Sewage Treatment (College Board AP® Environmental Science): Study Guide

Primary sewage treatment

• The process of removing harmful substances from wastewater is known as sewage treatment

• This safely returns wastewater to the environment and can be used for domestic purposes (e.g. for toilets, showers, drinking water)

• It prevents pollution, protects public health, and helps conserve water by recycling treated water

Primary treatment

• Purpose: Removes large solid materials (e.g. debris) and some suspended solids

• Process:

  • Screening: Large objects like sticks, rubbish, and plastic are filtered out using screens (usually made from metal bars)

  • Comminution: Any remaining large solids are ground up by a device called a comminutor to prevent pipe blockages

  • Grit removal: Sand and gravel settle to the bottom of the grit chamber

  • Sedimentation: Wastewater flows into sedimentation tanks (also known as primary clarifier) where smaller suspended solid particles settle at the bottom as sludge

  • Skimming: Floating materials like grease and oil are skimmed from the surface

• Result: Water is partially cleaned but still contains dissolved and smaller particles

Secondary sewage treatment

• Purpose: Breaks down organic matter using biological processes

• Process:

  • Aeration: Oxygen is pumped into the wastewater to encourage bacteria to break down organic pollutants (e.g. human waste, food)

  • Biological degradation: Microorganisms (mainly bacteria) consume the organic waste, converting it into harmless by-products like carbon dioxide, water, and 'activated sludge'

    • Activated sludge contains aerobic bacteria that decompose organic matter

  • Clarification: Water is again left to settle (in a secondary clarifier) so that any remaining sludge can be removed

    • A portion of this sludge is reused as activated sludge in the aeration tank to continue the process of organic matter decomposition

• Result: Water is significantly cleaner but may still contain some nutrients (e.g. nitrates, phosphates) and pathogens

Tertiary sewage treatment

• Purpose: Removes remaining nutrients, chemicals, and pathogens for maximum water quality

• Process:

  • Nutrient removal: Processes like chemical precipitation are used to remove excess nitrogen and phosphorus (in the form of nitrates and phosphates), which can cause eutrophication if released into water bodies

  • Filtration: Water may be filtered through sand, activated carbon, or other materials to remove any final impurities

• Result: Effluent (treated water) is cleaned

Final sewage treatment

• Prior to discharge, the treated water is exposed to one or more disinfectants

• Chemical treatment: Chlorine, ozone or ultraviolet light (UV) is used to disinfect the water and kill any remaining bacteria

• Result: Water can now be safely discharged into rivers, lakes, or reused