The activated sludge process is a wastewater treatment method in which the carbonaceous organic matter of wastewater provides an energy source for the production of new cells for a mixed population of microorganisms in an aquatic aerobic environment. The microbes convert carbon into cell tissue and oxidized end products that include carbon dioxide and water. In addition, a limited number of microorganisms may exist in activated sludge that obtain energy by oxidizing ammonia nitrogen to nitrate nitrogen in the process known as nitrification.

Bacteria constitute the majority of microorganisms present in activated sludge. Bacteria that require organic compounds for their supply of carbon and energy (heterotrophic bacteria) predominate, whereas bacteria that use inorganic compounds for cell growth (autotrophic bacteria) occur in proportion to concentrations of carbon and nitrogen. Both aerobic and anaerobic bacteria may exist in the activated sludge, but the preponderance of species are facultative, able to live in either the presence of or lack of dissolved oxygen.

Fungi, rotifers, and protozoans are also residents of activated sludge. The latter microorganisms are represented largely by ciliated species, but flagellated protozoans and amoebae may also be present. Protozoans serve as indicators of the activated sluge condition, and ciliated species are instrumental in removing Escherichia coli from sewage. Additionally, viruses of human origin may be found in raw sewage influent, but a large percentage appear to be removed by the activated-sludge process.

The success of the activated-sludge process is dependent upon establishing a mixed community of microorganisms that will remove and consume organic waste material, that will aggregate and adhere in a process known as bioflocculation, and that will settle in such a manner as to produce a concentrated sludge (return activated sludge, or RAS) for recycling. Any of several types of activated sludge solids separations problems indicate an imbalance in the biological component of this process. In the ideal "healthy" system, filamentous organisms grow within a floc (a large aggregate of adherent, or floc-forming, microorganisms, such as bacteria) and give it strength, with few filaments protruding out into the surrounding bulk solution. In such a system, there is no interference with the compaction and settling rates of the activated sludge prior to its recycling.
 
 

The activated-sludge process is a biological method of wastewater treatment that is performed by a variable and mixed community of microorganisms in an aerobic aquatic environment. These microorganisms derive energy from carbonaceous organic matter in aerated wastewater for the production of new cells in a process known as synthesis, while simultaneously releasing energy through the conversion of this organic matter into compounds that contain lower energy, such as carbon dioxide and water, in a process called respiration. As well, a variable number of microorganisms in the system obtain energy by converting ammonia nitrogen to nitrate nitrogen in a process termed nitrification. This consortium of microorganisms, the biological component of the process, is known collectively as activated sludge.

The overall goal of the activated-sludge process is to remove substances that have a demand for oxygen from the system. This is accomplished by the metabolic reactions (synthesis-respiration and nitrificaction) of the microorganisms, the separation and settling of activated-sludge solids to create an acceptable quality of secondary wastewater effluent, and the collection and recycling of microorganisms back into the system or removal of excess microorganisms from the system.
 
 

Before beginning a discussion of the biological component of the system, an overview of the physical components that comprise the activated-sludge process would seem to be in order. This will help the reader gain a better understanding of the environment in which a mixed community of microorganisms metabolizes organic (or in some cases inorganic) matter, settles to form a thickened sludge, and is recycled back into or removed from the system.

According to Activated Sludge, Manual of Practice #9 (Water Environment Association, 1987), the activated-sludge process contains five essential interrelated equipment components. The first is an aeration tank or tanks in which air or oxygen is introduced into the system to create an aerobic environment that meets the needs of the biological community and that keeps the activated sludge properly mixed. At least seven modifications in the shape and number of tanks exist to produce variations in the pattern of flow.
 

Second, an aeration source is required to ensure that adequate oxygen is fed into the tank(s) and that the appropriate mixing takes place. This source may be provided by pure oxygen, compressed air or mechanical aeration. Just as there are modifications in the shape and number of aeration tanks that can be used in the activated-sludge process, different equipment systems exist to deliver air or oxygen into aeration tanks.
 

 
CRYOGENIC AIR SEPARATION FACILITY (HYPERION TREATMENT PLANT)
 
 

 

AERATION TANK AGITATOR (HYPERION TREATMENT PLANT)
 
 

 
OXYGEN AND EFFLUENT INFLOW PIPES (HYPERION TREATMENT PLANT)

Third, in the activated-sludge process, aeration tanks are followed by secondary clarifiers. In secondary clarifiers, activated-sludge solids separate from the surrounding waterwater by the process of flocculation (the formation of large particle aggregates, or flocs, by the adherence of floc-forming organisms to filamentous organisms) and gravity sedimentation, in which flocs settle toward the bottom of the clarifier in a quiescent environment. This separation leads ideally to the formation of a secondary effluent (wastewater having a low level of activated-sludge solids in suspension) in the upper portion of the clarifier and a thickened sludge comprised of flocs, termed return activated sludge, or RAS, in the bottom portion of the clarifier.

Next, return activated sludge must be collected from the secondary clarifiers and pumped back to the aeration tank(s) before dissolved oxygen is depleted. In this way, the biological community needed to metabolize influent organic or inorganic matter in the wastewater stream is replenished.
 

Finally, activated sludge containing an overabundance of microorganisms must be removed, or wasted (waste activated sludge, or WAS), from the system. This is accomplished with the use of pumps and is done in part to control the food-to-microorganism ratio in the aeration tank(s).
 
 

The biological component of the activated sludge system is comprised of microorganisms. The composition of these microorganisms is 70 to 90 percent organic matter and 10 to 30 percent organic matter. Cell makeup depends on both the chemical composition of the wastewater and the specific characteristics of the organisms in the biological community. (Water Environment Association, 1987)

Bacteria, fungi, protozoa, and rotifers constitute the biological component, or biological mass, of activated sludge. In addition, some metazoa, such as nematode worms, may be present. However, the constant agitation in the aeration tanks and sludge recirculation are deterrents to the growth of higher organisms.

The species of microorganism that dominates a system depends on environmental conditions, process design, the mode of plant operation, and the characteristics of the secondary influent wastewater. (Water Environment Association, 1987) The microorganisms that are of greatest numerical importance in activated sludge are bacteria, which occur as microscopic individuals from one micron in size to visible aggregations or colonies of individuals. Some bacteria are strict aerobes (they can only live in the presence of oxygen), whereas others are anaerobes (they are active only in the absence of oxygen). The preponderance of bacteria living in activated sludge are facultative—able to live in either the presence or absence of oxygen, an important factor in the survival of activated sludge when dissolved oxygen concentrations are low or perhaps approaching depletion.

While both heterotrophic and autotrophic bacteria reside in activated sludge, the former predominate. Heterotrophic bacteria obtain energy from carbonaceous organic matter in influent wastewater for the synthesis of new cells. At the same time, they release energy via the conversion of organic matter into compounds such as carbon dioxide and water. Important genera of heterotrophic bacteria include Achromobacter, Alcaligenes, Arthrobacter, Citromonas, Flavobacterium, Pseudomonas, and Zoogloea. (Jenkins, et al., 1993)

Autotrophic bacteria in activated sludge reduce oxidized carbon compounds such as carbon dioxide for cell growth. These bacteria obtain their energy by oxidizing ammonia nitrogen to nitrate nitrogen in a two-stage conversion process known as nitrification. Due to the fact that very little energy is derived from these oxidization reactions, and because energy is required to convert carbon dioxide to cellular carbon, nitrifying bacteria represent a small percentage of the total population of microorganisms in activated sludge. In addition, autotrophic nitrifying bacteria have a slower rate of reproduction than heterotrophic, carbon-removing bacteria. Two genera of bacteria are responsible for the conversion of ammonia to nitrate in activated sludge, Nitrobacter and Nitrosomonas. (Water Environment Society, 1987)

Nitrification generally occurs when the time that the sludge stays in the system (called the mean cell residence time, or MCRT) is increased. A longer mean cell residence time, therefore, allows an adequate population of nitrifying bacteria to be built up. However, because the oxygen demand for complete nitrification is high, both the necessary oxygen supply and power requirements for the system will be increased. Moreover, optimum pH for the growth of nitrifying bacteria is in the 8 to 9 range, with pH levels below 7 causing a substantial reduction in nitrification activity. In the process of converting ammonia to nitrate, mineral acidity is produced. In instances when insufficient alkalinity exists, the pH in the system will drop, potentially inhibiting nitrification. Finally, though nitrification occurs over a wide range of temperatures, a reduction in temperature produces a slower rate of reaction. (Water Environment Society, 1987)

Some activated sludge systems have been designed specifically to promote the higher growth rate of bacteria that remove carbon from influent wastewater, and adding chemicals may suppress nitrification. Other systems are operated to achieve nitrification in the second stage of a two-stage activated-sludge system due to the longer mean cell residence time (MCRT) necessary for nitrification. Still other systems are designed to promote nitrification. (Water Environment Society, 1987)

Fungi are also a constituent of activated sludge. These multicellular organisms metabolize organic compounds and can successfully compete with bacteria under certain environmental conditions in a mixed culture. (Water Environment Society, 1987) In addition, a small number of fungi are capable of oxidizing ammonia to nitrite, and fewer still to nitrate. (Painter, 1970) The most common sewage fungus organisms are Sphaerotilus natans and Zoogloea sp. (Curtis, 1969)

A number of species of protozoa have been identified in activated sludge. Protozoa are single-celled organisms that can consume food such as bacteria and particulate matter. Ciliated protozoa are numerically the most common species in activated sludge, but flagellated protozoa and amoebae may also be present. The species of ciliated protozoa most commonly observed in wastewater treatment processes include Aspidisca costata, Carchesium polypinum, Chilodonella uncinata, Opercularia coarcta and O. microdiscum, Trachelophyllum pusillum, Vorticella convallaria and V. microstoma. (Curds and Cockburn, 1970)

Protozoa are a useful biological indicator of the condition of the activated sludge. Being strict aerobes, these microorganisms prove to be excellent indicators of an aerobic environment (though some protozoa are capable of surviving up to 12 hours in the absence of oxygen). Protozoa also act as indicators of a toxic environment, as they exhibit a greater sensitivity to toxicity than bacteria. A clue that toxicity may be a problem in a system is the absence of or a lack of mobility of these organisms in activated sludge. The hallmark of a well-operated, stable activated-sludge system is the existence of large numbers of highly evolved protozoa in the biological mass. (Water Environment Society, 1987)

Further, ciliated protozoa play the dominant role in the removal in the removal of Escherichia coli from wastewater by predation or flocculation. The E. coli population is generally reduced by 91 to 99 percent in the activated-sludge process. (Curds and Fey, 1969)

Rotifers are multicellular aquatic microorganisms that look like rapidly revolving wheels when they are in motion. This is due to the fact that the anterior end of the animal is modified into a retractible disc, or corona, bearing circles of strong cilia. 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 only in a very stable activated-sludge environment. (Water Environment Society, 1987)

Finally, viruses are also found in wastewater, particularly human viruses that are excreted in large quantities in feces. These human enteric viruses can be divided into six major subgroups: adenovirus, coxsackievirus, echovirus, infectious hepatitus, poliovirus, and reovirus. Viruses native to animals and plants exist in lesser quantities in wastewater, and bacterial viruses may also be present. (Grabow, 1968) While Grabow (1968) notes that there is a quantitative reduction of these viruses by the activated-sludge treatment process, the author states that the mechanism by which they are removed or deactivated remains to be clearly explained. Different mechanisms indicated by the work of various researchers included inactivation of viruses by biological antagonists in the sludge, adsorption, and reduction in which suspended solids, colloidal material, aeration, and perhaps toxic substances play a role.

Before ending my discussion of the biological component of the activated-sludge process and discussing solids separation, factors affecting the efficiency of the process in removing carbonaceous organic material and achieving nitrification should be mentioned. These factors have been summarized in Activated Sludge, Manual of Practice #9 (Water Environment Society, 1987) and include: how readily organic material and ammonia can be metabolized by the microorganisms; the mean cell retention time (MCRT) and food-to-microorganism ratio; how readily organic material can be oxidized and used for cell synthesis; the number and types of active microorganisms present in the aeration tank(s); environmental factors such as dissolved oxygen concentration, nutrients, pH, temperature, and presence of toxic materials; adequacy of the original design for mixing, RAS (return activated sludge) and WAS (waste activated sludge) pumping, and aeration capacity; proper maintenance of plant equipment; and adequate training of plant staff.
 
 

Microorganisms play a second important role in the activated-sludge process beyond removing carbonaceous organic material from and nitrifying ammonia in secondary influent wastewater. This is the process of solids separation in which activated-sludge solids separate by flocculation and gravity sedimentation from treated wastewater in secondary clarifers. The goal of this process is to create a secondary effluent low in suspended solids in the upper portion of a clarifier and a thickened activated sludge composed of flocs in the bottom portion of a clarifier that will be recycled back into the system as return activated sludge (RAS).

Activated sludge flocs, agglomerations of particles that may reach sizes of more than 1mm, are composed of the biological component discussed in the previous section and a nonbiological component. According to Jenkins et al. (1993), genera of heterotrophic bacteria, including Achromobacter, Alcaligenes, Arthrobacter, Citromonas, Flavobacterium, Pseudomonas, and Zoogloea, appear to be the primary floc-forming microorganisms. The nonbiological component of activated sludge flocs are organic and inorganic particles and extracellular microbial polymers that are generally made up largely of carbohydrates (polysaccharide [complex carbohydrate] and glycoprotein [a conjugated protein in which the nonprotein group is a carbohydrate] fibers). (Jenkins, et al., 1993)

Two levels of structure exist in activated-sludge flocs: microstructure and macrostructure. Microbial aggregation, adhesion, and bioflocculation are the basis of the microstructure. Although the mechanism of bioflocculation is not well understood, it is felt to be the result of bridging between extracellular microbial polymers functioning as polyelectrolytes (a substance of high molecular weight, such as a protein, that is an ionic conductor). These extracellular microbial polymers form felt-like envelopes around cells and groups of cells. (Jenkins, et al., 1993)

The macrostructure of activated sludge consists of filamentous organisms that form the network within a floc onto which floc-forming bacteria cling. This network of filamentous organisms provides activated-sludge flocs with strength and the attainment of large size. As a consequence, their integrity is preserved in the aeration basin, where conditions of increasing shear occur in a turbulent environment. (Jenkins, et al., 1993)

Water is also contained within activated-sludge flocs, and the amount varies with the size of the particles present. The three types of water found in flocs are the water within the organisms, capillary water within the particles, and stagnant water within the intertstices formed by the collection of particles into a mass. (Laubenberger and Hartmann, 1971)
 
 

Benedict, R. G. and Carlson, D. A. (1971) “Aerobic Heterotrophic Bacteria in Activated Sludge,” Water Research, v. 5, pp. 1023-1030.

Curds, C. R. and Cockburn, A. (1970) “Protozoa in Biological Sewage-Treatment Processes—I. A Survey of the Protozoan Fauna of British Percolating Filters and Activated-Sludge Plants,” Water Research, v. 4, pp. 225-236.

Curds, C. R. and Fey, G. J. (1969) “The Effect of Ciliated Protozoa on the Fate of Escherichia coli in the Activated-Sludge Process,” Water Research, v. 3, pp. 853-867.

Curtis, E. J. C. (1969) “Sewage Fungus: Its Nature and Effects,” Water Research, v. 3, pp. 289-311.

Grabow, W. O. K. (1968) “The Virology of Waste Water Treatment,” Water Research, v. 2, pp. 675-701.

Jenkins, D., Richard, M. G., and Daigger, G. T. (1993) Manual on the Causes and Control of Activated Sludge Bulking and Foaming, 2nd ed. Boca Raton: Lewis Publishers.

Laubenberger, G. and Hartmann, L. (1971) “Physical Structure of Activated Sludge in Aerobic Stabilization,” Water Research, v. 5, pp. 335-341.

Painter, H. A. (1970) “A Review of Literature on Inorganic Nitrogen Metabolism in Microorganisms,” Water Research, v. 4, pp. 393-450.

Siebert, M. L. and Toerien, D. F. (1969) “The Proteolytic Bacteria Present in the Aerobic Digestion of Raw Sewage Sludge,” Water Research, v. 3, pp. 241-250.

Spellman, F. R., Ph.D. (1997) Microbiology for Water/Wastewater Operators. Lancaster, PA: Technomic Publishing Co. Inc.

Toerien, D. F. (1967) “Direct-Isolation Studies on the Aerobic and Facultative Anaerobic Bacteria Flora of Anaerobic Digesters Receiving Raw Sewage Sludge,” Water Research, v. 1, pp. 55-59.

Toerien, D. F. (1970) “Population Description of the Non-methanogenic Phase of Anaerobic Digestion—I. Isolation characterization and identification of Numerically Important Bacteria,” Water Research, v. 4, pp. 129-148.

Water Environment Association. (1987) Activated Sludge, Manual of Practice #9.