Introduction
Sewage treatment is the process of removing contaminants from wastewater, primarily from household sewage. It includes physical, chemical, and biological processes to remove pollutants and produce an environmentally safe treated wastewater. The final product, called effluent, can be safely released into the environment, such as surface waters or reused for irrigation. The by-products of treatment, such as biosolids, can also be recycled and used as fertilizer.
Primary Sewage Treatment
Primary treatment is the first step in the process of sewage treatment. It involves the physical separation of solid and liquid waste using screens and sedimentation tanks. The goal of primary treatment is to remove large objects and debris, and to settle out heavy particles in the wastewater.
Secondary Sewage Treatment
Secondary treatment is the next step in the process, and involves the use of biological processes to break down the remaining organic matter in the wastewater. This is typically done using microorganisms, such as bacteria, that consume the organic matter and convert it into biomass. Secondary treatment is usually done in aerated lagoons or activated sludge tanks.
Tertiary Sewage Treatment
Tertiary treatment is the final step in the sewage treatment process. It involves the use of physical, chemical, or biological processes to remove remaining pollutants and pathogens from the wastewater. This can include methods such as filtration, disinfection, and nutrient removal. The goal of tertiary treatment is to produce an effluent that is of high enough quality to be safely discharged into the environment or reused for irrigation.
Purpose of Sewage Treatment Plans
The purpose of a sewage treatment plant is to remove pollutants and pathogens from wastewater generated by households, industries and commercial buildings, so that the treated water can be safely released into the environment or reused for irrigation. The main pollutants removed during the treatment process are organic matter, nutrients, pathogens, and other contaminants.
Sewage treatment plants also play an important role in protecting public health by reducing the spread of disease through the removal of pathogens from the wastewater. They also help to protect the environment by preventing pollution of surface waters, groundwater, and coastal areas. Additionally, sewage treatment plants also produce by-products such as biosolids that can be used as fertilizer to improve soil quality.
Moreover, sewage treatment plants also help to conserve water resources by treating and reusing wastewater, which is beneficial in areas where water is scarce. Additionally, they can generate energy through the process of anaerobic digestion of the organic matter present in the wastewater which can be used to power the treatment plant and even provide extra energy to the grid.
Septic Tank
A septic tank is a type of underground wastewater treatment system that is commonly used in areas where there is no connection to a centralized sewage treatment plant. It is essentially a large, underground container that receives and stores wastewater from a building, and allows for the separation and treatment of the waste.
The septic tank is designed to separate solid waste from liquid waste through a process of sedimentation and anaerobic digestion. The solid waste, called sludge, sinks to the bottom of the tank and accumulates over time. The liquid waste, called effluent, flows out of the tank and is typically discharged into a drainage field or leach field, where it is further treated through a process of filtration and bacterial decomposition.
Septic tanks are typically made of concrete, fiberglass, or plastic and are buried underground. They require regular maintenance to ensure they function properly and to prevent the buildup of sludge in the tank. This includes regular pumping of the tank to remove the accumulated sludge, and regular inspections of the tank and drainage field to ensure they are functioning properly.
Proper design, installation and maintenance of septic tanks are important to ensure that they function efficiently and effectively, and to prevent groundwater and surface water pollution.
Where are Septic Tanks Used?
Septic tanks are commonly used in rural areas, suburban areas, and small communities where there is no connection to a centralized sewage treatment plant. Here are a few examples of how septic tanks are used in real life:
- Residential homes: Septic tanks are often used in homes that are not connected to a municipal sewer system. The tank receives and treats the waste from the home's plumbing system, and the treated effluent is discharged into a drainage field or leach field.
- Small communities: In small towns and rural areas, septic tanks are commonly used to treat the wastewater from multiple homes or businesses. The tanks are typically connected to a common drainage field or leach field.
- Recreational areas: Septic tanks are also used in recreational areas, such as campgrounds, RV parks, and cabin communities. The tanks receive and treat the waste from the toilet and shower facilities, and the treated effluent is discharged into a drainage field or leach field.
- Commercial properties: Septic tanks can also be used to treat the waste from commercial properties such as small businesses, hotels, and motels. These tanks are typically larger than residential septic tanks to accommodate the greater volume of waste generated by the property.
- Remote or off-grid locations: Septic tanks can be used in remote or off-grid locations where there is no access to a centralized sewage treatment system. The tanks are designed to treat the waste generated by the occupants of the location and discharge the treated effluent into the environment.
Groundwater Pollution
Groundwater pollution, also known as groundwater contamination, occurs when pollutants or contaminants enter the groundwater and make it unsafe for human use or the environment. Groundwater is a vital source of drinking water for many communities and is also used for irrigation, industry and other purposes.
There are many sources of groundwater pollution, including agricultural activities, industrial and commercial operations, landfills, and septic systems. Chemicals such as pesticides, fertilizers, oil, gasoline, and industrial chemicals can leach into the groundwater and contaminate it. Additionally, pathogens and microorganisms can also pollute groundwater, making it a health hazard.
Groundwater pollution can have severe and long-lasting effects on the environment and human health. Contaminated groundwater can lead to the destruction of ecosystems, the loss of wildlife habitat, and the contamination of drinking water supplies. In addition, exposure to contaminated groundwater can cause a wide range of health problems, including cancer, birth defects, and neurological damage.
To prevent groundwater pollution, it is essential to properly manage and dispose of hazardous materials and waste, and to implement best management practices for agricultural activities, and industries. Additionally, regular testing and monitoring of groundwater quality is important to detect and address any contamination issues in a timely manner.
Clean Water Act
The Clean Water Act (CWA) is a federal law in the United States that was enacted in 1972 to regulate the discharge of pollutants into the nation's surface waters, including lakes, rivers, streams, wetlands, and coastal areas. The Act's primary goal is to restore and maintain the chemical, physical, and biological integrity of the nation's waters, and to ensure that they are safe for human use and the environment.
The CWA gives the Environmental Protection Agency (EPA) the authority to establish water quality standards for all contaminants in surface waters, and to regulate the discharge of pollutants from point sources, such as municipal and industrial wastewater treatment plants, and nonpoint sources, such as agricultural activities and stormwater runoff.
The CWA also established the National Pollutant Discharge Elimination System (NPDES) permit program, which requires that any discharge of pollutants from a point source into surface waters must have a permit issued by the EPA or an authorized state. The Act also provides for the protection and management of wetlands, and the Clean Water State Revolving Fund, which provides funding for municipalities to improve their wastewater treatment plants.
The Clean Water Act has been successful in improving the quality of surface waters in the United States. However, there are still many challenges ahead, such as addressing nonpoint source pollution, dealing with emerging contaminants, and protecting wetlands and other sensitive aquatic habitats.
The Safe Drinking Water Act
The Safe Drinking Water Act (SDWA) is a federal law in the United States that was enacted in 1974 to protect public health by regulating the nation's public drinking water supply. The Act's goal is to ensure that the drinking water provided by public water systems is safe to drink and meets the standards established by the Environmental Protection Agency (EPA).
The SDWA gives the EPA the authority to set national standards for the quality of drinking water, including standards for contaminants that may be present in drinking water and the treatment technologies that must be used to remove them. The Act also requires public water systems to regularly test and monitor the quality of their drinking water and to provide the results to their customers.
The SDWA also requires the EPA to identify and regulate contaminants that may have an adverse effect on public health. The EPA sets a maximum contaminant level (MCL) for each regulated contaminant, and it is the responsibility of the public water systems to meet these standards.
Additionally, the SDWA also requires public water systems to take steps to protect their sources of drinking water from contamination and to inform their customers of any violations of the standards or any other significant information about their water. The Act also provides funding for states, tribes and local governments to improve their drinking water infrastructure and to ensure access to safe drinking water.
Overall, the Safe Drinking Water Act has been successful in protecting public health by ensuring that the nation's public drinking water is safe to drink, but there is still work to be done to ensure that all Americans have access to safe and clean drinking water.
Frequently Asked Questions
What is sewage treatment and why do we need it?
Sewage treatment is the multi-step process that cleans wastewater so it can be safely returned to the environment. For the AP CED, know the three stages: primary treatment physically removes large objects and lets solids settle (screens, grates, sedimentation → sludge); secondary treatment uses bacteria in aeration/activated sludge tanks or trickling filters to break down organic matter (lowers biological oxygen demand and produces more sludge); tertiary treatment uses chemical or ecological methods to remove remaining pollutants like nitrogen and phosphorus (nutrient removal). Before discharge, water is disinfected (chlorine, ozone, or UV) to kill pathogens. We need treatment to protect human health, prevent oxygen depletion in aquatic systems (low BOD causes dead zones), and stop eutrophication from excess nutrients. For a quick review, check the Topic 8.11 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE) and Unit 8 resources (https://library.fiveable.me/ap-environmental-science/unit-8). Practice questions: (https://library.fiveable.me/practice/ap-environmental-science).
How does primary sewage treatment work step by step?
Primary sewage treatment is a physical process that removes large solids before biological steps. Step-by-step: 1. Influent enters the plant and passes through coarse screens and grates that catch rags, plastics, and large debris. 2. Flow then moves to a grit chamber where heavy inorganic particles (sand, gravel) settle out. 3. Water flows into primary sedimentation (clarifier) tanks where suspended organic solids slowly settle to the bottom as sludge. Lighter materials (oils, grease) float to the surface and are skimmed off. 4. Settled sludge is removed for further handling (often anaerobic digestion) while the clarified effluent moves on to secondary (biological) treatment. 5. Primary treatment reduces suspended solids and lowers BOD but doesn’t remove dissolved organics or nutrients—so secondary/tertiary steps and disinfection follow. This matches the AP CED essential knowledge (EK STB-3.N.1). For a short Topic 8.11 review, see Fiveable’s study guide (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE). For more practice, try the APES question bank (https://library.fiveable.me/practice/ap-environmental-science).
What's the difference between primary, secondary, and tertiary sewage treatment?
Primary, secondary, and tertiary are the three main stages in sewage treatment on the AP CED. - Primary (physical): removes large solids with screens and grates, then uses sedimentation so heavier solids settle as sludge in a tank (EK STB-3.N.1). - Secondary (biological): uses bacteria to break down dissolved organic matter into CO2 and more inorganic sludge; tanks are aerated (activated-sludge process, trickling filters) to lower BOD and speed decomposition (EK STB-3.N.2). - Tertiary (polishing): uses ecological or chemical processes to remove remaining pollutants—especially nutrients like nitrogen and phosphorus—and other contaminants (EK STB-3.N.3). Before discharge treated water is disinfected (chlorine, ozone, or UV) to kill pathogens (EK STB-3.N.4). These terms and keywords (BOD, sludge, aeration, nutrient removal, disinfection) show up on the AP exam—review the Topic 8.11 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE) and more Unit 8 resources (https://library.fiveable.me/ap-environmental-science/unit-8). For extra practice, try the APES practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Why do they use bacteria in secondary treatment instead of just chemicals?
Secondary treatment uses bacteria because it's a biological, efficient way to remove organic matter and lower BOD (biological oxygen demand). In aeration tanks (activated sludge or trickling filters), aerobic bacteria eat the dissolved organic wastes and convert them to CO2 and inorganic sludge that settles out (CED EK STB-3.N.2). That’s faster and cheaper long-term than trying to oxidize all organics with chemicals, which can create toxic byproducts, use lots of reagents, and not target biodegradable waste as effectively. Chemicals are used later in tertiary steps (nutrient polishing or disinfection) when needed (EK STB-3.N.3–4). Bacteria also let plants recover nutrients in sludge (anaerobic digestion) and reduce energy and chemical costs. For the AP exam, know terms: aeration tanks, activated sludge, BOD, sludge, and the role of tertiary chemical or UV disinfection. For a focused review, see the Topic 8.11 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE) and more practice questions at (https://library.fiveable.me/practice/ap-environmental-science).
I'm confused about what happens during secondary treatment - can someone explain it simply?
Secondary treatment is the biological step after primary screening and settling. Wastewater goes into aeration tanks where oxygen is added so aerobic bacteria can eat dissolved organic matter—this lowers biological oxygen demand (BOD). The common setups are the activated-sludge process (bacteria mixed with water, then allowed to settle) or trickling filters (wastewater trickles over media covered in microbes). As microbes consume organics they convert them to CO2 and biomass (inorganic/organic “sludge”) that later settles out. That settled sludge is removed for further treatment (often anaerobic digestion). The goal: remove most of the dissolved organics biologically so the water needs less polishing in tertiary treatment and disinfection before discharge (EK STB-3.N.2, EK STB-3.N.3, EK STB-3.N.4). For AP review, memorize terms: aeration tanks, activated sludge, trickling filters, BOD, sludge. More on Topic 8.11: Sewage Treatment is in the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE) and practice questions at (https://library.fiveable.me/practice/ap-environmental-science).
What are screens and grates used for in sewage treatment plants?
Screens and grates are part of primary treatment—they physically remove large objects (rags, sticks, trash, plastics) from incoming sewage so those things don’t clog pumps or damage downstream equipment. After screening, smaller solids settle out by sedimentation into sludge at the bottom of primary tanks. Removing big debris up front improves efficiency and protects aeration tanks and biological processes used in secondary treatment (activated sludge, trickling filters), and it helps reduce maintenance and safety hazards. This is an essential CED point (EK STB-3.N.1). For quick review, see the Topic 8.11 sewage treatment study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE). On the AP exam, expect questions to link screens/grates to primary, physical removal and sedimentation.
How does aeration help bacteria break down organic matter in secondary treatment?
Aeration speeds up secondary treatment by giving the bacteria the oxygen they need to do aerobic respiration. In aeration tanks (like in the activated sludge process) air is mixed into the wastewater so aerobic microbes can break organic matter into CO2 and inorganic sludge much faster than they would without oxygen. That lowers biological oxygen demand (BOD) in the effluent, keeps more organic material in a form that settles out as sludge, and suppresses smelly anaerobic reactions. Aeration also keeps microbes and solids suspended so they contact more food, improving breakdown rates and treatment efficiency (EK STB-3.N.2: secondary treatment is a biological process; aeration increases the rate). For AP review, remember terms: aeration tanks, activated sludge, BOD, sludge (see the Topic 8.11 study guide for a quick recap: https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE). For extra practice, check Fiveable’s APES practice problems (https://library.fiveable.me/practice/ap-environmental-science).
What kinds of pollutants are left after secondary treatment that need tertiary treatment?
After secondary (biological) treatment most of the big organic stuff and BOD are gone, but several pollutants remain that require tertiary polishing. Key ones: dissolved nutrients (nitrogen—nitrate/ammonium—and phosphorus) that drive eutrophication; fine suspended solids/turbidity; remaining pathogens and fecal coliforms (though disinfection follows); dissolved salts and some heavy metals; and trace contaminants like pharmaceuticals, personal-care chemicals, and endocrine disruptors. Tertiary processes target nutrient removal (nitrification/denitrification, chemical P precipitation), filtration/clarification for solids, advanced oxidation or activated carbon for micropollutants, and final disinfection (chlorine/ozone/UV). For the AP exam, be able to link secondary = biological BOD reduction and tertiary = chemical/ecological or advanced processes to remove nutrients and residual pollutants (see the Topic 8.11 study guide: https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE). For extra practice, check Unit 8 resources and practice questions (https://library.fiveable.me/ap-environmental-science/unit-8 and https://library.fiveable.me/practice/ap-environmental-science).
Why do they disinfect the water at the end if bacteria already cleaned it in secondary treatment?
Secondary treatment uses bacteria to break down most organic matter and lower BOD (EK STB-3.N.2), but it doesn’t remove all pathogens or guarantee safe levels of disease-causing microbes. Some bacteria, viruses, and protozoan cysts can survive biological treatment or regrow in distribution systems. That’s why facilities disinfect before discharge—to meet public-health standards and prevent waterborne disease (EK STB-3.N.4). Common disinfectants: chlorine gives a residual that protects water after release, while ozone and UV kill microbes but differ in whether they leave a lasting residual. Tertiary processes remove nutrients and other pollutants first, then disinfection finishes the job so effluent is safe for humans and ecosystems (EK STB-3.N.3–3.N.4). For AP review, see the Topic 8.11 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE) and extra practice (https://library.fiveable.me/practice/ap-environmental-science).
What's the difference between using chlorine, ozone, and UV light for disinfection?
Chlorine, ozone, and UV all disinfect final effluent, but they work differently and have trade-offs (EK STB-3.N.4). - Chlorine: cheap, leaves a residual in the water so pipes stay disinfected, and works well in turbid water. Downside: forms disinfection byproducts (DBPs) like trihalomethanes and can harm aquatic life if overdosed. Good for continuous protection before discharge. - Ozone: a very strong oxidizer that destroys microbes and organic pollutants quickly and doesn’t leave long-term chemical residues. Downside: no residual protection, must be generated on site (cost/energy), and can be more expensive. - UV light: inactivates microbes by damaging DNA/RNA (no chemicals, no DBPs). It’s immediate and safe, but clarity matters—UV is less effective in turbid or colored water and also provides no residual protection downstream. For AP exam focus: know these methods as common disinfectants and their pros/cons (see the Topic 8.11 study guide for a quick review: https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE). For extra practice, hit Fiveable’s unit page (https://library.fiveable.me/ap-environmental-science/unit-8) or practice problems (https://library.fiveable.me/practice/ap-environmental-science).
How do sewage treatment plants actually remove all the gross stuff from wastewater?
Sewage plants remove the “gross stuff” in three main steps that match the AP CED (STB-3.N): - Primary treatment (physical): screens and grates catch large objects, then solids settle out by sedimentation into sludge tanks. - Secondary treatment (biological): aerated tanks (activated sludge or trickling filters) let bacteria break down organic matter into CO2 and more inorganic sludge; aeration speeds this up and lowers BOD. - Tertiary treatment (polishing): chemical or ecological processes remove nutrients (N and P) and any remaining pollutants. Before discharge, water is disinfected with chlorine, ozone, or UV to kill pathogens (EK STB-3.N.1–4). Sludge is often treated further (anaerobic digestion) to stabilize it and capture methane. This sequence—physical → biological → chemical/disinfection—is exactly what the exam expects you to describe. For a quick CED-aligned review, see the Topic 8.11 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE). For extra practice, check Fiveable’s AP practice problems (https://library.fiveable.me/practice/ap-environmental-science).
What happens to the sludge that settles at the bottom of the tanks?
Sludge that settles in primary and secondary tanks is concentrated and then treated—not just thrown away. First it’s thickened and often sent to anaerobic (or sometimes aerobic) sludge digestion, where microbes break down organic material; that stabilizes the sludge and produces methane (which can be captured for energy). After digestion the sludge is dewatered (so less volume) and either disposed of (landfill or incineration) or reused as “biosolids” for soil application if it’s been properly treated and meets safety standards. On the AP exam you should know terms like primary sludge, secondary/activated sludge, aeration, and anaerobic sludge digestion—those appear in EK STB-3.N.1–3. For a concise CED-aligned review, see the Topic 8.11 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE). For more practice on related concepts check the Unit 8 overview (https://library.fiveable.me/ap-environmental-science/unit-8) or practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Can treated sewage water be used for drinking or is it just released into rivers?
Short answer: yes—treated sewage can be made safe to drink, but only after advanced tertiary treatment and disinfection; otherwise it’s usually released into rivers or used for nonpotable purposes. Explain: typical sewage plants do primary (screens, sedimentation) and secondary (aerated biological breakdown/activated sludge) treatment, then often discharge the effluent after disinfection (chlorine, ozone, or UV) per the CED (EK STB-3.N). To make water potable you need additional tertiary/advanced steps (nutrient removal, filtration, often reverse osmosis and advanced oxidation) plus reliable disinfection and monitoring—called indirect or direct potable reuse. Many places use indirect reuse (put treated effluent into reservoirs or rivers and later treat it for drinking) because it adds buffer and dilution. For AP prep: know primary/secondary/tertiary processes, disinfection options, and the idea of potable reuse (Topic 8.11). For the Fiveable study guide on sewage treatment see (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE) and grab practice questions at (https://library.fiveable.me/practice/ap-environmental-science).
I missed the lab on water treatment - what are the main steps I need to know for the AP exam?
For the AP exam, memorize the four main steps and the key terms from the CED (STB-3.N): - Primary treatment—physical removal: screens/grates remove large objects, then sedimentation lets solids settle as sludge. - Secondary treatment—biological: aerated tanks (activated sludge, trickling filters) let bacteria break down organic matter to CO2 and more sludge; this lowers BOD. - Tertiary treatment—polishing: ecological or chemical processes remove remaining pollutants (especially N and P removal, filtration). - Disinfection—before discharge: kill pathogens with chlorine, ozone, or UV. Also know anaerobic sludge digestion (for sludge) and why BOD and nutrient removal matter for preventing eutrophication. AP exam tips: expect short prompts asking you to describe these steps or explain why aeration lowers BOD (practice 1 and 7-style prompts). Review the Topic 8.11 study guide (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE) and do practice problems at (https://library.fiveable.me/practice/ap-environmental-science).
Why is tertiary treatment sometimes called advanced treatment and when is it necessary?
“Tertiary” is called advanced treatment because it goes beyond the physical (primary) and biological (secondary) steps to remove specific pollutants left in the effluent using chemical or ecological processes—e.g., nutrient removal (nitrogen and phosphorus), filtration, or chemical precipitation (EK STB-3.N.3). It’s necessary when discharged water must meet stricter limits to protect sensitive ecosystems, reduce eutrophication, or when reclaimed water will be reused (irrigation, drinking—after further treatment). Typical triggers: high nutrient loads causing algal blooms, groundwater recharge requirements, or local regulations that lower BOD/nutrient limits. Remember final disinfection (chlorine, ozone, UV) still occurs after treatment to kill pathogens (EK STB-3.N.4). For exam prep, you should be able to name examples (nutrient removal, activated carbon, constructed wetlands) and explain why they reduce eutrophication/BOD—see the Topic 8.11 study guide on Fiveable for quick review (https://library.fiveable.me/ap-environmental-science/unit-8/sewage-treatment/study-guide/k6tH0cz62Ysho1sbilgE).