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
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
A wastewater treatment plant operator should know
about microorganisms in wastewater treatment for two practical
- Some microorganisms are capable of causing serious
public health problems, and
- 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
, and oxidation
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
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.
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.
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
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
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
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
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