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Floc Structure

Size, shape, strength and background clarity of activated sludge floc.

Floc structure sketchA firm, rounded floc with dense center and a few short internal filaments.Line sketch for orientation only. Not to scale.
Sketch: floc structure as typically seen at 100x to 400x. Not to scale.

Under the microscope

Floc with finger like Zoogloea growth and a nematode inside it, phase contrast
Fingered Zoogloea and nematode. Fingered Zoogloea with a nematode in the same floc. Phase contrast. What this indicates: Fingered Zoogloea grows in high F/M systems where food is soluble or easily degraded and pH is low. Nematodes go with an older sludge.
Round, compact floc with only a few short filaments and a clear background, phase contrast
Compact floc. Round, firm floc with few extending filaments and a clear background. Phase contrast. What this indicates: Firm floc with a clear background settles well. If all the floc is this small and round, watch for pin floc.
Large floc with many filaments extending from its edges, phase contrast
Floc with extending filaments. Large floc with filaments growing out past its edges. Phase contrast. What this indicates: Filaments extending from floc are an early bulking warning. Rate their abundance and track SVI.
Many small, irregular flocs on a clear background, phase contrast at low power
Small, irregular floc. Small, irregular flocs with a clear background. Low power, phase contrast. What this indicates: Small floc settles slowly and may leave a hazy supernatant. Compare with the settleometer.
Small, scattered pin floc particles on a hazy background
Pin floc. Pin floc: small, compact particles scattered across the slide. What this indicates: Pin floc occurs at long sludge ages. SVI is often low but the supernatant is turbid.
Floc in India ink at 100x: bright floc on a black ink background
India ink stain. India ink reverse stain of a floc. 100x. What this indicates: Ink fills the background. Large clear areas inside the floc show excess exocellular slime, a sign to check for viscous bulking and nutrient deficiency.
Fingered Zoogloea growth with finger like projections, 400x
Fingered Zoogloea. Fingered Zoogloea. 400x. What this indicates: Found in high F/M systems with soluble, easily degraded food and low pH. It can cause viscous bulking.

Photomicrographs from Brad Musick's microscope slide collection.

See all 53 photos in the gallery

Illustrated

Inside a healthy flocAn irregular floc made of floc forming rod bacteria held together by sticky polymer, with a few short filaments running through it as a backbone, some trapped particles and stalked ciliates on its edge. The liquid around it is clear.Inside a healthy flocFilament backbone gives strengthFloc forming bacteria (mostly rods)EPS: sticky polymer glueTrapped particles and finesStalked ciliates graze on the edgeClear liquid, few dispersed cellsSize: about 50 to 1,000 micronsFirm, irregular, a few internal filaments, clear background: settles well and leaves a clear supernatant.
Inside a healthy floc. Floc forming rods held in sticky EPS around a short filament backbone, with trapped particles and stalked ciliates on the edge. What this indicates: Firm, irregular floc with a few internal filaments and a clear background settles well and leaves a clear supernatant. Original illustration for this site, not to scale.
Healthy floc, filamentous bulking and pin floc comparedThree microscope views with settleometers: healthy floc with a few internal filaments settles well with a clear supernatant; filamentous bulking with filaments bridging flocs settles slowly to a high blanket; pin floc of tiny particles settles fast but leaves a turbid supernatant.Three floc conditions comparedHealthy flocFilamentous bulkingPin flocSettles well, clear supernatantSVI usually 80 to 150Filaments mostly inside flocSettles slowly, compacts poorlyHigh SVI, blanket may riseFilaments bridge between flocsSettles fast, turbid supernatantLow SVI, cloudy effluentFew or no filaments, long SRTDrawings show a 100x view and a 30 minute settleometer for each case. Not to scale.
Healthy floc, bulking and pin floc. Three 100x views, each with its 30 minute settleometer result. What this indicates: Filaments bridging flocs mean high SVI and slow settling; tiny, filament free pin floc settles fast but leaves a cloudy supernatant, often from too long an SRT. Original illustration for this site, not to scale.

See all microscopy illustrations

What you see

  • Healthy floc: medium to large, irregular, firm with clear edges, a few internal filaments, clear background.
  • Weak floc: loose, open, breaking apart, many dispersed cells around it.
  • Pin floc: small (below about 50 to 150 microns), round, compact, few or no filaments.
  • Open, diffuse floc with filaments extending: bulking tendency.

What it may indicate

  • Healthy floc: good settling and clear effluent.
  • Weak floc: young sludge, low DO, toxicity, or nutrient issues.
  • Pin floc: long SRT, shear, absence of filaments.
  • Diffuse floc: filaments or viscous bulking.

What process data to check next

  • SRT and F/M.
  • SVI and settleometer supernatant.
  • DO.
  • Mixing energy and shear points.
  • Nutrients.
One slide is a snapshot. Look at trends over several days and compare with SRT, F/M, DO, SVI and effluent quality before you change anything.