With rising summer temperatures, wastewater plants often face the problem of sludge floating in secondary clarifiers. This not only affects effluent quality but may also cause unstable system operation.
So, why does sludge frequently float in summer? And how can we solve it effectively?
Here are the 5 most common causes and their solutions 👇
1️⃣ Denitrification Leads to Floating Sludge
🟤 Feature: Sludge floats in blocks on the water surface.
⚙️ Cause: Strong nitrification in aeration tanks. Nitrate flows into the secondary tank and triggers denitrification inside sludge layers. Released nitrogen gas makes sludge float.
Common reasons include:
- Low internal recirculation
- Thick sludge blanket
- Delayed sludge removal
- Scraper malfunction or dead zones
✅ Solutions:
- Optimize aeration intensity and time to avoid over-nitrification
- Control internal/external recirculation ratio
- Maintain timely sludge withdrawal
- Repair scraper and improve flow pattern
2️⃣ Anaerobic Decay Produces Gas
⚫ Feature: Sludge turns black, smells bad, and floats with bubbles.
⚙️ Cause: Anaerobic conditions in the secondary tank. Microbes produce gas during decomposition, lifting sludge upward. Often due to low aeration, high load, or sludge accumulation.
✅ Solutions:
- Strengthen aeration in front processes
- Avoid long sludge retention time
- Reduce load and keep microbial balance
3️⃣ Sludge Bulking Reduces Settling
⚪ Feature: Poor sludge-water separation, effluent carries sludge.
⚙️ Cause: Filamentous or non-filamentous bulking.
- Filamentous bulking: DO low, pH low, C/N imbalance
- Non-filamentous bulking: High load or shock load
✅ Solutions:
- Adjust nutrient ratio and aeration strategy
- Regular microscopic check for filamentous bacteria
- Apply selective biocides if needed
4️⃣ Sludge Aging or Disintegration
🟤 Feature: Sludge breaks into fine particles, floats with foam.
⚙️ Cause: Long sludge age, high DO, or low-load operation. This weakens sludge structure and breaks flocs, reducing settling ability.
✅ Solutions:
- Control proper sludge age and discharge regularly
- Avoid over-aeration
- Strengthen monitoring to keep sludge active
5️⃣ Hydraulic Shock Load
🔵 Feature: After sudden influent surge, effluent turns turbid and carries fine sludge.
⚙️ Cause: Instant high inflow reduces hydraulic retention time. Surface load increases, fine particles cannot settle in time.
✅ Solutions:
- Balance flow and load fluctuations
- Add PAC/PAM coagulants if needed to improve settling
- Use equalization tanks to buffer shock load
📝 Conclusion
Sludge floating in summer is not accidental. It results from multiple operational factors combined.
By analyzing causes, adjusting operational parameters, and applying proper measures, we can:
- Prevent floating sludge
- Ensure stable effluent quality
- Improve overall treatment efficiency
For sludge bulking and aging, precise system design and daily monitoring are even more important.
