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Identification: Free-living nematodes are non-segmented, terrestrial macroinvertebrates (no backbone or invertebrate).  They may be saprozoic, saprophytic, or coprozoic. They often they may look segmented due to the thickening of the cuticle or epidermis. Their bodies are cylindrical with tapering ends. Nematodes range in size from 0.5 to 3.0 mm in length to 0.02 to 0.05 mm in width.  Aquatic earthworms have setae along the body which allows them to tunnel through the floc particles, ingesting chunks of bacterial floc. They can be very common in old activated sludge.

Nematodes also secrete a sticky substance in order to  anchor themselves to a substrate (media)  or floc particles so that they can feed without interference by currents or turbulence. A lack of nematode activity  or dead and hollow nematodes can be one of the bio-indicators of a toxic condition that may be developing in the treatment process.

Additional Information: Free-living nematodes include eel worms, nemas, roundworms, and threadworms. Some types of free-living nematodes commonly found in wastewater are Butlerius, Caenorhabditis, and Rhabditis.

Where Found: They are found in many different types of wastewater. Nematodes are often observed in higher MCRT systems or where the sludge is older.  They thrive in aerobic wastewater treatment processes when the D.O. concentrations are high and bacterial food is abundant.  They are present in large numbers in secondary wastewater effluent, trickling bio-filters or rotating biological contactors (RBC's) where an older biofilm develops. They are usually the last to come ( as far as age) and the first to go( as far as increases in toxicity or BOD loading)  from an "indicator organism" standpoint.

They can indicate a system with a very old sludge age, high MLSS, low F/M ratio or high MCRT. Their burrowing action permits oxygen diffusion to the floc structures which promotes microbial activity. They crop the bacterial population and recycle nutrients.

Bristle worms also tolerate low oxygen conditions.  They are found in older activated sludge ages.  They are usually indicative of high nitrates present in the water.

How to Find Them: Microscopic examination of a wet mount. Nematodes crawl upon floc particles and move in whip-like fashion when in the free-swimming mode.

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