Floc characteristics (using the 10x objective)
With 10x objective (100x magnification) floc size, floc characteristics, protozoa and other organisms, non-biological and inorganic particles, bacterial colonies, cells dispersed in the bulk liquid between flocs, and the effect of filamentous organisms on the floc structure can be determined. The reverse India ink stain may be used at 10x objective to judge polysaccharide (slime) abundance. Often, training at 10x objective is sufficient for operational personnel. With minimal amounts of training, a general idea of the problem can be determined which allows the operator to decide if someone with further experience needs to get involved.
Higher life form organisms are beneficial from a toxicity standpoint (free swimming and stalked ciliates are most sensitive); they also provide a relative idea of the dispersed bacteria prey density. As there are less dispersed bacteria present, higher life form organisms become more efficient at capturing dispersed bacteria prey.
It is important to note that the higher life form organism populations can shift rapidly, depending on plant conditions. They should not be used in correlation to sludge age or in determining wasting rates. If a slug load enters a facility within a very brief period of time, the predominant higher life form organism may change (e.g., from stalked ciliates and rotifers to flagellates and amoeba). For this reason, using the 100x (oil) objective is necessary as filamentous bacteria, floc structure, and other characteristics change more slowly.
In determining sludge quality at 10x objective, one of the most important aspects is the strength of the floc structure. Stronger flocs have more weight associated with them and can support a higher abundance of filamentous bacteria, with less impact on the sludge volume index.
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Filamentous bacteria identification
Filamentous bacteria can be identified using table 2.18 of the 3rd Edition Manual on Cause and Control of Activated Sludge Bulking, Foaming, and Other Solids Separation Problems. This is entitled the Summary of Typical Morphological and Staining Characteristics of Filamentous Organisms Commonly Observed in Activated Sludge (Jenkins, 2004). Individual characteristics, such as branching, motility, filament shape and location, presence of attached bacteria, presence of a sheath, presence of septa, filament width, filament length, filament cell shape and size, are needed for identification.
Ultimately, filamentous bacteria identification becomes like “birdwatching” to the trained individual. After seeing a certain organism many times, most can be identified immediately. There is some subjectivity involved with this as organisms do not always fit neatly into different size and shape characteristics. Morphology may change depending on the type of substrate as well as the growth rate of the organism.
In industrial plants, it is common for a filament such as Thiothrix II to look slightly different than it may appear in a municipal plant. It is important to not “force fit” organisms and accept the fact that not every organism can be identified. It is important to look at the big picture. For example, if all the other indications present a “big picture” of septicity and the operator is unsure if the filament is type 0914 or type 0675, said organism is likely to be type 0914 because this grows due to organic acids and sulfide. Type 0675 grows at longer sludge ages and lower F/M ratios and is often found with filaments such as type 0041 and type 1851.
Organism ranking is critical in determining the big picture. Common abundance is classified as 1 – 5 organisms per floc. Because one may be looking at the tail end of one condition and the beginning of another, it is important to not put much stock into an organism and its associated cause (s), unless that organism is ranked at common abundance or greater.
Practical application
Common short-term control strategies for filamentous bulking include RAS chlorination, sludge juggling, coagulant/polymer addition, and change of process configuration, such as switching to step feed operation. Long-term control strategies involve identifying and eliminating the root cause of a problem. Every system is different and plant personnel should be involved in helping determine the best and most practical operational strategies.
Ryan Hennessy is a Microbiology and Operations Specialist with Midwest Contract Operations Inc.
