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Daniel L. Theobald

Known in the industry as “Wastewater Dan,” Daniel L. Theobald, proprietor of Environmental Services, is a professional wastewater and safety consultant/trainer. He has more than 24 years of hands-on industry experience operating many variants of wastewater treatment processing units and is eager to share with others his knowledge about water conservation. (

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How sludge settles is a key performance indicator

Someone recently asked, 'What are some procedures for measuring settle-ability of activated sludge?'

March 31, 2014
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Collecting samples, use of equipment for simulating settle-ability and the interpretation of results are all part of measuring activated-sludge settle-ability.

Multiple means for evaluating the activated sludge process exist. However, the settle-ability test is a key sludge-condition indicator. It simulates conditions in the secondary or final clarifier. The 1,000-milliliter/1-liter bench-scale settling jar known as the “settlometer” is a small version of a clarifier. Watching and measuring mixed-liquor behavior as it settles indicates how sludge is settling in the clarifier.

In an aeration tank, return activated-sludge (RAS) rates or waste activated-sludge (WAS) rates will encourage activated-sludge growth, with activated sludge settling properly in the secondary final clarifier by settling all of the BOD and solids down with it. The settling jar simulates the behaviors found in the secondary or final clarifier, unless clarifier design, condition or loading is problematic.  

Collect samples

The mixed liquor samples collected for the settle-ability test must represent the mixed liquor flowing out of the aeration tank to the secondary or final clarifier.

Samples must be collected either just before it leaves the aeration tank or some point prior to the secondary or final clarifier. If the mixed liquor or activated sludge is from two or more aeration tanks, brought together before flowing to the clarifiers, be sure the sample represents the combined flow.

However, it is also useful to measure the settle-ability in each aeration tank to check for differences in sludge condition amongst them. Also, collect mixed liquor/activated sludge. Be certain you are not collecting surface scum.

Collect mixed liquor/activated sludge samples in wide-mouth containers, rush the samples to the lab, and start the settle-ability test immediately. This is necessary because the settle-ability procedure is intended to simulate clarifier settling behavior.

Simulation equipment needed

Running the settle-ability test requires a settling jar “settlometer,” paddle and timer.

The settling jar or “settlometer” should be a clear-glass or plastic cylinder with a larger-diameter mouth. A short, wide “settlometer” mirrors the secondary or final clarifier shape.

A wide paddle gently stirs the mixed liquor in the “settlometer” as the procedure starts. The timer will alarm at 5-minute or 10-minute intervals.

Sampling-technique and procedure conditions strongly influence overall results. It should be done in a place free from vibration and direct sunlight. Temperature should not vary a great deal from one test to another, to the extent possible.

  • Agitate the mixed liquor sample carefully and rapidly with the least possible amount of additional aeration or turbulence. Stir the settlometer contents gently with a wide paddle.
  • As the paddle is smoothly and carefully removed from the settlometer, start the timer.
  • After five minutes, record the volume of the settlometer occupied by settled sludge volume in milliliters.
  • Record the settled sludge volume (SSV) at five-minute intervals for first thirty minutes and at 10-minute intervals for the next 30 minutes of the one-hour “settlometer” procedure.

Allow the sample to stand several hours and record sludge conditions.

Results interpretation

Two types of information follow from correct testing: numerical information corresponding to the settled sludge volume at various time intervals and visual information or observations, including sludge characteristics and sludge-blanket formation.

Observable sludge characteristics include whether it is granular, compact, fluffy or feathery. Are the agglomerating sludge particles large or small? Do the sludge particles settle individually or do they first form a blanket? Is the supernatant/clarified water clear or cloudy? Is the blanket ragged and lumpy, or uniform on the surface?

The numbers on settling include sludge volume. Visual inspection can identify sludge types and bacteria health. Ways of characterizing sludge settling, such as “normal settling,” “rapid settling” or “slow settling” help detail sludge behavior.

For a given plant, examining numeric-settling information and sludge characteristics can indicate treatment conditions and be used to identify process-control adjustments such as changing wasting rates.