At the SRWTF Laboratory, mixed liquor samples are analyzed microscopically on a daily basis along with Total Suspended Solids (TSS), 30 minute settling, and respiration rates. The microscopic examination can be as useful a tool for the successful operation of the activated sludge system as these other tests. Much can be learned about the state of the aeration basins based on the type of protozoa and metazoa present. Other helpful information is the description of the floc size and shape, and type and amount of filamentous bacteria present.
In a healthy mixed liquor sample for the SRWTF, the microscopic examination should yield a predominance of stalked ciliates with moderate populations of other ciliates such as crawlers (creepers) or free swimmers. Other protozoa such as flagellates and amoebae should be found in small to scarce numbers. Although they are found in small populations, it is important that these other organisms be present with the ciliates in order to provide a more stable environment. For example, if an upset occurred that affected the higher life form, such as the D.O. dropping, a lower life form that can tolerate a low D.O. will survive.
The protozoa and metazoa have two important functions at a treatment plant. The first is to serve as indicator organisms to allow the viewer to determine the treatment level of the plant by microscopic evaluation. The second function is to clarify the effluent by feeding on the free bacterial cells in the solution that have not flocculated.
THE PROTOZOAAmoebae: The Amoebae are a group of very simple organisms classified by the presence of a pseudopodia or a "false foot". The pseudopodia are used by the amoebae for eating and moving. At the SRWTF we usually see two types of amoebae, shelled and naked. For our purposes the presence of either type has the same meaning. Amoebae usually predominate during a start up period when there is young sludge or after a major upset. They need large amounts of soluble food present in order to survive.
Flagellates: Flagellates are another group of primitive organisms, related to the amoebae. Flagellates get their name from the "tail like" projection called a flagella that extends from their bodies. The flagella is used for movement through the liquid. Like their relatives the amoebae, flagellates are present when there are large amounts of soluble food available (high F:M). 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.
Ciliates: The Ciliates are more complex organisms than the amoebae and flagellates. They are classified as ciliates because of the tiny hairlike projections called cilia that are found on their bodies. In examination of an activated sludge sample the three important groups of ciliates are stalked, crawling and free-swimming.
Free-swimming ciliates: Free-swimming ciliates are identified by the cilia that surround most or all of their bodies. Free-swimmers swim faster than fiagellates so they can out compete them for food. Free-swimmers are usually found when no large flocs have been formed so that it is easier to swim around.
Crawling ciliates: Crawling ciliates have cilia mainly on the lower surface of their bodies that make them appear to be legs. In order for crawlers to dominate there must be large flocs present that impede the free-swimmers and flagellates movement and provide a surface for the crawlers to "walk" on. This means the F:M is low and the bacteria have started to flocculate as a survival response. Crawlers also require a high D.O. content in the mixed liquor.
Stalked ciliates: Stalked ciliates feed on the stray bacteria cells. They are easily identified from other ciliates because the main body of the organism is attached to a stalk that is usually implanted in the floc. Stalked ciliates are very efficient feeders and will predominate when the F:M is low. They also do best when there are high D.O. readings.
THE METAZOA
Rotifers: Rotifers are very large as compared to other organisms. Unlike the protozoa, they are multicellular organisms. The rotifers are only found as the sludge age increases because it takes three days for their eggs to hatch, so if you waste 25% of your sludge a day you can lose 75% of the rotifer eggs.
Nematodes: Nematodes are also large in size as compared to the protozoa. Their eggs also require time to hatch and therefore they predominate at an older sludge age.
FILAMENTOUS BACTERIA
Filamentous bacteria serve as the backbone of floc formation. Sludge settles most efficiently when it contains a moderate number of filaments which provide structure for the floc and aid in the stripping of the water column. The floc cannot form properly if there are too few filaments, and the floc cannot settle properly if there are too many. The filamentous bacteria are analyzed in two ways: their effect on floc structure and their abundance.