Definition and Sources of Wastewater
Definition of Wastewater
Wastewater is defined as water, which properties has been changed by domestic,
industrial, agricultural or other use and other water discharged with such water in dry
weather (sewage) and also as water from precipitation which runs off and is collected
from built-up and hard-surfaced areas (rainwater). Liquids discharged and collected from
facilities for the treatment, storage or dumping of waste are also defined as sewage.
The term “wastewater” is a broad, descriptive term. Generally it includes liquids and
waterborne solids from domestic, industrial or commercial uses as well as other waters
that have been used (or “fouled”) in man’s activities, which quality has been degraded,
and which are discharged to a sewage system.
From where it comes
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The term “sewage” has been used for many years. Wastewater is not just sewage. All
the water used in the home that flows down the drains or into the sewage collection
system is wastewater. This includes water from baths, showers, sinks, dishwashers,
washing machines, and toilets. Small businesses and industries often contribute large
amounts of wastewater to sewage collection systems; others operate their own
wastewater treatment systems. In combined municipal sewage systems, water from
storm drains is also added to the municipal wastewater stream. Wastewater is about 99
percent water by weight and is generally referred to as influent as it enters the
wastewater treatment facility.
Sources and Types of Wastewater
Human and Domestic Wastewater Sources
Human excretion, mostly faeces and urine, become part of the wastewater through toilet
flushing. These waste streams have obvious public health implications and are of
importance since they contain organisms which may cause diseases in man. The safe
and effective treatment of sanitary wastes constitutes a major objective in wastewater
disposal.
Common waterborne household waste streams are relate to processes as launder,
bathing, cooking. Most of these waste streams will contain synthetic detergents. Kitchen
wastes will consist of food residues as well as greases. “Domestic wastewater” is
wastewater that is primarily generated by individuals in households, and does not
generally include industrial or agricultural wastewater.
Discharge of domestic wastewater causes heavy pollution of rivers and often also of
urban groundwater aquifers. The latter is a particularly serious problem because
groundwater is an important source for drinking water, which is sometimes used without
any pre-treatment.
Typical water consumption pattern in typical Indian
Household in litres per day
9.7
12.1
17.2
115.4
29.9
66.5
81.8
76
Others
Cooking water
Drinking water
Cleaning house
Washing utensils
Washing clothes
Used in toilet
Bathing
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Source: house hold survey by TISS, May ,2005
Fig. 1: Types of wastewater from the household Black water is water from toilets with flush water, faecal (brown water) and Urine (yellow
water). It contains a high concentration of organic carbon, and concentrations of
nitrogen, phosphorus and pathogens. Washing water from sink unit, bathroom, laundry
etc., it’s called grey water. Its light contaminated wastewater with sometimes-high
volume streams. Grey water contains beside grease and oil from the kitchen sink,
chemicals from detergents, shampoos etc.
For a proper treatment of wastewater, it is essential to understand the nature of its
pollution. Domestic wastewater can be classified in four groups, which can be
characterized as follows.
Types of wastewater
Stream
Treatment
Yellow Water
(Urine)
Brown Water
(Faeces)
Hygienization by
storage and drying
Anaerobic digestion,
drying, compost,
Mixing with org. residues
Use
Liquid or solid
fertilizer
Biogas,
Soil conditioner
Gray Water
(Shower, Kitchen,
Laundry etc.)
Sand/gravel filter,
ponds,
biol. Treatment, EM
membrane technology
Irrigation,
Recharge ground water
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Physical characters of wastewater
Domestic wastewater is a turbid or cloudy appearing liquid containing solid material in
suspension. When fresh, it is grey in colour, and has a musty but not unpleasant odour.
Domestic wastewater will have present, in varying amounts, all kinds of floating matter
such as faecal solids, bit of food, oil, garbage, paper, rags, wood, plastics and other
materials disposed of in the daily life of a community. Under certain conditions, as a
result of biochemical changes caused by bacteria, the colours of the wastewater will
gradually change from grey to black. As this happens, foul and unpleasant odours
develop and black solids appear on the surface or throughout the liquid. A wastewater
that has undergone such a change is called septic.
Constituents in Wastewater
Wastewaters consist of water in which solids exist as settle able particles, dispersed as
colloids, which are materials that do not settle readily, or solids in a dissolved state. The
wastewater mixture will contain large numbers of microscopic organisms, mostly
bacteria, which are capable of consuming the organic component (fats, proteins and
carbohydrates) of the mixture, and bringing about rapid changes in the wastewater.
Since the sources of wastewater as well as the inputs are highly variable. Since there is
also an active microbial component, the composition of all wastewaters is constantly
changing. Prior to entering a wastewater treatment plant, a wastewater is sometimes
called raw wastewater or raw sewage.
Wastewater is characterised in terms of physical, chemical, and biological
composition.
Physical Characteristics
Physical characters of sewage
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•
•
•
Turbidity
Colour
Odour
Temperature
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The most important physical characteristic of wastewater is the total solid contend, which
is composed of floating matter, settable matter, colloidal matter, and matter in solution.
Other important physical characteristics include particle size distribution; turbidity; colour;
transmittance; temperature; conductivity; and density; specific gravity and weight. Odour,
sometimes considered as physical factor, is considered later in chapter 5.1.1.7 Sensory
Characteristics (Turbidity, Colour, Odours)
Solids in wastewater
Since solids are classified in a variety of ways, they should be discussed with regard to
the various categorisations that are used as well as with respect to their chemical makeup. There will, of course, be some overlap in the classification method.
Solids can be further classified
•
•
•
•
•
Suspended solids
Settleable solids
Colloidal suspended solids
Dissolved solids
Total solids
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Organic Solids
In domestic wastewater, solids are about 50 percent organic. This fraction is generally of
animal or vegetable life, dead animal matter, plant tissue or organisms, but may also
include synthetic (artificial) organic compounds. These are substances which contain
carbon, hydrogen and oxygen, some of which may be combined with nitrogen, sulphur
or phosphorous. The principal organic compounds present in domestic wastewater are
proteins, carbohydrates and fats together with the products of their decomposition.
These compounds are subject to decay or decomposition through the activity of bacteria
and other living organisms and are combustible, that is, they can be ignited or burned.
Since the organic fraction can be driven off at high temperatures, they are sometimes
called volatile solids.
Suspended solids
Suspended Solids are those which are visible and in suspension in the water. They are
the solids, which can be removed from the wastewater by physical or mechanical
means, such as sedimentation or filtration. More precisely, they are the solids, which are
retained on the filter mat or glass fibre pad. Suspended solids will include the larger
floating particles and consist of sand, grit, clay, faecal solids, paper, and pieces of wood,
particles of food and garbage, and similar materials. Suspended solids are
approximately 70 percent organic solids and 30 percent inorganic solids, the latter being
principally sand and grit. The suspended solids portion consists of settleable solids and
colloidal solids.
Settleable solids
Settleable solids are that portion of the suspended solids, which are of sufficient size and
weight to settle in a given period of time, usually one hour. There are those, which will
settle in an “Imhoff-Cone” in one hour. The results are reported as millilitres of settled
solids per litre of wastewater. Settleable solids are approximately 75 percent organic and
25 percent inorganic.
Colloidal suspended solids and Dissolved Solids Colloidal suspended solids are solids
that are not truly dissolved and yet do not settle readily. These are somewhat loosely
defined as the differences between the total suspended solids and the settleable solids.
There is, at present, no simple or standard laboratory test to specifically determine
colloidal matter. Most colloids will not settle out even after long quiescent periods of
settling. They constitute that portion of the total suspended solids (about 40 percent),
which are not readily removed by physical or mechanical treatment facilities but may be
filtered out in a Gooch Crucible. Colloidal solids are about 65 percent organic, 35
percent inorganic, subject to rapid decay, and are an important factor in the treatment
and disposal of wastewater.
Dissolved Solids: The term “dissolved solids” as commonly used in discussing
wastewater is not technically correct. Dissolved solids are smaller in size than
suspended and colloidal solids. As used, the term means all of the solids, which pass
through the filter pad of a Gooch Crucible. Of the total dissolved solids, about 90 percent
are in true solution and about 10 percent colloidal. Dissolved solids, as a whole, are
about 40 percent organic and 60 percent inorganic.
Total Solids
Total solids, as the term implies, includes all of the solid constituents of a wastewater.
Total solids are the total of the organic and inorganic solids or the total of the suspended
and dissolved solids. In an average domestic wastewater, total solids are about half
organic and half inorganic, and about two-thirds in solution (dissolved) and one-third in
suspension. The organic solids, which are subject to decay, constitute the main problem
in wastewater treatment.
Solids Determinations
The solid components of domestic wastewater can be classified in a number of ways.
For example, wastewater solids can be categorised on the basis of several operational
procedures used in the wastewater treatment laboratory. Total solids may be determined
by driving off the water fraction, and filtering out the solid fraction on a porous pad and
drying may determine suspended solids.
Settleable solids may be determined by permitting a sample to settle in a special “ImhoffCone-apparatus”. The categories used most often in the wastewater treatment field are
suspended solids and total solids. The colloidal fraction of domestic wastewaters
comprises about 20 percent of the solid component of an “average” wastewater. As
stated, this component is characterized by being non-settleable, that is, usually long
periods of time would be required for them to settle by gravity alone. Colloidal
components of wastewater do not fit neatly into a component that can be determined in
the laboratory and colloids are found in both the suspended and dissolved solid
fractions. Any estimate of wastewater composition can give only an average
composition. The amounts of solids indicated cannot be applied equally to all
wastewaters at all times.
Physical characters of sewage
•
•
•
•
Turbidity
Colour
Odour
Temperature
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Sensory Characteristics (Turbidity, Colour, Odours)
Most of wastewater are turbid because of the solids suspended in them break the light.
Therefore, highly turbid fluid indicates a high percentage of suspended solids. Turbidity
may cause the algae in surface waters not to produce oxygen during daytime, as would
otherwise be the case. Colour is an indication of how “clean” domestic wastewater is. A
black sample represents wastewater that is anaerobic and still needs significant
treatment. A clear sample indicates that BOD and TSS have been minimised. Fresh
domestic wastewater is grey while aerobically degraded water tends to yellow and water
after anaerobic digestion becomes blackish. A brownish colour is telling of incomplete
aerobic or facultative fermentation. Wastewater that does not smell probably contains
enough free oxygen to restrict anaerobic digestion or the organic matter that has long
since been degraded. A foul smell (“like rotten eggs”) comes from H2S (hydrogen
sulphur), which is produced during anaerobic digestion, especially at a low pH. Other
odours are related to fresh wastewater from various sources. Experience is the best
basis for conclusions: Dairy wastewater will smell like dairy wastewater, distillery
wastewater will smell like distillery wastewater, etc., etc..
The process
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However, to „smell the performance“ of a treatment plant is most important. A
wastewater engineer should be alert and „collect“ various odours and its causes, to build
up a repertoire of experience for future occasions.
Temperature and pH
Temperature of was tewater is a very important parameter because of its effect on biochemical reactions, because bacterial growth increases with higher temperature,
principally, limits notwithstanding. Due to low energy gains as a result of “incomplete”
anaerobic decomposition, aerobic processes are less sensitive to low temperatures than
anaerobic processes. This is obvious from the fact that biogas is still oxidiseable and is
therefore an energy-rich end product. Temperatures between 25° and 35°C are most
ideal for anaerobic digestion. 18° to 25°C is also good enough. In short, a digester
temperature above 18°C is acceptable in principal. Higher temperatures are also
favourable for aerobic bacteria growth, but are disadvantageous for oxygen transfer (Fig.
13.). The cooler the environment the more oxygen can be dissolved in water and
thereby; more oxygen will be absorbed from the air. This is the reason why ponds may
become anaerobic in the height of summer. The pH indicates whether a liquid is acidic
or alkaline. The scientific definition of the pH is rather complicated and of no interest to
practical engineering (it indicates the H-ion concentration). Pure water has a pH of 7,
which is considered to be neutral. An effluent of neutral pH is indicative of optimum
treatment system performance. Wastewater with a pH below 4 to 5 (acidic) and above 9
(alkaline) is difficult to treat; mixing tanks may be required to buffer or balance the pH
level. In case of a high pH, ammonia-N dominates, whereas as ammonium-N is
prevalent in case of low pH.
Fig. 9: Different between acidic and basic solutions
Inorganic and other Compounds
While many inorganic compounds are present in domestic wastewaters, they are not
often measured. This includes substances such as Sodium, bicarbonates, chlorides,
sulphates, calcium, and potassium, as well as others. Among those that have a
potential utility through measurement are chlorides and sulphates.
Chlorides originate from urine as well as from the original waster supply. Since chlorides
are relatively inert and are not affected by passage through the wastewater treatment
plant, their concentrations will essentially reach more or less predictable levels. Thus,
any deviation from these levels may indicate an unusual input to the treatment plant
such as from industrial waste streams or from infiltration of the sewer systems.
Sulphate levels in domestic wastewater treatment are relatively unimportant and are not
normally measured. Sulphates are readily broken down under anaerobic conditions to
hydrogen sulphide gas which causes safety, odour, and colour problems. The presence
of hydrogen sulphide can be dangerous to personnel since this gas is toxic. Generally,
when it is formed, being heavier than air, it will collect in manholes, pits or any stagnant
areas. When this gas is detected the area should be thoroughly ventilated prior to
entering.
While at higher concentrations, hydrogen sulphide affects and deadens the mucous
membranes in the nose and no odour can be detected even though the gas is present.
Hydrogen sulphide is directly responsible for the rotten-egg odour from septic
wastewater’s. Since it is a very soluble and reactive gas, only a small amount is
sufficient to cause an odour or to cause it to enter into reactions with other compounds.
Other elements
•
•
•
•
•
•
Chlorides
Fats, oil and grease
pH
H2S gas
Nitrates
Phosphates
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Nitrogen Compounds
Domestic wastewaters will contain a number of nitrogen containing compounds.
Nitrogen is a significant element in wastewater treatment since it is a necessary nutrient
for satisfactory bacterial growth during biological treatment. One group of nitrogen
containing compounds, proteins, and their breakdown products, amino acids, have
already been noted as an organic nutrients component of wastewater.
Another organic nitrogen compound that is commonly found in domestic wastewaters is
urea. Urea will breakdown readily in the presence of most microorganisms to ammonia
in domestic wastewaters. Ammonia, an inorganic nitrogen compound, has several
sources —humans as an excretory product and as a decomposition product from urea or
from protein breakdown.
Two forms of inorganic nitrogen not usually found in raw domestic wastewaters, but
which are significant are nitrate and its unstable precursor form, nitrite. Nitrate is actually
formed from the oxidation of ammonia in the presence of dissolved oxygen. This is
called nitrification and occurs in the course of biological wastewater treatment or in the
receiving stream. Because this ammonia oxidation consumes oxygen in the receiving
stream and since microorganisms as well as aquatic plants can use nitrates or nitrites as
a nutrient and thus be stimulated to excessive growth, the reduction of nitrogen levels in
wastewater discharges is sometimes required.
Phosphorous Compounds
Since early in the 1970’s, the presence of phosphorous in domestic wastewaters has
received increasing attention. Phosphorous, like nitrogen, is required for the nutrition of
bacteria in a wastewater treatment plant. In addition, phosphorous is one of the prime
nutrients for plant growth in lakes, ponds, and streams, and as such, can cause, under
certain conditions, excessive enrichment or plant growth. These considerations have led
to a close examination of phosphorous, its presence in wastewater discharges, and
its effect on receiving waters. Phosphorous is most commonly found in the form of
inorganic phosphates and originates from human urinary excretions, agricultural run-off
and from household detergents. Recent legislation in Germany and other states banning
or limiting phosphate containing detergents has limited the significance of phosphorous
in domestic wastewater
Other Compounds (oil, fats, grease, toxics)
Other significant compounds found in domestic wastewater will include various oils, fats,
and greases. Oils and greases can be the source of problems in the treatment plant
where they may accumulate in the form of “greaseballs” (grease congregation) or
floating oil layer. These layers can be a general nuisance to operations.
Domestic wastewater will also contain small and varying concentrations of dissolved
gases. Among the most significant of these are dissolved oxygen and hydrogen
sulphide. In addition, domestic wastewater may contain other gases, such as carbon
dioxide resulting from the decomposition of organic matter or nitrogen dissolved from the
atmosphere. These gases, although small in amount have important roles in the
decomposition and treatment of wastewater.
Domestic wastewater may also contain volatile liquids. These are, in general, organic
liquids that boil at less than 100 degrees C, as for example, gasoline or many solvents.
Toxic organic compounds: A number of toxic organic compounds that can cause
neurological, developmental, or other problems in humans and interfere with biological
processes in the environment can be found in septic tank effluent. The toxic organics
that have been found to be the most prevalent in wastewater are 1,4-dichlorobenzene,
methylbenzene (toluene), dimethylbenzenes (xylenes), 1,1-dichloroethane, 1,1,1trichloroethane, and dimethylketone (acetone). These compounds are usually found in
household products like solvents and cleaners.
Metals can be present in raw household wastewater because many commonly used
household products contain metals. Several metals have been found in domestic septic,
confirming their presence in wastewater. They primarily include cadmium, copper, lead,
and zinc. Aging interior plumbing systems can contribute lead, cadmium, and copper.
Metals like lead, mercury, cadmium, copper, and chromium can cause physical and
mental developmental delays, kidney disease, gastrointestinal illnesses, and
neurological problems.
Basics of biological treatment
• Numerous bacteria are found in wastewater.
• About 5 to 50 billion bacteria are generally present
per litre of sewage.
• Pathogenic and non pathogenic
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Biological Compounds in wastewater
Micro organisms
Domestic wastewater also contains countless numbers of living organisms, most of them
too small to be visible except with help of a microscope. Typically, a domestic
wastewater prior to entering the treatment plant will contain from 100,000 to 1,000,000
microorganisms per millilitres. These microbes have their origin from two general
sources: sanitary wastes and the soil.
Table 3: Important pathogens in Wastewater
Both wastewaters and soils contain large numbers of microorganisms— especially
bacteria. Generally the microorganisms can be regarded, as a natural living part of the
organic matter found in wastewaters and their presence is most important because they
serve a primary function in the degradation of wastes in biological wastewater treatment.
In a sense the successful operation of a biological wastewater treatment plant is
dependent upon knowledge of the activities of the microorganisms—especially the
bacteria. Efficient treatment then depends on understanding the requirements for optimal
growth as well as recognizing unfavourable conditions.
5.3.2
Parasitic Bacteria (Pathogens)
While the majority of the microorganisms found in wastewaters are not harmful to man—
that is non-pathogenic (do not cause disease), some microorganisms are pathogenic
(disease causing) and always are of great concern in wastewater treatment. Parasitic
bacteria are those which normally live off of another living organism, known as the host,
since they require a food supply already prepared for their consumption, and generally
do not develop outside the body of the host. The parasitic bacteria are of importance in
wastewater.
They originate in the intestinal tract of human beings and animals and reach the sewage
by means of body discharges. Included among the parasitic bacteria are certain specific
types, which, during their growth within the body of the host, produce toxic or poisonous
compounds that cause disease in the host. These bacteria are called pathogenic
bacteria. They may be present in sewage receiving the body discharges of persons ill
with such diseases as typhoid fever, dysentery, cholera, or other intestinal infections.
Source: BMZ, Environment handbook, pg. 3
Table 4: Ranges of pathogen concentration in sewage treatment plant inflows and
outflows in developing and industrialized countries
The most critical component, in terms of what must be removed from wastewater, is
pathogens. Pathogens are organisms that cause disease, including viruses, protozoan,
and bacteria. Pathogens may be found in wastewater from anywhere in the house. Any
human contact with water results in the potential to add pathogens to the environment.
The presence of pathogens in wastewater makes its treatment a public health issue,
because of the risk of spreading disease. Among the diseases that are associated with
wastewaters are typhoid fever, dysentery, cholera, and hepatitis. Faecal coliform
bacteria are pathogens used as an indicator of the presence of any pathogens in
wastewater. These bacteria are residents of human intestinal tracts. Faecal coliform
bacteria are fairly easy to test for, and their presence is an indication that other
pathogens, which are more difficult to isolate and identify, may also be present. An
average value for faecal coliform bacteria in septic tank effluent is 1,000,000 organisms
per 100 millilitres.
Biological Components of Sewage
Fungi
Bacteria
Protozoa
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Fungi (Saprophytic Bacteria) and Protozoa
The microorganisms found in wastewaters are commonly classified by their appearance
(morphology). While all microorganisms found in wastewater treatment plants have
some role in the decomposition of wastes, probably the three most significant microbial
groups in biological treatment are the bacteria, fungi, and protozoa.
Most fungi are saprophytic, obtaining their food from dead organic matter. Along with
bacteria, by this activity they are of utmost importance in sewage treatment methods
designed to facilitate or hasten natural decomposition of the organic solids in sewage.
Such processes of decomposition will not progress without their activity. In the absence
of bacterial life—sterility—decomposition will not take place. In addition to food and
oxygen, bacteria require moisture to remain alive. This is adequately provided in
wastewater by its water component. In order to function at maximum efficiency bacteria
require a favourable temperature. They are very susceptible to changes in temperature
in that their rate of growth and reproduction, which is directly proportional to the amount
of work done, is definitely and sharply affected by such variations. The larger proportion
of the saprophytic types thrive best at temperatures from 20°C to 40°C. These are
known as mesophilic types.
They need more
In addition to food and oxygen, bacteria
require moisture and temperature to like.
• Pshychrophylic bacteria – 0 to 5 degree
• Mesophilic bacteria – 20 to 40 degree
• Thermophilic bacteria – 55 to 60 degree
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Variations from this temperature range limit the activity of mesophilic bacteria, practically
eliminating it at extremely low temperatures and at high temperatures. Mesophilic sludge
digestion proceeds most rapidly at 35°C. Other bacteria live best at high temperatures,
in the range of 55°C to 60°C. These are known as thermophilic types. Thermophilic
bacteria function in sewage treatment principally in high temperature digestion of sludge
solids. All of the bacteria, parasitic and saprophytic, require in addition to food, oxygen
for respiration. Certain types of them can use only oxygen dissolved in water, termed
dissolved oxygen and sometimes called free or molecular oxygen. These organisms are
known as aerobic bacteria and the process of degradation of organic solids, which they
carry out, is termed aerobic decomposition, oxidation or decay. Other types of bacteria
cannot exist in the presence of dissolved oxygen but must obtain the required supply of
this element from the oxygen content of organic and some inorganic solids, which is
made available by their decomposition. Such microorganisms are termed anaerobic
bacteria and the process of degradation of solids that they bring about is called
anaerobic decomposition or putrefaction, that is, decomposition in the absence of
dissolved oxygen, which results in the production of foul odours and unsightly conditions.
Protozoa are single-celled micoorganism without cell membrane.
Protozoa of
importance for wastewater engineers include amoebas, flagellates, and free-swimming
and stalked ciliates. Protozoa feed on bacteria and other microorganisms and are
essential in the operation of biological treatment processes and in the self-purification
process of natural waters, because they maintain a nature balance among the different
groups of microorganisms.
When all the environmental conditions of food supply, oxygen, moisture, and
temperature are properly maintained at their optimum amounts for the full functioning of
the bacteria, decomposition of the wastewater solids proceeds in a natural orderly
manner.
As noted, it is the biological organisms that actually carry on the processes of biological
waste treatment. A prime responsibility of the operator is to provide the environmental
condition best suited for their growth.
BOD and COD
• Chemical Oxygen Demand (COD) -- A standard test
that measures the amount of the organic matter in
wastewater that can be oxidized (burned up) by a
very strong chemical oxidant. It is expressed in mg/l.
• The BOD (Biochemical Oxygen Demand) is always a
fraction of the COD.
• It denotes the amounts of oxygen required by the
microorganisms to decompose organic matter
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Measurement of Organic Content
Are two parameters extensively used to monitor the organic content of wastewater and
sewage and used extensively in the operation of water treatment plants. Off all
parameters, the Chemical Oxygen Demand (COD) is the most general parameter of
measure organic pollution. It describes how much oxygen is required to oxidize all
organic and inorganic matter found in water. The BOD (Biochemical Oxygen Demand) is
always a fraction of the Chemical Oxygen Demand.
Biochemical Oxygen Demand (BOD) and Dissolved Oxygen (DO)
The BOD test is one of the most basic tests used in the wastewater field. It is essentially
a measure of the biological and the chemical component of the waste in terms of the
dissolved oxygen needed by the natural aerobic biological systems in the wastewater to
break down the waste under defined conditions. Generally the BOD test is carried out by
determining the dissolved oxygen on the wastewater or a diluted mixture at the
beginning of the test period, incubating the wastewater mixture at 20°C, and determining
the dissolved oxygen at the end of 5 days (BOD5). The difference in dissolved oxygen
between the initial measurement and the fifth day measurement represents the
biochemical oxygen demand.
The standard BOD test is carried out over 5 days (BOD5) and gives the rate of oxygen
consumption for the sample under investigation as an overall rate constant k (day-1) that
is assumed to be first order.
Fig. 10: The standart BOD test is carried out over 5 days (BOD5)
As this method is a fairly long-term bioassay test (5 days), a more rapid (2-3 hour) test is
often used to estimate the BOD; it is known as the COD, or chemical oxygen demand
test.
The dissolved oxygen concentrations in a wastewater before and after treatment are
very important. While dissolved oxygen concentrations are necessary to carry out the
BOD determination, as described above, dissolved oxygen levels are also quite
important in determining how satisfactory a biological wastewater treatment plant is
operating.
For example, for BOD - loss of biodegradable organic matter (oxygen demand)
biodegradable oxygen demand that can be captured by defined biological
analysing method within 5 days satisfactory biological wastewater decomposition (i.e.
treatment) some dissolved oxygen must be present. If it is not, the system will be
inefficient and is said to be anaerobic. Septic conditions follow, accompanied by a
variety of nuisance conditions such as odor and color changes.
Generally, raw wastewater will have little if any dissolved oxygen present while
wastewater in final settling tanks, or in the final effluent will probably have at least
measurable dissolved oxygen concentrations
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Chemical Oxygen Demand (COD)
The COD test will oxidize non-biodegradable and recalcitrant (slowly degrading)
compounds. These compounds are not detected by the test for Biochemical Oxygen
Demand (BOD). A chemical measure of the amount of oxygen required oxidizing organic
substances in water or wastewater. A strong oxidizing agent (+acid and heat) is used to
oxidize all the carbon compounds in the sample.
COD and BOD Tests do not fully reflect the biochemical truth, but are reliable indicators
for practical use. The BOD5 is part of the total BOD, the total BOD may be understood
as part of the COD and the COD is part of the absolute real oxygen demand.
Source: L. Sasse (adapted)