Sewage Treatment
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AP Environmental Science › Sewage Treatment
A plant adds chlorine to the final effluent after filtration to reduce pathogens before discharge to a river. This action is part of which treatment stage?
Combined sewer overflow control (stormwater diversion)
Primary treatment (grit removal and settling)
Tertiary treatment (disinfection)
Secondary treatment (trickling filter biofilm)
Explanation
Tertiary treatment is the advanced treatment stage that occurs after secondary biological treatment and focuses on further polishing the effluent. Disinfection is a key component of tertiary treatment designed to reduce pathogenic microorganisms before discharge to receiving waters. Adding chlorine to the final effluent after filtration is a classic disinfection process that inactivates bacteria, viruses, and other pathogens. While disinfection can sometimes be considered a separate category, it is most commonly classified as part of tertiary treatment when it occurs as the final step before discharge to protect public health and meet water quality standards.
Which stage is best described as “advanced treatment beyond secondary, often including nutrient removal and disinfection to improve effluent quality for sensitive waters”?
Septic tank treatment
Primary treatment
Tertiary treatment
Secondary 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.
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
Collection system maintenance
Tertiary treatment
Secondary treatment
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.
A city adds a chlorine contact basin at the end of treatment. This addition most directly improves the effluent’s:
Settleable solids concentration
Influent flow rate
Grit content
Pathogen (microbial) levels
Explanation
A chlorine contact basin added at the end of treatment is specifically designed to reduce pathogen (microbial) levels through disinfection. Chlorine is a powerful disinfectant that inactivates bacteria, viruses, protozoa, and other disease-causing microorganisms through oxidation of cellular components. The contact basin provides adequate retention time for the disinfection reaction to occur, ensuring sufficient contact between chlorine and pathogens to achieve required log reductions. This pathogen reduction directly improves the microbiological quality of the effluent, making it safer for discharge to receiving waters or for reuse applications.
A wastewater plant uses rotating biological contactors (RBCs), where a biofilm on rotating disks alternately contacts wastewater and air to degrade organics. RBCs are an example of which stage?
Tertiary treatment
Primary treatment
Secondary treatment
Disinfection
Explanation
Secondary treatment utilizes biological processes to remove dissolved and colloidal organic matter through microbial decomposition. Rotating biological contactors (RBCs) are a type of attached-growth biological treatment system where microorganisms form biofilms on rotating media that alternately contact wastewater and air. This design provides both the organic substrate (from wastewater contact) and oxygen (from air contact) needed for aerobic biological treatment. RBCs achieve BOD reduction and nitrification through the same biological processes as other secondary treatment technologies like activated sludge or trickling filters.
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?
Screening
Secondary treatment
Tertiary treatment
Primary treatment
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.
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:
Screening
Secondary treatment only
Primary treatment
Tertiary treatment
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.
A plant wants to reduce biochemical oxygen demand (BOD) in effluent to prevent oxygen depletion in a river. Which stage is primarily responsible for BOD reduction via microbial activity?
Tertiary treatment
Primary treatment
Secondary treatment
Chlorine disinfection
Explanation
Secondary treatment is specifically designed to reduce biochemical oxygen demand (BOD) through biological processes that consume organic matter. Aerobic microorganisms in secondary treatment systems (activated sludge, trickling filters) metabolize dissolved and colloidal organic compounds that contribute to oxygen demand in receiving waters. This biological oxidation converts organic pollutants into biomass, carbon dioxide, and water, achieving 85-95% BOD reduction. Preventing oxygen depletion in rivers requires removing the organic matter that would otherwise consume dissolved oxygen as it decomposes naturally in the water body.
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?
Primary treatment
Secondary treatment
Tertiary treatment
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.
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?
Secondary treatment (biological decomposition with aeration and floc settling)
Tertiary treatment (filtration and nutrient removal after secondary)
Disinfection only (pathogen inactivation without BOD removal)
Primary treatment (gravity settling of raw sewage solids)
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.