The Microbial Workforce at PCWRA

PCWRA's Biological Nutrient Removal Depends Upon Millions of Microbes - Below are just a Few

Stalked Ciliates

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Stalked ciliates can be seen in single organism form or can grow in colonies. Each “head” in a colony of stalked ciliates is considered one organism. Therefore, when counting higher life forms for maturity index calculations every organism is counted in the colony. Colonies can range from three to over three hundred organisms each. Stalked ciliates usually attach them-selves to a piece of floc or inert material but can occasionally be seen moving through the water, with or without the stalk. Centrifuging a sample for observation can break the stalk off. Each species resembles a tulip or tube shape with cilia (small hairs) around the opening. The cilia trap bacteria, which are used as a food source, by creating a current that moves the bacteria toward the opening. The stalk ciliate then contracts in a quick motion, which pushes the food into the body where it can be utilized.

Stalked ciliates usually indicate a stable, healthy system with a moderate to high maturity index. Because stalked ciliates attach to pieces of floc, they usually imply that the biomass (bacteria) is forming well structured floc that is essential to settling and good effluent quality. However, one stalked ciliate, Vorticella microstoma, is often indicative of high turbidity and poor effluent quality. This is because they consume single bacteria cells in open water which means dispersed bacteria are present and turbidities are elevated. But, this particular stalked ciliate has a very small mouth opening compared to other species which enables them to be identified fairly easily.
EBS Environmental

Rotifers (Video Link)

Rotifers are metazoa and are the simplest of the multicellular animals. They are found in many different types of water including aeration stabilization basin systems, activated sludge, and some freshwater systems. Rotifers range in size from 40 to 500 µm. They are, in fact, microscopic crustaceans. Rotifers move by swimming freely through the bulk water or crawling. They have a cliliated area at the anterior end (mouth opening) that resembles a “rotating wheel.” This group of cilia at the mouth aids in the feeding and movement of the rotifer. They feed on suspended particles and bacteria. The food is passed into the gut via two grinding plates called “mastax”. Many rotifers also have a posterior podite (foot) which allows them to attach to floc. It usually looks like a forked tail. The main role of rotifers in wastewater systems is the removal of bacteria. They also aid in floc formation. Rotifers thrive in conditions with plenty of oxygen and are an indicator species for low Biochemical Oxygen Demand (BOD), low toxicity, and stable wastewater systems.

They are the most abundant macroinvertebrates in an activated sludge system. In the activated sludge system, rotifers have shown to be helpful in reducing bulking by preying on the filamentous bacteria “Microthrix” which is associated with bulking in wastewater systems. They are mostly found in very stable activated sludge systems. They are excellent indicators of the health of the sludge and suggest whether the sludge is of good quality, is stable, and has plenty of oxygen or not. They are also beneficial to the wastewater system because they stabilize the organic waste, enhance oxygen penetration into the floc, and recycle mineral nutrients. They contribute to floc formation and reduction of final effluent turbidity by removing (consuming) non-flocculated bacteria. Since they are more predominant in older sludge, some wastewater plants use rotifers to identify whether they need to increase wasting.
EBS Environmental

Water Bears (Tardigrade) (Video Link)

Tardigrades, also called water bears, are metazoan that overlap in size with protozoa, but are easy to distinguish when identifying organisms. They have a tubular pharynx, an eyespot, claws, and intestine. They resemble teddy bears with six legs. Tardigrades usually indicate low food to mass ratios and good BOD degradation.

Gastrotrichs are a group of poorly understood metazoan. Their size range overlaps that of ciliates, with which they are often confused because of the cilia and spines. They are distinguished by two adhesive structures, like a forked tail, at the posterior end of the cell and by the presence of a discrete pharynx. They usually glide rather than swim. Gastrotrichs are sensitive to hostile conditions and only appear when treatment conditions are stable.
EBS Environmental

Nemotode, Free-Swimming Ciliates & Flagellates

Flagellates belong to the class Mastigophora and range in size from 5-20 micrometers in diameter. They are commonly ovoid or pear-shaped with one to four flagella, hair-like projections used for locomotion, attached to one or both ends of the cell. The flagella can usually be observed at 1000X magnification. Some flagellates may form colonies in which the cell bodies are clumped together with their flagella projecting outward. Some flagellates contain chlorophyll and are capable of photosynthesis. Due to the particular characteristic of resembling plants they are often classified as flagellated algae rather than protozoa.

The locomotion of flagellates is usually fast and they seem to flip and twist as the flagella are “whipped” around to propel them. Some flagellates have multiple flagella. This makes them appear “bouncy” and unorganized due to their locomotion mechanism while other higher life forms such as free-swimming ciliates present a more organized locomotion mechanism. This is sometimes helpful in identifying flagellates under the microscope. When counting flagellates under the microscope, a 400X magnification should be used in order to identify them better due to their small size.

Flagellates feed on soluble organic matter and dispersed bacteria. Flagellates are more common in heavily loaded plants or during startups. They predominate when there is a high dispersed (single-celled) bacteria population density. They are sometimes associated with turbid effluents produced during toxic upsets. They also bloom when septic sludge or inadequate aeration causes anaerobic conditions in the aeration basin. Their presence in a wastewater treatment system indicates high soluble biochemical demand levels, low dissolved oxygen and high organic load. Typically higher life forms disappear when a chemical upset goes through a wastewater system. Flagellates are the first higher life form to come back after a chemical upset in the wastewater system. Flagellates can indicate when the wastewater system is getting healthier or if it is still overcoming a chemical upset.
EBS Environmental

Nematodes are common and widespread metazoa. Most are 2-3 mm long and have a long thin shape and a slightly blunt anterior end, resembling earthworms. They are rather stiff and can move either by writhing or gliding through a substrate. At high magnification, a strong muscular pharynx is apparent near the front and egg-bearing ovaries near the back end. Nematodes occur in lightly loaded plants operated at a low F/M (food to mass) ratio. They are commonly found in old activated sludge with ample dissolved oxygen and in biofilm reactors. They can be used as a type of bioindicator for determining when toxins have been introduced into a system. Their burrowing action promotes floc health by allowing oxygen to penetrate the larger pieces of floc. EBS Environmental
Nematode Video Link

The ciliates are so named because of the cilia, small hairs that are distributed over the entire body. Ciliates are generally ovoid or pear-shaped and maintain their shape by means of a tough but flexible pellicle. Cilia protrude through the pellicle in a variety of patterns. The term ciliate comes from the Latin word “ciliate” which means eyelash.

Free-swimming ciliates range in size from 40-100 micrometers. Rapid, rhythmic cilia movement propels them through the liquid. Some are completely covered with cilia while others have cilia in rows or spirals around the cell. Euplotes, Colpidium, and Paramecium are common examples of free-swimming ciliates.

Some ciliates have specialized cilia that look and function like legs, allowing them to crawl around on floc particles and “flick” up the bacteria so that they can consume them. These are called crawling ciliates and tend to stay on the floc more than free in the bulk water. Aspidisca is one example of a crawling ciliate. The main purposes of their cilia are to propel the organisms and to gather food into their mouths (cytostome). They feed mostly on bacteria and other single cell organisms. They are sometimes identified by their smooth gliding or “swimming” motion through a sample or by the “crawling” movements around a piece of floc.

Ciliates are typical colonizers of biological sludge. Under optimum conditions, their numbers range from 1,000 to100,000 cells/mL. A sudden reduction in the number of individuals, or occurrence of encysted, inflated or dead ciliates, is an indication of shock loading with toxic substances or organic overloading. Therefore they are indicators of toxicity in wastewater systems such as ASB’s or Activated Sludge. Since they move fast and they prey on bacteria, ciliates helps to produce a low TSS and turbidity effluent. Some plants use the presence of ciliates to predict the quality of the plant’s effluent. EBS Environmental


Amoebae range in size from 100 to 300 micrometers. They are unique in that they have no distinct shape. They continually range from round to oval to irregular shapes with protrusions periodically extending from the main cell mass and then withdrawing. They move by the use of foot-like protrusions called pseudopodia (false feet). A pseudopodia is pushed out and the main cell mass moves by flowing into the extended portion. They are sometimes harder to detect because they usually move extremely slowly. Bare, or naked, amoebae are commonly found during startups or in heavily loaded plants. Testate, or shelled, amoebae will usually indicate a stable, lightly loaded wastewater system. EBS Environmental

View the entire PCWRA Microbe Gallery HERE