Protozoa and metazoa are two significant groups of "higher" life forms in the activated sludge process. They enter the process through inflow and infiltration (I/I) as soil and water organisms and make up approximately 5% of the weight of the MLVSS. Ciliated protozoa may be present as high as 50,000 per milliliter. Metazoa usually are present in highly variable numbers. Unless the MCRT of the activated sludge process is >28 days, most metazoa are not provided with sufficient time to reproduce and usually are present in the activated sludge process in relatively small numbers (<200 per milliliter).
Protozoa and metazoa often are used as bioindicators of the "health" of the activated sludge process or mixed liquor. Protozoa are microscopic, single-celled organisms that are animal-like, fungus-like, and plant-like. Protozoa in the mixed liquor process commonly are classified into five groups. These groups are amoebae, flagellates, free-swimming ciliates, crawling ciliates, and stalked ciliates.
Generally, amoebae and flagellates dominate the mixed liquor under adverse or harsh operational conditions including low dissolved oxygen concentration, toxic-ity, and low MCRT, while the ciliated protozoa, especially crawling and stalked
ciliates, dominate under favorable conditions, including high dissolved oxygen concentration, low pollution, and absence of toxicity. With improving quality of the operational conditions within the activated sludge process, progression in dominant protozoan groups occurs from amoebae to flagellates to free-swimming ciliates to crawling ciliates to stalked ciliates. With decreasing quality of the operational conditions within the activated sludge process, regression in dominant protozoan groups occurs from stalked ciliates to crawling ciliates to free-swimming ciliates to flagellates to amoebae.
Metazoa are multicellular animals that may be microscopic or macroscopic in size. Commonly observed metazoa in the activated sludge process include bristle-worms, flatworms, free-living nematodes, rotifers, and waterbears. The most commonly observed metazoa are free-living nematodes and rotifers.
Because protozoa and metazoa are observed and counted easily during microscopic examinations of the mixed liquor, their numbers, activity, structure, and changes in dominant and recessive groups are often used as bioindicators of the presence of adverse or harsh operational conditions, including the presence of toxic wastes.
The presence of toxicity within the activated sludge process may be associated with the following microscopic bioindicators:
• Decrease in activity or loss of activity
• Decrease in numbers
• Regression in dominant protozoan groups
• Gassing or bubble production in stalked ciliated protozoa (Figure 19.5)
If the toxic wastes are surfactants, dispersion of rotifers (Figures 19.6 and 19.7) and free-living nematodes (Figures 19.8 and 19.9) and rotifers may occur.
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