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

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3 PRELIMINARY WASTE WATER TREATMENT STEPS

9

4 ACTIVATED SLUDGE

15

5 MICROORAGNISMS IN ACTIVATED SLUDGE

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6 GROWTH OF BACTERIA IN WASTE WATER TREATMENT PLANT

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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 PO43−) 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 (SO42−)

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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.

References:

Waste-water treatment technologies, United Nations New York, 2003

Waste-water Bacteria by Michael H. Geradi

Wastewater Engineering Treatment, Disposal, and Reuse by Metcalf & Eddy, 1991,

McGraw-Hill, New York

Wastewater Treatment by Sundstrom, D. W. and Klei, H. E., 1979, Wastewater Treatment

http://www.iwawaterwiki.org/xwiki/bin/view/Articles/CoagulationandFlocculationinWaterandWastewaterTreatment

http://www.sourcewatch.org/index.php?title=Sewage_sludge

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