Waste water treatment
Waste (sewage) water from homes and industry contains organic matter, harmful microbes and dissolved pollutants. Treatment plants clean it before returning it to rivers/seas.
Stages of waste water treatment
1. Screening
- Pass through metal grids/screens.
- Removes large solids (twigs, plastic, rags).
- Sand/grit also removed (sedimentation tanks).
2. Sedimentation
- Slow flow allows heavy solids to settle out as sludge (bottom).
- Lighter materials and clear liquid (effluent) above.
3. Anaerobic digestion of sludge
- The sludge is digested by anaerobic bacteria (no oxygen).
- Produces:
- Methane (used as biogas — energy source).
- Solid digested sludge (used as fertiliser/landfill).
4. Aerobic biological treatment of effluent
- The liquid effluent is sprayed over filter beds or stirred in tanks with air bubbled through.
- Aerobic bacteria digest organic matter and microbes.
- Produces water clean enough to discharge into rivers.
Comparing potable water vs waste water sources
- Sea water: large salt content; needs desalination (high energy).
- Ground/surface water: low salt; needs filtration + sterilisation.
- Waste water: more contaminants; needs screening, sedimentation, biological treatment.
In terms of energy demand, treating waste water is generally less energy-intensive than desalinating sea water.
Why two types of bacteria?
- Anaerobic in the sludge — produces useful biogas; works in absence of oxygen.
- Aerobic for effluent — cleans the liquid; needs oxygen.
The two work in different conditions and on different parts of the waste stream.
⚠Common mistakes
- Confusing screening with filtration — screening removes large objects; filtration removes finer particles.
- Saying both bacteria are anaerobic — only the sludge digesters are anaerobic.
- Forgetting biogas as a useful product — methane from anaerobic digestion is used as fuel.
- Treating "treated" waste water as drinking water — most waste water is returned to rivers/sea, not the tap.
Links
Builds on C10.2 (potable water). Connects to C9.4 (carbon footprint of energy use) and C10.6 (recycling/reuse).
AI-generated · claude-opus-4-7 · v3-deep-chemistry