AP Environmental Science Quiz: Sewage Treatment
20 questions · exam conditions
0:00
Sewage TreatmentQuestion 1 of 20

A plant's sequence is: (1) primary clarifier, (2) activated sludge, (3) sand filtration, (4) UV. Step (1) is which stage?

Secondary treatment
Primary treatment
Tertiary treatment
Disinfection
← Back to quizzes

AP Environmental Science Quiz

AP Environmental Science Quiz: Sewage Treatment

Practice Sewage Treatment in AP Environmental Science with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Sewage Treatment, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Environmental Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A plant's sequence is: (1) primary clarifier, (2) activated sludge, (3) sand filtration, (4) UV. Step (1) is which stage?

  1. Secondary treatment
  2. Primary treatment (correct answer)
  3. Tertiary treatment
  4. Disinfection
Explanation: Primary treatment is the first major treatment stage that uses physical processes to remove settleable solids from wastewater. Step (1) in the sequence, the primary clarifier, represents this initial treatment stage where gravity settling removes suspended solids and floating materials are skimmed. This physical separation process typically removes 50-60% of suspended solids and 25-40% of BOD before the wastewater proceeds to secondary biological treatment (activated sludge), tertiary filtration (sand filtration), and disinfection (UV).

Question 2

Which stage is best described as "advanced treatment beyond secondary, often including nutrient removal and disinfection to improve effluent quality for sensitive waters"?

  1. Primary treatment
  2. Secondary treatment
  3. Tertiary treatment (correct answer)
  4. Septic tank treatment
Explanation: Tertiary treatment is best described as advanced treatment beyond conventional secondary treatment that often includes enhanced nutrient removal and disinfection to improve effluent quality for discharge to sensitive waters. This stage addresses specific pollutants that remain after biological treatment, such as nitrogen and phosphorus (which can cause eutrophication), pathogens (which pose public health risks), and trace contaminants. Tertiary treatment is typically required when receiving waters are particularly sensitive to pollution or when effluent will be reused for applications requiring high water quality standards.

Question 3

A plant's manager says: "Our biggest concern is reducing suspended solids using settling tanks and skimmers." That concern aligns most closely with which stage's main purpose?

  1. Primary treatment (correct answer)
  2. Secondary treatment
  3. Tertiary treatment
  4. Disinfection
Explanation: Primary treatment's main purpose is the physical removal of suspended solids using settling tanks and skimmers, which aligns directly with the manager's stated concern. Primary treatment typically removes 50-60% of suspended solids through gravity settling in clarifiers, where heavier particles settle to the bottom and lighter materials like oils and grease float to the surface for skimming. This physical separation process is the foundation of wastewater treatment, preparing the wastewater for subsequent biological and advanced treatment stages by removing the bulk of settleable materials.

Question 4

A wastewater engineer says: "This step mainly uses gravity to separate solids from liquids; it does not rely on microbes to reduce BOD." Which stage is being described?

  1. Primary treatment (correct answer)
  2. Secondary treatment
  3. Tertiary treatment
  4. Disinfection
Explanation: Primary treatment is characterized by physical processes that rely primarily on gravity separation rather than biological activity. The engineer's description of using gravity to separate solids from liquids without relying on microbes to reduce BOD perfectly describes primary treatment processes. These include sedimentation in clarifiers where heavier solids settle and lighter materials float for skimming. Primary treatment typically achieves 50-60% suspended solids removal and 25-40% BOD reduction through purely physical means, setting it apart from the biological processes that define secondary treatment.

Question 5

A plant's sequence is: (1) primary settling, (2) trickling filter, (3) secondary clarifier, (4) ozonation. Step (4) is best categorized as:

  1. Primary treatment
  2. Secondary treatment
  3. Tertiary treatment (correct answer)
  4. Secondary clarification
Explanation: Tertiary treatment encompasses advanced processes that occur after conventional secondary biological treatment to further improve effluent quality. Step (4) in the sequence, ozonation, is an advanced oxidation and disinfection process characteristic of tertiary treatment. Ozone provides powerful disinfection capabilities and can also oxidize trace organic compounds that remain after biological treatment. When ozonation occurs after primary settling, trickling filter (secondary), and secondary clarifier, it serves as a tertiary treatment step to achieve high-quality effluent suitable for discharge to sensitive waters or reuse applications.

Question 6

A plant's monitoring shows a major drop in TSS immediately after the first clarifier, before any aeration occurs. That drop is most attributable to which stage?

  1. Primary treatment (correct answer)
  2. Secondary treatment
  3. Tertiary treatment
  4. Disinfection
Explanation: Primary treatment achieves its major suspended solids removal through gravity settling in clarifiers, which occurs before any biological treatment processes. The major drop in total suspended solids (TSS) immediately after the first clarifier, before aeration, is characteristic of primary treatment effectiveness. Primary clarifiers typically remove 50-60% of suspended solids through physical settling and flotation processes. This reduction occurs without biological activity and represents the primary treatment stage's main function of preparing wastewater for downstream biological treatment by removing settleable materials.

Question 7

A plant's sequence is: (1) settling tank, (2) aeration basin, (3) chlorine contact tank. Step (2) is which stage?

  1. Primary treatment
  2. Secondary treatment (correct answer)
  3. Tertiary treatment
  4. Tertiary sludge digestion
Explanation: Secondary treatment is the biological stage that follows primary treatment and precedes advanced tertiary processes. In the described sequence, step (2) is the aeration basin, which is the core component of secondary biological treatment. Aeration basins provide dissolved oxygen for aerobic microorganisms to decompose organic matter and reduce BOD. This biological process typically follows primary settling (step 1) and precedes final disinfection (step 3, which would be tertiary treatment). The aeration basin represents the heart of secondary treatment where biological oxidation occurs.

Question 8

A wastewater plant's effluent is intended for discharge into a nutrient-sensitive lake. Which stage is most likely to be emphasized or expanded to prevent algal blooms?

  1. Primary treatment
  2. Secondary treatment
  3. Tertiary treatment (correct answer)
  4. Screening
Explanation: Tertiary treatment is most likely to be emphasized or expanded when effluent will be discharged to nutrient-sensitive waters like lakes prone to algal blooms. Excess nutrients, particularly nitrogen and phosphorus, can trigger eutrophication in receiving waters, leading to algal growth, oxygen depletion, and ecosystem disruption. Tertiary treatment provides advanced nutrient removal capabilities through biological nutrient removal (BNR) systems, chemical precipitation, or other advanced processes that can achieve the low nutrient concentrations required to prevent eutrophication. This advanced treatment goes beyond what conventional secondary treatment can accomplish.

Question 9

A plant's final effluent shows very low BOD and TSS but elevated nitrate and phosphate. The plant adds an anoxic tank for denitrification and alum to precipitate phosphorus. Which stage is being expanded?

  1. Primary treatment
  2. Secondary treatment
  3. Tertiary treatment (correct answer)
  4. Collection system maintenance
Explanation: Tertiary treatment is the advanced treatment stage that addresses specific pollutants that remain after conventional secondary treatment. The scenario describes a plant with good BOD and TSS removal (indicating effective primary and secondary treatment) but with elevated nutrients (nitrate and phosphate). Adding an anoxic tank for denitrification (converting nitrate to nitrogen gas) and alum for phosphorus precipitation are classic tertiary treatment processes designed for nutrient removal. These advanced processes go beyond the basic biological treatment of secondary stage to meet more stringent discharge standards and prevent eutrophication in receiving waters.

Question 10

Wastewater is directed into a tank with slow mixing but no oxygen, followed by an aerated tank, to enhance nitrogen removal (denitrification then nitrification). This kind of advanced nutrient control is typically considered part of:

  1. Primary treatment
  2. Secondary treatment only
  3. Tertiary treatment (correct answer)
  4. Screening
Explanation: Tertiary treatment encompasses advanced biological nutrient removal processes that combine anoxic (no oxygen) and aerobic conditions to achieve enhanced nitrogen removal. The described sequence of anoxic tank followed by aerated tank represents biological nutrient removal (BNR) technology where denitrification (nitrate to nitrogen gas) occurs in the anoxic zone and nitrification (ammonia to nitrate) occurs in the aerobic zone. This advanced treatment goes beyond conventional secondary treatment to achieve low nitrogen effluent concentrations needed to prevent eutrophication in sensitive receiving waters.

Question 11

To reduce eutrophication risk in a downstream estuary, a plant installs a process to remove nitrogen and phosphorus beyond what typical secondary treatment achieves. This is most characteristic of which stage?

  1. Primary treatment
  2. Secondary treatment
  3. Tertiary treatment (correct answer)
  4. Screening
Explanation: Tertiary treatment is specifically designed to provide advanced nutrient removal beyond what conventional secondary treatment can achieve. The installation of processes to remove nitrogen and phosphorus to prevent eutrophication represents classic tertiary treatment objectives. Eutrophication occurs when excess nutrients (particularly nitrogen and phosphorus) stimulate algal growth in receiving waters, leading to oxygen depletion and ecosystem damage. Tertiary treatment may include biological nutrient removal (BNR) processes, chemical precipitation, or advanced oxidation to achieve the low nutrient levels required for discharge to sensitive waters like estuaries.

Question 12

After primary settling, a plant routes wastewater into an aeration basin where air is bubbled through and a mixed community of microbes forms flocs that consume dissolved and fine suspended organic matter, lowering biochemical oxygen demand (BOD). The mixture then goes to a clarifier to settle the microbial floc. Which stage is this?

  1. Secondary treatment (biological decomposition with aeration and floc settling) (correct answer)
  2. Primary treatment (gravity settling of raw sewage solids)
  3. Tertiary treatment (filtration and nutrient removal after secondary)
  4. Disinfection only (pathogen inactivation without BOD removal)
Explanation: Secondary treatment is the biological stage of wastewater treatment that follows primary settling and focuses on removing dissolved and fine suspended organic matter. In the activated sludge process described, wastewater enters aeration basins where air is bubbled through to maintain aerobic conditions for microorganisms. These microbes form flocs (clumps of bacteria and protozoa) that consume organic pollutants, converting them to CO2, water, and more microbial biomass. This biological oxidation significantly reduces BOD, typically achieving 85-95% removal. After aeration, the mixture flows to secondary clarifiers where the microbial flocs settle out, with some returned to the aeration basin as "activated sludge" to maintain the microbial population. The clear supernatant moves on to further treatment or discharge.

Question 13

After primary clarifiers, wastewater enters an aeration basin where air is bubbled through and microbes consume dissolved organic matter, forming flocs that will later settle in a secondary clarifier. Which treatment stage is this?

  1. Primary treatment (sedimentation of settleable solids)
  2. Secondary treatment (biological decomposition of organics) (correct answer)
  3. Tertiary treatment (filtration and disinfection)
  4. Industrial pretreatment (source control at factories)
Explanation: Secondary treatment is the biological stage of wastewater treatment where microorganisms are used to decompose dissolved and colloidal organic matter. The described process involves an aeration basin where air is bubbled through the wastewater to maintain aerobic conditions for bacteria to consume dissolved organic matter (reducing BOD). These bacteria form flocs (clumps) that will later settle in a secondary clarifier. This is the activated sludge process, the most common form of secondary treatment. Secondary treatment typically follows primary treatment and can achieve 85-95% BOD removal and 85-92% suspended solids removal through biological oxidation.

Question 14

A wastewater facility has already completed primary settling and secondary biological treatment. Before discharge to a lake prone to algal blooms, the plant adds a process to reduce nitrogen and phosphorus and then performs final disinfection. Which overall stage best describes the nutrient-removal step added after secondary treatment?

  1. Primary treatment (settling of suspended solids)
  2. Secondary treatment (microbial oxidation of organic matter)
  3. Tertiary treatment (advanced nutrient removal/polishing) (correct answer)
  4. Screening (removal of large debris with bar racks)
Explanation: Tertiary treatment, also called advanced treatment, encompasses any processes applied after secondary treatment to further improve effluent quality beyond conventional BOD and suspended solids removal. The primary purpose is often nutrient removal (nitrogen and phosphorus) to prevent eutrophication in receiving waters, particularly important when discharging to lakes prone to algal blooms. Nitrogen removal typically involves biological nitrification-denitrification processes, while phosphorus removal uses chemical precipitation or enhanced biological phosphorus removal. Tertiary treatment may also include filtration, membrane processes, or other polishing steps. This advanced stage is essential for meeting stringent discharge standards and protecting sensitive aquatic ecosystems from nutrient pollution that secondary treatment alone cannot adequately address.

Question 15

Wastewater passes through a circular tank where suspended solids settle and are scraped to the center; no aeration is provided and the goal is to remove settleable solids and reduce turbidity. Which stage is this?

  1. Secondary treatment (aerobic biological oxidation)
  2. Primary treatment (sedimentation/clarification) (correct answer)
  3. Tertiary treatment (advanced nutrient removal)
  4. Disinfection (UV irradiation)
Explanation: Primary treatment focuses on the physical separation of solids from wastewater through sedimentation and clarification processes. The described circular tank with settling and scraping mechanisms, where no aeration is provided, is a classic primary clarifier design. The goal of reducing turbidity and removing settleable solids through gravity settling is the fundamental purpose of primary treatment. The absence of aeration distinguishes this from secondary treatment, which relies on biological processes. Primary clarifiers typically use mechanical scrapers to collect settled solids at the center for removal as primary sludge.

Question 16

A municipal wastewater plant sends influent through bar screens and then into a large rectangular tank where flow slows and heavy solids settle to the bottom as sludge while oils and grease float and are skimmed. Which treatment stage is being described?

  1. Secondary treatment (activated sludge aeration)
  2. Tertiary treatment (nutrient removal and disinfection)
  3. Primary treatment (physical settling and skimming) (correct answer)
  4. Sludge digestion (anaerobic stabilization of biosolids)
Explanation: Primary treatment is the first major stage of wastewater treatment that relies primarily on physical processes to separate solids from liquids. The described process involves bar screens (preliminary treatment) followed by a large settling tank where flow velocity is reduced to allow gravity separation. Heavy solids settle to the bottom forming sludge, while lighter materials like oils and grease float to the surface where they can be skimmed off. This physical separation process is characteristic of primary clarification, which typically removes 50-60% of suspended solids and 25-40% of BOD through settling and flotation. No biological processes or aeration are involved at this stage.

Question 17

A plant uses a secondary clarifier after aeration to settle microbial flocs (activated sludge) and return some biomass to the aeration tank. This settling after biological treatment is associated with which stage overall?

  1. Primary treatment
  2. Secondary treatment (correct answer)
  3. Tertiary treatment
  4. Disinfection
Explanation: Secondary treatment encompasses both the biological processes and the associated clarification steps needed to separate the biological products from treated wastewater. The secondary clarifier described is an integral part of the secondary treatment stage, designed to settle the microbial flocs (activated sludge) produced during biological treatment. The return of some biomass to the aeration tank is essential for maintaining the biological treatment process. While this involves settling similar to primary treatment, it occurs after biological treatment and serves to separate biological solids rather than raw wastewater solids.

Question 18

A plant uses a membrane bioreactor (MBR) that combines biological treatment with membrane filtration to produce very low TSS effluent prior to disinfection. In typical classification, the membrane filtration/polishing function aligns most with:

  1. Primary treatment
  2. Secondary treatment only
  3. Tertiary treatment (correct answer)
  4. Collection system
Explanation: Membrane bioreactor (MBR) technology combines biological treatment with membrane filtration in a single system, where the membrane component provides tertiary-level polishing. While the biological treatment aspect aligns with secondary treatment, the membrane filtration function that produces very low TSS effluent prior to disinfection represents tertiary treatment capabilities. In typical classification systems, the advanced membrane separation that provides polishing beyond conventional secondary clarification is categorized as tertiary treatment. MBRs essentially combine secondary and tertiary functions, but the membrane polishing aspect that produces exceptionally high-quality effluent aligns most closely with tertiary treatment objectives.

Question 19

A plant adds a final step specifically to reduce remaining turbidity and microorganisms for water reuse in irrigation. Which stage most commonly includes both filtration and disinfection for this purpose?

  1. Primary treatment
  2. Secondary treatment
  3. Tertiary treatment (correct answer)
  4. Influent screening
Explanation: Tertiary treatment is specifically designed to produce high-quality effluent suitable for reuse applications by providing advanced filtration and disinfection beyond secondary treatment standards. For irrigation reuse, effluent must meet strict standards for turbidity (clarity) and pathogen reduction to protect public health and prevent clogging of irrigation systems. Tertiary treatment commonly includes multimedia filtration to reduce remaining suspended solids and turbidity, followed by disinfection (chlorine, UV, or ozone) to inactivate pathogens. This combination addresses both the physical and microbiological requirements for safe water reuse.

Question 20

A plant uses trickling filters: wastewater is sprayed over a bed of rocks/plastic media coated with biofilm; microbes in the biofilm oxidize organic matter, and the sloughed biomass is later settled out. This biological oxidation is an example of which stage?

  1. Primary treatment (physical settling)
  2. Secondary treatment (biological decomposition) (correct answer)
  3. Tertiary treatment (advanced chemical/physical polishing)
  4. Disinfection (pathogen kill step)
Explanation: Trickling filters represent an alternative form of secondary treatment that uses attached-growth biological processes rather than suspended-growth systems like activated sludge. In this system, wastewater is distributed over media (rocks, plastic modules) colonized by biofilms containing aerobic microorganisms. As water trickles down through the media, these microbes oxidize dissolved organic matter, converting it to CO2, water, and additional biomass. The biofilm periodically sloughs off and is removed in secondary clarifiers. This biological oxidation of organic matter is the defining characteristic of secondary treatment, whether accomplished through trickling filters, activated sludge, or other biological processes. Both systems achieve the same goal of BOD reduction through microbial metabolism, making this clearly a secondary treatment process.