Rotifers 

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Rotifers

When trying to determine species, stick to the basics, and focus on the causes and controls of the higher life forms present. The main point of any wastewater biomass identification is not to get a PhD, but to fix your plant!

Identification

The rotifers are the most abundant macro invertebrates found in the activated sludge process. There are about 2000 species, divided into three classes. Most are fresh water species, but there are a few found in marine environments.
Rotifers can be found in many different shapes and sizes. Important structural characteristics used to classify rotifers are body shape (sac, spherical or worm), size, number of gonads, foot development, number of toes and protective covering.  

 Most rotifers are colorless, except for the eyespot. However, ingested food may sometimes give the organism the appearance of having color. Rotifers are characterized by the possession of a ciliated area or a funnel-shaped structure at the anterior end that may look like rotating wheels and a specialized pharynx that is part of many pieces that act as jaws.  The mouth opening of the rotifer is surrounded by two bands of cilia. The beating of the cilia creates water current for locomotion and food gathering.

Rotifers move by swimming freely or crawling. Rotifers are able to consume both microbes and particulate matter. Like protozoa, these microorganisms are strict aerobes and are more sensitive to toxic conditions than bacteria. Rotifers are found usually in a very stable activated-sludge environment in wastewater treatment plants.

Rotifers range in size from 40 to 500 µm and have an average life span of 6 to 45 days. They are the smallest animals.

Additional Information:

Rotifers are metazoa in the phylum Rotifera. Rotifers do have Eyespots. They are often in the same size range as ciliates. Most rotifers have a contractile foot that allows them to attach to activated sludge floc. Some types of rotifers include Keratella, Brachionus, Conochilus, and Polyarthra

Environment

They are found in many different types of water, including waste water. They are beneficial in stabilizing organic wastes, stimulating microfloral activity and decomposition, enhancing oxygen penetration, and recycling mineral nutrients. Rotifers are often found in extended aeration systems and systems with a high MCRT.  The presence of rotifers in activated sludge generally means a good, stable sludge with plenty of oxygen. 

The principal role of rotifers in wastewater is the removal of bacteria and the development of floc. Rotifers contribute to the removal of effluent turbidity by removing non-flocculated bacteria. Mucous secreted by rotifers at either the mouth opening or from the foot aids in better floc formation. Rotifers usually are present in older sludge or longer MCRT. Rotifers indicate increasing stabilization of organic wastes and lower BOD and TSS.

Some plants use rotifers and worms as an indicator to when to increase wasting since it means their sludge is getting older.

How to find them:

Microscopic examination of a wet mount. Some of the larger Rotifers can be seen at 40-100x and 200x. Blow them up to 400x and 1000x though to see more details.  Rotifers are only found as the sludge age increases because it takes up to three days for their eggs to hatch.

What does it mean when I see an increase in Rotifers in my system?

It depends upon what the rest of the biomass looks like. Typically the presence of Rotifers indicates an older sludge age with low loading of food vs. the amount of biomass available to stabilize the organics. Rotifers indicate a relatively healthy, older system, no toxicity, progressing along in a sludge age towards stable to endogenous age. If mixed in with the right amount of stalked ciliates, and a few free swimming ciliates, you are probably doing very well.
 
Rotifers usually anchor themselves to a stable floc formation and create a vortex by swirling the water around with cilia on their posterior to filter in single celled bacteria. Usually, this means that the sludge is a bit stable  and TSS is clearer and more floc structures are developing.   Rotifers stimulate microfloral activity and decomposition by enhancing oxygen penetration in the floc structures by their grazing activities. They also help recycle some of the nutrients.

Rotifers  can reproduce sexually and lay eggs! 
It may mean the sludge is older if the floc is golden to dark brown, firm and compact.  Usually you can expect very few solids in the effluent and slightly lower BOD levels than if free swimming ciliates are present in significant numbers.  If you push the sludge age too old, though beware, the floc will start to break apart and water quality will deteriorate.  Ashing on the clarifier is a sign that the sludge is getting older or running out of air.  Another symptom will be very dark brown to black centers in the floc structure.

If you have once through lagoons, you may find many rotifers, as well as many different types in the same sample. Lagoons differ from activated sludge and the types of higher life forms may vary depending upon the holding time of the lagoon and if a very large lagoon, different sections will have different stages of higher life forms. Rotifers can tolerate lower D.O. than some of the stalks and free swimmers, so they are often present in lagoons in higher numbers.

Daily microscopic analyses is helpful in documenting where you are today, where you have changed since the previous day and how to react to changes proactively as opposed to when they have become critical!

What should I do if there is a significant change in my higher life forms and all of a sudden there is an increase in rotifers?
First check to see why they have increased? Is there a change in loading that might impact other areas?

You might want to adjust your wasting or RAS levels.
 
You might need to check the Bed levels in your clarifier. Check your TSS off your clarifiers.  You will have nowhere near the level of single celled bacteria that you see with amoeba or flagellates, but there still might be some. Usually rotifers mean you have an older sludge age but if you get too old, you may wind up with pin floc and some TSS. Check the floc and correlate all the parameters, but you are probably doing ok!   If all you have are rotifers, then you are progressing to an even higher sludge age. Stalked ciliate, suctoria and rotifers indicate a healthy sludge! Good work!

More to come soon! 

For more information on Higher Life Form Identification

 Rotifer Bug of the Month

 Collotheca Bug of the Month

More photos to come. . .