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Sewage
Treatment

Explore how wastewater is transformed from a public health hazard into safe effluent through physical, biological, and chemical processes.

AP Environmental Science

What Is Sewage?

Sewage (also called wastewater) is the liquid waste produced by households, industries, and stormwater runoff. It contains organic matter, pathogens, nutrients, and suspended solids that must be removed before discharge into the environment.

Domestic
Kitchen, bathroom, and laundry water from homes
Industrial
Chemical-laden discharge from factories and plants
Stormwater
Runoff carrying pollutants from roads and surfaces

Why Does Treatment Matter?

Untreated sewage introduces pathogens (cholera, typhoid), excess nutrients (nitrogen, phosphorus), and oxygen-demanding organic matter into water bodies: causing disease, eutrophication, and aquatic dead zones.

Essential Vocabulary

Influent
Raw sewage entering the treatment plant
Effluent
Treated water discharged from the plant
Sludge
Solid material settled out during treatment
BOD
Biochemical Oxygen Demand: oxygen needed by microbes to decompose organic matter

Three Stages of Treatment

Primary Physical Separation Secondary Biological Decomposition Tertiary Chemical / Advanced STAGE 1 STAGE 2 STAGE 3
\( \approx 60\% \) solids removed
\( \approx 85\% \) BOD removed
\( \approx 99\% \) pollutants

Primary Treatment: Physical Separation

Screening Remove large debris Grit Chamber Settle sand, gravel, grit Sedimentation Tank Clarified water Sludge settles
Removes \( \approx 60\% \)
suspended solids
Raw Influent

Secondary Treatment: Biological Processes

After primary treatment, the water still contains dissolved organic matter. Secondary treatment uses aerobic bacteria to decompose this material, dramatically reducing BOD.

Activated Sludge
Aeration tanks mix wastewater with bacteria-rich sludge and pump in air to accelerate decomposition
Trickling Filter
Water is sprayed over beds of rocks or plastic media coated with biofilm that consumes organics

The microbial floc stays while water moves on

A turbulent rectangular aeration basin feeds a circular clarifier with clear water above settled brown biomass. Separate piping returns bottom sludge to the aeration basin and sends a branch to wasting; a magnified floc contains varied microbial shapes.In activated-sludge treatment, settled biomass returns from the clarifier to aeration while effluent and wasted sludge leave. Hydraulic retention time tracks liquid volume and external flow; solids retention time tracks solids inventory and solids losses.

Water and solids residence differ

Water and solids residence differ. In activated-sludge treatment, settled biomass returns from the clarifier to aeration while effluent and wasted sludge leave. Hydraulic retention time tracks liquid volume and external flow; solids retention time tracks solids inventory and solids losses.In activated-sludge treatment, settled biomass returns from the clarifier to aeration while effluent and wasted sludge leave. Hydraulic retention time tracks liquid volume and external flow; solids retention time tracks solids inventory and solids losses.

Tertiary Treatment: Advanced Polishing

Tertiary treatment targets pollutants that survive primary and secondary stages: nitrogen, phosphorus, remaining pathogens, and trace chemicals.

1
Nutrient removal: Biological or chemical processes strip nitrogen (nitrification/denitrification) and phosphorus to prevent eutrophication.
2
Filtration: Sand filters or membrane filtration remove remaining fine particles and turbidity.
3
Disinfection: Chlorination, UV radiation, or ozone destroy remaining pathogens before discharge.

Explain why pathogen inactivation does not itself remove dissolved nutrient mass.

Use this illustration to explore the concept described in its caption.

Three detailed metal-and-water cutaways show an aerated chamber, a conical chamber collecting sediment, and a UV-sleeve channel. The editable companion distinguishes two nitrogen conditions, conserved phosphorus symbols entering solids, and organism inactivation without a nutrient-removal endpoint.
Distinct treatment modules illustrate aerobic nitrogen conversion, phosphorus transfer into collected solids, and UV inactivation of susceptible organisms. The companion separates aerobic ammonium-to-nitrite-to-nitrate conversion from anoxic nitrate-to-nitrogen-gas conversion. Phosphorus is retained in collected solids rather than destroyed; UV inactivation alone does not remove dissolved nutrient mass. The apparatus is illustrative, and its violet UV glow is a visibility cue; it is not a complete treatment train or a performance guarantee.
Aerobic ammonium-to-nitrate steps lead to anoxic nitrogen-gas production; six phosphorus symbols move into collected solids; UV marks organisms as inactivated.
Nitrogen transformation and removal, phosphorus transfer to solids, and UV inactivation are different treatment outcomes.

Treatment stages separate contaminants through different physical and biological mechanisms

Isometric treatment facility with screening channel, two clarifiers, an aerated basin, final channel and a separate solids-handling area.Screens, settling, biological processing and advanced treatment target different contaminants. Organic carbon can become CO₂ or biomass, while residual contaminants remain in effluent.

Treatment separates different pathways

Treatment separates different pathways. Screens, settling, biological processing and advanced treatment target different contaminants. Organic carbon can become CO₂ or biomass, while residual contaminants remain in effluent.Screens, settling, biological processing and advanced treatment target different contaminants. Organic carbon can become CO₂ or biomass, while residual contaminants remain in effluent.

Nitrogen removal and disinfection differ

Nitrogen removal and disinfection differ. Aerobic nitrification converts ammonium to nitrate; anoxic denitrification can remove nitrogen as N₂. Disinfection instead targets pathogens and does not substitute for nutrient removal.Aerobic nitrification converts ammonium to nitrate; anoxic denitrification can remove nitrogen as N₂. Disinfection instead targets pathogens and does not substitute for nutrient removal.

Settling and metabolism have different exits

Settling and metabolism have different exits. Settling separates scum and sludge. Biological processing divides organic carbon among CO₂, new biomass and residual effluent; solids and carbon follow different exit routes.Settling separates scum and sludge. Biological processing divides organic carbon among CO₂, new biomass and residual effluent; solids and carbon follow different exit routes.

Comparing the Three Stages

Primary Secondary Tertiary
Method Physical Biological Chemical / Advanced
Targets Solids, debris Dissolved organics Nutrients, pathogens
BOD Removal 25-40% 85-90% >95%
Key Process Sedimentation Activated sludge Disinfection
Required? Yes (minimum) Yes (most plants) Varies by regulation

BOD and Dissolved Oxygen

Concentration Distance Downstream from Sewage Discharge 0 Far Discharge BOD DO DO Sag Zone

What Happens to the Sludge?

Both primary and secondary treatment produce sludge: a concentrated mixture of solids and microorganisms. This sludge itself must be treated before final disposal.

1
Thickening: Gravity or flotation concentrates the sludge, reducing volume.
2
Anaerobic digestion: Bacteria break down organic matter without oxygen, producing methane (biogas) that can be captured for energy.
3
Dewatering: Mechanical presses or centrifuges remove water, creating a solid "cake."
4
Disposal: Treated sludge (biosolids) may be applied to farmland as fertilizer, incinerated, or sent to landfill.

Trace where removed pollutants and biomass go.

Use this illustration to explore the concept described in its caption.

A wastewater solids handling facility accompanies separate effluent and solids pathways. Residual solids are stabilized and dewatered before conditional use or disposal; residual contamination is monitored.
Treating wastewater separates residual solids that still need management. Stabilization, dewatering and subsequent use or disposal address different concerns; validated treatment reduces rather than universally eliminates pathogens.

Full Treatment Process: Step by Step

1
Screening: Remove large debris (rags, sticks, plastic) using bar screens.
2
Grit removal: Slow the flow to settle sand, gravel, and heavy inorganic particles.
3
Primary sedimentation: Organic solids settle as sludge; oils and grease float and are skimmed.
4
Aeration / Biological treatment: Aerobic bacteria consume dissolved organics in activated sludge or trickling filter systems.
5
Secondary clarification: Allow biological floc to settle; recycle some activated sludge back to the aeration tank.
6
Disinfection: Chlorinate, UV-treat, or ozonate the effluent to kill remaining pathogens before discharge.

Example: BOD Removal Calculation

A wastewater plant receives influent with a BOD of 250 mg/L. After primary treatment, the BOD drops to 150 mg/L. After secondary treatment, the BOD is 25 mg/L. What is the percent removal at each stage?

Primary
\( \frac{250 - 150}{250} \times 100 = \) 40% removal
Secondary
\( \frac{150 - 25}{150} \times 100 \approx \) 83% removal
Overall
\( \frac{250 - 25}{250} \times 100 = \) 90% total removal

Example: Eutrophication Link

Excess \( \mathrm{N} \) & \( \mathrm{P} \)
in effluent Algal Bloom rapid growth Algae Die decomposition BOD Spikes O2 consumed Dead Zone aquatic life dies Tertiary treatment prevents this cascade

Septic Systems vs. Municipal Plants

Septic System Septic Tank Anaerobic Drain Field On-site, decentralized Soil filters effluent No nutrient removal Risk: groundwater contamination Municipal Plant 1ry 2ry 3ry Centralized, regulated Multi-stage treatment Nutrient removal possible Result: high-quality effluent

Common Mistakes

Confusing Primary with Secondary
Primary is physical (settling), secondary is biological (bacteria). They are not interchangeable.
Assuming All Plants Use Tertiary
Many plants only do primary and secondary treatment. Tertiary is an additional step, not always required.
Forgetting Sludge Treatment
The sludge removed during treatment must also be processed: it doesn't just disappear. Anaerobic digestion produces useful biogas.

Summary: Treatment at a Glance

Process Type What It Removes Key Example
Screening Physical Large debris Bar screens
Sedimentation Physical Suspended solids Settling tanks
Activated Sludge Biological Dissolved organics Aeration basins
Nitrification Biological Ammonia/Nitrogen Nitrifying bacteria
Chlorination Chemical Pathogens \( \mathrm{Cl}_2 \) dosing
UV Disinfection Physical Pathogens UV lamps

Question 1 of 5

Which stage of sewage treatment relies on aerobic bacteria to decompose dissolved organic matter?

Question 2 of 5

A plant's influent has a BOD of 300 mg/L and the effluent BOD is 30 mg/L. What is the overall percent BOD removal?

Question 3 of 5

Which of the following is the primary environmental concern when nitrogen and phosphorus are not removed from sewage effluent?

Question 4 of 5

During anaerobic digestion of sewage sludge, what useful byproduct is generated?

Question 5 of 5

A septic system differs from a municipal sewage plant primarily because a septic system: