Protozoa
Flagellates
Flagellates are single-celled protists with one or more flagella, which are whip-like organelles often used for propulsion. The flagella is used for movement through the liquid. Eucaryotic flagella are not the same as flagella of bacteria. The flagella found in flagellates has an internal structure composed of small tubules of protein called microtubules.
Some flagellates live as colonial entities, while others function as a single cell. Most are free-living organisms, however, a number are parasitic or pathogenic for animals and humans. They multiply by binary fission and some species possess cyst stages. Flagellates range in size from 5-20 µm. Many flagellates are able to feed autotrophically (fix their own energy from inorganic sources) as well as heterotrophically(depend upon energy and carbon fixed by some other organism). Flagellated protozoa are oval in shape and have one or more whip-like structures or flagella. The whipping action propels the protozoa through the activated sludge in a "cork-screw" pattern of locomotion. While in motion, flagellates accidentally "hit" substrate. With decreasing numbers of suspended bacteria, flagellates find it more difficult to find substrate.
There are two primary groups of flagellates. The Peranema belongs to the group which ingests its food. The other group of flagellates is more like bacteria. They don't ingest whole food. They take in food that is already partially "digested." Dinoflagellates are important primary producers (photosynthesisers) in lakes and oceans, yet they can also ingest prey and feed in an animal-like fashion. Some types of flagellates commonly found in wastewater are Euglena, Trigonomonas, and Monas.
Like their relatives the amoebae, flagellates are usually present when there are large amounts of soluble food available (high F:M or high BOD). They are found during start up when the sludge is young or after an upset, but will quickly predominate over the amoebae because they are more efficient feeders. They are often found in trickling filters, oxidation ponds, lagoons and activated sludge. Flagellates are one of the few protozoan form present in sludges that are strongly loaded. Their presence may indicate high soluble BOD levels. Flagellates usually are present in very large numbers during initial start-up of a wastewater treatment plant, during recovery from a toxic discharge to the treatment plant, or at low D.O. levels. If flagellates are present as the dominant protozoan group, this could indicate an unstable wastewater environment and a sludge biomass that is very young. Usually found in low MCRT or low HRT for activated sludge systems. Lagoon systems are different and flagellates are often found in lagoons since it is harder to develop an older sludge in a lagoon with high flows.
Flagellates prefer soluble nutrients and dead or decaying material. They compete with bacteria for food in the activated sludge process, but can only dominate when the nutrient level is high. Some of the larget flagellates eat bacteria in the sludge but can not keep up with the logarithmic growth rate of the bacteria in the activated sludge. Therefore flagellates can only dominate early in the treatment process when the soluble organic nutrients are high enough for both flagellates and bacteria to eat. In a continuosly fed batch process, the dominance of these microorganisms is short lived.
Binary Fission
Flagellates reproduce through binary fission since it consists of a cell splitting in half. As you can see in the animation above, the microorganism first makes a second copy of its DNA in a process known as replication . Next, the cell begins to constrict in the middle, leaving one set of DNA and organelles on each side of the constriction. Eventually, the cell splits apart into two identical daughter cells. Once these daughter cells enlarge to adult size, each one is ready to split into two more daughter cells.
