Protozoa

Protozoa are unicellular organisms. Most protozoa are free-living and solitary, but some do form colonies. Most protozoa are strict aerobes, but some including amoebae and flagellates can survive anaerobic conditions.

In the activated sludge process, protozoa are placed commonly in five groups according to their means of locomotion. These groups are amoebae (Figure 1.3),

FIGURE 1.4 Flagellate. The flagellate is a single-celled organism that moves by the beating action of one (flagellum) or more (flagella) whip-like structures.

flagellates (Figure 1.4), free-swimming ciliates (Figure 1.5), crawling ciliates (Figure 1.6), and stalked ciliates (Figure 1.7).

Ciliated protozoa are the most important groups of protozoa in the activated sludge process. They possess short hair-like structures or cilia that beat in unison to produce a water current for locomotion and food gathering—that is, to bring bacteria into their mouth opening (Figure 1.8). Ciliated protozoa provide the following benefits to the activated sludge process:

Free Swiming Wastewater
FIGURE 1.5 Free-swimming ciliate. The free-swimming ciliate is a single-celled organism that moves by the beating action of hair-like structures or cilia that are found in rows that cover the entire surface of the organism.
FIGURE 1.6 Crawling ciliate. The crawling ciliate is a single-celled organism that moves by the beating action of hair-like structures or cilia that are found in rows that cover only the ventral or "belly" surface of the organism.

• Add weight to floc particles and improve their settleability.

• Consume dispersed cells and cleanse the waste stream.

• Produce and release secretions that coat and remove fine solids (colloids, dispersed cells, and particulate material) from the bulk solution to the surface of floc particles.

• Recycle nutrients (nitrogen and phosphorus) through their excretions.

Wastewater Organisms

FIGURE 1.7 Stalked ciliate. The stalked ciliate is a single-celled organisms that moves by the beating action of hair-like structures or cilia that are found in rows that surround only the mouth opening of the organism. Some stalked ciliates may grow in a colony, and some by "spring" by means of a contractile filament or myoneme in the posterior portion or "stalk" of the organism.

FIGURE 1.7 Stalked ciliate. The stalked ciliate is a single-celled organisms that moves by the beating action of hair-like structures or cilia that are found in rows that surround only the mouth opening of the organism. Some stalked ciliates may grow in a colony, and some by "spring" by means of a contractile filament or myoneme in the posterior portion or "stalk" of the organism.

FIGURE 1.8 Cilia surrounding the mouth opening of a stalk ciliated protozoa.

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