1. Global Environmental Lab (Private) Limited (GEL)
Waste Water Treatment
Theory
Submitted by: Mr. Gulfam raza Haidery
Submitted to: Mr. Umer Mehmood
Abstract: The document includes brief introduction to waste water, waste water
treatment methods, role of microorganisms in water as well as in activated sludge. A
concise description about the poisonous substances, which can be harmful for the
microorganisms, has also been added.

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Contents
Chapter Number Chapter Name Page Number
1 ORGANIC MATTERS IN H2O 3
2 WASTE WATER TREATMENT METHODS 6
3 PRELIMINARY WASTE WATER TREATMENT
STEPS
9
4 ACTIVATED SLUDGE 15
5 MICROORAGNISMS IN ACTIVATED SLUDGE 17
6 GROWTH OF BACTERIA IN WASTE WATER
TREATMENT PLANT
22

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Chapter 1
ORGANIC MATTERS IN H2O
1.1 CLASSIFICATION OF ORGANIC MATTER (OM)
1. Natural organic matters (NOM)
2. Anthropogenic organic matters
3. Particulate organic matters(POM)
4. Dissolved organic matters(DOM)
1.1.1 Sources of natural organic matter
 The OM is derived from natural sources; it is often referred to as natural organic
matter (NOM). The OM that is of natural origin is derived primarily from plant and
/ or microbial residues.
On land, plants grow; sometimes shed leaves, and die, leaving roots within the
upper soil layers and ‗litter‘ on the soil surface. Microorganisms also flourish
within the soil. When they die their biomass adds to the soil organic content.
 Organic matter is also produced in situ within a water body. Wetlands, both
natural and constructed, are a prime example. There, the luxuriant growth of
vegetation produces a thick mat of aerial material and roots that, upon death, are
deposited in the water.
 Other water bodies, rivers, lakes, and oceans, support the growth of aquatic
plants and animals to a smaller degree and their organic remains also become
part of the total aquatic system like algae, bacteria and other microorganisms.
 Microscopic animals also release soluble organic matter from their bodies.
 Humic material (HM) is a form of environmental organic matter of plant or
microbial origin.
1.1.2 Sources of Anthropogenic organic matters
 Besides the natural sources, there are human inputs that contribute to the
organic matter water. These include large volumes of poorly defined wastes,
such as domestic sewage or mill effluent, that are sometimes discharged directly
or after treatment into rivers, lakes, and oceans. Besides the bulk effluents,
anthropogenic sources also supply specific organic compounds—agricultural
chemicals, medicinal, and products or byproducts of industrial processes.

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 Release of nitrilotriacetic acid (NTA) into a waste stream from a detergent-
manufacturing facility is an example of an indisputably anthropogenic event.
 Trichloromethane (chloroform) is a chlorinated hydrocarbon that we might
assume is produced only via industrial processes.
1.1.3 Chemistry of dissolved organic matter
 About 50% of DOM is made up by Humic Material (HM).
 As an approximation, the remaining DOM consists of following;
o low molar mass acids(25%) such as;
 oxalic acids
 citric acids
 formic acids
 acetic acids
o Neutral compounds much of which is carbohydrate material (15%)
o On a global scale it is estimated that around 10% of microbial activity in
water goes to the production of DOM.
1.2 HUMIC MATERIAL
Humic material (HM) is a form of environmental organic matter of plant or microbial
origin.
1.2.1 Subdivision of humic material
Humic material (also called humate or humus) is subdivided in an operational sense into
three classes or categories;
 Fulvic acid (FA) is the fraction of humic matter that is soluble in aqueous
solutions that span all pH values.
 Humic acid (HA) is insoluble under acid conditions (pH 2) but soluble at
elevated pH.
 Humin (Hu) is insoluble in water at all pH values.
1.2.2 Forms of humic materials
Humic materials as a group are found in the aqueous and terrestrial environments in a
variety of forms and associations.
 Free HM consists of soluble or insoluble forms of the material itself.
 Complexes HM is chemically bound to metals, other inorganic species such as
phosphate, or organic molecules.
The complexes HM are either in solution or in particulate form.

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 Surface-bonded HM is chemically bonded to other solids such as clay minerals
or iron and aluminium oxides.
In this way the surface of the inorganic material is altered so that its chemical
properties are determined largely by the organic coating. The HM can then react
in a manner similar to that of the pure material itself.
1.3 TOXICITY OF SPECIFIC COMPOUNDS
Organic matter in water is of environmental importance for several reasons. For one
thing, particular compounds may be toxic in varying degrees to living organisms,
including humans. Following are all well-known;
 Polyaromatic hydrocarbons
 Polychlorinated biphenyls
 Dioxins

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Chapter 2
WASTE WATER TREATMENT METHODS
2.1 METHODS OF WASTE WATER TREATMENT
Wastewater refers to any water which has been used for some human activity and thus
has been contaminated and left it unsuitable for further use.
Or
Wastewater treatment is a process wherein the contaminants are removed from
wastewater - both domestic and industrial, in order to produce waste stream or solid
waste suitable for safe discharge or reuse.
They can be broadly categorized into three different groups (on the basis of tasks
involved).
1. Physical method
2. Chemical method
3. Biological method
2.1.1 Physical method
When it comes to physical wastewater treatment, following physical processes are used
for the treatment of water instead of resorting to chemicals or biological means.
 Sedimentation
Wherein coarse screening of waste water is done to remove contaminating
objects after allowing them to settle at the base, when heavy contaminants settle
down, the removal of cleared effluent or waste stream becomes relatively easy.
 Aeration
Wherein air is added to the wastewater physically in order to provide oxygen to
the contaminated water.
 Filtration
Wherein the contaminated water is passed through various filters to separate the
contaminating solids from the water. Sand filter is by far the most common filter
used in this process.

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2.1.1.1 Filtration problems
 Biofouling
Biofouling or biological fouling is the accumulation
of microorganisms, plants, algae, or animals on wetted surfaces. Such
accumulation is referred to as epibiosis when the host surface is another
organism and the relationship is not parasitic.
 Effect of biofouling
Biofouling essentially breaks down water filtration systems in water purification
facilities.
 Causes of biofouling
 In water, Natural Organic Matter (NOM) can still bind to metal ions and minerals.
 These bound molecules are not necessarily stopped by the purification process,
but do not cause harm to any humans, animals, or plants.
 However, because of the high level of reactivity of natural organic matter,
byproducts that do not contain nutrients can be made.
 These byproducts are much larger and can induce biofouling. The larger
molecules clog the water purification filters
 Treatment of biofouling
 The byproduct problem could be treated by the disinfection technique known as
chlorination, which often breaks down residual material clogging systems.
 Water with natural organic matter could be disinfected with ozone-initiated radical
reactions.
The ozone has very strong oxidation characteristics. It can form hydroxyl radicals
(OH) when it decomposes, which will react with the natural organic matter to shut
down the problem of biofouling.
 Anti-fouling is the process of removing or preventing these accumulations from
forming. In industrial processes, bio-dispersants can be used to control
biofouling.
2.1.2 Chemical method
As opposed to physical treatment of water, chemical treatment involves the use of
chemicals to get rid of contaminants in it.
Various methods are used to treat water chemically, they are following;
 Chlorination
In this process, chlorine - a strong oxidizing chemical, is used to kill the bacteria
which lead to decomposition of water.

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 Oxidization
This is a chemical water treatment method, wherein oxidizing agents - such as
ozone, are used to treat polluted water. These oxidizing agents make water
reusable by hampering the biological growth process of bacteria which happens
to be the main cause of decomposition of water.
 Neutralization
When it comes to industrial wastewater treatment, a chemical process known as
Neutralization is quite common.
This process involves adding acid or base to the water to adjust its pH value and
bring it back to neutral level.
 Lime (CaO), commonly known as quicklime or burnt lime, is one of the
best examples of base used in the process of neutralization to neutralize
acid wastes.
 Polyvalent metals, i.e. metals having more than one valence, are very
often used as coagulating chemicals in sewage treatment.
 Iron and other metals containing compounds like ferric sulfate and
aluminum sulfate are some of the best examples of coagulants.
2.1.3 Biological method
In biological water treatment processes, bacteria and other such microorganisms are
used to biochemically decomposes the wastewater and stabilizes the end product.
Biological water treatment is further categorized into two sub-divisions as following;
 Aerobic process
In the aerobic process, bacteria consume the organic matter and helps convert it
to carbon dioxide in the presence of oxygen.
 Anaerobic process
In the anaerobic process, on the other hand, sludge is fermented at a particular
temperature in the absence of oxygen.
References: http://www.buzzle.com/articles/wastewater-treatment-methods.html

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Chapter 3
PRELIMINARY WASTE WATER TREATMENT STEPS
3.1 BACKGROUND OF PRIMARY TREATMENT
The first waste water treatment systems, introduced by the end of the 19th
century, were
designed as units for the separation of solids and liquid by means of gravity settling: a
process known as the primary treatment of waste water.
A large fraction of the organic material in waste water is not settle able and therefore is
not removed by primary treatment.
3.2 BACKGROUND OF SECONDARY TREATMENT
Secondary treatment was introduced in the first decades of the 20th
century, With the
objective of improving the treatment efficiency of waste water treatment plants.
Secondary treatment is characterized by the use of biological methods to remove the
organic material present in the waste water.
In search of an efficient waste water treatment system, the activated sludge process
was developed in 1914 by Lockett and Ardern at the University of Manchester. They
noted that aeration of municipal sewage resulted in an increased removal rate of
organic material, while at the same time the formation of macroscopic flocs was
observed, which could be separated from the liquid phase by settling, forming a
biological sludge.
The capacity of the sludge to increase the removal rate of organic material led to the
common denomination ―activated sludge‖.
The basic principle of the activated sludge process has not changed since the
first application: organic material is still placed in contact with activated sludge in
an aerobic environment and let the end to come.
3.3 MODES OF PROCESS
In its original version, the activated sludge process was operated as a batch process:
The first important advance in the development of the activated sludge process was the
transformation of the original sequential batch process into a continuous process,
through the addition of a settling tank after the biological reactor.

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3.4 WASTE WATER TREATMENT AND PUIRFICATION STAGES
Treatment and purification stages are categorized as following;
1. Primary treatment
2. Secondary treatment
3. Disinfection
4. Sludge treatment
3.4.1 Primary treatment
Primary treatment uses both physical as well as chemical method.
 Screening chamber
The incoming wastewater, called influent, passes through screens consisting of
upright bars, spaced one to three inches apart. These bars remove large pieces
of trash including rags, sticks, newspaper, soft drink cans, bottles, plastic cups
and other similar items. This protects the main sewage pumps and other
equipment. The garbage is transported to landfills. The main sewage pumps then
lift the wastewater from the screening chamber to the surface level of the plant.
The wastewater enters primary settling tanks, also called sedimentation tanks,
for one to two hours.
 Coagulation
The materials which are suspended or found in the colloidal form in raw water or
influent are removed by coagulation.
Substance that is used to carry out coagulation is called coagulant.
 The most important coagulants are;
o Al2(SO4)3·14H2O or Al2(SO4)3·18H2O (alum)
o FeCl3
o FeCl3 (with lime)
o Fe2(SO4)3 (with lime)
o FeSO4·7H2O (copperas) (with lime)
 Alum or aluminium sulphate
When alum is added to waste water in alkaline medium,aluminium hydroxide is
precipitated out as reaction shows following;
K2SO4.Al2 (SO4)3·24H2O +3Ca(OH)2 3CaSO4+2Al(OH)3+K2SO4+24H2O
So, suspended particles get adsorbed on the surface of gelatinuous aluminium
hydroxide.

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 Ferric salts
Ferric salts are commonly used as coagulants but they are difficult to handle
because an insoluble ferric oxide is produced in the pH range from 3 to 13.
 Lime
The water may contain calcium and magnesium salts which make water hard. It
is treated by adding lime in waste water as shown by reactions;
 Partially treated waste water and primary sludge
The flow of the water is slowed, allowing heavier solids to settle to the bottom of
the tank and the lighter materials to float.
 The settled solids, called primary sludge, are then pumped through cyclone
degritters — devices that use centrifugal force to separate out sand, grit (such as
coffee grinds) and gravel. This grit is removed, washed and taken to landfills.
o The degritted primary sludge is pumped to the plant's sludge handling
facilities for further processing.
o The partially treated wastewater from the primary settling tanks then flows
to the secondary treatment system.

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3.4.2 Secondary treatment
Secondary treatment uses chemical, physical and biological methods.
The partially treated wastewater from the primary settling tanks then flows to the
secondary settling tanks which are termed to be as aeration tanks (also called biological
reactors or bubbling tanks).
Secondary treatment is also called the activated sludge process.
Aeration
Air pumped into large aeration tanks by means of aeration equipments which are in
following forms and any of them can be used according to process feasibility ;
 Surface aerators
 Compressors connected to submerged air diffusers
In this process, air is passed through partially treated waste water and following effects
occurs such as;
 Removal of dissolved foul smelling H2S
 Removal of Organosulpur compounds
 Removal of Volatile organic compounds
 Some organic materials are oxidized with air and CO2 is produced
 Removal of remaining organic materials by passing water over activated carbon
 Aeration process also oxidizes water soluble Fe+2
to Fe+3
which then forms
insoluble Fe(OH)3 and is separated as solid

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Aeration equipments mix the wastewater and sludge that stimulates the growth of
oxygen-using bacteria and other tiny organisms that are naturally present in the
sewage.
These beneficial microorganisms consume most of the remaining organic materials that
are polluting the water and this produces heavier particles that will settle later in the
treatment process.
Wastewater passes through these bubbling tanks in three to six hours, producing
sludge which is called activated sludge.
The aerated wastewater then flows to the final settling tanks called clarifier which are
similar to the primary settling tanks. Here the heavy particles and other solids settle to
the bottom as secondary sludge. Some of this sludge is re-circulated back to the
aeration tanks called return sludge which stimulates the activated sludge process.
The returned sludge contains millions of microorganisms that help maintain the right mix
of bacteria and air in the tank and contribute to the removal of as many pollutants as
possible.
The remaining secondary sludge is removed from the settling tanks and added to the
primary sludge for further processing in the sludge handling facilities.
Wastewater passes through the settling tanks in two to three hours and then flows to a
disinfection tank.
3.4.3 Disinfection
Even after primary and secondary treatment, disease causing organisms may remain in
the treated wastewater.
Chlorination
To disinfect and kill harmful organisms, the wastewater spends a minimum of 15-20
minutes in chlorine-contact tanks. Hypochlorous acid HOCl is used as disinfecting
agent.
Hypochlorous acid is not stable thus it cannot be stored. So it is generated by dissolving
the water with any of the following;
 Molecular chlorine gas
Cl2 + H2O HOCL + H+
+ Cl-
 Sodium hypochlorite

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 Calcium hypochlorite
The treated wastewater, or effluent, is then released into local waterways.
Harmful effects of chlorination
Reaction of chlorine with dissolved ammonia and organic matters produce detrimental
effects.
Hypochlorous acid reacts with dissolved ammonia to form chloramines NH2Cl, NHCl2
and the most important nitrogen trichloride NCl3 which is powerful eye-irritant. The
reaction is given below;
NH3 + 3HOCl NCl3 + 3H2O
The alkaline pH can prevent the formation of chloramines.
Production of toxic organic compounds in chlorination
 If phenol is present in water, chlorinated phenols are formed which are toxic in
taste and offensive in odor.
 When Hypochlorous acid reacts with humic acid, chloroform CHCl3 is formed.
Chloroform is a liver carcinogen. To avoid the formation of toxic compounds with
chlorine, ozone or chlorine dioxide is used for the disinfection of water.
3.5 TERMINOLOGIES
 Influent
The incoming wastewater, called influent.
 Effluent
The treated wastewater, or effluent.
 Primary sludge
The settled solids in the sedimentation tanks during primary treatment are called
primary sludge.
 Activated sludge
The settled solids in aeration tanks can be termed as activated sludge.
The activated sludge process is named so because it involved the production of
an activated mass of microorganisms capable of aerobically stabilizing the
organic content of a waste.
 Secondary sludge
The settled solids in final settling tanks (clarifiers) are called secondary sludge.
 Return sludge
Re-circulated sludge from clarifier to aeration tank is called return sludge.

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Chapter 4
ACTIVATED SLUDGE
4.1 INTRODUCTION
The activated sludge process was developed in England in 1914.
Activated sludge is probably the most versatile of the biological treatment processes
capable of producing an effluent with any desired Biological Oxygen Demand (BOD).
4.2 FORMATION OF ACTIVATED SLUDGE
The wastewater contains some suspended and colloidal solids and when agitated in the
presence of air, the suspended solids form nuclei on which biological life develops and
gradually build up to larger solids which are known as activated sludge.
Activated sludge is a brownish floc-like substance consisting of organic matter obtained
from the wastewater and inhabited by myriads of bacteria and other forms of biological
life.
Activated sludge with its living organisms has the property of absorbing or adsorbing
colloidal and dissolved organic matter.
The biological organisms utilize the absorbed material as food and convert it to inert
insoluble solids and new bacterial cells. Much of this conversion is a step-by-step
process.
Some bacteria attack the original complex substances to produce simpler compounds
as their waste products. Other bacteria use the waste products to produce still simpler
compounds and the process continues until the final waste products can no longer be
used as food for bacteria.
4.3 WHY RETURN SLUDGE IS NEEDED?
The generation of activated sludge or floc in wastewater is a slow process and the
amount so formed from any volume of wastewater during its period of treatment is small
and inadequate for the rapid and effective treatment of the wastewater which requires
large concentrations of activated sludge.
Such concentrations are built up by collecting the sludge produced from each volume of
wastewater treated and re-using it in the treatment of subsequent wastewater flows.
The sludge so re-used is known as returned sludge.
The purpose of return sludge is to maintain a concentration of activated sludge in the
aeration tank sufficient for the desired degree of treatment.

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Ample return sludge pump capacity should be provided since the return sludge volume
may range from 10 to 50 percent of the volume of wastewater being treated and
sometimes more. For a conventional plant, the percentage is usually between 20 and
30 percent.
The surplus, or excess activated sludge, is then permanently removed from the
treatment process and conditioned for ultimate disposal.
4.4 REMOVAL OF ACTIVATED SLUDGE
Excess activated sludge should be wasted as required to maintain the desired solids
concentration in the aeration tank.
This can be done by either withdrawing mixed liquor directly from the aeration tank or to
waste from the sludge return line.
4.5 TERMINOLOGIES
 On-site sludge
Sludge produced in septic tanks is termed as on-site sludge.
 Off-site sludge
Activated sludge in the aeration tanks is referred as off-site sludge.

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Chapter 5
MICROORAGNISMS IN ACTIVATED SLUDGE
5.1 CLASSIFICATION OF LIVING CELLS
All living cells can be classified into following two types;
 Prokaryotic
Prokaryotic cells lack a nucleus and other membrane-bound structures.
 Eukaryotic
Eukaryotic cells possess these structures. The nucleus is the primary membrane-
bound structure in eukaryotic cells.
5.2 CLASSIFICATION OF MICROORGANISMS
Based upon cellular structure and function, microorganisms are commonly classified as;
 Prokaryotes
 Eukaryotes
5.2.1 Prokaryotic organisms in the activated sludge process
The predominate type of bacteria present will be determined by following factors;
 The nature of the organic substances in the wastewater
 The mode of operation of the plant
 The environmental conditions present for the organisms in the process
The prokaryotes consist of following two types;
1. Eubacteria or ―true‖ bacteria
2. Archaebacteria or ―ancient‖ bacteria
The eubacteria and archaebacteria are the most important microorganisms in biological,
wastewater treatment plants.
Together, these two prokaryotes, commonly, are referred to as bacteria.
5.2.2 Eukaryotic organisms in the activated sludge process
There are four important eukaryotic organisms in the activated sludge process.
1. Fungi
2. Protozoa
3. Rotifers*

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4. Nematodes*
.
These free-living (non-disease-causing) eukaryotes enter wastewater treatment plants
through inflow and infiltration (I/I) as soil and water organisms.
* Rotifers and nematodes are collectively called metazoan.
Fungi
Fungi are relatively rare in activated sludge. When present, most of the fungi tend to be
of the filamentous forms which prevent good floc formation and therefore decrease the
efficiency of the plant.
Following factors stimulate fungi growth;
 High carbohydrate waste
 Unusual organic compounds
 Low pH
 Low dissolved oxygen concentrations
 Nutrient deficiencies
Most fungi are strict aerobes and can tolerate a low pH and a low nitrogen environment.
Although fungi grow over a wide range of pH values (2–9),
 The optimum pH for most species of fungi is 5.6.
 The nitrogen nutrient requirement for growth of fungi is approximately one-half as
much as that for bacteria.
 In the activated sludge process filamentous fungi may proliferate and contribute
to settle-ability problems in secondary clarifiers.
 The proliferation of filamentous fungi is associated with low pH (<6.5) and low
nutrients.

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Protozoa
Protozoa are unicellular organisms.
In the activated sludge process, protozoa are placed commonly in five groups according
to their means of locomotion. These groups are following;
 Amoebae
 Flagellates
 Free-swimming ciliates
 Crawling ciliates
 Stalked ciliates
Amoebae
Flagellates
Free-swimming ciliates
Crawling ciliates
Stalked ciliates

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Ciliated protozoa
Ciliated protozoa are the most important groups of protozoa in the activated sludge
process.
They possess short hair-like structures or cilia that beat in unison to produce a water
current for locomotion and food gathering—that is, to bring bacteria into their mouth
opening.
Ciliated protozoa provide the following benefits to the activated sludge process;
 Add weight to floc particles and improve their settleability
 Consume dispersed cells and cleanse the waste stream
 Produce and release secretions that coat and remove fine solids (colloids,
dispersed cells, and particulate material) from the bulk solution to the surface of
floc particles
 Recycle nutrients (nitrogen and phosphorus) through their excretions
Rotifers and nematodes
Rotifers and nematodes are multicellular microscopic animals (metazoan)
Rotifers
Nematodes
Those also provide numerous benefits to the activated sludge process. The metazoan
burrow into floc particles. The burrowing action promotes acceptable bacterial activity
for the degradation of substrates in the core of the floc particle by permitting the
penetration of dissolved oxygen, nitrate (NO-3
), substrates, and nutrients.

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5.3 SUMMARY
Activated sludge is a biological contact process where following microorganisms are
commonly found;
 Bacteria
 Fungi
 Protozoa
 Small organisms such as;
o Rotifers and
o Nematode worms
The bacteria are the most important group of microorganisms for they are the ones
responsible for the structural and functional activity of the activated sludge flocs. All
types of bacteria make up activated sludge.
References: Waste Water Bacteria by Micael H.Gerardi

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Chapter 6
GROWTH OF BACTERIA IN WASTE WATER TREATMENT PLANT
6.1 CELLELAR COMPOSITIONS OF BACTERIA
The chemical composition of bacteria is as following;
 80% water approximately
 20% dry material.
Of the dry material,
 90% is organic approximately
A simple organic formula for a bacterial cell that includes nitrogen is C5H7O2N.
 10% is inorganic
Inorganic compounds such as ionized ammonia (NH4+
), ammonium salts,
nitrate (NO3−
) and nitrite (NO2−
) are most often used.
Although the inorganic composition of bacterial cells is relatively small, a shortage of
any inorganic element can limit bacterial growth and wastewater treatment plant
efficiency.
6.2 ESSENTIAL ELEMENTS FOR BACTERIA GROWTH
The growth of bacteria in wastewater treatment plants is affected by many factors
including the presence of available nutrients like following;
 Major elements
The major elements (macroelements) in the composition of bacterial cells
include C, H, N, O, P and S. These elements are required in large quantities.
 Minor elements
The minor elements (microelements) such as Ca, Fe, K, Mg and Na are
required in small quantities.
 Trace elements
The trace elements including Co, Mn, Mo, Ni and Zn are required in relatively
small quantities for most bacteria.
These wastewaters often are nutrient deficient.
 The most commonly occurring deficiencies for nutrients in industrial
wastewaters are the major elements nitrogen and phosphorus,

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While
 Deficiencies for minor and trace elements (calcium, cobalt, iron and nickel) do
occur.
6.3 SPECIFIC BACTERIA AND CERTAIN ELEMENTS
 Methane-forming bacteria
Some bacteria such as methane-forming bacteria require major element like
sulfur and some minor and trace elements such as Co, Fe and Ni in quantities
2–5 times greater than other bacteria.
Some bacteria including a small group of methane-forming bacteria use
atmospheric or molecular nitrogen (N2).
 Halophiles
Halophiles require large quantities of chlorine (Cl) and sodium.
 Former of vitamin B12
Bacteria that synthesize vitamin B12 require Co in large amounts.
 Gram-positive bacteria
Calcium is required in large amounts by Gram-positive bacteria for the synthesis
of cell walls.
 Heterotrophs
Nearly all bacteria obtain carbon from organic compounds
 Autotrophs
Nearly all such bacteria obtain carbon from carbon dioxide Oxygen and
hydrogen requirements for cellular synthesis are often satisfied together by the
availability of organic compounds.
 Aerobic and facultative anaerobic bacteria
Sulfur is available to aerobic and facultative anaerobic bacteria in the oxidized
form as sulfate (SO42−
).
Sulfur is available to anaerobic bacteria in the reduced form as sulfide (HS−
).
Some bacteria are capable of using sulfur-containing amino acids as a source of
sulfur.
Phosphorus is available to bacteria as phosphate. The form of phosphate
(H2PO4
−
, HPO4
2−
, or PO4
3−
) used by bacteria is pH-dependent.
Consequently, nutrient addition to biological treatment units may be required when
soluble, cBOD-rich industrial wastewaters are being treated.

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6.4 FACTORS AFFECTING BACTERIA GROWTH
The growth of bacteria in wastewater treatment plants and consequently treatment
efficiency is influenced by a variety of followings;
 Nutritional factors
Nutritional factors include the availability of followings;
o Substrates
o Nutrients
 Physical factors
Physical factors include followings;
o pH
o Temperature
o response to free molecular oxygen
6.4.1 Physical factors
6.4.1.1 pH
It is studied by caterogization as following;
 Optimum pH
Bacteria have an optimum pH at which they grow best.
For most bacteria the optimum pH usually is near neutral (pH 7) and most
bacteria do not grow at values ±1 unit of their optimum pH and cannot tolerate
pH values below 4 or above 9.5.
 Operational pH
Most biological treatment units operate at pH values near neutral (6.8 to 7.2) and
these units may experience operational problems at pH values below or above a
near neutral pH value.
o Operational problems at pH values lower than 6.8
Operational problems that may occur in biological treatment units that
experience pH values lower than 6.8 include the following:
 Decreased enzymatic activity
 Increase in hydrogen sulfide (H2S) production
 Inhibition of nitrification
 Interruption of floc formation
 Undesired growth of filamentous fungi and some Nocardioforms
o Operational problems at pH values higher than 7.2
Operational problems that may occur in biological treatment units that
experience pH values higher than 7.2 include the following:
 Decreased enzymatic activity

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 Increase in ammonia (NH3) production
 Inhibition of nitrification
 Interruption of floc formation
6.4.1.1.1 Groups of bacteria w.r.t. acidity or alkalinity
There are three groups of bacteria with respect to the conditions of acidity or alkalinity
that they can tolerate. These groups include following;
 Acidophiles
Acidophiles or acid-loving organisms grow at pH values lower than 5.4.
Thiobacillus and Sulfolobus grow at pH values lower than 2, and many fungi
prefer pH values lower than 5.
 Neutrophiles
Neutrophiles grow at pH values from 5.4 to 8.5.
Most bacteria in wastewater treatment plants are neutrophiles.
 Alkalinophiles
Alkalinophiles or base-loving organisms grow at pH values from 7 to 11.5.
The nitrifying bacteria,Nitrosomonas and Nitrobacter are alkalinophiles.
6.4.1.1.2 Effect of pH upon the activity of bacteria
In addition to the effect that pH has upon the activity of bacteria, there are two pH-
related operational concerns.
 First, pH affects the degree of ionization of substrates, nutrients, and toxic
wastes and their transportation into bacterial cells.
 Second, the use of substrates and production of wastes by bacteria may
significantly change the pH of a biological treatment unit.
The change in pH may result in undesired bacterial activity and inefficient treatment of
wastewater or sludge.
Examples of pH change in biological treatment units due to bacterial activity include the
following:
 Denitrifying bacteria increase the pH of a biological treatment unit through the
release of hydroxyl ions (OH−
).
 Nitrifying bacteria decrease the pH of an aeration tank through the use and
destruction of alkalinity.
 Organotrophic bacteria decrease the pH of a biological treatment unit through the
production of carbonic acid (H2CO3) when they release carbon dioxide.
 Fermentative bacteria decrease the pH of an anaerobic digester through the
production of fatty acids.

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 Methane-forming bacteria increase the pH of an anaerobic digester through use
of fatty acids, especially acetate.
6.4.2 Temperature
Temperature exerts two significant effects upon a bacterial population.
 First, it affects the rate of diffusion of substrates and nutrients into bacterial cells.
 Second, it affects the rate of enzymatic activity. With increasing temperature the
rate of diffusion of substrates and nutrients into bacteria cells increases, and the
rate of enzymatic activity increases.
Therefore, with increasing bacterial activity during warm wastewater temperatures, an
operator of a wastewater treatment plant can decrease solids (bacteria) inventory and
still maintain acceptable treatment of wastewater.
However, with decreasing bacterial activity during cold wastewater temperatures, an
operator of a wastewater treatment plant may need to increase solids inventory in order
to maintain acceptable treatment of wastewater.
Optimum Temperature
The impact of temperature upon bacterial activity is significant.
For every 10°C rise in temperature, enzymatic activity nearly doubles.
However, once the optimum temperature for enzymatic activity and cellular growth has
been exceeded, enzymes become denatured (damaged) and can no longer efficiently
catalyze biochemical reactions.
Temperature range
There are three groups of bacteria with respect to the minimum and maximum
temperatures that they remain active.

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Bacterial group Range of temperature Best growth temperature
Psychrophiles(cold-loving) −10°C to 30°C 12°C to 18°C
Mesophiles*1
20°C to 50°C 25°C to 40°C
Thermopiles*2
(heat-loving) 35 to 75°C 50°C to 65°C
*1 Mesophiles are common inhabitants of the gastrointestinal tract of humans (body
temperature approximately 37°C) and enter wastewater treatment plants in large
numbers in human feces. They are present in very large numbers in the activated
sludge process and the mesophilic anaerobic digester.
*2
Thermophiles are common inhabitants of thermophilic anaerobic digesters and
thermophilic composting operations.
6.4.3 Response to free molecular oxygen
Bacteria grow in the presence or absence of free molecular oxygen and can be placed
in three groups according to their need for or response to free molecular oxygen. These
groups are following;
 Aerobes
Aerobes require oxygen for the degradation of substrate.
Examples of aerobic bacteria in activated sludge process include;
o Filamentous organisms haliscomenobacter hydrosis and sphaerotilus
natans
o Floc former zoogloea ramigera
o Nitrifying bacteria nitrosomonas and nitrobacter
 Anaerobes
Anaerobic bacteria do not use free molecular oxygen for the degradation of
substrates.
These organisms include;
o Sulfate-reducing bacteria that use sulfate (SO4
2−
)

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o Methane-forming bacteria (O2 intolerant) that use carbon dioxide.
 Facultative anaerobes
The term ―facultative‖ implies the ability to live under different conditions.
Facultative anaerobic bacteria have the ability to use free molecular oxygen or
another molecule such as nitrate (NO3
−
) to degrade substrate.
Denitrifying bacteria including Bacillus, Escherichia and Pseudomonas are
facultative anaerobic bacteria.
With respect to the quantity of oxygen necessary in activated sludge process to ensure
acceptable biological activity by aerobe and facultative anaerobes, there are four
activities of concern.