Lesson 7: Bacteriological Processes

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Bacteria and Other Microorganisms

 The science of biology is concerned with the study of living organisms, their habits, food requirements and general functions.

The science of microbiology is concerned with those organisms that are of microscopic dimensions and thus cannot be seen except with the aid of a microscope.  The study of biology and microbiology is of great importance since these sciences are the foundation upon which sanitation and wastewater treatment are based.  Without knowledge of the fundamental factors concerning these organisms and their relationships to one another and to human beings, it would be difficult to fully comprehend the principles of effective wastewater treatment.

Wastewaters normally carry many microorganisms as well as a variety of compounds that serve as food for these microorganisms.

To fully understand wastewater treatment, it is necessary to know what microorganisms do and how they break down the components in the wastewater.  We should also know the best conditions for these processes to occur as well as how environmental changes and other factors affect microbiological activity.

A wastewater treatment plant operator should know about microorganisms in wastewater treatment for two practical reasons:

  1. Some microorganisms are capable of causing serious public health problems, and
  2. Microorganisms are responsible for the success of biological wastewater treatment, as well as for many problems associated with wastewater treatment operation.
Biological changes occur in wastewater during treatment and after discharge.  Biological wastewater treatment is carried out by such treatment methods as the activated sludge process, trickling filtration, and oxidation ponds, etc.


At the technical level, bacteria can be described as minute, living organisms, each consisting of a single cell but which can also grow either in suspended masses, as in the activated sludge process, or attached to a fixed media, as in the trickling filter or rotating disc slimes.

Bacteria, besides being present in large numbers in raw wastewaters, biological treatment plants, effluents, and natural waters, are found everywhere in our environment.  They are present in the soil in large numbers and can also be found in various forms suspended in the air.  Thus, bacteria are also found in water as the result of sources flowing through and over the ground.  Bacteria are also present in and on the bodies of all living creatures, including man.  Most bacteria are not pathogenic, that is, they do not cause disease.  Many bacteria in fact have been shown to carry on very useful and necessary functions related to the life of larger organisms.

Identification of Bacteria

Bacteria may be identified through a systematic application of procedures which are designed to grow, isolate, and identify the individual bacteria.  These procedures are highly specialized and technical.  Ultimately, the bacteria is characterized by gross appearance on growth, morphology (its form and structure), and a variety of biochemical and other specialized tests.  On the basis of these observations and experience, a bacteriologist uses a standard reference text for comparative descriptions.  Identification of a common species is not difficult, but requires considerable training and experience.

The Bacterial Cell

Bacteria are so small that individual cells can only be seen when they are magnified under a microscope.  The unit used for measuring these microscopic sizes is called the micron and measures one-thousandth (1/1000) of a millimeter.  A typical bacterial cell, such as a coliform bacteria that is rod shaped, is about 2 um long and about 0.7 microns wide.  It would take approximately 13,000 of these organisms lying end to end to stretch to the length of one inch (2.54 cm.).

Despite its small size, the bacterial cell is both structurally and functionally complex.  The bacterial cell consists of a rigid cell wall made of a complex chemical composition that surrounds the cell.  This wall consists of proteins, polysaccharides, and lipids (fats).  Lying immediately next to the cell wall is a very thin balloon-like membrane, the cell membrane, which has a number of functions;  among them, regulating all material passing in and out of the cell.  Beneath the membrane, the bacteria cell will contain the non-rigid cytoplasm which contains a dispersed nucleus and various smaller bodies vital to the cell functions.  Among these are ribosomes, vacuoles, plastids, and inclusion bodies.  Food assimilation, waste excretion, respiration, growth and all other activities are carried on through the action of the one single cell.

Most bacteria, especially those found in wastewater, are also immersed, covered or surrounded by a sticky, thick, gelatinous material called "slimes".  This material causes clumping of many organisms into "flocs" or aggregations which can be settled.  These slimes, as well as the more consolidated versions called "capsules", are significant since they offer a protective coating to the bacteria.  In order for disinfecting agents such as chlorine to be effective, they must penetrate this protective slime layer.

Many bacteria are also motile, that is, capable of movement.  Movement is by means of flagellum the hair-like projection on the organisms.  The number and location of flagella varies with species.  Motile bacteria may contain from one to many flagella which may be on opposite ends of the cell or as tufts at either end of the cell.

Physical Forms of Bacteria

The three general morphological categories into which all bacteria fall are "cocci", "bacilli", and "spirilla", although there are modifications of these forms.

Cocci are round cells, sometimes slightly flattened when they are adjacent to one another.  They exist in pairs, as diplococci;  in chains as streptococco;  in groups of 4, as tetrads;  and in packets of 8 as sarcinae.

Bacilli are rod shaped bacteria.  The length of the cell varies, even in a single species, under the influence of age or environmental conditions.  They too may occur singly or in chains.  Bacteria of the coliform group (normally used as an index of wastewater pollution of water), as well as those responsible for typhoid fever, are rod shaped.

Spirilla are curved bacteria.  The length of the cell and the number of convolutions varies with each organism.  Some are nothing more than gently curved shapes, such as the causative organism of cholera, whereas others are shaped somewhat like a corkscrew.  Many of them are rigid and motile.  A special group of spirilla known as spirochetes are not rigid, but flexible and ar long and slender.

Some bacteria also are capable of forming spores, a thickened consolidation of nucleus material.  The spore state is a dormant, inactive form of the bacteria that is very resistant to adverse conditions such as drying, heating and disinfection.

 Physical and Chemical Factors

Many other conditions affect the growth and survival of bacteria.  Since these factors are often used as bacterial control agents, some consideration of them should be made.  Included are heat, temperature, drying, bacterial agents, bacteriostatic agents, and antimetabolites of various kinds.


Heat and Temperature:

Bacteria are not usually destroyed by low temperatures.  Most bacteria will reproduce either very slowly or not at all under these conditions.  When transferred from the frozen state to a more suitable environment, they immediately carry on a normal life cycle.  Extreme heat, on the other hand, readily destroys all bacterial species, although those which are in the spore state will withstand much more heat than those which are not.  Moist heat (steam) is more efficient in destruction of bacteria than dry heat and is often used in sterilization (killing all microorganisms) of laboratory equipment for the coliform test.  A temperature above which no cells of a bacterial species can survive is considered the "thermal death point" of the species.


Certain wave-lengths of light are very destructive to bacterial cells.  These lie in the region of the spectrum known as the ultraviolet (UV).  Light rays below 2,900 angstrom units (290 millimicrons wave-length) are especially destructive.  A necessary condition, however, is that the light ray strike the cell directly.  Thus, cells in a flask or test tube cannot be used.  Ultraviolet light may be used to destroy bacteria in water provided the water is passed by the UV light as a thin layer and is sufficiently free of suspended matter to permit the light ray to strike the bacterial cells.  Water itself has some absorptive effect on ultraviolet light and therefore even in clear water the distance between the bacteria cells and the light source must be kept short.


Bacteria cannot reproduce without moisture.  Drying of food materials is therefore a time honored method for preservation from decomposition by bacteria.  Spore-forming bacteria may survive in a dry environment, but they cannot function normally.  If moisture becomes available, bacterial spores will vegetate and resume a normal life cycle.

Osmosis and Salts:

Osmosis is a physical phenomenon that depends upon the relative concentration of soluble substances, usually salts, inside and outside the bacterial cell walls.  If the concentrations are in equilibrium, the cell is said to be in an "isotonic" environment, which is favorable.  If the concentration of electrolytes outside the wall is greater than that within, there is a tendency for water to pass out of the cell to restore equilibrium.  This may result in destruction of the cell through the effect of shrinking or "plasmolysis".  On the other hand, if the concentration of soluble substances outside the cell walls is lower than that within , water will tend to pass through the cell membrane from the outside and cause the cell to swell and perhaps burst.  This effect is called "plasmoptysis".  A common practice for preservation of meat and vegetables is pickling the product in a strong salt solution.  Under these conditions bacteria present in the food are destroyed by plasmolysis.  There is no practical application of this principle in the treatment of water.  Undoubtedly, however, the length of time that intestinal bacteria can survive in water is dependent in some degree on the effects of osmosis on the bacterial cells.

Disinfectants, Germicides and Bacteriostatic Agents:

A disinfectant is a compound that will kill disease causing pathogens.  A germicide is a substance which destroys a bacterial cell on contact, and a bacteriostatic situation indirectly brings about the destruction of the bacterial culture since without reproduction there can be no continuation of life.  The most common disinfectant and germicide used in the wastewater treatment field is, of course, chlorine.  Bacteriostatic agents, although useful in some fields, are not used in wastewater treatment.


Parasites are bacteria that cannot live an independent existence, cannot find their own food supply, and must remain in close association with some other living organisms, from which they can obtain food already prepared.  Parasites are dependent on the body of the host organisms to secure the environmental conditions upon which their existence and growth depend.  However, they carry on a similar type of decay and decomposition of this food supply, producing as a result the end products which are necessary for the nourishment of the host.  Most of these parasitic bacteria are beneficial and are necessary for the proper functioning of the living organism with which they are associated.

Coliform Organisms

Because the methods for the finding and identifying pathogens are specific, time-consuming and complicated, pathogens are not normally identified in wastewaters.  Instead, a more commonly occurring group of wastewater bacteria, called coliforms, are identified.  All warm-blooded animals harbor in their intestinal tract parasitic bacteria of various types.  All members of this one specific group are designated as the coliform group of bacteria.  These microorganisms function in the digestive processes of the host organism and are not normally pathogenic.  They are discharged from the intestinal tract in tremendous numbers and will always be present in large numbers in sewage.  If sewage enters a water, the various coliform bacteria are carried with it and will survive there for long periods of time.  Thus, their presence provides positive evidence of pollution and indicates the possible presence of the pathogenic bacteria from the discharge of the animal bodies.  Their detection by laboratory examination is relatively simple, by an elevated temperature test known as the examination for "fecal coliforms".

Index of Pollution

The number of these coliform bacteria that are present in any definite volume of water is a measure of the amount of sewage or waste which has been discharged into that water, and can be interpreted as a measure of the safety of the water for human consumption, recreation, and other water uses.  If large numbers of these bacteria are present, there will be a large amount of pollution and the water is unsatisfactory and potentially unsafe.  A smaller number of these microorganisms, of course, shows a lesser concentration of pollution.


Among the parasitic bacteria are some whose growth produces end products that are poisonous to the host organism and which produce a condition that is called disease.  Some of these are pathogenic only to human beings, that is, they produce disease only in the body of human beings.  Others are pathogenic only to certain types of warm-blooded animals while some are pathogenic only to plants.  There are a few types of saprophytic bacteria which have all of the characteristics of that class but which can, if they find entry into the body of an animal, produce end products which cause disease, such as anthrax or tetanus, in the body of the invaded animal.  These particular saprophytic bacteria are also termed pathogenic.

Other Microorganisms

A wide variety of other microorganisms are found in wastewaters in addition to the types previously described.  Among the principal minor categories of microorganisms that may often be found are the algae, viruses, rotifers and worms.  However, depending on the wastewater being treated and the process characteristics, almost any microorganism can be found in a biological wastewater treatment plant.


Algae contain the green pigment chlorophyll.  Like plants, algae carry out photosynthesis which is a biochemical process requiring sunlight, carbon dioxide, and raw mineral nutrients.  As an end product, algae produce oxygen.  Since algae require light for their growth, they are restricted mostly to the top surfaces of trickling filters and ponds.  While algae can usually be seen in wastewater treatment plant waters, they are not usually found in large numbers except in tertiary treatment units, such as clear wells, nor do they play a significant role in treatment.  However, in oxidation ponds the algae may represent a substantial portion of the population and may play a significant role in treatment.

Since algae grow readily in water or soil whenever conditions are appropriate, they can be found ass transients in wastewaters.  More importantly, algae grow in inland receiving waters in direct response to mineral nutrients, such as nitrogen (usually nitrates, ammonia and even nitrogen gas), phosphorous, and others, but especially phosphorous.  The limitation on phosphorous in some wastewater effluents in New York State stems directly from this concern.

When algae grow in receiving waters, they may grow attached or dispersed as plankton.  In either case they can cause nuisance conditions as well as affect dissolved oxygen relationships from night to day (cycling), since they produce dissolved oxygen during the day as a result of their photosynthesis in sunlight and use up the oxygen at night in respiration.

Another algae nuisance condition that is seen in some lakes and inland bodies of water which is indirectly attributable to mineral nutrient inputs, is a sudden rapid growth of algae called an algae "bloom".  Algae blooms are quite noticeable and often the water becomes colored and turbid or has floating accumulations of algal mats or scums.  The waters may become supersaturated with dissolved oxygen during the day.  A rise in pH (over 8 and as high as 9.5) can also occur.  Frequently, an algae bloom will consume itself.  This happens when local nutrient depletions occur as a result of the rapid growth of the algae.  When the algae die the organic matter from the algal cells, plus the availability of dissolved oxygen, make the conditions appropriate for bacterial growth which usually follows the "bloom".  Bacteria, like algae, grow in response to the availability of food and nutrients and they too undergo a rapid growth phase, sometimes depleting dissolved oxygen and causing the system to go anaerobic.

Wastewater treatment plant effluents contain mineral nutrients that are utilized by algae - especially phosphorous, whose availability in sufficient quantity can greatly enhance their growth.


Viruses are too small to be observed by the ordinary light microscope and can only be recognized by the harm they cause.  Since many viruses are associated with feces, they are generally expected to be present in domestic wastes.  All viruses are obligate parasites and must grow on living tissues.

There are many different viruses, each specific in terms of its activity.  Viruses cause a variety of diseases, such as infectious hepatitis, polio, influenza, smallpox and a variety of intestinal disturbances.  Because many viruses can survive for extended periods of time in natural waters and also because many viruses are not removed in wastewater treatment or killed by the normal methods of disinfection, they are a public health concern.


Rotifers are a well defined group of multi-cellular microorganisms that are often  associated with aerobic biological wastewater treatment plants.  Usually the rotifers that are seen are either grazing on smaller microbes or attached  to debris by their forked tail.

At their head end, rotifers have actively moving cilia (short, hair-like appendages), that frequently beat in a circular motion and in such a way that food is drawn into the organism.


Flat worms or nematodes are two varieties of worms that may be found in water and in wastewater.  Flat worms feed principally on algae and are found in the lower depth of ponds because of their aversion to light.  They range in size from a fraction of a millimeter to several centimeters in length.  Nematodes are parasitic worms that live on other organisms, including man.  It is believed that those which are parasitic in man are usually associated with contaminated food rather than contaminated water.  Nematodes are very hardy and will survive  over wide variations in temperature and humidity.  They even survive under prolonged drying.  They are very abundant in sewage sludge and are believed to play an important part in the stabilization of sludge.

 Relative Number of Microorganisms  vs.  Sludge Quality