What is the correct sequence of stages in a conventional municipal plant?

Primary, preliminary, secondary, tertiary

Secondary, primary, preliminary, tertiary

Preliminary, secondary, primary, tertiary

Why are screens and grit chambers placed at the start of the plant?

To remove rags and sand/gravel that would damage pumps and pipes

To remove nitrogen and phosphorus

What happens in the aeration tank of the activated sludge process?

Air is supplied so bacteria in recycled sludge oxidise organic matter and form flocs

Solids settle by gravity with no air

Chlorine is added to kill bacteria

Why is part of the settled sludge from the secondary clarifier returned to the aeration tank?

To maintain the biomass (MLSS) needed to treat incoming sewage

To reduce the air requirement to zero

Practice · 8 marks

Municipal wastewater treatment plant

Short answer

Conventional treatment includes preliminary (screens, grit removal), primary sedimentation, secondary biological treatment (activated sludge with aeration tank and clarifier, return sludge) and tertiary disinfection. The activated sludge process mixes wastewater with recycled microbial flocs and air for 4–8 hours, converting organics to settleable biomass and achieving 85–95% BOD removal.

Describe with a flow diagram the stages of a conventional municipal wastewater treatment plant (preliminary, primary, secondary and tertiary treatment) and explain the working of the activated sludge process.

Municipal wastewater (sewage) is treated to remove solids, organic matter, nutrients and pathogens before discharge or reuse.

Raw sewage
   |
 [Screen] -> [Grit chamber]          PRELIMINARY
   |
 [Primary sedimentation tank] -> sludge ---+   PRIMARY
   |                                       |
 [Aeration tank] <--return sludge--+       |   SECONDARY
   |                              |        |
 [Secondary clarifier] -> excess sludge ->[Digester]->drying bed
   |
 [Disinfection (chlorine/UV)]               TERTIARY
   |
 Treated effluent -> river / reuse

Preliminary treatment

Screens (bar racks) remove rags, plastics and large floating matter.
Grit chamber (velocity about 0.3 m/s) settles sand and gravel; skimming removes oil and grease. It protects pumps and pipes.

Primary treatment

Primary sedimentation (detention 1.5-2.5 h) settles suspended solids by gravity. It removes about 60% of SS and 30-35% of BOD. The settled sludge goes to the digester.

Secondary (biological) treatment

Removes dissolved and colloidal organic matter using microorganisms: activated sludge process, trickling filter, oxidation ponds, aerated lagoons. It removes 85-95% of BOD.

Activated sludge process: effluent from the primary tank is mixed with a recycled "activated" sludge rich in bacteria in the aeration tank, to which air is supplied by diffusers or surface aerators for 4-8 h. Bacteria oxidise the organic matter and form flocs. The mixture goes to a secondary clarifier where the flocs settle; part of the sludge (return activated sludge, RAS) goes back to maintain the biomass (MLSS about 2000-3000 mg/L), and the rest (waste sludge) is treated. Key design parameters: F/M ratio, sludge age and detention time.

Tertiary (advanced) treatment

Removes nutrients (nitrogen and phosphorus), remaining solids (sand filtration) and pathogens by disinfection (chlorination, UV, ozone). Used when effluent standards are strict or the water is reused.

Sludge treatment

Thickening, anaerobic digestion (producing biogas), drying beds and then land application or disposal.

✓ Answer checked by Claude Opus 5.5 (high effort)

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