02 leaflet biogas basics eng

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Biogas Basic principles of fermentation

Table of contents

2 | C o n t e n t s | 3

1. Introduction........................................................................ 4

2. Basic principles of fermentation........................................ 6

2.1 Composition and quality of biogas ................................... 8

2.2 Generation of biogas ....................................................... 10

2.2.1 Hydrolysis ................................................................ 11

2.2.2 Acidification.............................................................. 14

2.2.3 Generation of acetic acid .......................................... 14

2.2.4 Generation of methane............................................. 18

2.3 Generation times of the bacteria ...................................... 20

3. Creating the right environment for the bacteria ................. 24

3.1 Temperature ................................................................... 25

3.2 pH value and buffer capacity ........................................... 26

3.3 Supplying the bacteria with nutrients ................................ 28

3.4 Impurities and inhibitors................................................... 29

4. Process monitoring ............................................................ 32 4.1 Determination of the buffer capacity using the

‘VOA/TAC’ method ........................................................ 32

4.2 Instructions for performing the VOA/TAC analysis ............ 33

4.2.1 Preparation: ............................................................. 36

4.2.2 Execution: ................................................................ 36

4.2.3 Calculation of the VOA/TAC value ............................ 38

5. Summary............................................................................. 39

Methane molecule

C

1. Introduction

Besides the daily checking and feeding of the reactor, the

prerequisite for the successful operation of a biogas reactor is an

exact understanding of the biological processes and coherences

taking place inside the fermenter. The generation of biogas, i.e. the

fermentation of biogenic substrates, is an extremely complex

process. Numerous different types of bacteria, whose metabolisms

have not yet been fully researched, are involved in the process. The

environmental conditions and nutrients required by the various

bacteria are in part widely different. The reactor operator must

satisfy these requirements as well as he can in order to achieve a high

gas yield and, hence, ultimately economic success. The better he is

acquainted with the microbiological processes and the more

purposefully he can react to impairments, the more efficiently he

will be able to manage his biogas reactor. The ‘management’

factor is therefore accorded very great importance in the biogas

sector.

The intention of this brochure is to examine more closely the

biological sequences within the process of the fermentation of

organic substances, as they occur in agricultural biogas reactors.

First of all, the composition of the biogas and the various phases

of its formation are described. This is followed by a description of

the measurement variables that are crucial to the fermentation

process and their importance to the bacterial flora in the fermenter.

4 |Introduction

The consideration of inhibitors, which can negatively influence

the biogas process, deepens the insight into the ‘black box’,

i.e. the fermenter, and provides information for the creation and

maintenance of an optimum environment for the bacteria.

Finally, the possibilities to check and control the process are

presented. The determination of the so-called ‘VOA-TAC’ value,

which is considered to be an important measurement variable in

the daily operation of a reactor, is examined in detail here.

There are a great many factors that influence the biogas process.

No reactor is completely comparable to another and each operator

is individual. This brochure is intended to provide suggestions for

the encouragement of optimised process control and a better

understanding of the processes taking place in the biogas reactor. .

Introduction | 5

II. Basic principles of fermentation

The generation of biogas is a process that is significantly older

than the idea of exploiting this natural product. Biogas has been

generated for millions of years in decomposition processes that chiefly

occur in swamps, lakes and pools under the exclusion of oxygen.

The bacteria involved in this process are some of the oldest living

creatures in the world and have had to adapt themselves

outstandingly well to the widest variety of conditions over long

periods of time. They are capable of breaking down organic

substances into their individual parts and ultimately of converting them

into combustible methane that can be used for generating electricity in

combined heating and power plants or, after appropriate treatment, for

direct injection into gas networks.

The energy released in the combustion process thereby

originates from the sun, which similarly stores its ‘power’

intermediately in the fermentable energy plants. As opposed to

the burning of fossil fuels such as coal, oil or natural gas, the use of

biogas is CO2-neutral, since the carbon dioxide generated by the

process circulates in a natural cycle and is consumed by the plants

in the course of photosynthesis as they grow again. Furthermore,

the putrefied fermentation residues can be used as a high quality

fertiliser for the energy plants. Biogas is therefore a renewable energy

source, which completely satisfies the requirements for the protection

of the climate.

Fig. 1: The biogas cycle

6 | Basic principles of fermentation Basic principles of fermentation | 7

Source: original illustration

Solar energy

Energy plants Biogas reactor

CO2 + fertiliser

Substrate

Heat & electricity


2.1 Composition and quality of biogas

Biogas is generated by the anaerobic degradation of organic

substances, i.e. in the absence of oxygen. In principle, any

organic material is suitable for the production of biogas, but not

all constituents are degradable to the same degree by the

bacterial strains. For example, strongly lignified plants can only be

degraded very slowly due to their high content of stored lignin and

are therefore less suitable for economical fermentation in biogas

reactors. The lignin proportion of the substrate increases as the

energy plants become riper, making timely harvesting and

preservation necessary. The target variable of the fermentation

process is combustible methane, whose proportion of the biogas

fluctuates between 50 and 75 % depending on the substrate.

However, methane contents of the order of 75 % can barely be

achieved with renewable raw materials; the addition of a suitable

co-substrate is necessary for this. A methane content of < 50 %

leads to problems in the combustion of the biogas in the combined

heating and power plant, since the correct functioning of the engine

is no longer ensured in the case of such low methane contents.

Carbon dioxide is the second main constituent of the biogas,

besides methane, and represents 25 – 50 % of the gas mixture.

Various trace gases can also be determined. Table 1 provides a

summary of the composition of biogas.

Table 1: Composition of biogas

Constituent Formula Concentration

Methane CH4 50 – 75 %

Carbon dioxide CO2 25 – 50 %

Water H2O 2 – 7 %

Hydrogen sulphide H2S approx. 2 %

Nitrogen N2 < 2 %

Hydrogen H2 < 1 % Source: Top Agrar biogas reference book, 2002

The quality of the biogas is determined first and foremost by

the ratio of combustible methane to non-combustible carbon

dioxide. The carbon dioxide has a ‘diluting’ effect and causes

additional costs, above all with regard to the storage of the gas.

The highest possible methane content must therefore be aimed for.

The methane content of the biogas is directly influenced by the

following factors:

1. Nutrient composition of the substrate

2. Process control

3. Temperature

Besides methane and carbon dioxide, hydrogen sulphide,

nitrogen (elementary and in the form of ammonia (NH3)) and

water are constituents of the biogas mixture. Particular attention

must be paid to the hydrogen sulphide content, since this gas can

cause damage to gas pipelines and the combined heating and

power plant due to its corrosive properties. It is therefore

recommended to install facilities for desulphurisation of the gas.

Furthermore, the quality of the gas can be increased by removing

water by means of condensation. A large part of the similarly

damaging ammonia can also be removed in this manner.

2.2 Generation of biogas

The process of the generation of biogas can basically be

divided into four consecutively occurring sub-stages.


These are: hydrolysis, acidification (acidogenesis), the

formation of acetic acid (acetogenesis) and finally the

formation of methane (methanogenesis). Different groups of

bacteria, which are strongly dependent on one another for their

action, are involved in the respective stages of the transformation of

the organic material. Of vital importance here is the fact that the

bacterial strains differ from one another with respect to their ideal

living conditions. The individual process steps will be explained

below. Figure 2 shows the phases of the process of biogas

formation.

Fig. 2: Phases of the process of biogas formation

Source: own illustration

2.2.1 Hydrolysis

The organic substance that is brought into the biogas reactor is in

the form of undissolved compounds with a high molecular weight.

In the first step, hydrolysis, these macromolecular compounds

weight must be broken down into their individual constituents so

that they can be degraded by the bacteria in the subsequent

process steps. During hydrolysis, these ‘large building blocks’ of the

substrate, namely carbohydrates, proteins and fats, are broken

down biochemically into compounds of low molecular weight. Hence

carbohydrates become monosaccharides, proteins become amino

acids and fats become fatty acids. This is performed by the

aforementioned hydrolytic bacteria, which excrete special enzymes.

These are capable of attacking large macromolecules and

splitting them into small, water-soluble molecules. Not all

ingredients of the input substrate can be hydrolysed as well or as

quickly. Hence celluloses, as significant constituents of vegetable

Formation of acetic acid (acetogenesis)

Phase 4 Formation of methane (methanogenesisis)


cell walls, and starches are decomposed only relatively slowly. In

contrast, hemicelluloses and sugars allow very rapid

transformation. Lignin, which is chiefly stored in strongly lignified

plant parts, is not degradable by hydrolysis. In general, it can be

said that the speed of hydrolysis reduces in the order: sugars,

hemicelluloses, fats, celluloses and proteins.

Hydrolysis represents the step that determines the speed of

the biogas process. A slowly proceeding hydrolysis therefore

results in correspondingly slow subsequent process phases.

Accordingly, it is important for the substrate used to be easily

hydrolysable; this is referred to as the bioavailability of the

substrate. Figure 3 illustrates the course of the hydrolysis with

the bacteria involved and the enzymes produced.

Fig. 3: Course of the hydrolysis

Macromolecules

Carbohydrates, celluloses Proteins Fats

Clostridium

spp.

Bascillus spp.

Pseudomonas

spp.

Bacteriodes

spp.

Cellulases,

amylases

Proteases

Lipases

Monosaccharides Amino acids Long-chain fatty acids

Short-chain peptides Bacteria

Enzymes

Glycerine



2.2.2 Acidification

The hydrolysis products are now further decomposed in the

following acidification phase, the acidogenesis. Here, the low-

weight molecular compounds are taken into the interior of the cells

of the bacteria that have already partly been involved in the

hydrolysis. Further decomposition takes place there, mainly to

propionic acid, butyric acid, valeric acid and lactic acid. Alcohols,

aldehydes, acetic acid, formic acid, hydrogen and carbon dioxide

are additionally generated. Of importance here is that the

bacteria consume the remaining oxygen in transforming the

hydrolysis fragments, hence creating the anaerobic, i.e. oxygen-

free environment for the generation of methane. Fig. 4 provides

an overview of the acidogenesis.

Fig. 4: Course of the acidogenesis

Monosaccharides Amino acids Long-chain fatty acids

Short-chain peptides

Glycerine

Clostridium spp. Acetivibrio spps. Bacteroides spp. Butyrivibrio spp.

Acetic acid Aldehydes Alcohols Ammonia Hydrogen sulphide

Butyric acid Formic acid Carbon dioxide Hydrogen Other carboxylic acids


2.2.3 Generation of acetic acid

The substances generated in the preceding acidification phase

are now converted to acetic acid, hydrogen and carbon dioxide

in the acetogenesis process step. The most important raw

materials here are propionic acid, valeric acid, butyric acid and

formic acid. The acetogenic phase is strongly connected with

the subsequently described generation of methane.

The hydrogen produced during the acetic acid generation would in fact inhibit a large proportion of the bacteria if it were not consumed instantaneously by the methane bacteria for the methanogenesis. Furthermore, the bacteria that generate the acetic acid require energy, which is released during the course of the generation of methane (see fig. 6).

The two most important reactions in the acetogenic phase are

shown in short form below, namely the formation of acetic acid from

butyric acid and from propionic acid. The acetic acid resulting

from this reaction is the raw material for the subsequent

methane generation.


Fig. 3: Formation of acetic acid from butyric acid

Butyric Acid + Water ______________________ Acetic Acid + Hydrogen

CH3CH2CH2COO + 2H2O 2CH3COOH + 2H2


Fig. 4: Formation of acetic acid from propionic acid

Propionic Acid + Water Acetic Acid + Hydrogen + Carbon Dioxide

CH3CH2COOH + 2H2O CH3COOH + 3H2 + CO2


The formation of the acetic acid can be illustrated schematically

as follows:

Fig. 5: Course of the acetic acid formation phase


2.2.4 Generation of methane

The methane generation phase is the last step in the biogas

generation process. Methane is produced strictly anaerobically

by the corresponding bacteria. The presence of oxygen would

inhibit or even kill the methanogenic bacteria. All species of

methanogenic bacteria are able to transform carbon dioxide and

several can transform hydrogen, but only a few can transform

acetic acid. Only one single species can process methanol.

Around 70 % of the methane produced is generated by the

utilisation of the acetic acid formed chiefly in the acetogenic phase,

the remaining 30 % comes from the methanation of carbon dioxide

and water. The consumption of hydrogen by the methanogenic

bacteria keeps the hydrogen partial pressure low, which should

be considered to be an important prerequisite for the reactions

within the context of the acetogenesis. The following equations

illustrate the two essential reactions for the formation of methane:

Fig. 6: Formation of methane from acetic acid

Acetic Acid Methane + Carbon Dioxide

CH3COOH CH4 + CO2


Clostridium Acetobacterium Synthrophomonas Synthrophobacter Desulfovibrio

spp. spp. spp. spp. spp.

Propionic acid

Valeric acid Buyric acid

Formic acid

Acetic acid


Fig. 7: Formation of methane from carbon dioxide

and hydrogen

Carbon dioxide + Hydrogen _________________________ Methane + Water

CO2 + 4H2 CH4 + 2H2O


The formation of methane from the remaining substances such as

alcohols plays only a subordinated role. The methanogenesis

should also be illustrated schematically once more:

Fig. 8: Course of the methane formation


2.3 Generation times of the bacteria

If one considers the formation of the biogas in the process steps

explained above, it should be noted that the bacterial strains

involved work at different speeds and do not multiply at the same

rates. A unit for this speed is the so-called generation time, which

indicates the time period during which the bacteria are able to

double their cell count and, hence, their working speed. In the

case of the bacteria that work during the hydrolysis and

acidification phases, the generation time is significantly shorter

than for the methanogenic micro-organisms.

Table 2: Bacteria generation times Bacterial group Generation time

Hydrolytic and acidogenic bacteria

Bacteriodes

Clostridia

< 24 hours

24 – 36 hours

Acetogenic bacteria Syntrophobacter

Syntrophomonas

40 – 60 hours

72 – 132 hours

Methanogenic bacteria

Methanobacterium

Methanosarcina Methanococcus/Metanosaeta

12 – 60 hours

120 – 360 hours

240 hours

Source: amended according to Weiland 2001

Methane

Carbon dioxide Acetic acid Hydrogen

Methanosarcina spp.

Methanobac-

terium spp.

Methanococcus spp.

Methanosaeta spp.

Carbon dioxide

Water


This fact is of critical importance when feeding the biogas reactor.

Since the acid-generating bacteria perform their work very fast

and can multiply rapidly, there is a danger of producing an

excess of the acids that are to be decomposed by the

methanogenic bacteria. The methanogenic bacteria cannot

cope with this excess, which ultimately leads to acidification of

the biogas reactor. The sinking pH value restricts the activity of

the bacteria involved in the process more and more; the

methane yield sinks until finally the process breaks down.

This ‘vicious circle’ must be counteracted in the case

of a fall in the pH value or a low gas yield respectively

by immediately reducing or stopping the feed of

substrate in order to give the methanogenic bacteria

time to decompose the acids.

The feeding of the substrate is often increased by mistake if a

lower gas yield is observed. The establishment of a dynamic

equilibrium between nutrient supply and decomposition should be

judged as optimum. Above all, the rates of decomposition of the

ingredients of the substrates used (see above) need to be

taken into account here. The introduction of substrates with a

high sugar content will hence lead to a rapid increase in the acid

concentration. In this case, the decomposition of propionic acid

and butyric acid by acetogenic and methanogenic bacteria

becomes the speed-determining step in the biogas process. In

contrast, celluloses and hemicelluloses are decomposed to

acids more slowly on account of their more complex molecular

structures.

In the fermentation of renewable raw materials, the problem of

a rapid increase in acidity or excess acids will probably not be

very significant, since the content of rapidly degradable

ingredients in the substrates is relatively low. The hydrolysis

determines the speed of the fermentation process.

13 | Creating the right environment for the bacteria Creating the right environment for the bacteria | 29

3. Creating the right environment for the bacteria

14 | Creating the right environment of the bacteria Creating the right environment of the bacteria | 29

The micro-organisms involved in the generation of biogas

require favourable living conditions in order to be able to work

efficiently. If the bacteria do not ‘feel well’ in their surroundings,

they will only be able to make a correspondingly small

contribution to a high gas yield. The problem is that the bacterial

strains in the process steps described in chapter 2 have

differing optimum environmental requirements, so that, for

example, the requirements of the hydrolytic bacteria do not

correlate with those of the methanogenic bacteria. If the biogas

reactor is operated in a single stage, i.e. all phases of the biogas

generation proceed in one container, a constant compromise

needs to be entered into with regard to the environment. The

environment is usually adjusted to the requirements of the

methanogenic bacteria, since these react most sensitively and

exhibit the longest generation times.

The significant parameters of the environmental conditions will be

examined in more detail below. These are: the temperature, the

pH value and the buffer capacity. Furthermore, the nutrients

required by the micro-organisms will be dealt with briefly, as

well as impurities and inhibitors that have a detrimental effect

on the biogas process. With regard to the overall process, a

sufficiently high water content of at least 50 % is required so that

the bacterial strains can work and multiply. Furthermore, it must

be remembered that micro-organisms are inhibited in their work

by light; the incidence of light must accordingly be excluded.

3.1 Temperature

In general it can be said that an increase in the ambient

temperature leads to the acceleration of chemical reactions.

However, since micro-organisms and enzymes are involved in

the biogas process, the temperature cannot be increased at will

in order to quicken events in the biogas reactor, i.e. the

formation of gas. In fact, the bacterial strains and the enzymes

that act in the hydrolysis phase have optimum temperatures.

If the temperature exceeds or falls short of these optimum

temperatures, this will lead to the process being inhibited or, in

the case of a large deviation, even to the micro-organisms dying

off. The bacterial strains can basically be classified into the

following groups, depending on the temperature:

1. Psychrophilic strains (up to 25 °C)

2. Mesophilic strains (32 – 42 °C)

3. Thermophilic strains (50 – 57 °C)

The psychrophilic temperature range plays no great part in the

biogas reactor. Accordingly, most biogas reactors are operated in

the mesophilic or thermophilic temperature ranges. These forms of

operation are in turn adjusted first and foremost to suit the

methanogenic bacteria, whose optimum temperatures lie either

in the mesophilic range or the thermophilic range.


In contrast, the acidifying bacteria consistently prefer temperatures

around 30 °C. The significant advantages of the mesophilic

process are the high process stability and the relatively low

expenditure of control and process energy. Nevertheless, longer

dwell times of the substrate in the reactor, a lesser degree of

decomposition of the material and, ultimately, a lower gas yield

must be accepted.

In thermophilic reactors, higher gas yields can be achieved and

any harmful germs that are present can be killed more

effectively. However, the process is considerably more

sensitive. Even daily temperature fluctuations of 1 °C around

the middle value can result in considerable impairment of the

bacteria; the mesophilic process management allows

fluctuations of 2 – 4 °C. Against this background, the mesophilic

method of operation is currently setting the pace. That said,

improved and automated process control have resulted in a

visible trend towards higher temperatures in order to ultimately

achieve higher gas yields.

3.2 pH value and buffer capacity

As in the case of the temperature, it should be pointed out when

considering the pH value that the bacteria involved in the

fermentation process do not all have the same requirements

with regard to this factor. The hydrolysing and acidifying bacteria

thrive best as a pH value of 4.5 to 6.3 and can accordingly work

most efficiently within this range. However, deviations from this

optimum value do not significantly inhibit these micro-organisms

in their function.

The bacteria responsible for the formation of acetic acid and

methane are a different case altogether. Their optimum pH

value lies within a relatively narrow window between 6.8 and

7.8, i.e. in the neutral area. Deviations are hardly tolerated at all

by these bacterial strains, and the process must be tailored to

their requirements for this reason.

The quantity and the properties of the substrate fed to the

fermenter influence the pH value. Substrates that are too easily

degradable lead to a lowering of the pH value due to rapid

acidification and should therefore be fed with particular care.

In this context, the term ‘buffer capacity’ plays a further

important part. The buffer capacity is a measure of how far an

acidification of the biogas reactor can be 'held off', i.e. buffered,

until a lowering of the pH value actually occurs. If there is a high

buffer capacity in the fermenter, a relatively large amount of

substrate can be fed in without the pH value sinking and the

bacteria being inhibited or even damaged. If the buffer capacity is

low, appropriate care must be taken when adding substrate in

order to avoid ‘overfeeding’.

The pH value is therefore less useful for short-term process

control, since a reversal may be too late under certain

circumstances, i.e. the reactor operator can no longer react in

good time. The determination of the buffer capacity, on the

other hand, makes it possible at all times to determine how

'hungry' the biogas reactor is and how much substrate can be


fed in order to utilise the bacteria to the optimum. A procedure

for determining the buffer capacity that anyone can perform will be

presented later in this brochure. Nonetheless, the pH value does

deliver important information on the stability of the biogas

process, provided it is measured constantly.

3.3 Supplying the bacteria with nutrients

So that the survival and rapid multiplication of the bacteria is

ensured at all times, particular nutrients must be made available

in the correct ratio. This also takes place via the substrate that is

fed in. Animal excrement and maize or grass silage principally

cover the micro-organisms’ nutrient requirements and can thus be

fermented with no further additives. The following is intended

merely to briefly mention the essential guiding values.

According to reference books, the optimum ratio of carbon to

nitrogen (C/N ratio) lies within a range from 10:1 to 45:1. The best

decomposition rates can be achieved here. If the ratio is too

low, the resulting excess nitrogen leads to overproduction of

ammonia, which is dangerous to the bacteria in high

concentrations. Alongside carbon and nitrogen, phosphor and

sulphur must also be available; a C:N:P:S ratio of 600:15:5:1 is

considered to be ideal. The trace elements nickel, cobalt and

selenium, namely in concentrations of approx. 0.1 mg/l, are

essential, especially for the methanogenic bacteria.

3.4 Impurities and inhibitors

The activities of the bacteria in the biogas process may be

inhibited due to the entry of certain substances or the

generation of negatively acting substances during the

fermentation.

When adding substrates, particular care should be taken to

ensure that these are free of substances such as antibiotics,

solvents or disinfectants, herbicides or heavy metals. The trace

elements essential to the bacteria can also have a toxic effect in

high concentrations inhibiting the decomposition process.


Exact values to describe the maximum bearable levels are very

difficult to specify, since the bacteria are apparently able to adapt

themselves within certain limits to the inhibitors entering the reactor.

It should be obvious that substrate infested with mould is of no use

to the biogas process.

It one considers the inhibitors that are generated during the biogas

process itself, then hydrogen sulphide (H2S) and ammonia (NH3)

should be mentioned primarily.

Hydrogen sulphide is generated in particularly large amounts

by the decomposition of substrates that contain sulphur or are

rich in proteins, and has an extremely toxic effect on the

bacteria involved in the process. Moreover, hydrogen sulphide

leads to damage to gas pipelines and the combined heating and

power plant due to its corrosive properties. The danger of the

formation of hydrogen sulphide increases as the pH value

decreases.

Ammonia is also produced by the decomposition of substrates

rich in protein, and is similarly highly toxic. An increased

production of ammonia can be observed in particular when

fermenting raw materials containing nitrogen, e.g. legumes. The

ammonia concentration increases as the pH value falls and the

temperature increases. The reactor operator can counteract this

by feeding in carbon in the form of material rich in crude fibres

in order to increase the C/N ratio.

The fact that an agricultural biogas reactor is more or less a ‘black

box’, which one cannot simply look inside, forces the reactor

operator to rely on a series of measured values.

The performance of the biogas reactor can be measured first

and foremost by the quantity and quality of the biogas produced.

‘Good’ biogas distinguishes itself by the highest possible methane

content and the lowest possible carbon dioxide content. The lower

the proportions of hydrogen sulphide and ammonia, the higher

the quality of the gas will be. These values can be measured and

recorded relatively easily using mobile gas measuring instruments.

A continuously high production of gas with the smallest possible

fluctuations points to a stable process. An arithmetic comparison of

the theoretically achievable quantity of biogas from the substrate

used to the actual quantity of gas produced by the reactor should

be performed regularly in order to check the efficiency of the

reactor.

If the production of gas is decreasing, or if the biogas is no longer of

the desired quality, the following possible sources of error should

be checked:


1. Is the volume load of the fermenter too high, i.e. is the

biogas reactor being ‘overfed’?

2. Has the reactor been fed sufficiently, i.e. is it under certain

circumstances a case of underfeeding?

3. Is the composition of the substrate and the nutrient supply

OK?

In general it can be said that if the process is unstable or does

not run optimally then the aforementioned environmental

conditions are not ideal. For this reason it is particularly important

to measure the essential process parameters, namely the

temperature, the pH value and the buffer capacity, on a regular

basis.

More important than the individual values is the continual

recording and analysis of the measured data in order to be

able to derive trends from it. This provides information as to

whether or not the reactor is running steadily with no large

fluctuations.

19 | Process monitoring Process monitoring | 33

4.1 Determination of the buffer capacity using the

‘VOA/TAC’ method

Temperature and pH values can be determined in a simple manner

with commercially available measuring instruments. For the

determination of the buffer capacity, which enables conclusions

to be drawn about the actual level of utilisation of the bacteria in

the fermenter, the so-called VOA/TAC method will be presented

in the following. This can be carried out easily by anybody,

requires little expenditure of time and enables more accurate

feeding of the biogas reactor.

The ‘VOA/TAC analysis’, is used to determine the ratio of volatile

organic acids (VOAs) to the buffer capacity (TAC = Total

Anorganic Carbon) in the fermentation substrate. This ratio

indicates how great the danger of acidification of the fermenter is.

In order to produce the maximum amount of biogas, the bacteria

should work close to their performance limits, i.e. they should be

working virtually at full capacity. Acidification indicates that the

bacteria have been overloaded; the feeding of substrate must

therefore be reduced. If there are too few acids, however, their

proportion must be increased by adding more substrate in order to

load the bacteria more intensively. The rule of thumb is that over-

acidification of the reactor is impending if the VOA/TAC value is

greater than 0.3.

4. Process monitoring


Volatile Organic Acids VOA/TAC = ___________________________________________ = max. 0.4

Total Anorganic Carbon (buffer capacity)

4.2 Instructions for performing the VOA/TAC analysis

Safety information:

It is imperative to wear protective clothing when

performing the test described below.

This includes gloves, a lab coat and protective goggles!

The following items are required for the analysis:

• 1 pH meter

• 1 titration burette

• 1 magnetic stirrer

• 1 glass beaker

• a kitchen sieve

• sulphuric acid (0.1 molar)

Fig. 9: Material arrangement

pH meter

Burette with sulphuric acid

Glass beaker with fermentation substrate

Magnetic stirrer


Source: Fermenterdoktor, 2006

22 | Specialist information from KWS Notes | 39

Process monitoring | 35 34 | Process monitoring

4.2.1 Preparation:

1. Put 20 ml of the fermentation substrate sample into

the glass beaker through the kitchen sieve. 2. Fill the burette with sulphuric acid and read off the fill

level.

4.2.2 Execution:

3. Open the burette and allow acid to drip slowly into the glass beaker.

4. When a pH value of 5.0 has been reached, close the burette

and note the amount of sulphuric acid used. (Quantity A, e.g. 15 ml)

Fig. 11: Adding the acid

1. Place the glass beaker on the magnetic stirrer and

begin stirring. 2. Measure the initial pH value.

Fig. 10: Determination of the initial pH value

5. Open the burette again. 6. Add acid until the pH value reaches 4.3, then close the

burette and note once more the amount of sulphuric acid

used. (Quantity B, e.g. 2 ml)

4.2.3 Calculation of the VOA/TAC value

The VOA/TAC value can now be determined arithmetically from the

values of the titration performed:

VOA = ((Quantity B x 1.66) – 0.15) x 500

TAC = Quantity A x 250

Quantity A =

Consumption of sulphuric acid from beginning to pH 5 = 15 ml

Quantity B =

Consumption of sulphuric acid from pH 5 to pH 4.3 = 2 ml

These values are now inserted in these equations:

VOA = ((2 ml x 1.66) – 0.15) x 500 = 4,905

TAC = 15 ml x 250 = 11,250

VOA/TAC = 4,905 / 11,250 = 0.42

The calculated VOA/TAC value is 0.42. This shows that the

bacteria in the reactor are being exploited very well or are

almost at the limit of their capability. Feeding should not be

further increased, but rather slightly reduced.

5. Summary

24 | Summary

A great many factors influence the biogas process. No reactor is

completely comparable to another. Each individual reactor

operator must develop a high degree of intuition for his fermenter

in order to achieve the maximum performance. This brochure is

intended to show how complex the process of biogas

generation is and to help in understanding the events occurring

inside the container. A thorough knowledge of the biological

interrelationships is the basis for a stable process and, hence,

for successful operation of the biogas reactor.

KWS has been researching into the breeding of energy plants for

many years and offers a wide range of renewable raw materials for

the production of biogas. A worldwide unique breeding programme

for energy maize began in 2002 in order to cater for the demands

of the biogas reactor on maize varieties . In other special breeding

programmes, additional crops such as sugar beet, rye, sorghum

and sunflowers are being bred for energy use in biogas reactors in

order to secure the supply of substrates through many different

crops. Due to this multi-crop approach by KWS, it will be possible to

establish energy crop rotations and to secure sufficient biodiversity.


Specialist information from KWS


Notes: iExtensive information on the subject of energy plants, their cultivation

and utilisation in biogas reactors can be found in our energy plant

guide.

, For a copy of the guide, or to speak to one of our team of specialists, Please contact Tel.: 0 55 61 / 311-543, Fax: 0 55 61 / 311-447 or E-Mail: [email protected].

02 leaflet biogas basics eng

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