Settleability, Problem or Not?

Settleability Test – Interpretation

One of the most useful and easy tests for operators to perform is the settleability test. This test provides valuable information about the settling characteristics of the mixed liquor suspended solids (MLSS) in the secondary clarifier. Understanding how to interpret the results is crucial for proper system operation.

What is Settleability?

Settleability refers to the ability of the mixed liquor to settle and compact in the secondary clarifier. Good settleability means the solids settle quickly and form a dense sludge blanket, while poor settleability can lead to solids loss in the effluent and operational problems.

Conducting the Settleability Test

The settleability test is performed by:

1. Taking a sample of mixed liquor from the aeration tank
2. Pouring it into a 1-liter graduated cylinder
3. Allowing it to settle for 30 minutes
4. Recording the volume of settled sludge

Interpreting Results

The settleability test results are typically expressed as milliliters per liter (mL/L). Here's how to interpret them:

Common Problems and Solutions

Bulking Sludge

When settleability is poor (high mL/L values), it often indicates bulking sludge. This can be caused by:

• Filamentous bacteria overgrowth
• Low dissolved oxygen concentrations
• Nutrient deficiencies
• High organic loading
• Low pH conditions

Pin Floc

Small, pinpoint floc particles that don't settle well can indicate:

• Young sludge age
• Insufficient mixing
• Chemical imbalances
• Shock loading conditions

Corrective Actions

For Bulking Sludge

• Increase dissolved oxygen to >2.0 mg/L
• Add nutrients (nitrogen and phosphorus) if deficient
• Adjust pH to 6.5-8.0
• Reduce organic loading if possible
• Consider chlorination of return activated sludge

For Pin Floc

• Increase sludge age by reducing wasting
• Improve mixing in aeration tanks
• Add settling aids if necessary
• Investigate and correct shock loading

Monitoring and Trending

Regular settleability testing (daily or every shift) helps operators:

• Identify problems early before they affect effluent quality
• Track the effectiveness of corrective actions
• Establish baseline performance for their system
• Predict potential upsets

Remember that settleability results should be considered along with other operational parameters such as dissolved oxygen, pH, temperature, and effluent quality to get a complete picture of system performance.