Causes of low MLSS

 Low MLSS (Mixed Liquor Suspended Solids) in an STP can compromise the biological treatment process, leading to poor organic matter removal and suboptimal effluent quality. Below are the common causes of low MLSS:

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1. Excessive Sludge Wasting (Over-Wasting)

• Description: Excessive removal of sludge reduces the concentration of active biomass in the aeration tank.
• Cause:
  • Improper sludge wasting rates due to incorrect operational settings or mismanagement.

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2. Insufficient Influent Organic Load

• Description: A low organic load (BOD/COD) leads to limited microbial growth, resulting in lower biomass levels.
• Cause:
  • Dilution of influent wastewater (e.g., during rainy seasons or due to industrial discharges of low-strength wastewater).

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3. Hydraulic Overloading

• Description: High flow rates dilute the mixed liquor, reducing MLSS concentration in the aeration tank.
• Cause:
  • Stormwater inflows or infiltration into the sewer system, causing a high hydraulic load.

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4. Poor Microbial Growth

• Description: Conditions unfavorable for microbial growth result in inadequate biomass production.
• Cause:
  • Insufficient nutrients (e.g., nitrogen, phosphorus).
  • Inhibitory substances (e.g., heavy metals, toxic chemicals) in the influent.
  • Low pH or temperature affecting microbial activity.

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5. Sludge Washout

• Description: Sludge is physically washed out of the aeration tank or clarifier due to high flow rates or poor settling.
• Cause:
  • Insufficient retention time or hydraulic surges.
  • Malfunctioning secondary clarifier or improper sludge recycling.

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6. Inadequate Return Activated Sludge (RAS)

• Description: Returning insufficient sludge from the secondary clarifier reduces the MLSS concentration in the aeration tank.
• Cause:
  • Improper RAS pump settings or operational errors.

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7. Shock Load Events

• Description: Sudden changes in influent composition or flow disrupt the microbial balance, reducing biomass levels.
• Cause:
  • Toxic shocks (e.g., chemicals, industrial discharges).
  • Rapid changes in temperature or pH.

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8. Equipment Malfunction

• Description: Aeration equipment failure affects microbial growth and MLSS concentration.
• Cause:
  • Aerator breakdown, leading to insufficient oxygen supply.
  • Pump failures affecting sludge recycling or wasting.

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9. Low Sludge Retention Time (SRT)

• Description: Short retention times do not allow sufficient microbial growth, resulting in low biomass levels.
• Cause:
  • High sludge wasting or low RAS rates.

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Impacts of Low MLSS

1. Reduced Treatment Efficiency:
   • Insufficient biomass leads to poor BOD and COD removal.
2. Poor Effluent Quality:
   • Higher organic load in the treated water due to inadequate biological treatment.
3. System Instability:
   • Difficulty maintaining process stability during fluctuations in influent load.

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Solutions for Low MLSS

1. Optimize Sludge Wasting:
   • Adjust wasting rates to retain sufficient biomass in the aeration tank.
2. Improve Influent Load:
   • Prevent dilution by controlling stormwater inflow and infiltration.
3. Maintain Proper Aeration:
   • Ensure adequate oxygen levels to promote microbial growth.
4. Adjust RAS Flow:
   • Increase sludge recycling to maintain MLSS levels in the aeration tank.
5. Provide Adequate Nutrients:
   • Ensure proper nitrogen and phosphorus levels to support microbial activity.
6. Minimize Toxic Shocks:
   • Identify and control sources of toxic chemicals or inhibitory substances.
7. Upgrade Infrastructure:
   • Install equalization tanks to handle hydraulic surges.
   • Repair faulty aeration or pumping equipment.

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By addressing these causes, STP operators can maintain optimal MLSS levels, ensuring efficient treatment and high-quality effluent.