bio energy biological processes

8/10/2019 Bio Energy Biological Processes

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Energy from Biomass

Anaerobic biological processes

(Biogas plant Midlum near Bremerhaven)

Material for the lecture

Hochschule Bremerhaven, SS 2010

Prof. Dr.-Ing. Dieter Lompe


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Seite 1

Content page

1 Structure of micro organisms 2

1.1 Pro- and eukaryotic cells 2

1.2 Prokaryotic cells 2

1.3 Classification of bacteria regarding their metabolism 5

2 The metabolism of bacteria 5

2.1 Basic metabolic processes 5

2.2 Respiration, fermentation, methanogenesis 8

2.2.1 The respiration 8

2.2.2 The fermentation 10

2.2.3 Acetogenesis and methanogenesis 11

3 Kinetics of growth and metabolic processes 13

3.1 Cell division and growth 13

3.2 Influence of Temperature 16

3.3 Influence of pH-value 17

3.4 Growth kinetics 19

3.4.1 Michaelis-Menten kinetic 19

3.4.2 Inhibitions 21

3.4.3 Monod-kinetic 24

4 Bioreactor design 26

5 Applications 28

5.1 General process design 28

5.2 Gas production 31

5.2.1 Gas quality 31

5.2.2 Gas yield 33

5.2.3 Reaction rates 34

5.2.4 Example: Digestion of slaughterhouse waste 39

Literature 40


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1 Structure of micro organisms

1.1 Pro- and eukaryotic cellsLiving cells can be subdivided in two types, the smaller and more simple prokaryotic cells

(e.g. bacteria) and the bigger and more complex eukaryotic cells (e.g., protozoas, fungi incl.

yeasts, plants, animals) (Fig. 1).

Fig. 1: Sketch of a pro- and eukaryotic cell (in red: Membranes) (Fritsche 1990)

1.2 Prokaryotic cells

Fig. 2 shows the inner and outer structure of a bacterial cell. In spite of very different types of

bacteria the general structure of bacteria is very similar.


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Fig. 2: Structure of a bacterial cell (Fritsche 1990)

Fig. 3: a) basic shapes of bacterial cells, b) basic forms of flagella (Fritsche 1990)


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The nucleus contains the genetic information coded in a conglomerate of DNA. This

macromolecule has a length of 1200 m at Escherichia coli, which means 500 times longer

than the cell.

Plasmids are small DNA molecules with additional information for individual genes.

Proteins are produced within ribosomes (app. 5000 to 50.000 per cell). Ribosomes receive

the information from DNA via mRNA, material is taken from cell plasma and energy is used

as ATP (see below).

Sometimes cells contain granula, that are stored substances like PHB (poly-hydroxo butyric

acid) or poly-phosphates.

The bacterial material contains in average the following main elements:

C 50%

O 20%

N 14%

H 8%

P 3%

S 1%

K 1%

Ca 0,5%

Fe 0,2%

These figures may vary very much depending on individual types of bacteria and on growth

conditions.


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1.3 Classification of bacteria regarding their metabolismBacteria metabolism is classified into groups regarding their carbon source, their energy

source and their electron acceptor (s. fig. 4)

Fig. 4: Classification of metabolisms

If bacteria use oxygen as final electron acceptor of their metabolism they are called aerobic

or anoxic bacteria. Anaerobic bacteria use other electron acceptors.

2 The metabolism of bacteria

2.1 Basic metabolic processes

Metabolism is used to get energy from substrates and to produce new cell material for

growth. For a better understanding the metabolism is subdivided in degradation metabolism

or Catabolism, e.g. oxidation of glucose or other organic substances. From this oxidation the

CO2autotroph

organic Subst.heterotroph

Radiationphototroph

Energy-source

Organ. Subst.chemo-

organotroph

Inorgan. Subst.chemo-

lithotroph

Chemoorgano-heterotroph

animals, fungi,many bakteria

Chemoorgano-autotroph

some methanebakteria

Photoheterotrophsome Purpur-

bakteria a. Algae

PhotoautotrophPlants, AlgaeCyanobacteria

Chemolitho-autotroph

nitrifying bacteriairon oxidizing bact.

Chemolitho-heterotroph

some methanebakteria

Carbon-source


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reaction enthalpy is used to produce ATP (Adenosine-tri-phosphate) from ADP (Adenosine-

di-phosphate) as an energy storage. This energy is used on the other hand to synthesize

new material for cell growth in the Anabolism.

There are many links between catabolism and anabolism, e.g. by using substances from a

partially degradation already for the anabolism, sometimes by converting these intermediate

products within the intermediate metabolism.

In order to use energy from reactions in amounts that enable the storage of energy in

molecules like ATP, e.g. from the oxidation of glucose, micro organisms use a lot of small

reaction steps which led normally to an transport of electrons. Therefore those reactions are

also named redox-reactions, because of they are a combination of reduction and oxidation.Often these reactions are called also hydrogen-transfer-reactions, because of hydrogen is a

combination of one proton and one electron.

As an example, the very exothermic oxidation of hydrogen with oxygen can be written in

general as in (eq.2.1),

O2+ H2H2O (2.1)

but in detail as a transfer of electrons and hydrogen to oxygen(eq. 2.2).

O2+ 2H++ 2e-H2O (2.2)

In this reaction, oxygen was reduced and hydrogen was oxidized.

The free energy from this reaction G0can be calculated by using the difference of the redox

potential R between the electron-donator (Hydrogen, -0,42V) and the electron-acceptor

(Oxygen, +0,81 V) and the Faraday-Constant F (96480 As/mol). This calculation leads to a

free energy from this reaction of G0= -237,4 kJ/mol (eq. 2.3).

(2.3)

For the storage and transport of energy and hydrogen mainly two substances are used by

cells: ATP and NADH respectively (s. fig.5).

FRG =0


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Fig. 5: Structure of ATP and NADH (Fritsche 1990)

The free energy for the ATP ADP-reaction has an amount of 34 KJ/mol. If organism are

able to portion the free energy from the degradation of substrates by using many small

redox-reactions with this energy amount of 34 KJ/mol each of these reactions let cells

produce one ATP from ADP.

NADH is often used in cells to store and transport hydrogen (s. fig. 5), but there are also

other substances for this purpose (e.g. FAD).


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2.2 Respiration, fermentation, methanogenesis

Fig. 6 shows the main types of bacterial metabolism where as an example of organic

substrates, but also for cell-material a unit of [CH2O] is used.

Respiration

Fermentation

Metabolism of phototrophic organisms

Metab. of chemolithotrophic organisms

Methanogenesis

Fig. 6: Main types of bacterial metabolism (Fritsche 1990)

2.2.1 The respiration

In the respiration-metabolism substrates (e.g. organic substrates, e.g. glucose) are oxidized

by using oxygen. Normally molecular oxygen O2is used, if there is dissolved oxygen in the


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aqueous system available which is named aerobic metabolism or metabolism under aerobic

condition. If there is no molecular oxygen available, but oxygen within substances like nitrate

or sulphate a respiration-metabolism under anoxic conditions takes place, but with a small

difference in the available free energy:

aerobic conditions:

C6H12O6+ 6 O2 6 CO2+ 6 H2O, G0= -2870 KJ (2.4)

anoxic conditions with nitrate:

C6H12O6+4,8 HNO3 6 CO2+ 4,8 H2O, G0= -2669 KJ (2.5)

The respirative degradation of glucose consists of mainly four steps:

1. Conversion of 1 mol glucose into 2 mol pyruvate C3H4O3 (Yield: 2 mol ATP, 2 mol

NADH)

2. Decarboxylation (split off of carbon dioxide) of pyruvate (Yield: 1 mol NADH)

3. Further degradation within the citric-acid-cycle or Krebs-cycle (Yield: 2 mol ATP, 7

mol NADH, 2 mol FADH)

4. The respiratory chain, where all hydrogen from NADH or FADH is transferred to

oxygen (Yield: 34 mol ATP by using 6 mol oxygen)

In total, 38 mol ATP are yielded from the aerobic oxidation of 1 mol glucose. If other ways of

transferring energy within a bacteria are neglected an efficiency hof the aerobic biological

oxidation of glucose can be calculated:

(2.6)

with: nATP: moles ATP per mol glucose

Using eq. 2.4 the efficiency of the aerobic oxidation of glucose results in (38*34/2870):

h= 45%,

which is more than normal power plants achieve.

0

0

Gl

ATPATP

G

Gn

=


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But the result shows also, that 55% of the input-energy is converted to heat. That leads at

aerobic processes with high reaction rates to a high temperature in the reactor.

2.2.2 The fermentation

Fermentation processes consist often of the same first three steps like the respiration

process, but due to a lack of oxygen hydrogen cannot be transferred to oxygen. Therefore

other products with a high hydrogen content were produced and released by those micro

organisms.

Depending on the intermediate products to which hydrogen is transferred, different

fermentation products are yielded. Often organic acids like acetic acid, propionic acid, butyric

acid, or lactic acid or alcohols like methanol or ethanol are produced (s. fig. 7).

As an example ethanol is produced be the yeast Saccaromyces cerevisaefrom glucose by

transferring hydrogen from NADH to acetaldehyde.

C6H12O6 2 CO2+ 2 C2H5OH, G0= -197 KJ (2.7)

Some organisms are able to release molecular hydrogen. This process can be used for

biological production of hydrogen from organic waste.

Organisms which produce lactic acid are widely used for the conservation of food like

sauerkraut, sour milk, yoghurt, cucumbers, some cheese or animal food like silage.

Because of the respiratory chain cannot be used in fermentation processes the energy yield

per amount of substrate and therefore also the growth rate are much lower than in aerobic

processes.

Many fermentative bacteria and yeasts are fastidious anaerobic, but there are alsoorganisms which are able to adapt to anaerobic and aerobic conditions or which are tolerant

to some oxygen.


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Fig. 7: fermentative production of alcohol (left) and lactic acid (right) (Fritsche 1990)

2.2.3 Acetogenesis and methanogenesis

The production of methane containing biogas is a complex process consisting of four main

steps where many different micro organisms are involved.

1. If solid substrates are used from organic waste or agricultural products as a first step

hydrolysis is necessary in order to transfer the material into a dissolved phase to

make them accessible to other organisms. Some micro organisms often

fermentative organisms are able to produce special enzymes and to secrete these

enzymes (eponyms). These extra cellular enzymes brake up large molecules like

polymers (e.g. starch) to smaller components. These smaller components are soluble

and can be transferred via the cell-membrane into the cell.


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2. These smaller hydrolysis products are used by fermentative bacteria for their

metabolism. As described above those bacteria release alcohols, organic acids,

hydrogen and carbon dioxide. Due to the production of organic acids this phase is

also called acidogenic phase.

3. In a third phase other fermentative bacteria use alcohols and higher organic acids like

propionic acid as substrates and produce the smallest organic acid, acetic acid, as

well as hydrogen. Due to the main product of this phase it is named acetic phase.

The bacteria Syntrophobacter woliniidegrades e.g. propionic acid to acetic acid and

hydrogen:

CH3CH2COOH + 2 H2OCH3COOH + CO2+ H2, G0= -76,1 kJ/mol (2.8)

The degradation of butyric acid by Synthrophomonas wolfei is an example for the

degradation of a long-chain fatty acid:

CH3CH2CH2COOH + 2 H2O2 CH3COOH + 2 H2, G0= -48,1 kJ/mol (2.9)

4. Finally, in the fourth phase special methanogenic bacteria (archae bacteria) produce

methane via two different ways. Firstly methane is produced from carbon dioxide and

hydrogen, e.g. by Methanosarcina:

H2+ CO2 CH4+ 2 H2O , G0= -135,6 kJ/mol (2.10)

This hydrogen-consuming reaction is important for the whole process because of

hydrogen is an inhibiting substance of reactions of the acetic phase.

Secondly, methane is produced from acetic acid, e.g. by Methanotrix:

CH3COOH CH4+ CO2, G0= -31 kJ/mol (2.11)

Normally, in biogas-reactors methane is mainly produced via acetic acid, but

hydrogen plays an important role in the regulation of these processes.

The steps of biogas production are illustrated in fig. 8. Fig. 9 shows the interaction between

organisms of the acetic phase and the methanogenesis.


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Fig. 8 and 9: the four steps of methane production and the role of hydrogen in

methanogenesis (Fritsche 1990)

The very low values of free energy obtained from reactions (eq. 2.10 and 2.11) are the

reason for the very low growth rate of methane producing bacteria.

It is possible to use the steps one to three including fermentation to produce hydrogen from

organic substances like organic waste biologically (eq. 2.9) . In spite of only a small portion of

the substrate is converted to hydrogen some research is done in this respect (Fischer 2009).

3 Kinetics of growth and metabolic processes

3.1 Cell division and growth

If there is enough substrate available bacteria grow by means of cell division. Under

conditions where substrates are limited very much, some bacteria produce spores instead of

the cell division process. Spores have no metabolism and are very resistant against extreme

conditions like temperatures, pressures, radiation, drought and chemical disinfection and

they are able to outlive decades. under good environmental conditions spores are able toswitch back to normal metabolism and cell division.


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If there is no limitation of substrates under good environmental conditions bacteria double

their quantity and their mass by producing a new cell generation by cell division within a

characteristic period, the generation time. Fast growing bacteria have a generation time of

down to 15 or 20 minutes. Due to this duplication within a certain period cells grow

exponentially under this conditions.

Therefore the quantity of cells N after n generations starting with a quantity of N0 can be

calculated to

N = N02n (3.1)

or

(3.2)

respectively

(3.3)

If a balance of the cell quantity for a batch reactor under exponential growth is done the

maximum growth rate maxis normally used to describe the process.

Based on the general form of a balance

Ac = Tr + Re (3.4)

with Ac: Accumulation

Tr: Transport across the system boundaries

Re: Reaction of the balanced material

the balance of the quantity of cells for the system described is:

dN/dt = maxN (3.5)

After integration and use of the initial condition N (t=0)= N0follows:

ln(N/N0) = maxt (3.6)

With (eq 3.3) follows:

t = n ln2 / maxt (3.7)

The generation time or duplication time tDfollows for one generation or n=1:

tD= ln2 / maxt (3.8)

n

N

N 20

=

)2ln(ln0

nN

N=


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From eq. (3.6) can be derived the exponential character of unlimited bacterial growth:

N/N0= exp(maxt) (3.9)

Example:

Many aerobic heterotrophic bacteria have a generation time of approximately 30 minutes.

From eq. (3.8) the maximum growth rate can be derived as:

max= ln2 / 0,5 h = 1,4 h-1 (3.10)

How fast a lack of substrate is a limitation for growth shows the following calculation: If a

bacteria has the size of a cube with the dimension of 1 m its volume is 10 -18m3. If a density

like water is assumed (106g/m3) the bacteria has a mass of 10 -12g.

After 67 hours of exponential growth the bacterial mass can be calculated from eq. (3.9), set

up for the mass m instead of the quantity N to:

m = 10-12exp(1,4 *67) = 2,18 * 1028g (3.11)

Compared with the earth-mass of 6 * 1027g more than three times of the earth mass would

have been grown.

Due to the limited amount of substrates and nutrients the growth of bacteria in a closed

system as a batch-reactor consists of four phases. During the lag-phase the cell adapts its

enzymes to available substrates without growth (s. fig. 3.1).

Fig. 3.1: Phases of bacterial growth (Fritsche 1990)


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Then exponential growth takes place as long as substrate is not limiting. Under substrate

limitation bacteria growth slowlier use more internal substances for their metabolism and

more bacteria die. That results during this stationary phase in a approximately constant

biomass concentration. After all substrate is consumed the quantity and mass of bacteria is

reduced during the decay phase due to the death of bacteria and the metabolism of all

remaining internal substrates.

3.2 Influence of Temperature

If for a chemical reaction of the substrates A and B result in a product C

A + B -> C

a pseudo-first-order reaction rate can be defined as

r = kAcA (3.12)

with

kA: rate-coefficient

cA: concentration of substrate A

the rate-coefficient depends on temperature often according to the Arrhenius-equation:

(3.13)

with

k0 rate coefficient at reference temperature

eAactivation energy

R universal gas constant

T Kelvin temperature

Biological reaction are enzyme-catalysed reactions. Enzymes are proteins which catalyses

reactions in a certain range of temperatures. Regarding theses ranges bacteria can be

subdivided into mesophilic bacteria with a temperature range between 5 and 45C and a

range for optimal growth between 35 and 40C. THermophilic bacteria grow between 40 and

85C with an optimum between 0 and 60C. There also other species as e.g. psychrophilic

)exp(0T

ekk

A

=


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bacteria (optimum at app. 5 to 10C) and extreme thermophilic bacteria (optimum at app. 90

to 100C).

Example:

The influence of temperature can be calculated by using eq. (3.13). The relation of reaction

rates for two temperatures k1and k2can be derived as follows:

(3.14)

with

eA= 50 KJ/mol (Block 1992) and

R= 8,314 KJ/(kmol K) follows for usual temperatures 10, 20, and 30 C:

k30/20= 1,97 and

k10/20= 0,48.

It can be seen that a temperature increase or decrease of 10 C within the temperaturerange of mesophilic bacteria results in twice respectively half of the reaction rate.

3.3 Influence of pH-value

All metabolic processes in cells are controlled by enzymes that consist of amino acids.

Amino acids contain different pH-sensitive side chains and therefore the activity of enzymes

depends on the pH-Value. The pH-Value inside cells is often regulated, but membrane

proteins and exoenzymes depend directly on the conditions of the surrounding medium. All

organisms produce enzymes for a certain range of pH-Value and are not able to grow below

or above a specific pH-limit.

Bacteria that produce e.g. lactic acid are able to grow at low pH-values down to pH4. Many

other bacteria cannot grow under this conditions that makes this process useful for

conservation of food products like some cheese, sour cucumbers, or sauerkraut.

In Table 3.1 some examples for bacteria are mentioned which grow at different optimal pH-

values.

=

21

12

1

2)(

expTT

TTe

k

kA


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pH-value Medium Organism

1 volcanic wells Thermoplasma acidophilum

2 citric acid, mine water Aspergillus, Thiobacillus

3 vinegar Acetobacter

4 sauerkraut lactic acid producing bacteria

5 cheese, sourdough lactic acid producing bacteria, yeasts

6 to 8 fresh water, sea water many micro organisms

9 alkaline soil, dung Nitrosomonas, Nitrobacter, urea degrading

bacteria, e.g. Bacillus pasteurii

10 fermented indigo leaves Bacillus alcalophilus

Tab. 3.1: pH-values, media, and micro organisms able to grow under these conditions

(Fritsche 1990, translated)

Additionally, the pH-value influences the dissociation of dissolved substances which are

often substrates for bacteria. The species Nitrosomonas for example oxidises ammonia to

nitrite. It could be shown (Dombrowski 1991) that only neutral molecule ammonia (NH3) can

be used, but not the ion ammonium (NH4+) which is the product of the dissociation of

ammonia in water.

Fig. 3.2 shows the strong influence of pH-value of the concentration of the substrate

ammonia.

In biogas-reactors there is a specific risk, if bacteria producing organic acids are faster than

those consuming acids with the consequence of decreasing pH-values. At low pH-values

methane producing and acid consuming bacteria are not able any more to grow which results

in even lower pH-values to a level where acid producing bacteria stop their metabolism also.

Then no biological reaction takes place any more in the reactor which is a main accident for

a biogas plant.


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pH-value

Fig. 3.2: Influence of temperature and pH-value on the ammonia-ammonium-equilibrium

(Dombrowski 1991)

3.4 Growth kinetics

3.4.1 Michaelis-Menten kinetic

In 1913 in Berlin Leonor Michaelis and Maud Menten set up a theory of enzyme catalysed

reactions that allows to calculate quantitatively the influence of the substrate concentration

on reaction rates.

Michealis and Menten assumed an equilibrium reaction between the substrate S and an

enzyme E with an enzyme-substrate-complex ES and a complete reaction from ES to the

product P and enzyme E:

(3.15)

Further on it was assumed a first order reaction for k2and k3and a second order reaction for

k1. Then the reaction rate r can be defined as

(3.16)

with concentrations in [ ]-brackets and calculated to

PEESSE kkk

+ + 321,

dt

Pdr

][=


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(3.17)

Further on for the ES-complex can be written:

(3.18)

For steady-state-conditions for the ES-complex it follows:

(3.19)

The unknown concentration [ES] can be expressed by using the total enzyme concentration

[E0]:

(3.20)

Putting (3.20) and (3.19) into (3.18) results in

(3.21)

using the Michaelis-Menten-constant kMas

(3.22)

Putting (3.21) and (3.17) into (3.16) results in:

(3.23)

If all enzyme E0is active as ES-complex the maximum production rate (see 3.17, 3.16) can

be reached:

(3.24)

Putting (3.24) into (3.23) results in the Michaelis-Menton-kinetic:

(3.25)

][][

3 ESkdt

Pd=

])[(]][[][

321 ESkkSEkdt

ESd+=

0][

=dt

ESd

][][][ 0 EEES =

][

]][[][ 0

Sk

SEES

M +

=

1

32

k

kkk

M

+=

][

][][3

Sk

SEkr

M

o+

=

][3max oEkr =

][

][max

Sk

Srr

M +

=


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Fig. 3.3: Michaelis-Menten-kinetic with kM= 50

Fig. 3.3 shows the meaning of the Michaelis-Menton-constant as that concentration where

the reaction rate reaches 50% of the maximum reaction rate. The constant indicates the

range of very low substrate concentration and indicates a range of very low concentrations

where a first-order-kinetic fits approximately to the Michaelis-Menten-kinetic. There thereaction rate is limited due to a lack of substrate (substrate limitation). At very high

concentrations a zero-order-kinetic describes the Michaelis-Menten-kinetic roughly.

3.4.2 Inhibitions

3.4.2.1 Substrate inhibition

At a high substrate concentration level sometimes the enzyme-substrate-complex ES (s. eq.3.15) reacts additionally with another substrate molecule to another complex ES2.

(3.26)

Because of the portion of enzyme bound in ES2 is not available for producing product the

production rate of the product decreases with increasing substrate concentration. A kinetic

modelling the whole reaction results in the substrate reaction rate rS:

(3.27)

0,000

0,250

0,500

0,750

1,000

0 1 2 3 4 5 6 7 8 9 10 11 12

[S] / kM

v/vma

first-order reaction

zero-order reaction

2ESSES +

H

S

SM

S

SS

k

cck

crr

2max,

++

=


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This kinetic formulation is also named Haldane-kinetic. It is to see that the reaction rate r sis

always lower than rs,max.

Under the condition

cs>> kM

and cs= kH

eq. (3.27) approximately results in

(3.28)

Fig. 3.4: Haldane kinetic (kM=50, kH=500)

An well known application is the conservation of marmalade by adding glucose up to high

concentrations in order to avoid glucose degrading bacteria and yeasts.

3.4.2.2 Other inhibitions

There are several other types of inhibitions. Two of them are the competitive inhibition and

the non-competitive inhibition. Inhibitors are often heavy metal ions, toxic organic

substances, or the product of a biochemical reaction like alcohol.

2

max,S

S

rr =

0,000

0,250

0,500

0,750

1,000

0 4 8 12 16 20

[S] / kM

r/rmax No Inhibition

Substrate Inhibition


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A model of competitive inhibition consists of a further reaction to eq. (3.15) where the

inhibitor I reacts with free enzyme E to another complex EI. Hence there is a competition

between reaction E with S and E with I.

Fig. 3.5: competitive product inhibition with variation of product concentration cP(Kus 1993)

With increasing inhibitor concentration the reaction seems to increase their Michaelis-

Menten-coefficient, but the maximum reaction rate stays constant and can be reached if the

substrate concentration is high enough.

On the other hand the non-competitive inhibition is modelled by adding a reaction to eq.

(3.15) where the inhibitor reacts with the ES-complex to an ESI-complex. Therefore the

Haldane kinetic can be understood as a special form of non-competitive inhibition with the

substrate as inhibitor.

Fig 3.6 shows a non-competitive product inhibition with a variation of the product

concentration which seems to result in a decreasing maximum reaction rate at constant

Michaelis-Menten-coefficient.


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Fig. 3.6: Non-competitive product inhibition with variation of product concentration cP

(Kus 1993)

3.4.3 Monod-kinetic

In 1942, the French scientist Jaques Lucien Monod (1910 1976, Nobel price in 1965) did

investigations of the growth of Escherichia coliwith glucose. He found out that the growth of

these bacteria can be described very well with the kinetic of Michaelis and Menten in spite of

the fact that bacterial growth is much more complex than the simple enzyme reaction of

Michaelis and Menten. But because of that mathematical formulation fits very well to the

experimental results of many investigations with different bacteria it is well known and widely

used for many bacteria (by far not for all) as Monod-kinetic for bacterial growth under normal

growth-conditions without inhibition and without bacterial decay.

The normal formulation is shown in eq. (3.29):

(3.29)

with

m

: specific growth rate

SS

S

ck

c

+= max


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kS: Monod-constant or saturation constant

The growth rate of bacteria rBthen can be expressed as:

(3.30)

or with eq. (3.29):

(3.31)

During exponential growth often the relation between growth rate and the rate of substrate

degradation or product production is a constant value which are named yield-coefficients,

e.g.:

(3.32)

with

YB/S: Yield-coefficient between bacterial growth and substrate degradation.

(3.33)

with

YB/P: Yield-coefficient between bacterial growth and product production

(3.34)

with

YS/P: Yield-coefficient between substrate degradation and product production.

Yield coefficients are constants under conditions of exponential growth. During this growth

bacteria do a metabolism specialised to maximum growth. Under other conditions, e.g.

substrate limitation or inhibition many bacteria are able to change and adapt their metabolism

which results in other values of yield coefficients. Therefore yield coefficients from literature

has to be checked seriously whether they were obtained under appropriate conditions for

own purposes.

BB cr =

B

SS

S

B c

ck

cr

+= max

S

B

SB

r

rY =/

P

B

PB

r

rY =/

P

S

PS

r

rY =/


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4 Bioreactor design

Most of anaerobic bioreactors are designed as a so called chemostat reactor or CSTR

(continuously stirred tank reactor), which is a completely mixed reactor (fig. 4.1).

Fig. 4.1: Chemostat reactor

If steady state conditions are assumed and if there are no bacteria in the influent a mass

balance for bacteria results in the following formulation:

(4.1)

with

VR: reactor volume

With the definition of the dilution rate D and the hydraulic residence time tV

(4.2)

follows from eq. (4.1):

(4.3)

Using the Monod formulation eq. (3.29) eq. (4.3) results in:

(4.4)

V0

cB,a

.

V0

cB,a

.

RaBaB VccV +=

,,00

RV V

V

tD

==01

D=

1max,

=

D

Kc

S

aS


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Fig. 4.2: Results of eq. 4.4 as well as bacteria concentration

(with SS: effluent substrate concentration, XS: effluent biomass concentration,

S0: influent substrate concentration

max= 1,0 h-1, kS= 0,005 g/L, YX/S= 0,5)

It is seen that for =D no biomass any more is in the reactor and no substrate degradationtakes place. At this wash-out-point the transport of bacteria out of the reactor is equal to the

maximum growth rate of bacteria. A slight increase of the dilution rate increases the transport

term resulting of a loss of all bacteria after steady state conditions are met again.

Therefore for a stable operation under real variations of operating conditions it is necessary

to set a dilution rate sufficient lower than the critical wash-out-dilution-rate.


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5 Applications

5.1 General process designOne stage CSTR-types of reactors are widely used for decades for anaerobic treatment of

sewage sludge. A typical egg-shaped reactor is shown in fig. 5.1. Egg-shaped reactors were

developed to reduce the risk of sand sedimentation on the bottom and foam formation on the

top. But other more cylindrical reactors are used as well.

Fig. 5.1: Mesophilic anaerobic sewage sludge treatment on the waste water treatment plantBochum-lbachtal: Volume 2 x 8720 m3, 3 x 770 KW cogeneration plant (Oswald-Schulze)

If the content of dry matter is chosen in the range of 15 to 50% processes are sometimes

called dry fermentation (s. fig. 5.2) which need other transport and mixing devices. Those

systems are also offered as container-based fermentation systems where in particular

attention should be given on the heat loss across the container surface.

Fig. 5.2: So-called dry fermentation system with plug-flow-behaviour (Linde, BRV-process)


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Fig. 5.3: Digester with CSTR- or plug-flow-behaviour for substrates with low solid content,

e.g. waste water (Schwarting-Uhde)

By adjusting a recirculation pump the behaviour of the Schwarting-Uhde reactor can be

chosen as CSTR (high recirculation) or as plug-flow-reactor (no circulation) (s. fig. 5.3).

Two stage systems are sometimes used to reach high performance systems by adjusting the

conditions in stage 1 for hydrolysis and acidification (pH 5 to 6,5) and in stage 2 for

acetogenesis and methanogenesis (pH 7 to 7,5) (Kaltschmitt 2001). This can be done with all

the material in two separate e.g. CSTR-systems or solid and liquid phases are separated in

between, like in the Wherle-system (s. fig. 5.4).

Other systems are available with low investment costs for discontinuous operation (batch

systems) (s. fig. 5.5).

An overview of various designs of anaerobic reactors is shown in Fig. 5.6.


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Fig. 5.4: Two-phase and two-stage fermentation (Wherle)

Fig. 5.5: Batch system for discontinuous use without mixing (Aschmann in Biogas 2007)


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Fig. 5.6: Overview of various anaerobic continuously operated reactor systems (Kaltschmitt

2001)

5.2 Gas production

5.2.1 Gas quality

The gas quality depends mainly on the substrate composition. The theoretical portion of

methane and carbon-dioxide for pure known substrates can be calculated stoichiometrically

(Biogas 2007):

(5.1)

Results of this calculation related to real, but pure substrates are shown in fig. 5.7. The

fermentation of real mixed substrates results often in a methane content of 50 to 60%.

42248248224

CHbau

CObau

OHba

uOHCbau

++

+

+


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Fig. 5.7: Theoretical gas composition of pure substrates (Kaltschmitt 2001)

Besides the main components methane and CO2 there other components present in low

concentrations (s. tab. 5.1). In particular hydrogen sulphide is important as a hazardous

component and as a substance that reacts to sulphur dioxide in incineration processes and

which results therefore in air pollution.

Tab. 5.1: Components of biogas (Kaltschmitt 2001)


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5.2.2 Gas yield

Also the amount of gas that can be produced from substrates can be calculated

stoichiometrically. Tab. 5.2 shows some figures related to the input mass of dry matter(Trockenmasse, TM)

Tab. 5.2: Gas yield of various substances (Biogas 2007)

In literature there are many data of gas yields for real substrates available that are normally

measured at laboratory conditions and long duration of the fermentation. Therefore in real

plants the gas yield under less optimum conditions sometimes might be lower than expected.

The following tables 5.3 to 5.7 show some gas yields and other data of real substrates. In

these tables mean

TM: dry matter (Trockenmasse), FM: wet matter (Feuchtmasse)

oTM organic dry matter

Tab. 5.3: Data of some manure substrates (Biogas 2007)

Tab. 5.4: Data of some agricultural products (Biogas 2007)


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Tab: 5.5: Data of same waste material (Biogas 2007)

Tab. 5.6: Data of some food waste material (Biogas 2007)

The gas yield of unknown substrates has to be investigated in laboratory tests.

5.2.3 Reaction rates

There are many results available regarding one of the specific steps of anaerobic

degradation that are necessary for models and simulation. Due to the limited duration of this

lecture only some examples and general results can be discussed here.

Because of hydrolysis often is the slowest step of the degradation chain, anaerobic digestion

of solid material. On the other hand in solid material often organic substances are much

more concentrated than in liquid material.

Fig. 5.8 shows the influence of hydraulic residence time tVon the COD-conversion a for a

fixed bed bioreactor in order to achieve a high bacteria concentration (using plastic foam

cubes of 0,7 and 1,0 cm ). A highly concentrated wastewater from the treatment of sewage

sludge was used as substrate containing approximately 30% of COD as not degradable

under this conditions.


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Fig. 5.8: COD conversion rate depending on hydraulic residence time for highly concentrated

wastewater (Spie 1991)

Fig. 5.9: Methane production rate depending on hydraulic residence time (Spie 1991)


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For the same investigations fig. 5.9 shows the methane production rate which could be

achieved. The results show the typical behaviour of a pseudo first-order reaction at constant

biomass concentration. At low residence times that means operating the reactor at high loadconditions the specific reaction rate per reactor volume reaches highest values. But only a

small portion of the input material is converted to biogas at low residence times. Therefore

often lower specific production rates are chosen in order to achieve a better use of the

substrate which means a higher conversion. In general an economically design results in an

optimum operation point of a certain conversion rate at as high as possible reaction rates

under this specific conditions.

If reactors are operated at production rates high concentrations are present in the reactor

which might result in inhibition effects. Fig. 5.10 shows the inhibition of the degradation of

propionic acid by high concentrations of propionic acid. Due to other investigations it is

known that the undissociated acid is used as substrate by bacteria. Therefore the substrate

concentration in fig. 5.10 is expressed as undissociated propionic acid.

Fig. 5.10: Conversion rate of dissolved organic matter depending the concentration of

undissociated propionic acid (Kus 1993)


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Due to slow hydrolysis residence times for the conversion of solid material are considerably

longer. Results from the digestion of sewage sludge show a conversion of up to 50% of

organic matter at residence times of 20 days in CSTR-reactors (s. fig. 5.11).

Fig. 5.11: Conversion of organic matter from sewage sludge by anaerobic digestion at

mesophilic conditions ( Pfeiffer 1989).

Depending on substrate quality sometimes far higher residence times are necessary in order

to achieve a high conversion.

Fig. 5.12 shows lab-scale results of anaerobic digestion of organic waste after aerobic pre-

treatment. Black squares indicate results under controlled temperature with a high

conversion after 40 days. Even if treatment periods from batch tests cannot be interpreted as

residence times under continuous operation, results show that far longer residence times

than 20 days are necessary to convert a portion of input material to biogas. In technical

applications hydraulic residence times of approximately 60 days are already realized.


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Fig. 5.12: Gas yield of organic waste (Linke 1999)

If inhibition play a role in an individual application growth rates of bacteria and therefore the

production rate of biogas is lower than without inhibitions. That might result in higher

residence times necessary to achieve a high conversion of the input material.

Kaltschmitt reported results (Kaltschmitt 2001) of the inhibition by sodium at concentrations

of some g/L in some cases. In other cases bacteria were able to adapt to concentrations up

to 60 g/L without inhibition effects.

It is known that hydrogen as an intermediate product inhibits the degradation of organic acids

at concentrations of some mol/L.

Heavy metals are known also as inhibitor if they are not precipitated by sulphide. Kaltschmitt

reports inhibition effects at some 10 mg/L depending on the type of metal.

Also ammonia (not ammonium!)is known as inhibitor. Due to the dissociation equilibrium

between ammonium and ammonia the pH-value influences inhibition effects very much.

Kaltschmitt reports an inhibition of ammonia at concentration of 150 mg/L.


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5.2.4 Example: Digestion of slaughterhouse waste

Fig. 5.13 shows a simplified example of anaerobic digestion of slaughterhouse waste. Gas

yield, conversion, nutrient concentration, and other parameters depend on the mixture ofdifferent slaughterhouse wastes like flotation waste (high fat content), blood waste (high

protein and nitrogen content), or stomach contents (other organic materials) in any individual

case.

In this example a high conversion of 85% is assumed due to the quality of raw material and a

residence time of approximately 40 days. Depending on the water content of the input

material and pH-value high ammonia concentration might occur. Therefore attention should

be paid to inhibition of the process by ammonia. If the output material should be used as

fertilizer also attention has to be paid to a limitation of nutrients applied on agricultural areas

in order to prevent ground water contamination by e.g. nitrate.

Fig. 5.12: Energy and nutrient balance of anaerobic digestion of slaughterhouse waste

Further aspects that were not mentioned yet are biogas desulphurisation, off-gas cleaning,

gas storage, output storage, excess-heat utilization, the conversion of biogas to bio-methane,

or a post-treatment of output material solid-liquid separation, composting of solids, further

processing of the liquid phase, or nutrient separation from output material.

Biogasreactor

gasengine

Input:

2800 Mg/a dry matter (slaughterhouse waste)

124 Mg/a N

17 Mg/a P

9 Mg/a K

Ouput:

400 Mg/a dry matter

124 Mg/a N

17 Mg/a P

9 Mg/a K

Biogas

5000 m3/d

60% Methane

6 KWh/m3:

Energy: 1250 KW

Electricity: 500 KW

(40 % efficiency):

Heat: 537 KW

(43% efficiency)

Off-gas


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Literature

Biogas 2007 Biogashandbuch Bayern, Materialienband, Juli 2007

Block 1992 Block,J.: Untersuchungen zur Kinetik der Abwasserreinigung durch ae-

rob thermophile Bakterien. In: VDI (Hrsg.): VDI-Fortschrittsberichte Rei-

he 15, Nr. 98, VDI-Verlag Dsseldorf 1992

Dombrowski 1991 Dombrowski, T.: Kinetik der Nitrifikation und Reaktionstechnik der Stick-

stoffelimination aus hochbelasteten Abwssern. In: VDI (Hrsg.): VDI-Fortschrittsberichte Reihe 15, Nr. 87, VDI-Verlag Dsseldorf 1991

Fischer 2009 Fischer, L.: Wasserstoff aus Biomll. Spektrum der Wissenschaft, Juni

2009, p.14.

Fritsche 1990 Fritsche, W.: Mikrobiologie, Gustav-Fischer-Verlag, Jena 1990

Kaltschmitt 2001 Kaltschmitt, M., Hartmann, H. (Hrsg.): Energie aus Biomasse. Springer

Verlag Berlin Heidelberg New York, 2001

Kus 1993 Kus, F.: Kinetik des anaeroben Abbaus von Essig- und Propionsure in

Bioreaktoren mit immobilisierten Bakterien. In: VDI (Hrsg.): VDI-

Fortschrittsberichte Reihe 15, Nr. 115, VDI-Verlag Dsseldorf 1993

Linke 1999 Linke, B, Schelle, H.: Verfahren zur Biomethanisierung von Feststoffen.

in: Frdergesellschaft erneuerbare Energien e.V. (HRSG.): Stoffliche

und energetische Nutzung von Biomasse im Land Brandenburg. Tagung

im Mai 1999 in Paaren, Berlin 1999

Pfeiffer 1989 Pfeiffer, W.: Verfahrensvarianten der Faulung und Entseuchung Leis-

tungsvergleich. Dissertation an der TU Mnchen 1989, in: Roediger, M.:

Bemessungsvorschlag fr Schlammfaulungsanlagen. Korrespondenz

Abwasser 1997 (44), S.1856

Schgerl 1985 Schgerl, K.: Bioreaktionstechnik Band 1. Otto-Salle-Verlag Frankfurt,

Berlin, Mnchen 1985, S.132


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Spie 1991 Spie, A.: Anaerobe Abwasserreinigung in neuen Bioreaktore mit Polyu-

rethan-Schaumstoffpartikeln zur Immobilisierung der Bakterien. In: VDI

(Hrsg.): VDI-Fortschrittsberichte Reihe 15, Nr. 85, VDI-Verlag Dsseldorf1991

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