Young sludge: Amoebae and flagellates predominate with a few free swimming ciliates.
Mature sludge: For municipal wastewater treatment plants the most desirable stage of the ecological succession is the stage where both stalked ciliates and free swimming ciliates are observed with possibly a few rotifers. This stage is is correlated with rapid settling of solids and easy separation of solids from water. The resulting effluent is correspondingly low in turbidity, suspended solids and biochemical oxygen demand (BOD).
Older sludge are characterized by increasing numbers of rotifers and nematodes with fewer stalked ciliates.
Both the young and the old sludge are associated with poor settling and high effluent turbidity. Both are conditions municipal wastewater treatment operators strive to avoid.
Filamentous bacteria, or bulking bacteria occur in activated sludge plants or lagoons. Low numbers of filaments are thought to be helpful by pulling pin flocs together so flocs become heavy enough to settle.
Amoebae are some of the earliest organisms to be observed as the activated sludge process develops. They are associated with "young sludge". They move and eat by sending out "false feet", pseudopods, that serve both for locomtion and to engulf small particles of organic material and bacteria.
See pictures of amoebae.
flagellated protozoans range in size, shape and number of flagella. A flagellum is a tail-like structure comprised of protein that whips back and forth causing the organism to move through the water. Most flagella push the protozoan; a few protozoans have flagella that pull them through the water. One or more flagella may occur at either end of the protozoan.
Ciliated protozoans have hair-like structures all over the surface of the cell, also made of protein, that wave back and forth causing the organism to move. These organisms engulf bacteria and small particles for food, with some of the larger ciliates that pry on smaller ciliates and flagellates. They occur at about the same time in sludge succession as the flagellated protozoans.
See pictures of ciliated protozoans.
Stalked Ciliated Protozoans
Stalked ciliated protozoans are the organisms most highly regarded by sewage waste treatment operators. They are associated with sludge that settles readily and with low suspended solids in the supernatant from the clarifier. The stalk is stuck to the floc so the organism is "rooted" in the floc. The body of the ciliate is perched on the end of the stalk with cilia at the opposite end from the stalk. The cilia wave back and forth producing a current that sweeps small praticles and bacteria into the cell. Te stalk is actually spring-like and coils slowly into a spring. Periodically the spring releases and the coil stretches in a new direction taking the body of the protozoan to a new area where more particles and bacteria can be swept into the body of the ciliate by the cilia. Some forms have immature stages that lack a stalk, but have cilia.
See pictures of Vorticellar, one type of typical ciliated stalked protozoans. It has one cell at the end of stalk.
When environmental conditions (pH, temperature, concentrations of nutrients, waste products and toxic compounds) are adverse, many protozoan have the ability to foam cysts. Cysts have cell walls resistant to low or high pH, low or high temperature, toxins and drying. Since the cell is not metabolizing, food and other materials outside the cell are not being taken into the cell. the strategy is for the organism to avoid contact with the environment and remain alive until the environment improves. Cysts can also occur as a result of reproduction.
See pictures of protozoan cysts.
Rotifers are multicellular organisms with an alimentary tract, muscles and a reproductive system with most being female. At the head end are cilia that sweep bacteria and particles into the animal. Some rotifers have heavier cilia that provide locomotion in addition to sweeping small particles into the mouth. One prominent feature of the digestive system is the mastax which is comprised of muscles surrounding a series of sclerotized jaws that macerate the food particles. Some sewage plant staff prefer to see more rotifers in their activated sludge tan others do. For these plants settling is optimized when more totifers are observed. In some plants more than a few rotifers are associated with old sludge and plant staff increase sludge wasting.
Pictures of rotifers.
Nematodes are roundworms with a mouth at one end, an alimentary tract and anus at the other. In the wastewater treatment plant they feed on detritus - small particles of organic material including bacteria. Some forms are predators that eat protozoa, rotifers and other nematodes. Nematodes are associated with old sludge. When seen in the sludge in substantial numbers operating staff typically increase wastage rates.
Pictures of netmatodes.
Pretreatment facilities associated with food plants typically have evolved over many year. Food canning, freezing, package dinners, milk and cheese plants, and slaughter houses are often located near rural areas where their raw materials are raised. Land costs were relatively inexpensive, particularly at the time many plants were constructed. In the 1960s and 1970s it was common practice to dig another hole to store dirty water when another restriction occurred. The result is often a series of lagoons or ponds frequently totaling several millions of gallons. Over the years discharge regulations have become more strict and suburbia has often grown closer to the plant. The lagoons are aerated and the first one or two lagoons behave as an activated sludge treatment system and contain the organisms described above. Subsequent lagoons in the system often are green as a result of algae growth. The algae contribute to total suspended solids and may be responsible for total suspended solids being over the discharge limit. For ponds with a goal to remove phosphorous from solution, the algae may serve a purpose. In these cases a method to collect and harvest the algae is necessary so the phosphorus is removed. Another alternative is to consider a constructed wetland and growing aquatic plants as well as algae.
See pictures of algae.
Filamentous fungi have also been collected from food plants with lagoons. Compared to bacteria, fungi require long periods of time to grown, a doubling time of a few hours for bacteria compared to days for fungi. Fungi grow from a spore or a piece of vegetative hypha. The walls of many fungi contain the biopolymer chitin, the same material that comprises the tough exoskeleton of insects. They knot together as strands of spaghetti or a mat of long hair. The knot or mat serves as a net to catch smaller particles with the result that equipment can be clogged. Fungi play an exceedingly important role in decomposing trees and fallen leaves in forests. Thermalphilic (heat-loving) fungi are important in composting operations. In wastewater treatment lagoons they appear to indicate a long retention time and some opportunities to improve the operation of the wastewater treatment scheme.
See pictures of fungi.
Difference Between Bacteria (procaryotes) and Protozoans, Algae and Fugi (eucaryotes)
Intracellular organelles (small organs inside the single cell) can be seen on many of these photomicrographs of amoebae and protozoans. Intracellular organelles include the nucleus of the cell, food vacuole, and other specialized structures. Organelles are surrounded by a membrane that separates them from the rest of the cell. Digestive enzymes are secreted into food vacuoles to digest the food particle and avoid exposure of the rest of the cell to the enzymes. Bacteria lack intracellular structures that are visible with light microscopy.
The values of specific growth rates of various microorganisms are listed in the following table. [source: Wastewater Treatment with Microbial Films, by Shigehisa Iwai, Takane Kitao, P15, Technomic Publishing Company, Inc. 1994]
Growth Rates of Microbes
|Growth Rate Parameters||
|Dry Weight of a Cell|
"Hawkes comparing the biotas of trickling filter slime and activated sludge, have shown in the figure above. As metazoas (Rotatoria, Nematoda, Insecta, Shellfish, Oligochaeta, etc) are all large-sized, whose length ranges from several millimeters to a few centimeters, and who prey on film microbes diligently, resulting in a remarkable decrease of excess sludge productions. Moreover, an ecosystem with highly diverse biota is a stable system which enables a stable treatment effect inevitably. Those bacteria, who utilize substrates slowly assimilated or substrates with low value of growth yield, always have relatively small specific growth rates. Therefore, mcirobial film processes have excellent performance in removing such substrates."
source: Wastewater Treatment with Microbial Films, by Shigehisa Iwai, Takane Kitao, P16， Technomic Publishing Company, Inc. 1994
Process Conditions Versus Organisms Present/Population
|Poor BOD5 and TSS Removal
Mainly dispersed bacteria
No floc formation
Very cloudy effluent
|Predominance of amoeba and flagellates
A few ciliates present
|Poor Quality Effluent
Some floc formation
|Predominance of amoeba and flagellates
Some free-swimming ciliates
Good floc formation
|Predominance of free-swimming ciliates
Few amoeba and flagellates
|High Quality Effluent
Excellent floc formation
High effluent clarity
|Predominance of stalked ciliates
Some free-swimming ciliates
A few rotifers
A few flagellates
|Effluent High TSS and Low BOD5
High settled sludge volume
|Predominance of rotifers
Large numbers of stalked ciliates
A few free-swimming ciliates
Source: Frank R. Spellman, Spellman矾s Standard Handbook for Wastewater Operators, Fundamental Level, Volume 1, Technomic Publishing Company, 199, Page 157
Particle size can be determined from the photomicrographs using the following equation:
Particle size = (Dimension/Magnification) x 1000
The 1000 multiplier converts mm to micrometers (microns). Particle size is expressed in micrometers.
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