Introduction to Microbiology...
Classification of Microorganisms...
( 1 ) By Kingdom...
Microorganisms are organized into five broad groups based on their structural and functional differences. The groups are called kingdoms. The five kingdom are animals, plants, protista, fungi and bacteria.
( 2 ) By Energy and Carbon Source...
The relationship between the source of carbon and the source of energy for the microorganism is important. Carbon is the basic building block for cell synthesis. A source of energy must be obtained from outside the cell to enable synthesis to proceed.

Our goal in wastewater treatment is to convert both the carbon and energy in the wastewater in the cells of microorganisms, which we can remove from water by settling. Therefore, we wish to encourage the growth of organisms that use organic material for both their carbon and energy source.

If the microorganism uses organic material as a supply of carbon, it is called heterotrophic. Autotrophs require only CO₂ to supply their carbon needs. Organisms that rely only on the sun for energy are called phototrophs. Chemotrophs extract energy from organic or inorganic oxidation/reduction reactions. Organotrophs use organic materials, while lithotrophs oxidize inorganic compounds.
( 3 ) By Their Relationship to Oxygen...
Bacteria also are classified by their ability or inability to utilize oxygen as a terminal electron acceptor in oxidation/reductions. Obligate aerobes are microorganisms that must have oxygen as the terminal electron acceptor. When wastewater contains oxygen and can support obligate aerobes, it is called aerobic. Obligate anaerobes are microorganisms that cannot survive in the presence of oxygen. They cannot use oxygen as a terminal electron acceptor. Wastewater that is devoid of oxygen is called anaerobic. Facultative anaerobes can use oxygen as the terminal electron acceptor and under certain conditions ; they can also grow in the absence of oxygen. Under anoxic conditions, a group of facultative anaerobes called denitrifiers utilizes nitrites (NO₂^-) and nitrates (NO₃^-) as the terminal electron acceptor. Nitrate nitrogen is converted to nitrogen gas in the absence of oxygen. This process is called anoxic denitrification.
Some Microorganisms of Interest in Wastewater Treatment...
( 1 ) Bacteria...
Bacteria (singular, bacterium) are the simplest forms of plant life which use soluble food and are capable of self-reproduction. They are single-celled organisms. No particular species is selected as the best. The highest population of microorganisms in a wastewater treatment plant will belong to the bacteria. Bacteria are fundamental microorganisms in the stabilization of organic wastes and therefore of basic importance in biological treatment. Individual bacterial cells range in size from approximately 0.5 to 5 micro-m in rod, sphere and spiral shapes, and occur in a variety of forms : individual, pairs, packets and chains. In most species, the process of reproduction – growth, maturation, and fission-occurs in 20-30 minutes under ideal environmental conditions.

Based on nutritive requirement, bacteria are classified as heterotrophic or autotrophic bacteria, although several species may function both.
( a ) Heterotrophic...
Heterotrophic bacteria use organic compounds as an energy and carbon source for synthesis. A term commonly used instead of heterotroph is “saprophyte” which refers to an organism that lives on dead or decaying organic matter. The heterotrophic bacteria are grouped into three classifications, depending upon their action toward free oxygen.

1. Aerobes : Require free dissolved oxygen to live and multiply.

2. Anaerobes : Oxidize organic matter in the complete absence of dissolved oxygen.

3. Facultative : Bacteria are a class of bacteria which use free dissolved oxygen when available but can also respire and multiply in its absence, e.g. Escheichia coli.
( b ) Autotrophic...
Autotrophic bacteria use carbon dioxide as a carbon source and oxidize inorganic compound for energy. Autotrophs of greatest significance in sanitary engineering are the nitrifying, sulfur, and iron bacteria.

1. Nitrifying bacteria : They perform the following reactions :



2. Sulfur bacteria : They perform the following reaction e.g. Thiobacillus :



This bacterial production of sulfuric acid occurs in the moisture of condensation on side walls and crowns of sewers conveying septic wastewater. Since thiobacilli can tolerate pH levels less than 1.00, sanitary sewers constructed on flat grades in warm climates should be built using corrosion – resistant materials.

3. Iron bacteria : True iron bacteria are autotrophs which oxidize inorganic ferrous iron as a source of energy. These filamentous bacteria occur in iron-bearing waters and deposit the oxidized iron, Fe(OH)₃, in their sheath. These bacteria are truly iron-accumulating bacteria and thrive in water pipes conveying water containing dissolved iron and form yellow or reddish – colored slimes. When mature bacteria die, they may decompose imparting foul tastes and odors to water.


( 2 ) Fungi...
Fungi (singular, fungus) is a common term used to refer to microscopic nonphotosynthetic plants, including yeasts, molds, and bacteria. Fungi are multicelluar. Because of their importance, bacteria are frequently excluded from the fungi classification. Their cells require only half as much nitrogen as bacteria so that in nitrogen – deficient wastewater, they predominate over the bacteria. The most important group of yeasts for industrial fermentations is the genus Saccharomyces. Saccharomyces cerevisiae is single-celled. Under anaerobic conditions, this yeast produces alcohol as an end product. Saccharomyces cerevisiae is facultative and performs the following reactions :



Molds are filamentous fungi which resemble higher plants in structure, composed of branched, filamentous, threadlike growths called hyphae. Molds grow best in low pH solutions (pH 2-5) high in sugar content. Molds are undesirable growths in activated sludge and can be created by low-pH conditions. The operation of an activated sludge wastewater-treatment system relies on gravity separation of microorganisms from the wastewater effluent. A large growth of molds creates a filamentous activated sludge which does not settle easily.
( 3 ) Algae...
Algae (singular, alga) are microscopic photosynthetic plants. Because of the chorophyll contained in most species, they produce oxygen through photosynthesis. In the presence of sunlight, the photosynthetic production of oxygen is greater than the amount used in respiration. At night they use up oxygen in respiration. If the daylight hours exceed the night hours by a reasonable amount, there is a net production of oxygen. The process of photosynthesis is illustrated by the equation :



The overall effect of this reaction is to produce new plant life, thereby increasing the number of algae. By-products oxygen results from the biochemical conversion of water. Algae are autotrophic, using carbon dioxide as a carbon source. The nutrients of phosphorus (as phosphate) and nitrogen (as ammonia, nitrite, or nitrate) are necessary for growth. In addition, certain trace nutrients are required, such as Mg, sulfur, B, Co, Ca, K, Fe, Mn, Zn and Cu. In natural waters the nutrients most frequently limiting algal growth are inorganic phosphorous and nitrogen. Energy for photosynthesis is derived from sunlight. Photosynthetic pigments biochemically convert the energy in sun’s rays to useful energy for plant synthesis.

In the dark reaction, the algae degrade stored food or their own protoplasm for energy to perform essential biochemical reactions for survival. The rate of this endogeneous reaction is significantly slower than photosynthetic reaction. Algal growth in rivers and lakes is not something mysterious or unknown but a simple natural process. Given a suitable environment (temperature, pH and sunlight) and a proper nutrient supply (phosphate, nitrogen and trace nutrients) algae will grow and multiply in abundance. Algae which grow unattached in the water are referred to as “phytoplankton”. If the supply of nutrients is not a limiting factor in natural waters, excessive growth of phytoplankton creates a “pea-soup” condition referred to as “bloom”, during which the algal populations multiply rapidly to several thousands per ml. The algae Oscillatoria rubescens has been identified as one which creates blooms in eutrophic lakes. Filamentous algae, such as Anabaena, float on the water surface, forming mats which are unsightly and wash onto bathing beaches. Water supplies from eutrophic rivers and lakes can have periodic serious problems, clogging filters, and producing tastes and odors caused by excessive algal growth. Algae grow in abundance in stabilization ponds rich in inorganic nutrients and carbon dioxide released from bacterial decomposition of waste organics. Green algae Chlorella are commonly found in oxidation ponds.
( 4 ) Protozoans...
Protozoa are single-celled organisms. The protozoans of significance in biological treatment systems are strict aerobics found in activated sludge, trickling filters, and oxidation ponds. These microscopic animals have complex digestive systems and use solid organic matter as an energy and carbon source. Protozoans are a vital link in the aquatic since they ingest bacteria and algae. They are desirable in wastewater effluents because they act as polishers in consuming the bacteria.

The species with hairlike cilia are the most prevalent forms found in activated sludge. Protozoans with cilia may be categorized as free-swimming and stalked. Free-swimming forms move rapidly in the water, ingesting organic matter at a very high rate. The stalked forms attach by a stalk to particles of matter and use cilia to propel their head about and bring in food.
( 5 ) Rotifers and Crustaceans...
Both rotifers and crustaceans are animals-aerobic and multicellular. The rotifer derives its name from the apparent rotating motion of two sets of cilia on its head. The cilia provide mobility and a mechanism for catching food. Rotifers consume bacteria and small particles of organic matter. Crustaceans, a group that includes shrimp, lobsters, and barnacles, are characterized by their shell structure. They are a source of food for fish and are not found in wastewater treatment systems to any extent except in under loaded lagoons. Their presence is indicative of a high level of dissolved oxygen and a very low level of organic matter.
Metabolism, Energy and Synthesis...
Metabolism is the biochemical process performed by living organisms to yield energy for synthesis, motility, and respiration to remain viable.

( a ) Metabolism of autotrophic bacteria : The metabolism of autotrophic bacteria is illustrated in Eqs. 1,2 and 3. In these reactions, the reduced inorganic compounds are oxidized, yielding energy for synthesis of carbon from carbon dioxide, producing organic cell tissue.

( b ) Metabolism of heterotrophic bacteria : In heterotrophic metabolism, organic matter is the substrate (food) used as an energy source. However, the majority of organic matter in wastewater is in the form of large molecules which cannot penetrate the bacterial cell membrane. The bacteria, in order to metabolize high-molecular-weight substances, must be capable of hydrolyzing the large molecules into diffusible reactions for assimilation into their cells. Therefore, the first biochemical reactions are hydrolysis of :

(1) Complex carbohydrates into soluble sugar units,
(2) Protein into amino acids,
(3) Insoluble fats into fatty acids.

Under aerobic conditions the reduced soluble organic compounds are oxidized to end products of carbon dioxide and water (Eq. given below).



Under anaerobic conditions, soluble organics are decomposed to intermediate end products, such as organic acids and alcohols, along with the production of carbon dioxide and water (Eq. given below). Many intermediates, such as butyric acid, mercaptons (organic compounds with SH radicals), and hydrogen sulfide have foul odors.



Under anaerobic conditions, if excess organic acids are produced, the pH of the solution will drop sufficiently to “pickle” the fermentation process. This is the principle used for preservation of silage. Bacteria produce an over-abundance of organic acids in the anaerobic decomposition of the green fodder stored in the silo, inhibit further bacterial decomposition, and preserve the food value of the fodder. However, of proper environmental conditions exist to prevent excess acidity from the production of organic acid intermediates, populations of acid – splitting methane-forming bacteria will develop and use the organic acids as substrate (Eq. given below). The combined biological processes of anaerobic decomposition of raw organic matter to soluble organic intermediates and the gasification of the intermediates to carbon dioxide and methane is referred to as digestion.



Growth and survival of microorganisms is dependent upon their ability to obtain energy from the metabolism of substrate. Biochemical metabolic processes of heterotrophs are energy-yielding oxidation- reduction reactions in which reduced organic compounds serve as hydrogen donors and oxidized organic or inorganic compounds act as hydrogen acceptors. Oxidation is the addition of oxygen, removal of hydrogen, or the removal of electrons. Reduction is the removal of oxygen, addition of hydrogen, or addition of electrons.

Energy stored in organic matter (AH₂) is released in the process of biological oxidation by dehydrogenation of substrate followed by transfer of hydrogen, or electrons, to an ultimate acceptor. The higher the ultimate hydrogen acceptor is on the energy scale, the greater will be the energy yield from oxidation of 1 mole of a given substrate. Aerobic metabolism using oxygen as the ultimate hydrogen acceptor yields the greatest amount of energy. Facultative respiration, using oxygen bound in nitrates and sulfates, yields less energy than aerobic metabolism. The least energy yield results from strict anaerobic respiration, where the oxidation of (AH₂) is coupled with reduction of B (an oxidized organic compound) to BH₂ (a reduced organic compound).

Hydrogen acceptors are used in the sequence of dissolved oxygen first, followed by nitrates, sulfates and oxidized organic compounds, in this general order. Thus hydrogen sulfide odor formation follows nitrate reduction and precedes methane formation.

The biochemical reactions are performed by oxidation-reduction enzymes. The coenzyme component of the enzyme determines what chemical reaction will occur. Coenzymes diphosphopyridine nucleotide (DPN) and flavoproteins (FP) are responsible for hydrogen transfer. Cytochromes are respiratory pigments that can undergo oxidation and reduction and serve as hydrogen carriers.

Microorganisms process organic matter to create new cells (Synthesis). Relationships between metabolism, energy and synthesis are important in understanding biological – treatment systems. The primary product of metabolism is energy, and the chief use of this energy is for synthesis. Energy release and synthesis are coupled biochemical process which cannot be separated. The maximum rate of synthesis occurs simultaneously with the maximum rate of energy yield (maximum rate of metabolism). Therefore, in heterotrophic metabolism of wastewater organics, maximum rate of removal of organic matter, for a given population of microorganisms, occurs during biological growth. Conversely, the lowest rate of removal of organic matter occurs when growth ceases.

The major features of anaerobic metabolism are :

Incomplete metabolism, small quantity of biological growth and production of high energy products such as acetic acid and methane.

The major features of aerobic metabolism are :

Complete metabolism and synthesis of the substrate, ending in a large quantity of biological growth.

Growth...
As an illustration, let us examine hypothetical situation in which 1,400 bacteria of a single species are introduced into a synthetic liquid medium. Initially nothing appears to happen. The bacteria must adjust to their new environment and begin to synthesize new protoplasm. On a plot of bacterial growth versus time, this phase of growth is called the lag phase. At the end of the lag phase the bacteria begin to divide. Since all of the organisms do not divide at the same time, there is a gradual increase in population. This phase is labeled accelerated growth on the growth plot. At the end of the accelerated growth phase, the population of organisms is large enough and the differences in generation time are small enough that the cells appear to divide at a regular rate. Since reproduction is by binary fission (each cell divides producing two new cells), the increase in population follows in geometric progression :



and so fourth. The population of bacteria (P) after the n^th generation is given by the following expression :



Where P₀ is the initial population at the end of the accelerated growth phase. If we take the log of both sides of the previous equation, we obtain the following :



This means that if we plot bacteria population on a logarithmic scale, this phase of growth would plot as a straight line of slope n and intercept P₀ at t₀ equal to the end of the accelerated growth phase. Thus, this phase of growth is called log growth phase. The log growth phase tapers off as the substrate becomes exhausted or as toxic by-products build up. Thus, at some point the population becomes constant either as a result of cessation of fission or a balance in dearth and reproduction rates. This is depicted by the stationary phase on the growth curve. Following the stationary phase, the bacteria begin to die faster than they reproduce. This death phase is due to a variety of causes that are basically an extension of those which lead to the stationary phase.

The batch – culture growth pattern is not directly applicable to biological-treatment processes which are continuous – flow systems. For example, an activated sludge system is fed continuously, and excess microorganisms are withdrawn, either continuously or intermittently, to maintain the desired mass of microorganisms for metabolizing incoming organic wastes.

A schematic diagram, illustrates the flow pattern for food (organic matter) and microorganisms in an activated-sludge system. Food (influent wastewater) is aerated with a mixed culture of microorganisms for a sufficient period of time to permit synthesis of the waste organics into biological cells. The microorganisms are then settled out of solution, removed from the bottom of the settling tank, and returned to the aeration tank to metabolize additional waste organics. Unused food, the nonsettleable fraction of the aeration tank effluent, passes out in the system effluent. Metabolism of the organic matter results in an increased mass of microorganisms in the system. Excess microorganisms are removed (wasted) from the system to maintain proper balance between food supply and mass of microorganisms in the aeration tank. This balance is referred to as the food to microorganism ratio (F/M).
The Monod Equation...
For the large numbers and mixed cultures of microorganisms found in waste treatment systems, it is convenient to measure biomass rather than numbers of organisms. In the log-growth phase, the rate expression for biomass increase is :



Because of the difficulty of direct measurement of in mixed cultures, Monod developed a model equation that assumes that the rate of food utilization, and therefore the rate of biomass production, is limited by the rate of enzyme reactions involving the food compound that is in shortest supply relative of its need. The Monod equation is :



Equation ;



assumes only growth of microorganisms and dose not takes into account natural die-off. It is generally assumed that the death or decay of the microbial mass is a first-order expression in biomass and hence equations ;



and



are expanded to ;



If all of the food in the system were converted to biomass, the rate of food utilization (ds/dt) would equal the rate of biomass production. Because of the inefficiency of the conversion process, the rate of food utilization will be greater than the rate of biomass utilization, so ;




Factors Affecting Growth...
Several factors affect the growth of microorganisms. The most important are temperature, pH, availability of nutrients, oxygen supply, presence of toxins, types of substrate, and, in the case of photosynthetic plants, sunlight.
( 1 ) Temperature...
Bacteria are classified as psychrophilic, mesophilic or thermophilic, depending upon their optimum temperature range for growth.

1. Psychrophilic : Of least significance to sanitary engineers are the psychrophilic (cold-loving) bacteria, which grow best at temperatures slightly above freezing (4 – 10^OC).

2. Thermophilic : Heat – loving bacteria like an optimum temperature range of 50 - 55^OC. They hold sanitary significance in food preservation, and attempts have been made to use a thermophilic temperature range for the anaerobic digestion of waste sludge. Thermophilic digestion has not been successful in practice because thermophilic bacteria are sensitive to small temperature changes, and it is difficult to maintain the required high operating temperature in a digestion tank.

3. Mesophilic : These bacteria grow best in the temperature range 20-40^OC. Most bacterial pathogens are mesophilic and thrive at human body temperature, 37^OC. The vast majority of biological – treatment operate in the mesophilic temperature range. Anaerobic digestion tanks are normally heated to near the optimum level of 35^OC. Aeration tanks and trickling operate at the temperature of the wastewater as modified by that of the air. Generally, this is within the 15-25^OC range.

A high temperature increases biological activity in the treatment process but rarely causes any severe operating problems. At high temperatures odor problems may be more pronounced at a wastewater plant, and, in one case, the increased metabolism rate during high loading periods of an activated – sludge system at elevated temperature resulted in serious dissolved – oxygen depletions in the aeration tanks.

Cold wastewater can reduce BOD removal efficiency of biological processes. The efficiency of trickling filters is definitely decreased during cold weather and increased during warm periods.

As a general rule, the rate of biological activity doubles for every 10-15^OC temperature rise within the range 5-35^OC. Above 40^OC mesophilic bacterial metabolism drops off sharply and thermophilic growth starts. Thermophilic bacteria have a range of approximately 45-75^OC, with an optimum near 55^OC.
( 2 ) pH...
The hydrogen – ion concentration of the culture medium has a direct influence on microbial growth. Most biological – treatment systems operate best in a neutral environment. The general range for operation of activated – sludge systems is between pH 6.5 and 8.5. At pH 9.0 and above, microbial activity is inhibited. Below 6.5, fungi are favored over bacteria in the competition for food. The methane-forming bacteria in anaerobic digestion have a much smaller pH tolerance range. General limits for anaerobic digestion are pH 6.7-7.4 with optimum operation a pH 7.0-7.1.
Population Dynamics...
Previous sections described the important characteristics of each group of microorganisms (bacteria, fungi, algae and protozoa) independently. But in biological – waste-treatment systems, the naturally occurring cultures are mixtures of bacteria growing in mutual association and with other microscopic plants and animals. A general knowledge of the relationships, both cooperative and competitive, between various microbial populations in mixed cultures is essential to understanding biological – treatment processes. When organic matter is made available to a mixed population of microorganisms, competition arises for this food between the various species. Primary feeders that are most competitive become the dominant microorganisms. Under normal operating conditions, bacteria are the dominant primary feeders in activated – sludge – treating municipal wastewater. Saprobic protozoans, those that feed on dead organic matter are not effective competitors against bacteria.

Species of dominant primary bacteria depend chiefly upon the nature of the organic waste and environmental conditions in aeration tanks. Primary bacteria in an activated sludge system are maintained in the declining growth phases. Under these conditions, the primary bacteria die and lyse releasing their cell contents to solution. In this process raw organic matter is synthesized and resynthesized by various groups of bacteria.

Holozoic protozoans, which feed on living organic matter, are common in activated sludge. They grow in association with the bacteria in a prey-predator relationship; that is, the bacteria (plants) synthesize the organic matter, and the protozoans (animals) consume the bacteria. For a single reproduction a protozoan consumes thousands of bacteria, with two major beneficial effects of the prey – predator action. Removal of the bacterial stimulates further bacterial growth, resulting in accelerated extraction of organic matter from solution. Second, the flocculation characteristics of activated sludge are improved by reducing the number of free bacteria in solution, and a biological floc with improved settling characteristics results.

There is also competition for food between the secondary feeders. In a solution with high bacterial populations, free-swimming protozoans are dominant, but when food becomes scarce, stalked protozoans increase in numbers. Stalked protozoas do not require as much as free-swimming protozoans; therefore, they complete more effectively in a system with low bacterial concentrations.
Biological Process...
Biological treatment systems are “living” systems which rely on mixed biological cultures to break down waste organics and remove organic matter from solution.
( 1 ) Anaerobic Decomposition...
The process of anaerobic digestion is carried out by a wide variety of bacteria, which can be categorized into main groups, acid-forming bacteria and methane – forming bacteria. The acid formers are facultative or anaerobic bacteria, which metabolize organic matter, forming organic acids as an end product, along with carbon dioxide and methane (associated with oxidation of fats to organic acids). Acid – splitting methane formers use organic acids as substrate and produce gaseous end products of carbon dioxide and methane. These methane bacteria are strict anaerobic inactivated by the presence of dissolved oxygen and inhibited by the presence of oxidized compounds. The growth medium must contain a reducing agent such as hydrogen sulfide. Acid – splitting methane bacteria are sensitive to pH changes and other environmental conditions.

A simplified diagram portrays relationship between the two bacterial stages in digestion or organic matter. Both major groups of bacteria must cooperate to perform the overall gasification of organic matter. The first stage creates food (organic acids) for the second stage where these organic acids are consumed, preventing excess acid accumulation. In addition to producing food for the methane bacteria, acid-formers also reduce the environment to one of strict anaerobiosis by using the oxidized compounds and excreting reducing agents.

Problems in operating anaerobic treatment systems result when an inbalance occurs in the population dynamics. For example, if a sudden excess of organic matter is fed to a digester, acid formers very rapidly process this food developing excess organic acids. The methane formers, are unable to metabolize the organic acids fast enough to prevent a drop in pH. When the pH drops, the methane bacteria are affected first, further reducing their capacity to breakdown the acids. Under severe or prolonged overloading, the contents of the digester “pickles” in excess acids, and all bacterial activity is inhibited. In addition to organic overloading, the digestion process can be upset by a sudden increase in temperature, a significant shift in the type of substrate, or additions of toxic or inhibiting substances from industrial wastes.
( 2 ) Oxidation Pond...
A unique relationship exists between bacteria and algae in small ponds and streams. The bacteria metabolize organic matter, releasing nitrogen and phosphorous nutrients and carbon dioxide. Algae use these compounds, along with energy from sunlight, for synthesis, releasing oxygen into solution. Oxygen released by the algae is taken up by the bacteria, thus closing the cycle. This type of association between organisms is referred to as symbiosis, a relationship where two or more species live together for mutual benefits such that the association stimulates more vigorous growth of each species than if growth were separate.

In a shallow oxidation pond with adequate sunlight and moderate temperatures, the bacteria-algae relationship depicts the primary biological reactions which take place. In addition, a variety of predators (protozoans, rotifers, and higher animals) feed on the plant growth (algae and bacteria).
( 3 ) Facultative Lagoons...
At the liquid depth commonly used in stabilization-pond design, bottom waters may become anaerobic while the surface remains aerobic. In terms of general oxygen conditions, these lagoons are commonly referred to as facultative lagoons. During periods when the dissolved oxygen is less than saturation level, the surface water is aerated through wind action. During the winter both bacterial metabolism and algal synthesis are slowed by cold temperatures. The lagoon generally remains aerobic, even under a transparent ice cover. If the sunlight is blocked by a snow cover, the algae cannot produce oxygen, and the lagoon becomes anaerobic. The result is odorous conditions during the spring thaw until the algae become reestablished.

This may take from a few days to weeks, depending upon climate conditions and the amount of organic matter accumulated in the lagoon during the winter.
( 4 ) Aerobic Decomposition...
From our discussion of bacterial metabolism you will recall that molecular oxygen (O₂) must be present as the terminal electron acceptor for decomposition to proceed by aerobic oxidation. As in natural water bodies, the oxygen is measured as DO. When oxygen is present, it is the only terminal electron acceptor used. Hence, the chemical end products of decomposition are primarily carbon dioxide, water and new cell material. Odiferous gaseous end products are kept to a minimum. In healthy natural water systems, aerobic decomposition is the principal means of self-purification. A wider spectrum of organic material can be oxidized aerobically than by any other type o decomposition. This fact, coupled with the fact that the final end products are oxidized to a very low energy level, results in a more stable end product (that is, one that can be disposed of without damage to the environment and without creating a nuisance condition) than can be achieved by the other oxidation systems.

Because of the large amount of energy released in aerobic oxidation, most aerobic organisms are capable of high growth rates. Consequently, there is a relatively large production of new cells in comparison with the other oxidation systems. This means that more biological sludge is generated in aerobic oxidation than in the other oxidation systems.

Aerobic decomposition is the method of choice for large quantities of dilute wastewater (BOD₅ less than 500 mg/l) because decomposition is rapid, efficient, and has a low odor potential. For high – strength wastewater (BOD₅ is greater than 1,000 mg/l), aerobic decomposition is not suitable because of the difficulty in supplying enough oxygen and because of the large amount of biological sludge produced. In small communities and in special industrial applications where aerated lagoons are used, wastewaters with BOD₅ up to 3,000 mg/l may be treated satisfactory by aerobic decomposition.
( 5 ) Anoxic Decomposition...
Some microorganisms can use nitrate (NO₃^-) as the terminal electron acceptor in the absence of molecular oxygen. Oxidation by this route is called denitrification. The end products from denitrification are nitrogen gas, carbon dioxide, water, and new cell material. The amount of energy made available to the cell during dentrification is about the same as that made available during aerobic decomposition. As a consequence, the rate of production of new cells, although not as high as in aerobic decomposition, is relatively high.

Denitrification is of importance in wastewater treatment where nitrogen must be removed to protect the receiving body. In this case, a special treatment step is added to the conventional process for removal of carbonaceous material. Denitrification will be discussed in detailed later.

One other important aspect of denitrification is in relation to final clarification of the treated wastewater. If the environment of final clarifier becomes anoxic, the formation of nitrogen gas will cause large gobs of sludge to float to the surface and escape from the treatment plant into the receiving water. Thus, it is necessary to ensure that anoxic conditions do not develop in the final clarifier.