Wastewater treatment by microorganisms – activated sludge tank
BiologyBiotechnologyAges 15–16
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Biotechnology
Wastewater rich in organic matter (high BOD) flows into a tank, where aerobic bacteria feed on the organic matter using oxygen supplied by an aerator. Adjust the influent BOD, aeration level, sludge (biomass) concentration and retention time to see BOD fall, dissolved oxygen change, and when the effluent meets discharge standards; turn the aeration off to see the tank turn anaerobic, treatment slow down, and odors appear.
Lesson: Microbial technology in environmental pollution treatment
What it shows
Biochemical oxygen demand (BOD) is the amount of oxygen microorganisms need to break down the organic matter in water, so it indicates how polluted wastewater is. In an activated sludge tank, aerobic bacteria living in sludge flocs use this organic matter as their carbon and energy source, oxidising it to CO₂ and water while producing new biomass. The treatment rate depends on the amount of sludge, the remaining BOD and the dissolved oxygen supplied by aeration. Without enough oxygen the tank turns anaerobic, breakdown slows, and foul-smelling hydrogen sulfide forms along with methane.
How to use
Set Influent BOD for the incoming wastewater, then adjust Aeration, Sludge and Retention time while watching the BOD and dissolved oxygen curves. Compare the treated water with the strict and basic discharge limits marked on the graph. Challenge students to find the shortest retention time that still meets the strict limit, then turn Aeration down to zero and discuss what happens in the tank.
Parameters you can change
- Influent wastewater BOD 50–800 mg/L
- Aeration level 0–100 %
- Sludge (biomass) concentration 0.5–5 g/L
- Hydraulic retention time 1–24 h
Questions to explore
- What does a high BOD tell you about a sample of wastewater?
- Does a higher sludge concentration shorten the time needed to meet the strict discharge limit?
- Why does a tank without aeration begin to smell bad?
