Bacteria

Bacteria are unicellular organisms that grow as individual cells, pairs, groups of four (tetrads), cubes (sarcinae), irregular clusters or clumps, or chains (Figure 4.1). Individual bacterial cells may be spherical (coccus), rod-shaped (bacillus), or helical (spirillum) (Figure 4.2). Most bacteria are 0.5 to 2.5 microns (mm) in diameter or width and 1 to 20 mm in length. Because of the very small size of bacteria, they can be examined only by using a microscope at high-power magnification. Often, a staining technique, such as Gram staining, is used to examine the bacteria during microscopic work. The Gram staining technique is provided in Appendix I.

All bacteria possess a cell wall, a cell membrane, cytoplasm, meso-somes, ribosomes, and inclusions or storage granules (Figure 4.3). The cell wall surrounds the bacterium and gives the organism its stiffness and shape. The cell wall also provides protection and helps to regulate the movement of compounds in and out of the cell. The cell membrane is a very thin and flexible structure located immediately beneath the cell wall. The cell membrane also helps to regulate the movement of compounds in and out of the cell. Invaginations of the cell membrane are the mesosomes. The invaginations may take the shape of tubules, vesicles, or lamellae. The function of the meso-somes is unknown.

The cytoplasm fills the interior of the cell and is surrounded by the cell membrane. The cytoplasm makes up the bulk content of the cell. It contains a variety of colloids and fluids as well as storage granules

Figure 4.1 Patterns of bacterial growth. There are several common patterns of growth for bacteria. These patterns include individual (a), pairs (b), irregular clusters (c), chains or filamentous (d), groups of four or tetrads (e), and cubes or sarcinae (f).

or inclusions. The cytoplasm also contains the mitochondria and ribosomes. The mitochondria are the sites where substrate (food) is degraded, while the ribosomes are the sites where protein synthesis occurs. Inclusions or storage granules consist of food, oils, polyphosphates, and sulfur.

Some bacteria contain a capsule and flagellum (Figure 4.4). The capsule is a gelatinous slime and provides additional protection for

Figure 4.2 Shapes of bacterial cells. Most bacterial cells have one of three shapes. These shapes are spherical or coccus (a), rod-shaped or bacillus (b), and helical or spirillum (c). The helical shape is the least rigid of the three bacterial shapes.

Figure 4.3 Major structural components of the bacterial cell. Significant, structural features of the bacterial cell consist of the cell wall, the cell membrane, the cytoplasm, the mesosome, and the ribosomes. Also contained within the cytoplasm are a variety of granules that may consist of stored food such as starch or inorganic materials such as sulfur deposits.

Figure 4.3 Major structural components of the bacterial cell. Significant, structural features of the bacterial cell consist of the cell wall, the cell membrane, the cytoplasm, the mesosome, and the ribosomes. Also contained within the cytoplasm are a variety of granules that may consist of stored food such as starch or inorganic materials such as sulfur deposits.

Figure 4.4 Bacterial capsule and flagellum. In addition to the major structural components of the cell, some bacteria may possess a capsule and a flagellum. The capsule provides additional protection for the cell, while the flagellum provides locomotion.

Nitrifying Bacteria

Nitrifying Bacteria

Organotrophs

Organotrophs

Figure 4.5 The floc particle. The basis structure of a floc particle consists of numerous bacteria that ''stick'' together or agglutinate. The bacteria consist of floc-formers such as organotrophs, non-floc formers such as nitrifying bacteria, and filamentous bacteria. The non-floc formers are adsorbed to the floc particle through the coating action from secretions by ciliated protozoa and other higher life forms. Filamentous bacteria grow within the floc particle and extend from the perimeter of the floc particle into the bulk solution. The filamentous bacteria provide the floc particle with strength and permit the floc particle to grow in size.

the cell. The slime helps to flocculate bacterial cells together, and it also holds particulate and colloidal wastes to the surface of the bacterium.

The flagellum is a proteinaceous, whiplike structure that provides locomotion for the cell. Most bacteria are highly motile when they are young. This locomotive ability is lost when bacteria are incorporated into floc particles (Figure 4.5) or the flagellum is lost through aging.

Bacteria enter the activated sludge process through inflow and infiltration (I/I) as soil and water organisms and as fecal organisms in domestic wastewater. Bacteria in the activated sludge process may be suspended in the water or bulk solution surrounding the floc particles or incorporated into floc particles. Bacteria are present in the activated sludge process at relatively high numbers. They are commonly found in millions per milliliter of bulk solution or billions per gram of floc particles or solids.

TABLE 4.1 Examples of Organic Compounds Oxidized by Organotrophs

Chemical Name

Common Name

Chemical Formula

Acetic acid

Vinegar

CH3COOH

Acetone

Nail polish remover

CH3COCH3

Ethyl alcohol

Drinking alcohol

CH3CH2OH

Glucose

Sugar

C6H12O6

Isopropyl alcohol

Rubbing alcohol

CH3CHOHCH3

Stearic acid

Fatty acid

CH3(CH2)16COOH

A large diversity of bacterial types is found in the activated sludge process. The type of bacteria or identification of the bacteria present is based on several characteristics of the organism. These characteristics include structural features of the organism, environmental conditions tolerated by the organism, biochemical reactions that are capable of the organism, substrates that the organism can degrade, and what molecule, such as O2, is used by the organism to degrade its substrate. The last two characteristics of bacterial identification are of critical importance in a review of nitrification and denitrifica-tion and the successful operation of an activated sludge process.

Substrate refers to the food used by bacteria to obtain carbon and energy for cellular activity and cellular growth or reproduction. The degradation of substrate to obtain carbon and energy is cellular respiration.

Bacteria that use organic compounds or carbonaceous wastes to obtain carbon and energy are organotrophs. The term ''organo'' implies organic, while the term ''troph'' implies nourishment. Organo-trophic bacteria feed upon organic compounds to obtain carbon and energy (Table 4.1). When bacteria use organic compounds to obtain carbon and energy, these compounds or substrates are degraded or oxidized. When these compounds are oxidized in an activated sludge process, the concentration of carbonaceous waste is decreased.

Bacteria that use minerals or inorganic compounds or wastes to obtain energy for cellular activity and cellular growth or reproduction are chemolithotrophs. The term ''chemo'' refers to chemicals, and the term ''litho'' refers to minerals. Therefore chemolithotrophs obtain their energy nourishment from chemical minerals. Examples of minerals degraded or oxidized by chemolithotrophs include iron by iron-oxidizing bacteria, sulfur by sulfur-oxidizing bacteria, and nitrogen by nitrifying bacteria (Table 4.2). The bacteria that oxidize inorganic

TABLE 4.2 Examples of Minerals or Inorganic Compounds Oxidized by Chemolithotrophs

Chemical

Compound

Formula

Biochemcial

Containing

of the

(Oxidative)

Oxidizing

Mineral

the Mineral

Compound

Reaction

Bacteria

Nitrogen

Ammonium

NH+

NH+ + I.5O2 !

Nitrosomonas

ion

NO2 + H2O + 2H+

Nitrogen

Nitrite ion

no2

NO2 + O.5O2 ! NO3

Nitrobacter

Sulfur

Sulfite ion

SOf-

SO2~ + 1.5O2 + 4H+

Sulfolobus

! SO23 + 2H2O

minerals are essential in the recycling of several critical elements between living and nonliving components of the environment.

Some chemolithotrophs, including the bacteria that oxidize nitrogen in the form of ammonium ions and nitrite ions, obtain their carbon from inorganic carbon. The inorganic-carbon source for these organisms is carbon dioxide. When carbon dioxide dissolves in wastewater, carbonic acid (H2CO3) is formed (Equation 4.1). In wastewater some of the carbonic acid disassociates and forms the bicarbonate ion (HCO^) and the hydrogen ion (Equation 4.2). It is the bicarbonate ion that is used as the inorganic carbon source.

When organisms, like green plants, use inorganic carbon or carbon dioxide, they are referred to as autotrophs. The term ''auto'' refers to self. Therefore organisms that obtain carbon from carbon dioxide are self-nourishing. Bacteria that obtain carbon from carbon dioxide and energy from the oxidation of chemical minerals are referred to as chemolithoautotrophs.

When bacteria degrade substrate to obtain energy, the chemical bonds in the organic and inorganic compounds are broken and electrons are released (Figure 4.6). The bacteria capture energy from the released electrons and stored it in the form of high-energy, phosphate bonds in the molecule adenosine triphosphate (ATP). After energy has been captured from the released electrons, the electrons must be removed from the cell. A molecule called the final, electron carrier molecule, removes the electrons from the cell (Table 4.3).

Figure 4.6 Energy capture by bacterial cell. When the bacterial cell absorbs soluble cBOD, enzymes within the cell split the chemical bonds within the cBOD molecules to release and capture energy. Here the chemical bond between carbon (C) and hydrogen (H) is broken, and two electrons from the bond are released. The released electrons are quickly captured by a series of cellular molecules (1, 2, and 3) that efficiently transport the electrons to an oxygen molecule that carries the electrons out of the cell. As the electrons are passed from one cellular molecule to another, some of the energy from the captured electrons is used to made high-energy phosphates bonds (ADP to ATP). Wastes from the degradation of the cBOD consist of carbon dioxide (CO2) and water (H2O). The oxygen is used to carry waste and electrons from the cell.

02 02

Figure 4.6 Energy capture by bacterial cell. When the bacterial cell absorbs soluble cBOD, enzymes within the cell split the chemical bonds within the cBOD molecules to release and capture energy. Here the chemical bond between carbon (C) and hydrogen (H) is broken, and two electrons from the bond are released. The released electrons are quickly captured by a series of cellular molecules (1, 2, and 3) that efficiently transport the electrons to an oxygen molecule that carries the electrons out of the cell. As the electrons are passed from one cellular molecule to another, some of the energy from the captured electrons is used to made high-energy phosphates bonds (ADP to ATP). Wastes from the degradation of the cBOD consist of carbon dioxide (CO2) and water (H2O). The oxygen is used to carry waste and electrons from the cell.

The carrier molecule may be dissolved oxygen, nitrite ions, or nitrate ions. The molecule that is used by the bacterium in the cell in to degrade substrate determines the type of respiration that occurs in the cell. Degradation of substrate that occurs with the use of dissolved oxygen is aerobic respiration. For aerobic respiration to occur, the environment of the bacteria must contain dissolved oxygen; that

TABLE 4.3 Final Electron Carrier Molecules and Types of Respiration

Carrier

Bacterial

Example of

Molecule

Respiration

Environment

Respiration

O2

Aerobic

Oxic

Nitrification

NOg

Anaerobic

Anoxic

Denitrification

no3

Anaerobic

Anoxic

Denitrification

is, the environment is "oxic." Degradation of substrate that occurs without the use of free molecular oxygen is anaerobic respiration.

For anaerobic respiration to occur, the environment of the bacteria must contain no dissolved oxygen; that is, the environment is anaerobic. Denitrification is one form of anaerobic respiration. During denitrification nitrite ions or nitrate ions are used to degrade substrate. Therefore the environment of the bacteria must contain nitrite ions or nitrate ions. This environment is referred to as an ''anoxic'' environment.

The activated sludge process is the most commonly used system for the treatment of municipal wastewater, and it is probably the most versatile and effective of all wastewater treatment processes. The treatment of wastes is biological in that it uses microscopic organisms to degrade or remove wastes. The process consists of at least one aeration tank and one clarifier (Figure 5.1).

Often a sedimentation tank or clarifier is placed upstream of the activated sludge process. The purpose of this first or primary clarifier is to remove floating materials such as oils and greases and heavy solids that settle to the bottom of the clarifier. If a primary clarifier is placed upstream of the activated sludge process, the clarifier following the aeration tank is secondary clarifier.

The aeration tank is a biological reactor or amplifier where relatively large numbers of bacteria are provided with dissolved oxygen and carbonaceous and nitrogenous wastes. In the presence of dissolved oxygen, the bacteria degrade the carbonaceous and nitrogenous wastes. The degradation of the wastes by the bacteria (biological reactor) results in the growth of the bacterial population (biological amplifier).

The wastes that are degraded by the bacteria are the substrates used to obtain carbon and energy. The term given for the substrates is biochemical oxygen demand (BOD). The BOD is the amount of dissolved oxygen measured in milligrams per liter (mg/l) required by the organisms, primarily bacteria, to oxidize (degrade) the wastes to simple inorganic compounds and more bacterial cells.

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