Waste Water Treatment Plant

Cleaning sewage and recovering energy

A wastewater plant removes solids and dissolved organic matter so the water can safely return to the environment, and treats the sludge that is produced along the way.

1. Preliminary treatment: a bar screen removes rags and plastics, and an aerated grit chamber removes sand and gravel.
2. Primary clarification: settleable solids sink and are pumped away as primary sludge.
3. Aeration (activated sludge): air supplied through diffusers lets bacteria consume dissolved organic matter and ammonia.
4. Secondary clarification: the bacterial flocs settle. Most are returned as RAS; the surplus (WAS) goes to the digester.
5. Disinfection: UV light (or chlorine) inactivates pathogens before the effluent is discharged.
6. Sludge treatment: an anaerobic digester converts sludge into more stable solids and biogas, which fuels an engine or a flare.

Typical numbers (design and regulation differ)

• Primary settling removes about 50–60% of suspended solids; conventional activated sludge runs at about 1,500–3,000 mg/L solids, a 4–8 hour detention and a sludge age of about 5–15 days.
• Return sludge flow is about 25–50% of plant flow; aeration is usually the biggest energy user.
• Digesters run at about 35–38 °C for about 15–20 days and yield roughly 0.5–0.9 m³ of biogas per kg of volatile solids destroyed.

Click any equipment in the animation, or a name above, to open a detailed animation of it.