The settleometer is the simplest and most useful process control test in an activated sludge plant. It costs almost nothing, takes 30 to 60 minutes, and shows you in a clear cylinder what your secondary clarifier is doing in the dark. Operators who run it well can often see a process change days before it shows up in effluent numbers.
This article covers the procedure, what to observe, how to calculate the common results, and the errors that make the test misleading.
Equipment
- A settleometer: a 2 liter clear cylinder or square settling container with graduations in mL/L (the Mallory type is common). A 1 liter graduated cylinder also works, but narrow cylinders hold sludge up on the walls and give higher readings.
- A clean sample bucket or dipper.
- A timer.
- A light source behind the cylinder, and a dark background card for viewing the supernatant.
- A stirring rod (optional, for the stir test).
- A data sheet or log.
Sampling
The sample should represent the mixed liquor going to the clarifier.
- Sample at the aeration basin effluent or the mixed liquor channel to the clarifiers, where the liquid is well mixed.
- Take the sample at the same location and time each day so results are comparable.
- Collect at least 2 liters in a clean bucket.
- Start the test within a few minutes. Mixed liquor changes quickly once it stops being aerated: it loses DO, starts to flocculate in the bucket, and may begin to denitrify.
Also collect a sample for MLSS from the same bucket at the same time. You need it for SVI.
Procedure
- Gently mix the sample in the bucket so solids are evenly suspended. Do not whip it, which breaks floc.
- Pour smoothly into the settleometer to the 1,000 mL/L mark.
- Start the timer immediately.
- Do not move or bump the settleometer during the test. Keep it out of direct sun, which warms the sample and can cause gas formation.
- Record the settled sludge level at 5, 10, 15, 20, 25, 30, 45 and 60 minutes. The 30 minute value (SSV30) is the standard, but the curve is more informative.
- Note the appearance of the supernatant and blanket at each reading.
- In warm weather, leave the test for 2 hours or more and note if and when the sludge rises (denitrification).
What to observe
The settling curve
Plot settled volume versus time. Typical shapes:
- Normal: rapid drop in the first 5 to 10 minutes, then gradual compaction. SSV30 roughly 200 to 400 mL/L at typical MLSS.
- Bulking: slow, steady settling that never really compacts. SSV30 high, often above 600 mL/L.
- Fast and dense: very rapid drop to a small volume. Often old sludge, may come with pin floc.
- Slow start (lag): sludge holds up for a few minutes before settling. Can mean a young or dispersed sludge, or a sample that was not mixed well.
The interface
A sharp, clean line between sludge and supernatant is a good sign. A fuzzy or ragged interface suggests filaments or straggler floc.
The supernatant
Hold the dark card behind the cylinder:
- Clear: good.
- Hazy with tiny particles (pin floc): old sludge or shear.
- Fluffy particles drifting (straggler floc): young sludge or gas.
- Cloudy, uniform turbidity: dispersed growth, possible toxicity or very young sludge.
Color and odor
Healthy mixed liquor is a medium chocolate brown with an earthy smell. Very light tan suggests young sludge; very dark brown or black suggests old or septic sludge. A septic or sulfide smell is a red flag.
Rising sludge
If sludge settles and then floats up in clumps after an hour or two, nitrate is denitrifying in the blanket. Note the time; it tells you how long sludge can stay in your clarifier blanket before it starts to rise. See What Causes Rising Sludge?.
Calculations
SSV and SSC
SSV30 is the settled volume in mL/L at 30 minutes.
Settled sludge concentration (SSC) estimates how thick the sludge got in the cylinder:
SSC (mg/L) = MLSS x 1,000 / SSV
At MLSS 2,620 mg/L and SSV30 310 mL/L: SSC = 2,620 x 1,000 / 310 = 8,450 mg/L.
SVI
SVI (mL/g) = SSV30 x 1,000 / MLSS
SVI = 310 x 1,000 / 2,620 = 118 mL/g. See Why Is My SVI Increasing? for interpretation.
Diluted SVI
When SSV30 is above about 800 mL/L, the test is crowded. Dilute 1:1 with clarified secondary effluent, run the test, multiply the settled volume by 2 (the dilution factor), and calculate SVI with the original MLSS. The SVI calculator has a dilution factor input.
RAS ratio estimate
RAS ratio = SSV30 / (1,000 minus SSV30)
310 / 690 = 0.45. At 8.0 MGD: RAS flow about 3.6 MGD. See How Much RAS Should I Return?.
Common errors
Old sample. Waiting 15 minutes before starting changes the result. Start within a few minutes.
Not mixing before pouring. Solids settle in the bucket and the cylinder gets a thin or thick portion.
Over-mixing. Breaking floc gives slow settling and cloudy supernatant.
Mismatched MLSS. SVI must use MLSS from the same sample.
Wrong container. A narrow 1 L graduated cylinder can read 10 to 30% higher than a 2 L settleometer for bulky sludge. Use the same container every time.
Temperature. A sample left in the sun or a hot lab warms quickly. Keep it near basin temperature.
Reading the wrong line. Read the top of the sludge blanket, not the top of a fluffy straggler layer.
How the cylinder differs from the clarifier
The settleometer is a model, not a copy, of the clarifier. Keep the differences in mind:
- Depth. A clarifier is 12 to 16 ft deep. The weight of a deep blanket compresses sludge more than a few inches of cylinder can, so real RAS is often thicker than the SSC predicts.
- Flow. The cylinder is still. A clarifier has inlet currents, density currents and a collector moving through the blanket.
- Time. Sludge may sit in a clarifier blanket for one to three hours, longer than the standard test.
- Temperature. The cylinder warms or cools toward room temperature.
That is why the settleometer is best used as a trend tool. Compare it with your own clarifier data (blanket depth, RAS concentration) over weeks, and you will learn how your clarifiers relate to the cylinder.
Making it a habit
- Run the test every day at the same time and place.
- Record the full curve, not just SSV30.
- Photograph the cylinder at 5 and 30 minutes. A photo log is surprisingly useful.
- Trend SSV30, SVI and supernatant clarity with SRT, F/M and DO.
- Run a second test at peak flow or when something looks different.
The settleometer will not tell you everything, but it will tell you something every day. Combined with the microscope, blanket depths and your SRT trend, it gives you a complete picture of how well the process is set up to settle.
Example log entry
White River WRRF (fictional), September 12, 8:30 AM. MLSS 2,560 mg/L. SSV: 5 min 780, 10 min 640, 15 min 530, 20 min 460, 30 min 385 mL/L. Supernatant clear, interface slightly fuzzy. SVI = 385 x 1,000 / 2,560 = 150 mL/g. Microscope shows filaments common (abundance 4), S. natans type. Action: blower repair scheduled, RAS raised, state point checked.