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Rotifers

Multicellular animals with a crown of cilia and a forked foot. A sign of older, stable sludge.

Rotifer sketchA rotifer with a two-lobed crown of cilia, a body and a forked foot.Line sketch for orientation only. Not to scale.
Sketch: rotifers as typically seen at 100x to 400x. Not to scale.

Under the microscope

Rotifer anchored to a floc, with another floc at its head end, phase contrast at 400x
Rotifer on floc. Rotifer anchored to a floc by its foot. Phase contrast, 400x. What this indicates: Active rotifers point to a mature, stable sludge at moderate to long SRT.
Rotifer body showing internal organs, with a label pointing to the rotifer, phase contrast
Rotifer. Rotifer showing its internal organs. Phase contrast. What this indicates: Rotifers need stable conditions. Large numbers with small, dense floc can mean the SRT is very long.
Sick rotifer and a rotifer cyst among sulfur granules, bacterial floc and a filament, labeled, phase contrast at 400x
Rotifer and cyst, sulfide toxicity. Sick rotifer and a rotifer cyst with sulfur granules nearby, from a plant with sulfide toxicity. Phase contrast, 400x. What this indicates: Rotifers forming cysts or losing activity, with sulfur granules on the slide, point to sulfide or another toxic stress. Check septicity and influent sulfide.

Photomicrographs from Brad Musick's microscope slide collection.

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Illustrated

Higher life formsFour panels: a rotifer with a two lobed ciliated crown and forked foot, a smooth nematode worm, a segmented Aeolosoma bristle worm with bristle tufts and colored spots, and a water bear with eight clawed legs.Higher life forms in activated sludgeRotifer100 to 500 micronsCiliated crown, forked footNematode0.5 to 3 mmSmooth roundworm, thrashesBristle wormAeolosoma, up to 10 mmSegments, bristle tufts, color spotsWater bearTardigrade, 0.1 to 1 mmEight stubby clawed legsAll are multicellular and usually point to an older, stable sludge with a longer SRT.
Higher life forms. Rotifer, nematode, bristle worm (Aeolosoma) and water bear (tardigrade), with typical sizes. What this indicates: These multicellular animals grow slowly, so steady numbers usually mean a long SRT and a stable, well oxidized sludge. Original illustration for this site, not to scale.

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What you see

  • Multicellular animals about 100 to 500 microns, with a ciliated crown (corona) that looks like spinning wheels.
  • A visible chewing organ (mastax) moves inside the body.
  • Many attach to floc with a forked "foot" or crawl like an inchworm.

What it may indicate

  • Mature, stable sludge with good floc and low dispersed bacteria.
  • Common at moderate to long SRT.
  • Very large numbers with small, dense floc: possibly very long SRT and pin floc.
  • Rotifers are fairly sensitive; sudden loss of activity can be an early toxicity sign.

What process data to check next

  • SRT trend: has it crept up?
  • Settleometer supernatant: pin floc?
  • If inactive or missing suddenly: possible toxicity, check OUR and influent.
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.