Higher life form organisms such as stalked ciliates and rotifers may be easily viewed under brightfield microscopes. They are fascinating, motile, and easy to recognize however process control changes should not be based strictly upon higher life form organisms and their abundance. The best analogy to use is that “you are what you eat” and if a higher life form organism type is predominant the root cause is that the environmental conditions and the substrate availability favor the selection of this organism.
Common Examples of why there is NO DIRECT CORRELATION of HIGHER LIFE FORM ORGANISMS ON FM RATIO and SLUDGE AGE
While some general correlations (such as predominance of flagellates often correlating with left over available substrate) often hold true, there are many common situations that may lead operators in the wrong direction. A common example is that rotifers often prey on small floc “fragments”. At higher SRT values when pin floc is present, rotifers are often common, however, if there is a stress that creates de-flocculation it is common for rotifers to proliferate. Nematodes (often associated with high sludge age values) may grow within treatment plants, but also commonly enter treatment processes through the influent, and have been viewed in systems with sludge ages as low as 2 days. There are many other examples that may be viewed at: https://www.tpomag.com/tags/bug-of-the-month.
How Fast do Higher Life Form Organisms Grow?
The mix of higher life-form organisms can change quickly depending on their available prey. The longest it takes protozoa to reproduce is 1 day at 20 degrees C (Curds, 1975) so sludge retention time rarely limits their competition. Certain species of rotifers can grow in sludge ages of less than even 4 days (Richard, 2018). In short, there is no proven correlation between higher life form organisms and sludge age, and because there are many other factors besides organic loading that can contribute to changes in sludge condition, basing process control decisions solely on higher life form organisms is not recommended. (Jenkins, 2004)
Impact of Environmental Stresses
From a stress perspective, free swimming and stalked ciliate species are typically the most sensitive to stresses and toxicity while testate amoebae and rotifers can compete well in tougher environments. Note that not all potential stresses are equal and depending on what type of stress is present these can have varying, and sometimes unusual impacts on the microbes present. In most instances, the higher life form organisms are the first to be impacted when there is stress or toxicity.
Summary
In summary, there is value in looking at the higher life form organisms but is also important to put their emphasis in perspective as they only represent a “piece of the puzzle”. A shift in predominant higher life form organisms should generally suggest a “deeper look” at what is going on under the microscope with an evaluation that documents factors such as floc structure, dispersed growth, zoogloea bacteria type abundance, filament bacteria type rank, abundance, and suspected growth causes, health of filamentous bacteria, impact of filaments on the floc structure, and bacterial viability within the mixed liquor.
Source: https://rhwastewatermicrobiology.com/wastewater-microbiology-book/
