles stay mixed. When it slows, particles begin separating by density. Heavier solids settle to the bottom. Lighter materials such as oils and grease rise to the surface. The water in the middle becomes clearer than the incoming wastewater and can move on to secondary treatment.

> **Key Term: primary clarifier**
> A tank where wastewater flow slows so that heavier solids settle and lighter materials float for removal.

This process is called **sedimentation**. It is not biological digestion. It is not disinfection. It is not advanced chemical treatment. It is gravity-based separation.

> **Key Term: sedimentation**
> The settling of suspended particles out of water when flow slows and gravity acts on the particles.

Inside the clarifier, three zones effectively form:

- a bottom zone where heavy solids accumulate
- a surface zone where floatable materials gather
- a middle liquid zone where clarified effluent flows onward

The efficiency of this process depends on time and calm conditions. If wastewater rushes through too fast, solids do not have time to settle. If turbulence is high, already-settled materials can mix back in. That is why clarifiers are built to reduce flow speed and promote even movement.

A useful cause-and-effect chain for exams is this:

1. wastewater enters the clarifier
2. flow slows
3. gravity separates materials by density
4. settleable solids sink and floatables rise
5. suspended solids in the liquid decrease
6. the next treatment stage receives a smaller pollution load

That final point is especially important. Primary treatment is not only about what it removes. It also changes the work that secondary treatment must do.

### Sludge, scum, TSS, and BOD: what is removed and why it matters

The solids that settle to the bottom of the clarifier are called **primary sludge**. The materials that float to the top are called **scum**. These are not interchangeable terms.

> **Key Term: sludge**
> The heavier solids removed from wastewater after they settle to the bottom.
>
> **Key Term: scum**
> The lighter floatable material, such as oil and grease, that rises to the surface of wastewater and is skimmed off.

This is one of the common exam distinctions. Sludge **sinks**. Scum **floats**.

Primary treatment removes a substantial fraction of **total suspended solids**, or **TSS**. TSS measures the amount of suspended particulate matter in the water. Since the clarifier is designed to settle and skim such matter, TSS falls sharply during this stage.

> **Key Term: total suspended solids (TSS)**
> A measure of suspended particles in water.

Typical primary treatment removes about **50–70% of TSS**. Some textbook summaries give figures around **60%**, which fits inside that normal range.

Primary treatment also lowers **biochemical oxygen demand**, or **BOD**. BOD measures how much oxygen microorganisms would need to break down biodegradable organic matter in the water. If wastewater has lots of organic material, microbes use more oxygen. That is a problem when untreated or poorly treated sewage enters rivers, because microbial decomposition can deplete dissolved oxygen and harm aquatic life.

> **Key Term: biochemical oxygen demand (BOD)**
> A measure of the oxygen needed by microorganisms to decompose biodegradable organic matter in water.

Primary treatment usually removes about **25–40% of BOD**. Some AP-style materials summarise this as around **30% of organic waste** removed at the primary stage.

Why is BOD reduction lower than TSS reduction? Because not all oxygen-demanding waste is in settleable particles. A lot of organic matter is dissolved or in very fine particles that do not settle easily. Primary treatment removes the easier fraction. Secondary treatment removes much more by using aerobic bacteria to digest biodegradable material.

This is exactly why students should avoid overclaiming what primary treatment can do. It is effective, but limited.

### What primary treatment does not remove

Primary sewage treatment is only a first stage. Even after settling and skimming, the wastewater still contains many pollutants.

Materials that commonly remain include:

- dissolved organic matter
- many pathogens
- significant nitrogen and phosphorus
- dissolved salts
- persistent organic pollutants
- many pharmaceuticals
- radioactive substances
- much of the finer suspended material

This explains why the water leaving the primary clarifier is not safe to drink and usually not safe to discharge without further treatment. It still has a high enough organic load to require biological treatment, and it may still contain disease-causing organisms.

A very common mistake is to think “primary” means “basic but complete.” It does not. The water is only **clarified**, not fully treated.

> **Test Tip:** If an FRQ asks for a pollutant not removed by basic sewage treatment, strong answers include persistent organic pollutants, pharmaceuticals, or radioactive isotopes.

### Primary treatment versus secondary treatment

Students often confuse primary and secondary treatment because both involve the removal of solids and both may produce sludge. The difference is in the main mechanism.

Primary treatment is mainly **physical**:

- screening
- grit removal
- settling
- skimming

Secondary treatment is mainly **biological**:

- aerobic bacteria digest biodegradable waste
- solids then settle again after biological action

A side-by-side comparison helps:

| Feature             | Primary treatment               | Secondary treatment                         |
| ------------------- | ------------------------------- | ------------------------------------------- |
| Main method         | Physical separation             | Biological decomposition                    |
| Main location       | Clarifier/settling tank         | Trickling filter or activated sludge system |
| Main target         | Settleable and floatable solids | Biodegradable organic matter                |
| Typical TSS removal | 50–70%                          | Much higher overall after this stage        |
| Typical BOD removal | 25–40%                          | Much higher overall after this stage        |
| Key idea            | Gravity separates materials     | Aerobic bacteria digest waste               |

This distinction is high-yield for APES because questions often ask how treatment plants lower **BOD** or reduce environmental damage. A complete answer usually mentions both the early physical removal of solids and the later biological digestion of organic matter.

### Why primary treatment matters for the environments and treatment plants

It is easy to describe primary treatment as a mechanical step and move on. But for AP Environmental Science, you should also understand why it matters environmentally.

First, it lowers the risk that untreated sewage will cause severe oxygen depletion in receiving waters. By removing part of the suspended solids and organic waste, primary treatment reduces the amount of material that would otherwise be decomposed by bacteria in rivers or coastal waters. Less decomposition means less oxygen demand.

Second, it protects later treatment stages. Secondary treatment depends on aerobic bacteria working efficiently. If the incoming wastewater is overloaded with debris and heavy solids, those bacteria systems become less effective and more expensive to operate.

Third, it reduces damage to infrastructure. Grit and large debris increase wear, block pipes, and create maintenance problems. Early removal makes the whole plant function better.

Fourth, it creates byproducts that can be processed further. Sludge may later be digested anaerobically, producing methane that can be used as fuel, while treated biosolids may be processed for fertilizer use if contamination levels are low enough.

So the environmental value of primary treatment is both direct and indirect:

- direct, because it removes pollutants from wastewater
- indirect, because it makes later pollution control more effective

### How this appears on the AP exam

On multiple-choice questions, primary treatment is often tested through definitions, process order, or removal percentages. You may be asked which step removes settleable solids, what a clarifier does, or which pollutant is least likely to be removed.

On free-response questions, command words matter:

- **Define** primary treatment: say it is the mainly physical stage that removes large debris, settleable solids, and floatable materials.
- **Describe** a primary clarifier: explain slowed flow, settling, sludge, floating scum, and removal.
- **Explain** why primary treatment is necessary: discuss pollutant reduction, equipment protection, and reduced load on secondary treatment.

A high-scoring response usually includes technical terms accurately, instead of vague phrases such as “the water is filtered and cleaned.”

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Primary sewage treatment is the early, mainly physical stage of wastewater treatment
- Screening removes large debris such as rags, plastics, and sticks
- Grit removal removes dense inorganic particles such as sand and gravel
- The main primary treatment step occurs in a primary clarifier or settling tank
- In the clarifier, heavier solids settle by gravity and lighter materials float
- Settled bottom material is called sludge, while floating top material is called scum
- Primary treatment typically removes about 50–70% of total suspended solids
- Primary treatment typically removes about 25–40% of biochemical oxygen demand
- TSS measures suspended particles, while BOD measures oxygen demand from biodegradable waste
- Primary treatment reduces the load on secondary biological treatment
- Primary treatment does not disinfect wastewater
- Many dissolved pollutants, pathogens, pharmaceuticals, and persistent pollutants remain after primary treatment

## Key Terms and Concepts

- wastewater
- primary sewage treatment
- screening
- grit removal
- primary clarifier
- sedimentation
- sludge
- scum
- total suspended solids (TSS)
- biochemical oxygen demand (BOD)


Sewage Treatment - Primary sewage treatment