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

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?

Primary treatment
Secondary treatment
Tertiary treatment
Collection system maintenance
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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 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 2

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 3

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 4

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 5

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 6

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 7

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 8

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 9

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 10

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 11

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 12

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.

Question 13

A municipal wastewater plant receives raw influent containing grit, rags, and suspended organic solids. The first major tank after screening is designed to slow the flow so that heavier solids sink to the bottom as sludge and oils/grease float for removal by skimmers, with no intentional microbial aeration. Which treatment stage is being described?

  1. Tertiary treatment (advanced nutrient removal and polishing)
  2. Secondary treatment (activated sludge biological oxidation)
  3. Primary treatment (physical settling and skimming) (correct answer)
  4. Disinfection step (chlorine/UV/ozone to kill pathogens)

Explanation: Primary treatment is the first major treatment stage after preliminary screening, designed to remove settleable solids and floatable materials through physical processes. In this stage, wastewater flows into large settling tanks (primary clarifiers) where the flow velocity is reduced, allowing gravity to separate materials by density. Heavy particles like grit and organic solids settle to the bottom as primary sludge, while lighter materials like oils and grease float to the surface where skimmers remove them. This process is purely physical with no intentional biological activity or aeration. Primary treatment typically removes 50-70% of suspended solids and 25-40% of BOD, preparing the wastewater for subsequent biological treatment.

Question 14

A wastewater plant is upgraded because the receiving stream is nutrient-sensitive. The upgrade adds an anoxic tank (no dissolved oxygen) where bacteria convert nitrate (NO3−\text{NO}_3^-) to nitrogen gas (N2\text{N}_2), followed by filtration and then UV disinfection. The anoxic tank is best described as part of which stage of treatment?

  1. Primary treatment (physical settling in a clarifier)
  2. Secondary treatment (standard aerobic activated sludge only, not nutrient removal)
  3. Tertiary treatment (advanced nutrient removal such as denitrification) (correct answer)
  4. Disinfection (UV/chlorine/ozone to kill pathogens)

Explanation: The anoxic tank described is performing biological denitrification, a key process in advanced nitrogen removal that is classified as tertiary treatment. In this process, facultative bacteria use nitrate (NO₃⁻) as an oxygen source in the absence of dissolved oxygen, converting it to nitrogen gas (N₂) which escapes to the atmosphere. This is a critical upgrade for nutrient-sensitive receiving waters because excess nitrogen (along with phosphorus) causes eutrophication, leading to algal blooms and oxygen depletion. Denitrification requires specific conditions (no dissolved oxygen, presence of nitrate, and a carbon source) and represents treatment beyond standard secondary processes. Since this anoxic tank is specifically designed for nutrient removal and goes beyond conventional secondary treatment's organic matter reduction, it is correctly identified as tertiary treatment (C), not standard secondary treatment which focuses on BOD removal.

Question 15

A plant adds a final polishing pond and then disinfects with UV before discharge. The polishing and UV steps are most associated with which stage?

  1. Primary treatment
  2. Secondary treatment
  3. Tertiary treatment (correct answer)
  4. Grit removal

Explanation: Tertiary treatment includes advanced polishing and disinfection processes that occur after secondary biological treatment to produce high-quality effluent. A polishing pond provides additional settling and biological polishing to reduce remaining suspended solids and nutrients, while UV disinfection inactivates pathogens without chemical addition. These processes work together to achieve effluent quality suitable for discharge to sensitive waters or for reuse applications. The combination of polishing and disinfection represents the tertiary treatment approach of providing final effluent improvement beyond conventional secondary treatment capabilities.

Question 16

In a clarifier, wastewater velocity is reduced so that sand, grit, and heavier suspended solids drop out by gravity, reducing wear on downstream equipment. This process is characteristic of which stage?

  1. Secondary treatment (microbial respiration)
  2. Primary treatment (physical removal by settling) (correct answer)
  3. Tertiary treatment (membrane filtration)
  4. Disinfection (chlorination)

Explanation: Primary treatment is characterized by physical processes that remove solids through gravity settling and flotation. The described process in a clarifier where wastewater velocity is reduced to allow sand, grit, and heavier suspended solids to settle out by gravity is a fundamental primary treatment operation. This physical separation protects downstream equipment from wear and removes the larger, more easily settleable materials. The mention of reducing wear on downstream equipment is particularly indicative of primary treatment, as it prepares the wastewater for more sensitive biological treatment processes that follow.

Question 17

A plant's effluent is clear but still contains disease-causing microorganisms. Which additional step, typically categorized as tertiary treatment, would directly address this issue?

  1. Primary sedimentation
  2. Activated sludge aeration
  3. Chlorination or UV disinfection (correct answer)
  4. Grit chamber settling

Explanation: Chlorination or UV disinfection are tertiary treatment processes specifically designed to address pathogenic microorganisms in wastewater effluent. If the effluent is clear (indicating good solids removal) but still contains disease-causing microorganisms, disinfection is the appropriate solution. These processes inactivate bacteria, viruses, protozoa, and other pathogens through chemical oxidation (chlorine) or physical disruption (UV radiation). Disinfection is typically classified as tertiary treatment when it follows secondary biological treatment and is designed to meet specific pathogen reduction standards for public health protection.

Question 18

A plant's first major tank is designed so that lighter materials float for skimming and heavier materials settle; the effluent then moves on for biological oxidation. The first tank is part of:

  1. Primary treatment (correct answer)
  2. Secondary treatment
  3. Tertiary treatment
  4. Disinfection

Explanation: Primary treatment is the first major treatment stage that uses physical separation processes to remove solids from wastewater through settling and flotation. The described tank where lighter materials float for skimming and heavier materials settle by gravity is a classic primary clarifier design. This physical separation process removes 50-60% of suspended solids and floating materials like oils and grease before the wastewater proceeds to biological treatment (secondary stage). The separation of materials by density differences without biological processes is characteristic of primary treatment's physical approach to contaminant removal.

Question 19

Which stage is most associated with removing remaining fine particles and some dissolved contaminants after biological treatment, often using filtration, membranes, or chemical processes?

  1. Primary treatment
  2. Secondary treatment
  3. Tertiary treatment (correct answer)
  4. Influent pumping

Explanation: Tertiary treatment is specifically designed to provide advanced polishing of effluent after conventional secondary biological treatment has been completed. This stage removes remaining fine particles through processes like filtration, membrane treatment, or advanced settling, and can also remove dissolved contaminants through chemical precipitation, adsorption, or advanced oxidation. Common tertiary processes include sand filtration, membrane bioreactors, nutrient removal systems, and disinfection. The goal is to produce high-quality effluent that meets stringent discharge standards or reuse criteria that cannot be achieved through primary and secondary treatment alone.

Question 20

A city upgrades its plant by adding rapid sand filters after secondary clarifiers to further reduce fine suspended solids before disinfection. The sand filtration step is part of which stage?

  1. Primary treatment (sedimentation)
  2. Secondary treatment (activated sludge)
  3. Tertiary treatment (advanced filtration) (correct answer)
  4. Primary treatment (screening)

Explanation: Tertiary treatment encompasses advanced processes that occur after secondary treatment to further improve effluent quality beyond conventional standards. Rapid sand filtration after secondary clarifiers is a classic tertiary treatment process designed to remove fine suspended solids that remain after biological treatment. This physical-chemical polishing step reduces turbidity and provides additional particle removal before disinfection. The placement of sand filters after secondary clarifiers and before disinfection is characteristic of tertiary treatment's role in producing high-quality effluent for discharge to sensitive waters or for reuse applications.