Causes of high MLSS

 High MLSS (Mixed Liquor Suspended Solids) in an STP can lead to operational issues, such as poor oxygen transfer, reduced treatment efficiency, and difficulties in sludge settling. Understanding the causes is essential for addressing and preventing problems. Here are the common causes of high MLSS:

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1. Inadequate Sludge Wasting (Under-Wasting)

• Description: Sludge wasting (also called waste activated sludge, or WAS) removes excess biomass from the system. If insufficient sludge is wasted, MLSS levels will increase.
• Cause:
  • Failure to monitor and adjust sludge wasting rates according to influent loads or process requirements.

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2. High Influent Organic Load

• Description: Excess organic matter in the influent promotes rapid microbial growth, leading to an increase in biomass.
• Cause:
  • Sudden surges in biochemical oxygen demand (BOD) or chemical oxygen demand (COD) from industrial discharges, septage, or unexpected loads.

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3. Extended Sludge Retention Time (SRT)

• Description: A long sludge age allows for the accumulation of inert solids and older, less active biomass.
• Cause:
  • Poorly calibrated control systems or improper operational settings for sludge age.

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4. Inefficient Aeration

• Description: Insufficient oxygen supply reduces microbial activity, causing biomass accumulation as microorganisms struggle to degrade organic matter effectively.
• Cause:
  • Malfunctioning aeration systems or inadequate dissolved oxygen (DO) levels in the aeration tank.

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5. Inorganic Solids Accumulation

• Description: Inert solids (non-biodegradable materials) entering the system can increase the overall MLSS concentration without contributing to treatment.
• Cause:
  • High levels of silt, sand, or other inorganic materials in the influent.

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6. Poor Sludge Settling (Bulking Sludge)

• Description: Sludge that fails to settle properly in the secondary clarifier increases MLSS in the aeration tank due to sludge recycle.
• Cause:
  • Filamentous bacteria overgrowth (sludge bulking), caused by nutrient imbalances or low DO.

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7. Process Upsets

• Description: Sudden disruptions in flow, load, or operational parameters can lead to a temporary imbalance in sludge wasting and MLSS control.
• Cause:
  • Equipment failure, influent variability, or operator error.

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8. Excessive Recycling

• Description: Returning too much sludge from the secondary clarifier to the aeration tank can increase MLSS levels.
• Cause:
  • Improper settings on return activated sludge (RAS) pumps or control systems.

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Impacts of High MLSS

• Reduced Oxygen Transfer:
  • High solids concentration increases the viscosity of the mixed liquor, making aeration less efficient.
• Poor Sludge Settling:
  • Causes carryover of solids in the treated effluent.
• High Energy Costs:
  • Aeration systems must work harder to maintain DO levels in thick sludge.
• Reduced Treatment Efficiency:
  • Excess MLSS can lead to uneven microbial activity and poor organic removal.

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Solutions for High MLSS

1. Increase Sludge Wasting:
   • Regularly monitor and adjust sludge wasting rates based on system conditions.
2. Monitor Influent Loads:
   • Identify and mitigate sources of high organic or inorganic loads.
3. Optimize Aeration:
   • Maintain proper DO levels to support microbial activity and avoid sludge buildup.
4. Control Sludge Recycling:
   • Adjust RAS flow rates to balance MLSS levels.
5. Perform Regular Maintenance:
   • Ensure aeration and clarifier systems function efficiently to prevent process upsets.
6. Implement Pre-Treatment:
   • Remove inorganic solids or grit upstream to prevent their accumulation in the system.

Maintaining an optimal MLSS range ensures effective biological treatment and prevents operational challenges in the STP.