24731 bacteria

© Cegelec 2007 24731: Bacteria 1

BACTERIA

Aerobic Bacteria

Anaerobic Bacteria

Bio-Fouling

Sulphate Reducing Bacteria (SRB's)


Lesson Objectives

At the end of this Lesson You will be Able to:

Explain the environmental conditions for Aerobic Bacteria

to thrive and corrective actions

Explain the environmental conditions for Anaerobic

Bacteria to thrive and corrective actions

Describe the conditions necessary for Bio-Fouling

Describe the conditions for Sulphate Reducing Bacteria to

exist and corrective measures to eliminate SRB’s


Microorganisms - Bacteria

Bacteria (singular: bacterium) are

unicellular microorganisms. They are

typically a few micrometers long and

have many shapes including

spheres, rods, and spirals.

Microorganisms fall into three main categories:

• Aerobic Bacteria - Require oxygen to thrive.

• Anaerobic Bacteria - Thrive best in the

absence of oxygen.

• Facultative Bacteria - Thrive in the absence or

presence of oxygen.


Aerobic Bacteria

An aerobic organism or aerobe

is an organism that has an

oxygen based metabolism.

Aerobes, in a process known as

cellular respiration, use oxygen

to oxidize substrates (for

example sugars and fats) in

order to obtain energy.

• Obligate aerobes require oxygen for aerobic cellular

respiration.

• Facultative aerobes can use oxygen, but also have

anaerobic methods of energy production.

• Microaerophiles are organisms that may use oxygen, but

only at low concentrations.

• Aerotolerant organisms can survive in the presence of

oxygen, but they are anaerobic because they do not use it

as a terminal electron acceptor.


Anaerobic Bacteria

An anaerobic organism is any organism

that does not require oxygen for growth,

such as:

• Obligate anaerobes will die when exposed

to atmospheric levels of oxygen.

• Facultative anaerobes can use oxygen

when it is present.

• Aerotolerant organisms can survive in the

presence of oxygen, but they are anaerobic

because they do not use oxygen as a

terminal electron acceptor.

Bacteria can be a major source of trouble in a waste water disposal

system. Given the right conditions, they can double their

population in twenty minutes or less. They grow best in a pH range

of 5 - 9 and at temperatures between -18ºC and 82ºC. They prefer

warm, fresh water but can survive in the Arctic and in sea water.


Sulphate Reducing Bacteria (SRB)

Sulphate Reducing Bacteria (SRB) are very

corrosive in an anaerobic environment. They can

also live in an aerated environment by

congregating in anaerobic places created by

deposits or crevices. The sulphur reducing

bacteria are able to reduce sulphates in the water

to sulphides, resulting in the production of

hydrogen sulphide, which can cause four different

corrosion problems:

a. Pitting directly under a colony of bacteria;

b. The generation of H2S can make the water more

acidic, raising the level of general corrosion.

c. Sulphide cracking (a form of stress corrosion

cracking) and sulphide blistering.

d. "Sour corrosion" which forms insoluble iron

sulphide which itself forms more sites for further

pitting.


SRB (cont’d)

Other factors that govern the proliferation of SRB are:

• Temperature: SRB prefer temperatures below 45'C but have been reported

up to temperatures as high as 170oC.

• pH: SRB are very tolerant of pH and if not suited to them, can self-adjust the

pH.

• Salinity: Some types of SRB only grow in waters below 1 0 000 mg/ 1; others

require a minimum salinity of 25 000 mg/I. Many SRB, however, can tolerate

salinities up to 100 000 mg/l and have been reported as existing at 150 000

mg/I.

• Nutrients: SRB require sulphate ions to grow; sulphite and other oxidation

levels of sulphur can equally supply the necessary nutrition. A carbon source

is also required and the by-products from the breakdown of hydrocarbons

are an ideal food source (although not the hydrocarbons themselves).

• Oxygen Deficiency: Though preferring and developing under anaerobic

conditions, SRB will survive in the presence of oxygen. SRB thrive in

colonies on metal surfaces (known as sessile bacteria rather than as freely

floating planktonic bacteria). They produce a protective polysaccharide film

known as a biofilm which surrounds and covers the bacteria, hence their

ability to create their own anaerobic environment in an aerobic system


Control of SRB’s

Increasing the temperature of the water above the

level where SRB can tolerate

Increasing aeration of the water (O2 may kill the SRB’s

but may cause other forms of corrosion

Biocide treatment (100% kill is not easily achieved

with biocides because SRB’s quickly become immune

to the chemicals, hence biocides must be varied from

time to time

Removing the food supply is the preferred method for

SRB control but is in practice not easily achieved.

Only where other bacteria provide this feed stock a

control may be practicable by killing these food

suppliers.


Bio-Fouling

Bio-fouling is divided into:

Micro fouling — biofilm formation and bacterial

adhesion — and

Macro fouling — attachment of larger organisms, of

which the main culprits are barnacles, mussels,

polychaete worms, bryozoans, and seaweed

Macro fouling is caused by coarse matter of biological origin or by

industrially produced refuse. Such matter enters into the cooling water

circuit through the cooling water pumps from sources like the open sea,

rivers or lakes. Even in closed circuits, like cooling towers, the ingress of

macro fouling into the cooling tower basin is possible through open

canals or by the wind. Sometimes, parts of the cooling tower internals

detach themselves and are carried into the cooling water circuit.

All such substances foul the heat-transferring surfaces of the coolers and

heat exchangers and deteriorate the relevant heat transfer coefficient.


Bio-Fouling


Combating Bacteria

Micro fouling - distinctions are made

between:

• Biological fouling, like settlements of

bacteria and algae;

• Sedimentation of sand and mud;

• Scaling as crystallization of salts.

Chemicals used to control bacteria are classified as follows:

1. Bactericide - Kills bacteria.

2. Bacteriostat - Inhibits bacteria growth.

3. Biocide - Kills many forms of life as well as bacteria.

4. Biostat - Inhibits growth of bacteria and other forms of

life.


Lesson Objectives

You should now be Able to:

Explain the environmental conditions for Aerobic Bacteria

to thrive and corrective actions

Explain the environmental conditions for Anaerobic

Bacteria to thrive and corrective actions

Describe the conditions necessary for Bio-Fouling

Describe the conditions for Sulphate Reducing Bacteria to

exist and corrective measures to eliminate SRB’s

24731 bacteria

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