AP Syllabus focus:
‘Tertiary treatment uses ecological or chemical processes to remove remaining pollutants. Before discharge, treated water is disinfected (often with chlorine, ozone, or UV) to kill bacteria.’
Tertiary treatment and disinfection are “polishing” steps in wastewater treatment.
Plant overview diagram showing wastewater treatment stages from headworks through tertiary filtration and chlorine contact basins. It visually situates tertiary “polishing” and disinfection as the final barriers before effluent is released or reused. The numbered layout helps connect unit processes to the pollutants they target (solids, nutrients, pathogens). Source
They target leftover nutrients, particles, and pathogens so effluent can be safely released or reused with reduced risks to ecosystems and public health.
Where tertiary treatment fits and what it targets
After earlier treatment steps remove most solids and biodegradable organic matter, some remaining pollutants can persist, including fine suspended particles, nutrients (nitrogen and phosphorus), dissolved chemicals, and microorganisms.
Tertiary treatment: Additional wastewater treatment that uses ecological or chemical processes (and often advanced physical separation) to remove remaining pollutants after secondary treatment.
Tertiary treatment is selected based on the receiving waters’ sensitivity and the intended use of the effluent (discharge vs. reuse).
Common “ecological” tertiary processes
Ecological approaches use living systems or ecosystem-like processes to improve water quality.
Constructed wetlands
Wastewater flows through shallow basins with plants and microbial communities
Removes pollutants via sedimentation, plant uptake, and microbial transformations
Wastewater stabilization ponds/lagoons
Longer residence time allows settling and natural biological processing
Sunlight and microbial competition can reduce some pathogens
Ecological methods can be effective but typically require more land area and can be sensitive to temperature and seasonal changes.
Common “chemical” (and advanced) tertiary processes
Chemical processes directly transform or capture pollutants.
Chemical precipitation (especially phosphorus)
Adding coagulants (e.g., alum or iron salts) forms insoluble compounds that can be separated as sludge
Coagulation/flocculation and filtration
Coagulants aggregate fine particles into flocs that settle or filter out
Activated carbon adsorption
Removes some dissolved organic chemicals by binding them to carbon surfaces
Membrane filtration (e.g., microfiltration/ultrafiltration)
Physically removes very small particles and many microbes (used in advanced reuse systems)
These options can improve clarity and reduce chemical contaminants, but they increase cost, energy use, and/or sludge production that must be managed.
High-resolution diagram and photos illustrating tertiary filtration using a sand media filter bed. The schematic labels key components (influent ports, media support, backwash system), showing how fine suspended solids are physically removed after secondary treatment. This connects “polishing” to improved clarity and lower particle loads before final disinfection. Source
Disinfection: purpose and main methods
Even after tertiary “polishing,” pathogens may remain.
Aerial photos and explanatory labels showing chlorine disinfection using sodium hypochlorite in chlorine contact basins. The layout emphasizes why contact time matters for inactivating pathogens and how disinfection follows filtration in a treatment train. It also introduces the idea of post-disinfection dechlorination prior to discharge, which is commonly used to reduce residual chlorine impacts. Source
Disinfection is therefore used just before discharge or distribution.
Disinfection: A treatment step that kills or inactivates disease-causing microorganisms in treated wastewater to reduce infection risk after release.
Disinfection effectiveness depends on water quality (e.g., turbidity can shield microbes), correct dosing, and sufficient mixing/contact.
Chlorine disinfection
Chlorine (gas, sodium hypochlorite, or calcium hypochlorite) is widely used because it is effective and can be easier to apply at large scale.
Strengths
Strong germicidal action for many bacteria and viruses
Can provide a short-lived residual that continues working downstream
Limitations
Some organisms are more resistant than others
Chlorine can react with remaining organic matter, so upstream “polishing” helps
Ozone disinfection
Ozone (O₃) is a powerful oxidant generated on-site and bubbled through water.
Strengths
Rapid inactivation of many pathogens
Also oxidizes some unwanted dissolved compounds (site-dependent benefit)
Limitations
No long-lasting residual; effectiveness is mainly within the contact system
Equipment and energy requirements can be higher than chlorination
UV disinfection
Ultraviolet (UV) systems pass treated water past UV lamps.
Strengths
Inactivates microbes by damaging genetic material
No chemical addition, which can simplify downstream water chemistry
Limitations
Effectiveness drops if water contains particles that block light
Requires reliable electricity and lamp maintenance
Why these steps matter for environmental discharge
When tertiary treatment and disinfection are appropriately matched to the wastewater and receiving environment, they reduce:
Eutrophication pressure by lowering nutrient loads (where nutrient removal is part of tertiary treatment)
Disease risk from bacterial contamination, especially near recreational waters and shellfish-growing areas
Ecological stress from residual pollutants that could affect aquatic organisms
FAQ
UV photons disrupt microbial DNA/RNA, preventing replication.
If organisms cannot reproduce, they become effectively non-infectious even if some cells remain intact for a time.
Chlorine can react with dissolved organic matter to form by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs).
These are monitored because long-term exposure is associated with health risks, so utilities balance microbial control with by-product minimisation.
Ozone is highly reactive and unstable, so it decomposes quickly.
As a result, it is typically produced as needed using electrical equipment, then immediately applied in a contact chamber.
Common factors include:
High turbidity or suspended particles shielding microbes
Poor mixing or short contact time
Equipment issues (e.g., UV lamp fouling, incorrect chlorine feed rates)
These factors can cause uneven exposure and leave pockets of surviving organisms.
Decisions are driven by discharge permits and local risk:
Nutrient limits tighten where waters are sensitive to algal growth
Stronger pathogen controls are prioritised near bathing waters or shellfish areas
Budget, available space, and operator capacity also influence technology choice.
Practice Questions
State two methods used to disinfect treated wastewater before discharge, and explain the main purpose of disinfection. (3 marks)
Any two of: chlorine, ozone, UV (1 mark each, max 2)
Purpose: to kill or inactivate pathogenic microorganisms (e.g., bacteria) to reduce infection risk after discharge (1 mark)
A wastewater plant discharges effluent into a river used for recreation. Monitoring shows low levels of remaining nutrients and occasional bacterial contamination. Explain how tertiary treatment and disinfection could be used to address these issues, and describe one trade-off of each step. (6 marks)
Tertiary treatment removes remaining pollutants beyond secondary treatment (1)
Identifies a suitable tertiary approach for nutrients (e.g., chemical precipitation of phosphorus and/or filtration/coagulation) (1)
Links tertiary treatment to reducing impacts of nutrient loading (e.g., lowered nutrient input to river) (1)
Disinfection used before discharge to kill/inactivate bacteria/pathogens (1)
Names a disinfection method (chlorine/ozone/UV) (1)
One trade-off for each step (any two distinct points) (1)
Tertiary trade-offs: higher cost/energy, added sludge or chemical use, land area for ecological methods
Disinfection trade-offs: chemical handling needs, energy/maintenance (UV/ozone), reduced effectiveness if water is turbid
