082 Biomethanation II (2003)

Prévia do material em texto

82 
Advances in Biochemical 
Engineering / Biotech nology 
Series Editor: T. Scheper 
E d i t o r i a l B o a r d : 
W. Babel . H. W. Blanch. I. Endo. S.-O. E n f o r s 
A. F i e c h t e r • M. H o a r e • B. M a t t i a s s o n • H. S a h m 
K. Sch i ige r l • G. S t e p h a n o p o u l o s • U. v o n S t o c k a r 
G. T. T s a o . J. V i l l a d s e n • C. W a n d r e y • J.-J. Z h o n g 
Springer 
Berlin 
Heidelberg 
New York 
Hong Kong 
London 
Milan 
Paris 
Tokyo 
Biomethanat ion II 
Volume Editor: Birgitte K.Ahring 
With contributions by 
B. K. Ahring, I. Angelidaki, J. Dolfing, L. EUegaard, 
H. N. Gavala, F. Haagensen, A. S. Mogensen, G. Lyberatos, 
P. E Pind, ]. E. Schmidt, I.V. Skiadas, K. Stamatelatou 
~ Springer 
Advances in Biochemical Engineering/Biotechnology reviews actual trends in modern 
biotechnology. Its aim is to cover all aspects of this interdisciplinary technology where 
knowledge, methods and expertise are required for chemistry, biochemistry, micro- 
biology, genetics, chemical engineering and computer science. Special volumes are dedi- 
cated to selected topics which focus on new biotechnological products and new pro- 
cesses for their synthesis and purification. They give the state-of-the-art of a topic in a 
comprehensive way thus being a valuable source for the next 3-5 years. It also discusses 
new discoveries and applications. 
In general, special volumes are edited by well known guest editors. The series editor and 
publisher will however always be pleased to receive suggestions and supplementary infor- 
mation. Manuscripts are accepted in English. 
In references Advances in Biochemical Engineering/Biotechnology is abbreviated as 
Adv Biochem Engin/Biotechnol as a journal. 
Visit the ABE home page at http:lllink.springer.de/series/abel 
http:l/link.Springer-ny.comlseries/abel 
I S S N 0 7 2 4 - 6 1 4 5 
I S B N 3 - 5 4 0 - 4 4 3 2 1 - 5 
S p r i n g e r - V e r l a g B e r l i n H e i d e l b e r g N e w Y o r k 
Library of Congress Catalog Card Number 72-152360 
This work is subject to copyright. All rights are reserved, whether the whole or part of 
the material is concerned, specifically the rights of translation, reprinting, reuse of 
illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, 
and storage in data banks. Duplication of this publication or parts thereof is permitted 
only under the provisions of the German Copyright Law of September 9, 1965, in its 
current version, and permission for use must always be obtained from Springer-Verlag. 
Violations are liable for prosecution under the German Copyright Law. 
Springer-Verlag Berlin Heidelberg New York 
a member of BertelsmannSpringer Science+Business Media GmbH 
http://www.springer.de 
© Springer-Verlag Berlin Heidelberg 2003 
Printed in Germany 
The use of general descriptive names, registered names, trademarks, etc. in this pub- 
lication does not imply, even in the absence of a specific statement, that such names 
are exempt from the relevant protective laws and regulations and therefore free for 
general use. 
Typesetting: Fotosatz-Service K6hler GmbH, Wiirzburg 
Cover: KiinkelLopka GmbH, Heidelberg/design & production, Heidelberg 
Printed on acid-free paper 02/3020mh - 5 4 3 2 1 0 
Series Editor 
Professor Dr. T. Scheper 
Institute of Technical Chemistry 
University of Hannover 
Callinstrafle 3 
30167 Hannover, Germany 
E-mail: scheper@iftc.uni-hannover.de 
Volume Editor 
Professor Birgitte K. Ahring 
Biocentrum 
The Technical University of Denmark 
DTU, Block 227 
2800 Lyngby 
Denmark 
E-mail: bka@biocentrum.dtu.dk 
Editorial Board 
Prof. Dr. W. Babel 
Section of Environmental Microbiology 
Leipzig-Halle GmbH 
Permoserstrafle 15 
04318 Leipzig, Germany 
E-mail: babel@umb.ufz.de 
Prof. Dr. I. Endo 
Faculty of Agriculture 
Dept. of Bioproductive Science 
Laboratory of Applied Microbiology 
Utsunomiya University 
Mine-cho 350, Utsunomiya-shi 
Tochigi 321-8505, Japan 
E-mail: endo@cel.riken.go.jp 
Prof. Dr. A. Fiechter 
Institute of Biotechnology 
Eidgen6ssische Technische Hochschule 
ETH-H6nggerberg 
8093 Ztirich, Switzerland 
E-mail: ae.~echter@bluewin.ch 
Prof. Dr. H.W. Blanch 
Department of Chemical Engineering 
University of California 
Berkely, CA 94720-9989, USA 
E-mail: blanch@socrates.berkeley.edu 
Prof. Dr. S.-O. Enfors 
Department of Biochemistry and 
Biotechnology 
Royal Institute of Technology 
Teknikringen 34, 
100 44 Stockholm, Sweden 
E-mail: enfors@biotech.kth.se 
Prof. Dr. M. Hoare 
Department of Biochemical Engineering 
University College London 
Torrington Place 
London, WC1E 7JE, UK 
E-mail: m.hoare@ucl.ac, uk 
VI Editorial Board 
Prof. Dr. B. Mattiasson 
Department of Biotechnology 
Chemical Center, Lund University 
P.O. Box 124, 221 00 Lund, Sweden 
E-mail: bo.mattiasson@biotek.lu.se 
Prof. Dr. K. Schiigerl 
Institute of Technical Chemistry 
University of Hannover 
Callinstrat3e 3 
30167 Hannover, Germany 
E-maih schuegerl@iftc.uni-hannover.de 
Prof. Dr. U. von Stockar 
Laboratoire de G~nie Chimique et 
Biologique (LGCB) 
D~partment de Chimie 
Swiss Federal Institute 
of Technology Lausanne 
1015 Lausanne, Switzerland 
E-mail: urs.stockar@epfl.ch 
Prof. Dr. ]. Villadsen 
Center for Process of Biotechnology 
Technical University of Denmark 
Building 223 
2800 Lyngby, Denmark 
E-maih john. villadsen@biocentrum.dtu.dk 
Prof. Dr. J.-J. Zhong 
State Key Laboratory 
of Bioreactor Engineering 
East China University of Science 
and Technology 
130 Meilong Road 
Shanghai 200237, China 
E-maih jjzhong@ecust.edu.cn 
Prof. Dr. H. Sahm 
Institute of Biotechnolgy 
Forschungszentrum ]filich GmbH 
52425 ]filich, Germany 
E-maih h.sahm@fz-juelich.de 
Prof. Dr. G. Stephanopoulos 
Department of Chemical Engineering 
Massachusetts Institute of Technology 
Cambridge, MA 02139-4307, USA 
E-maih gregstep@mit.edu 
Prof. Dr. G. T. Tsao 
Director 
Lab. of Renewable Resources Eng. 
A.A. Potter Eng. Center 
Purdue University 
West Lafayette, IN 47907, USA 
E-maih tsaogt@ecn.purdue.edu 
Prof. Dr. C. Wandrey 
Institute of Biotechnology 
Forschungszentrum ]filich GmbH 
52425 ]/Llich, Germany 
E-maih c. wandrey@fz-juelich.de 
Advances in Biochemical Engineering/Biotechnology 
also Available Electronically 
For all customers with a standing order for Advances in Biochemical Engineer- 
ing/Biotechnology we offer the electronic form via SpringerLink free of charge. 
Please contact your librarian who can receive a password for free access to the 
full articles. By registration at: 
http://www.springer.de/series/abe/reg_form.htm 
If you do not have a standard order you can nevertheless browse through the 
table of contents of the volumes and the abstracts of each article at: 
http:/ /link.springer.delserieslabel 
http://link.springer_ny.com/series/abe/ 
There you will find also information about the 
- Editorial Board 
- Aims and Scope 
- Instructions for Authors 
Attention all Users 
of the "Springer Handbook of Enzymes" 
Information on this handbook can be found on the internet at 
http:/ Iwww.springer.de/ enzym esl 
A complete list of all enzyme entries either as an alphabetical Name Index or as 
the EC-Number Index is available at the above mentioned URL. You can down- 
load and print them free of charge. 
A complete list of all synonyms (more than 25,000 entries) used for the enyzmes 
is available in print form (ISBN 3-540-41830-X). 
Save 15 % 
We recommend a standing order for the series to ensure you automatically 
receive all volumes and all supplements and save 15 % on the list price. 
Preface 
In November 1776, Alessandro Volta performed his classic experiment disturb- 
ing the sediment of a shallow lake, collecting the gas and demonstrating that this 
gas was flammable. The science of Biomethanation was born and, ever since, sci- 
entists and engineers have worked at understanding this complex anaerobic bio- 
logical process and harvesting the valuable methane gas produced duringanaer- 
obic decomposition. Two lines of exploitation have developed mainly during the 
last century: the use of anaerobic digestion for stabilization of sewage sludge, 
and biogas production from animal manure and/or household waste. Lately, the 
emphasis has been on the hygienic benefit of anaerobic treatment and its effect 
on pathogens or other infectious elements. The importance of producing a safe 
effluent suitable for recirculation to agricultural land has become a task just as 
important as producing the maximum yield of biogas from a given type of 
waste. Therefore, anaerobic digestion at elevated temperatures has become the 
main area of interest and has been growing during the last few years 
Anaerobic digestion demands the concerted action of many groups of 
microbes each performing their special role in the overall degradation process. 
Both Bacteria and Archaea are involved in the anaerobic process while the 
importance, if any, of eukaryotic microorganisms outside the rumen environ- 
ment is still unknown. The basic understanding of the dynamics of the complex 
microflora was elucidated during the latter part of the last century where the 
concept of inter-species hydrogen transfer was introduced and tested. The isola- 
tion of syntrophic bacteria specialized in oxidation of intermediates such as 
volatile fatty acids gave strength to the theories. Lately the use of molecular tech- 
niques has provided tools for studying the microflora during the biomethana- 
tion process in situ. However, until now the main focus has been on probing the 
dynamic changes of specific groups of microorganisms in anaerobic bioreactors 
and less emphasis has been devoted to evaluating the specific activities of the 
different groups of microbes during biomethanation. In the future we can expect 
that the molecular techniques will be developed to allow more dynamic studies 
of the action of specific microbes in the over-all process. From the present 
studies we know that many unknown microbes are found in anaerobic bio- 
reactors. Especially within the domain of Archaea, there are whole phyla of 
microbes such as the Crenarchaeota, which make up significant fractions of 
microbes in a reactor but without cultured representatives. Improving the 
techniques for the isolation of presently unculturable microbes is a major task 
for the future. 
X Preface 
Anaerobic digestion of waste has been implemented throughout the wodd for 
treatment of wastewater, manure and solid waste and most countries have sci- 
entists, engineers and companies engaged in various aspects of this technology. 
Even though the implementation of anaerobic digestion has moved out of the 
experimental phase, there is still plenty of room for improvements. The basic 
understanding of the granulation process, the basis for the immobilization of 
anaerobic microbes to each other without support material in UASB reactors, is 
still lacking. Like any other bioprocess, anaerobic digestion needs further con- 
trol and regulation for optimization. However, until now suitable sensors for 
direct evaluation of the biological process have been lacking and anaerobic 
bioreactors have generally been controlled by indirect measurements of biogas 
or methane production along with measurements of pH and temperature. The 
newly development of an on-line monitoring system for volatile fatty acids could 
be a major step in the right direction and the use of infra-red monitoring sys- 
tems could bring the price down to a reasonable level. A better performance of 
large-scale anaerobic bioreactor systems for treatment of complex mixtures of 
waste can be expected to be based on on-line monitoring of the process in the 
future along with controlling software for qualified management of these plants. 
Besides treatment of waste, anaerobic digestion possesses a major potential 
for adding value to other biomass converting processes such as gasification, 
bioethanol or hydrogen from ligno-cellulosic materials. Conversion of ligno-cel- 
lulosic biomass will often leave a large fraction of the raw material untouched 
which will be a burden for the over-all economy of the process and will demand 
further treatment. Anaerobic digestion will on the other hand be capable of con- 
verting the residues from the primary conversion into valuable methane, which 
will decrease the cost and the environmental burden of the primary production. 
Biomethanation is an area in which both basic and applied research is 
involved. Major new developments will demand that both disciplines work 
together closely and take advantage of each other's field of competence. The two 
volumes on Biomethanation within the series of Advances in Biochemical Engi- 
neering and Biotechnology have been constructed with this basic idea in mind 
and, therefore, both angles have been combined to give a full picture of the area. 
The first volume is devoted to giving an overview of the more fundamental 
aspects of anaerobic digestion while the second volume concentrates on some 
major applications and the potential of using anaerobic processes. The two vol- 
umes will therefore be of value for both scientists and practitioners within the 
field of environmental microbiology, anaerobic biotechnology, and environ- 
mental engineering. The general nature of most of the chapters along with the 
unique combination of new basic knowledge and practical experiences should, 
in addition, make the books valuable for teaching purposes. 
The volume editor is indebted to all the authors for their excellent contribu- 
tions and their devotion and cooperation in preparing these two volumes on 
Biomethanation. 
Lyngby, January 2003 Birgitte K. Ahring 
Contents 
Applications of the Anaerobic Digestion Process 
I. Angelidaki, L. EUegaard, B. K. Ahring . . . . . . . . . . . . . . . . . . . . 1 
Anaerobic Granular Sludge and Biofilm Reactors 
I.V. Skiadas, H. N. Gavala, ]. E. Schmidt, B. K. Ahring . . . . . . . . . . . . . 35 
Potential for Anaerobic Conversion of Xenobiotics 
A. S. Mogensen, L Dolfing, F. Haagensen, B. K. Ahring . . . . . . . . . . . . 69 
Monitoring and Control of Anaerobic Reactors 
P. F. Pind, I. Angelidaki, B. K. Ahring, K. Stamatelatou, G. Lyberatos . . . . . 135 
Author Index Volumes 51-82 . . . . . . . . . . . . . . . . . . . . . . . . . 183 
Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 
Contents of Volume 81 
Biomethanantion I 
Volume Editor: Birgitte K. Ahring 
Perspectives for Anaerobic Digestion 
B. K. Ahring 
Metabolic Interactions Between Methanogenic Consortia 
and Anaerobic Respiring Bacteria 
A. J. M. Stams, S. J. W. H. Oude Elferink, P. Westermann 
Kinetics and Modeling of Anaerobic Digestion Process 
H. N. Gavala, I. Angelidaki, B. K. Ahring 
Molecular Biology of Stress Genes in Methanogens: 
Potential for Bioreactor Technology 
E. Conway de Macario, A. J. L. Macario 
Molecular Ecology of Anaerobic Reactor Systems 
J. Hofman-Bang, D. Zheng, E Westermann, B. K. Ahring, L. Raskin 
Applications of the Anaerobic Digestion Process
Irini Angelidaki 1 · Lars Ellegaard 2 · Birgitte Kiœr Ahring 3
1 Environment & Resources, The Technical University of Denmark, Block 115, 2800 Lyngby,
Denmark. E-mail: ria@er.dtu.dk
2 BWSC, Gydevang 11, 3450 Allerød, Denmark
3 Environmental Microbiology & Biotechnology, Biocentrum, The Technical University 
of Denmark, Block 227, 2800 Lyngby, Denmark
At the start of the new millennium waste management has become a political priority in many
countries. One of the main problems today is to cope with an increasing amount of primary
waste in an environmentally acceptable way. Biowastes, i.e., municipal, agricultural or indus-
trial organic waste, as well as contaminated soils etc., have traditionally been deposited in land-
fills or even dumped into the sea or lakes without much environmental concern. In recent
times, environmental standards of waste incineration and controlled land fillinghave gradu-
ally improved, and new methods of waste sorting and resource/energy recovery have been de-
veloped. Treatment of biowastes by anaerobic digestion processes is in many cases the optimal
way to convert organic waste into useful products such as energy (in the form of biogas) and
a fertilizer product. Other waste management options, such as land filling and incineration of
organic waste has become less desirable, and legislation, both in Europe and elsewhere, tends
to favor biological treatment as a way of recycling minerals and nutrients of organic wastes
from society back to the food production and supply chain. Removing the relatively wet organic
waste from the general waste streams also results in a better calorific value of the remainder
for incineration, and a more stable fraction for land filling.
Keywords. Anaerobic digestion, Reactors, Codigestion, Biowastes, Solid waste, Slurries, Manure,
Industrial waste
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Factors Influencing the Biogas Process . . . . . . . . . . . . . . . 4
2.1 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 Toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3 Digestion of Slurries . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Process and Plant Configuration . . . . . . . . . . . . . . . . . . 8
3.3 Veterinarian Aspects . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.4 Combined Digestion of Livestock Waste and Industrial Waste . . 11
3.5 Process Tank Configuration . . . . . . . . . . . . . . . . . . . . . 13
3.6 Equipment Characteristics . . . . . . . . . . . . . . . . . . . . . 14
CHAPTER 1
Advances in Biochemical Engineering/
Biotechnology, Vol. 82
Series Editor: T. Scheper
© Springer-Verlag Berlin Heidelberg 2003
3.6.1 Mixing Technique . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.6.2 Pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.6.3 Heat Exchanging . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.6.4 Biogas Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.6.5 Biogas Transmission . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.6.6 Fiber Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.6.7 Odor Control Systems . . . . . . . . . . . . . . . . . . . . . . . . 22
3.7 Operational Experience and Results . . . . . . . . . . . . . . . . 22
3.7.1 Production Results . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4 Digestion of High-Solid Wastes . . . . . . . . . . . . . . . . . . . 23
4.1 Pretreatment of Municipal Solid Waste . . . . . . . . . . . . . . . 24
4.2 Post Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3 Biological Treatment . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3.1 Wet Digestion Systems . . . . . . . . . . . . . . . . . . . . . . . . 26
4.3.2 Dry digestion Systems . . . . . . . . . . . . . . . . . . . . . . . . 27
4.3.3 Multi-stage Anaerobic Digestion Systems . . . . . . . . . . . . . . 29
4.4 Summary of Processes Used for Anaerobic Treatment of Solid 
Wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
1
Introduction
Anaerobic degradation or digestion is a biological process where organic carbon
is converted by subsequent oxidations and reductions to its most oxidized state
(CO2), and its most reduced state (CH4).A wide range of microorganisms catalyze
the process in the absence of oxygen. The main products of the process are car-
bon dioxide and methane, but minor quantities of nitrogen, hydrogen, ammonia
and hydrogen sulfide (usually less than 1% of the total gas volume) are also gen-
erated. The mixture of gaseous products is termed biogas and the anaerobic
degradation process is often also termed the biogas process. As the result of the
removal of carbon, organic bound minerals and salts are released to their solu-
ble inorganic form.
The biogas process is a natural process and occurs in a variety of anaerobic en-
vironments. Such environments are, marine and fresh water sediment, sewage
sludge, mud, etc.
The interest in the process is mainly due to the following two reasons:
– A high degree of reduction of organic matter is achieved with a small increase
– in comparison to the aerobic process – in the bacterial biomass.
– The production of biogas, which can be utilized to generate different forms of
energy (heat and electricity) or be processed for automotive fuel.
Biogas has a lower calorific value than natural gas, and in specific applications,
such as automotive fuel, treatment of the biogas to improve its quality is required.
2 I. Angelidaki et al.
In Table 1, the upper and lower calorific value of biogas in comparison to natural
gas is shown.
The biogas process has been known and utilized for many years, but especially
after the rise of energy prices in the 1970s, the process received renewed atten-
tion due to the need to find alternative energy sources to reduce the dependency
on fossil fuels.Although the price of fossil fuels decreased in 1985, the interest in
the biogas process still remains due to the environmental benefits of anaerobic
waste degradation. Additionally, the biomass used for biogas production was
originally produced by photosynthetic fixation of carbon dioxide from the at-
mosphere, and combustion of biogas thus does not add extra carbon dioxide 
to the atmosphere as it does when combusting fossil fuels formed millions of
years ago.
The anaerobic degradation process has been used for years for energy pro-
duction and waste treatment. It is used in closed systems where optimal and con-
trolled conditions can be maintained for the microorganisms. The process can be
utilized for the fast and efficient degradation of different waste materials. The
anaerobic process is today mainly utilized in four sectors of waste treatment.
1. Treatment of primary and secondary sludge produced during aerobic treat-
ment of municipal sewage. The process is utilized to stabilize and reduce the
final amount of sludge and at the same time biogas is produced, which can be
used to partly cover the need for energy in the sewage treatment plant. This
application is widespread in the industrialized world in connection with the
establishment of advanced treatment systems for domestic wastewater.
2. Treatment of industrial wastewater produced from biomass-, food-processing
or fermentation industries. These wastewater types are often highly loaded
and can successfully be treated anaerobically before disposal directly to the en-
vironment or sewage system. The produced biogas can often be utilized to
cover the need for process energy. With the environmental concerns and cost
of alternative disposal this application of the anaerobic process is increasing.
3. Treatment of livestock waste in order to produce energy and improve the fer-
tilizing qualities of manure. Due to more strict rules concerning the usage, dis-
tribution, and storage of manure this application is growing especially in
countries with a high animal production density.
4. A relatively new sector for use of the anaerobic processes on an industrial scale
is treatment of the organic fraction of municipal solid waste (OFMSW). The
aim of this process is first of all to reduce the amount of waste in other treat-
ment systems, i.e. landfills and incineration plants, and secondly to recycle the
nutrients from this type of waste to the agricultural sector.
Applications of the Anaerobic Digestion Process 3
Table 1. Calorific value of biogas and natural gas
Gas composition Biogas 65% CH4 Biogas 55% CH4 Natural gas 
Upper calorific value KWh/m3STP a 7.1 6.0 12.0 
Lower calorific value KWh/m3STP a 6.55.5 10.8 
a STP (standard temperature and pressure), i.e. the volume at 0 °C and 1 bar pressure.
2
Factors Influencing the Biogas Process
The biogas process as a complex biological process is influenced by several en-
vironmental factors.
The interdependence of the bacteria is a key factor of the biogas process. Un-
der conditions of unstable operation, intermediates such as volatile fatty acids
and alcohols accumulate at different rates depending on the substrate and the
type of perturbation causing instability. Thus, changes in the concentration of in-
termediates indicate disturbance of the biogas process.
The most important environmental factors that can influence the balance of
the system are temperature, pH, substrate composition and toxins.
2.1
Temperature
Temperature is one of the main environmental factors affecting bacterial growth.
Anaerobic bacteria are affected in the same way as the aerobic ones. Growth rates
often increase with increasing temperature up to a certain limit, while there is a
rapid decrease in growth as the temperature approaches the upper limit for sur-
vival of the bacterium. Besides the influence on growth rates of bacteria, tem-
perature also influences physical parameters such as viscosity, surface tension
and mass transfer properties.Apart from the general steady state dependency on
temperature, also temperature stability is important, since even relatively small
changes in the temperature result in an efficiency drop until adaptation has 
occurred.
Treatment of waste in anaerobic reactors is normally carried out within two
temperature ranges: around 25–40 °C, known as the mesophilic range, and
higher than 45 °C, known as the thermophilic range.
Several advantages of anaerobic waste digestion at thermophilic temperatures
have been reported previously.
At higher temperatures:
– the rate of digestion is faster, and thus shorter retention times are required,
– smaller reactor volumes are required for treating the same amount of waste,
– higher rate and efficiency of particulate matter hydrolysis,
– more efficient destruction of pathogens.
Poor stability was previously believed to be associated with thermophilic tem-
peratures. However, many years of experience with full-scale biogas plants op-
erating at thermophilic temperature, have demonstrated that this is not the case.
Temperature has a positive effect on the digestion rate, resulting in higher 
volumetric methane production rates. Casali and Senior found that the
methanogenic rate in refuse increased 2.6 times when the temperature was in-
creased from ambient temperature to 30 °C and further 3 times when the tem-
perature was increased from 30 to 40 °C.
However, the ultimate methane yield from organic matter is not influenced by
temperature in the range 30 to 60 °C. Consequently the effect of temperature on
4 I. Angelidaki et al.
growth rates can only be seen when the loading rates are high or retention times
short. Varel et al. found with waste of beef-cattle in semi continuous reactor ex-
periments that the effect of temperature on the rate of methane production was
more noticeable at short retention times.As the retention time was decreased, ac-
tive fermentation could only be achieved at thermophilic temperatures.At 3 days
RT, active fermentation could only be achieved at temperatures between 45 and
60 °C and at 2.5 days RT only at 60 °C.
Methanogenesis is also possible under psychrophilic conditions (below 25 °C)
but at lower process rates. Anaerobic bacteria can adapt quite easily to low tem-
peratures, and high rate anaerobic treatment has been achieved at psychrophilic
conditions, when bacteria have been immobilized or otherwise retained in the
process.
It has been found that when shifting from mesophilic to psychrophilic tem-
peratures the microbial populations still have the same composition as under
mesophilic temperatures. This could indicate that at these temperatures psy-
chrotolerant rather than true psychrophilic organisms are active.
2.2
pH
The anaerobic digestion process is limited to a relatively narrow pH interval from
approx. 6.0 to 8.5; a pH value outside this range can lead to imbalance.
Each of the microbial groups involved in anaerobic degradation has a specific
pH optimum and can grow in a specific pH range. The methanogens and aceto-
gens have pH optimum at approx. 7, while acidogens have lower pH optimum
around 6. Methanogens at pH lower than 6.6 grow very slowly.
In an anaerobic reactor, instability will as a rule lead to accumulation of VFA,
which can lead to a drop in pH (acidification). However, accumulation of VFA will
not always be expressed as a pH drop due to the buffer capacity of some waste
types. In manure there is a surplus of alkalinity, which means that the VFA ac-
cumulation shall exceed a certain point before this can be detected as a signifi-
cant change in pH. This means that when a drop in pH in the reactor is eventu-
ally observed, the concentration of volatile fatty acids is most probably very high
and the process may already have been affected.
There are many factors which influence pH. It is especially organic acids and
carbon dioxide which lower pH, while ammonia will contribute to an increase of
pH. Other compounds contributing to the buffering capacity are hydrogen sul-
fide and phosphate.
2.3
Toxicity
A number of compounds are toxic to the anaerobic microorganisms. Methan-
ogens are commonly considered to be the most sensitive to toxicity of the micro-
organisms in anaerobic digestion. However, the process can acclimatize, and
higher concentrations of the toxicant can be tolerated after a period of adapta-
tion. The most common inhibitor for the anaerobic process is ammonia. In anaer-
Applications of the Anaerobic Digestion Process 5
obic digestion ammonia originates from soluble ammonia in the influent, from
protein degradation and other compounds such as urea. Many substrates used for
anaerobic treatment often contain ammonia in toxic concentrations. Such sub-
strates include pig and poultry manure, slaughterhouse waste, potato, juice,
highly proteinaceous sludge, wastewater from shale oil and coal liquefaction
processes. The results concerning ammonia-N inhibitory level are conflicting, as
they depend on parameters such as pH, temperature and adaptation of the in-
ocula. It is generally accepted that it is the non-ionized form of ammonia that is
responsible for inhibition, pH has a significant effect on the level of ammonia in-
hibition, as the pH value determine the degree of ionization. The free ammonia
ratio to total ammonia/ammonium ratio can be calculated from the equilibrium
relation as follows:
[NH3]/[T – NH3] = 1/(1+[H+]/Ka)
where, [NH3] and [T – NH3] are the free ammonia and the total ammonia/am-
monium concentrations respectively, and Ka is the dissociation constant, which
is temperature dependent.
In Fig. 1 this dependency is shown graphically. As pH and temperature in-
crease the free ammonia fraction also increases. Especially pH has a strong in-
fluence on the degree of ionization of ammonia. Bhattacharya and Parkin found
the maximum tolerable free-ammonia concentration to be only 55 mg-N/l. Most
reported inhibitory levels are, however, higher.Angelidaki and Ahring found that 
the biogas process could be adapted to tolerate free ammonia concentration 
of 800 mg-N/l. The inhibitory level of free ammonia depends strongly on the 
degree of acclimatization of inoculum to ammonia.
As the free ammonia concentration decreases with decreasing pH it could be
expected that even a slight reduction of pH would have a positive effect on ammo-
6 I. Angelidaki et al.
Fig. 1. pH and temperature influence on NH3 dissociation
nia inhibition. It was indeed found by several investigators that a pH decrease 
reduced ammonia inhibition. Free ammonia inhibition result in VFA accumula-
tion, which in turn lower pH and decrease the ratio of free ammonia, with the 
result that free ammonia inhibition is relieved. Due tothis self-stabilizing mech-
anism, processes can be maintained in a stable ammonia inhibited state, where a
balance between VFA concentration and ammonia loading exist.
The carbon/nitrogen (C/N) ratio is also important for process stability.A C/N
ratio of 25 to 32 has been reported to have a positive effect on the methane yield.
At lower C/N ratios the risk of excess nitrogen not needed for biomass synthesis
and therefore becoming inhibitory increases. Opposite, a very high C/N ratio
would lead to N deficiency for biomass synthesis.Waste with very high COD con-
centration and low content of nitrogen such as olive mill effluents has been
shown not to be able to be degraded alone. Addition of either nitrogen or codi-
gestion with wastes with a lower C/N ratio was needed in order to digest olive mill
effluents successfully.
Anaerobic treatment of wastewater containing high sulfate concentrations can
cause inhibition as a result of the formation of hydrogen sulfide. It has been 
reported that total hydrogen sulfide concentrations of 100 to 300 mg/l or free 
hydrogen sulfide concentrations of 50 to 150 mg/l caused severe inhibition re-
sulting in complete cessation of biogas production.
Long chain fatty acids, such as oleate and stearate, have been found to be toxic
to the anaerobic process. No adaptation to the fatty-acid toxicity was observed.
However, the presence of particulate material can increase the resistance of the
process to long-chain fatty acids as LCFA are absorbed on the particulate mate-
rial and thus not active as inhibitor. For other organic compounds such as phe-
nols, chloroform and formaldehyde a reversible toxicity has been observed.
Heavy metals are toxic for anaerobic microorganisms in concentrations in the
range 10–3 to 10–4 M. However, experiments have shown that acclimatization to
high heavy metal concentrations can occur and often the level of heavy metals
would become an environmental problem before affecting the process. In a re-
actor, the actual concentration of soluble metal ions is normally low due to pre-
cipitation of insoluble metal salts, e.g., as sulfides. It has been shown that less than
2% of the metals may be in the soluble form.
Although anaerobic bacteria are inhibited by several toxicants they can also
tolerate a number of toxicants. Complete anaerobic degradation of pen-
tachlorophenol has been reported. A number of other toxicants have been re-
ported to be detoxified in anaerobic reactors such as nitroaromatic compounds,
chlorinated aliphates, N-substituted aromatics and azo dyers.
3
Digestion of Slurries
3.1
General
One type of anaerobic waste treatment plants which have emerged since the mid
1980s in Denmark and elsewhere, are large scale manure treatment plants serv-
Applications of the Anaerobic Digestion Process 7
ing a number of farmers, treating a slurry of manure, often with a fraction of
other waste as supplementary substrate.
The main objective is to extract energy and to improve the fertilizing quality
of manure, resulting in less nutrient leakage to ground and surface waters from
the agricultural sector.
The description below gives an impression of the most important elements of
such a slurry treating biogas plant, also illustrating some of the practical design
considerations not directly related to the reactor system itself, as well as illus-
trating the gradual technical development waste treatment plants often undergo.
3.2
Process and Plant Configuration
The main elements in a typical large-scale manure biogas plant, are shown in
Fig. 2. Raw material is transported by special constructed lorries from/to farm-
ers and is kept in pre-storage buffer tanks. It is then pumped through heat ex-
changers into the reactor at the desired flow rate. The effluent is pumped to an
8 I. Angelidaki et al.
Fig. 2. Concept of manure biogas plants top Farm biogas plant bottom Centralized biogas plant
after-storage tank, before it is brought back to the farmers and finally utilized on
fields as fertilizer.
A biogas plant is in reality much more complex than described above. Many
unit operations are or may be utilized in such a plant i.e.:
– transport/pumping
– stirring/mixing
– macerating/grinding
– heat exchanging
– biogas treatment and cleaning
– biogas compression and transportation
– biogas storage
– filtration/separation
Figure 3a shows a picture of a biogas plant (Blaabjerg biogas plant, Denmark),
where some of these unit operations are employed, is shown.
In Fig. 4 the general lay-out of a full-scale biogas plant (Blaabjerg, Denmark)
is shown.
Many process and lay-out variations are possible, often dictated by local con-
ditions and needs.
3.3
Veterinarian Aspects
Mixing and redistribution of manure, from several farms, requires adoption of
a sufficient level of sanitation, in order to avoid the risk of spreading pathogens.
The veterinarian requirements in Denmark concerning manure are to ensure
that the manure is kept at a thermophilic temperature (> 50 °C) for a minimum
Applications of the Anaerobic Digestion Process 9
Fig. 3. A centralized biogas plant (Blaabjerg, Denmark)
of 4 hours. The required sanitation can be obtained directly in a thermophilic
process by observing special pumping routines, whereas a mesophilic process re-
quires a passive pre- or after sanitation stage (Fig. 5).
It has been documented that the above requirements ensure an effective
pathogen reduction, reaching a level equal to or better than 4 log-units when us-
ing Faecal streptococcus as an indicator organism. Certain types of supplemen-
tary waste suitable for codigestion in manure based biogas plants, such as
sewage-sludge, require more strict sanitation, due to the potential content of hu-
man pathogens. Originally such types of waste required heating to 70 °C for a
minimum of 1 hour.A special Danish veterinarian research effort, utilizing high
temperature resistant bacteria and vira as indicators, has resulted in the defini-
tion of alternative temperature/time combinations, which in conjunction with a
biogas process, are considered equivalent to a 70 °C/1 h heating. For a ther-
mophilic biogas process (minimum 52 °C), a guaranteed temperature/retention
time of 52 °C/10 hours, 53.5 °C/8 hours or 55 °C/6 hours is considered equivalent
to 70 °C/1 hour. The retention can take place in separate sanitation tanks before
or after the reactor stage, or directly in the reactor if pumping sequences allow
the necessary pauses. For mesophilic biogas plants slightly higher retention times
are required and sanitation must take place in a separate tank at a thermophilic
temperature level.While inclusion of “problematic” waste in the past required a
separate pretreatment step, all the biomass can now be sufficiently sanitized in
a single thermophilic process, provided the longer guaranteed retention times is
built into the design.
10 I. Angelidaki et al.
Fig. 4. Typical lay-out of a centralized biogas plant (Blaabjerg, Denmark). This plant is treat-
ing approx. 100,000 ton of manure and industrial waste per year, and producing approx. 3 mil-
lion m3 of biogas per year
3.4
Combined Digestion of Livestock Waste and Industrial Waste
Construction of Joint Biogas Plants gives the possibility for combined anaerobic
treatment and utilization of livestock waste and several types of organic waste
from the food processing industry.
Organic industrial waste is usually characterized by high pollution loads and
often contains high concentrations of rapidly degradable substrates such as sac-
charides, starches, lipids and proteins.
Manure usually has a rather low solids concentration (typically 3–5% total
solids (TS) for pigs and 6–9% for cattle and dairy cows). In addition the manure
contain particles and straw/fibers with a content of ligno-cellulose. This fiber
fraction is highly recalcitrant to degradation and will often pass through the re-
actor mainly undigested. The high content of water together with the high frac-
tion of fibers in manure is the reasonfor the low methane yields of manure, typ-
ically ranging from 10–20 m3 CH4/ton of manure treated.
However, manure is an excellent basic substrate for co-digestion of industrial
waste, which could otherwise be difficult to process alone. The reasons for this
are:
– the high content of water in manure acts as a solvent for the dryer types of
wastes, solving problems of pumping and mechanical treatment of solid
wastes.
Applications of the Anaerobic Digestion Process 11
Fig. 5. Plant configuration examples: 1 Thermophilic plant with two reactors and heat ex-
changing; 2 Mesophilic plant with two reactors, heat exchanging and thermophilic post sani-
tation
– the high buffering capacity of manure protects the process against failure due
to pH drop if the VFA concentration increase.
– manure is rich in a wide variety of nutrients necessary for optimal bacterial
growth.
By combining different types of waste, such as manure, municipal solid waste 
and organic industrial waste, a much higher gas yield can be obtained from 
biogas reactors since organic industrial wastes in particular are more easily 
degraded and have a higher gas potential than manure. As most types of indus-
trial organic waste result in methane yields varying from 30 to 500 m3/ton, these
sources constitute a very attractive supplementary substrate for manure based
biogas plants.
The gas yields from different types of organic industrial waste is shown in
Table 2.
Besides increasing the yield, addition of easily degradable material has been
shown to stabilize the anaerobic digestion process if added in a controlled fash-
ion. This effect could be partly due to a higher active biomass concentration in
the reactor, which will be more resistant to inhibitory compounds. Furthermore,
the inorganic parts of some organic wastes, such as clays and iron compounds,
12 I. Angelidaki et al.
Table 2. Methane yield from different types of industrial waste
Type of organic waste Composition of the Organic content Methane yield
organic material (%) (m3/ton) 
Stomach and Carbohydrates, proteins and lipids 15–20 40–60 
intestine content 
Flotation sludge 65–70% proteins, 30–35% lipids 13–18 80–130 
(dewatered) 
Bentonite- 70–75% lipids, 40–45 350–450
bound oil 25–30% other organic matter 
Fish-oil sludge 30–50% lipids and other 80–85 450–600
organic matter 
Source sorted organic Carbohydrates, proteins, and lipids 20–30 150–240
household waste
Whey 75–80% lactose and 20–25% protein 7–10 40–55
Concentrated whey 75–80% lactose and 20–25% protein 18–22 100–130 
Size water 70% proteins and 30% lipids 10–15 70–100
Marmelade 90% sugar, fruit organic acids 50 300 
Soya oil/Margarine 90% vegetable oil 90 800–1000
Methylated spirits 40% alcohol 40 240 
Sewage sludge Carbohydrates, lipids, proteins 3–4 17–22 
Concentrated sewage Carbohydrates, lipids, proteins 15–20 85–110
sludge 
have been shown to counteract the inhibitory effect of ammonia and sulfide, re-
spectively.
3.5
Process Tank Configuration
Continuous feeding/pumping is often preferred in order to optimize heat 
exchangers. In order to obtain the necessary guaranteed retention time for 
sanitation purposes, configurations with two or three reactors (or sanitation
tanks) operated in parallel are often seen, Fig. 6. At any given time, one reactor
is being fed, one emptied and one resting, i.e., ensuring the sanitation retention
time.
As fewer larger tanks are often preferred for economical reasons, various
schemes with discontinuous pumping or combinations with smaller buffer tanks
can also be seen.
Applications of the Anaerobic Digestion Process 13
Fig. 6. A biogas plant with two reactors and post sanitation
3.6
Equipment Characteristics
3.6.1
Mixing Technique
Initially, submerged electrically driven medium speed mixers, mounted on guid-
ing rails were utilized for most mixing tasks. These mixers soon proved rather
costly to operate, especially where access and routine service inspection is diffi-
cult as is the case for digesters or other closed process tanks. Slow moving, top
mounted central mixers with a freely suspended shaft with two propellers have
become the preferred solution for reactor mixing. The mixers usually work con-
tinuously with a mixing power input of 3–4 W/m3. Service requirements are very
limited, but a correct reactor level relative to the upper impeller is very critical
in order to avoid annoying floating layer formation. In storage tanks several types
of mixers are in use, including improved submerged models. The mixing energy
input is often discontinuous (high power for a short period) and average mixing
power input typically varies from 10 W/m3 in prestorage/mixing tanks to 1 W/m3
in after storage tanks. The position and type of mixers in combination with tank
geometry have proven to be very critical. Hydrodynamic favorable solutions, al-
lowing the material to flow to the mixers, generally work best in combination
with mixing at different depths. Figure 7 schematically shows the most common
mixer configurations.
14 I. Angelidaki et al.
Fig. 7. Common mixer configurations: A Top mounted reactor mixer; B Side mounted mixer
in process tank/storage; C Submerged mixer on guiding rail; D Top mounted mixer; E Top
mounted mixer with submerged angle gear
In Fig. 8 a picture of the reactor top with a centrally mounted top mixer is
shown.
3.6.2
Pumping
Eccentric worm pumps have been used for continuous feeding/extraction of
biomass (low flow, high pressure) while centrifugal pumps with open impellers
and cutters have been used for biomass transfer between storage tanks (high
flow, low pressure). Feeding pumps, in particular those handling fresh biomass
have proven to be rather costly to operate due to high wear rates, but, despite 
several efforts, no viable alternative has been found so far. Low pump speed
(< 300 rpm) and a pressure drop as low as possible can somewhat alleviate 
the problems.
Applications of the Anaerobic Digestion Process 15
Fig. 8. Reactor top, showing centrally mounted top mixer. Mixer shaft extends to the bottom of
the reactor (approx. 17 m) with two propellers (3.2 m) placed at the top (just below liquid sur-
face) and at the bottom
3.6.3
Heat Exchanging
Most biogas plants in Denmark utilize heat exchanging between in-going fresh
biomass and outgoing digested biomass, although a few plants have been built
with no or limited heat exchanging. The choice depends on the value of heat, i.e.,
the possibility of utilizing heat for other purposes.
Due to the high viscosity of manure and other biomass, especially when fresh,
it is seldom possible to obtain a turbulent flow without generating an excessive
pressure drop. Laminar or transition flow is the result with rather poor heat
transfer coefficients. The most common types of exchangers utilize curved flow
channels to break up the thermal boundary layers by secondary flow or multi
pass design to break up the flow at regular intervals. Most designs attain heat
transmission values of approx. 300 W/m2/°C with flowing velocities up to approx.
1 m/s, when exchanging fresh biomass against digested biomass, or approx. two
times as much if exchanging against water. Dimensioning, especially for pressure
drop, relies very much on experience, as prediction of biomass viscosity is very
uncertain.A good design margin and spare pump capacity is necessary to avoid
problems. In designs with rather narrow flow passages (< approx. 30 mm) it is
considered wise to macerate the fresh biomass, which is also likely to improve the
biogas yield slightly.
Different heat exchanger configurations are shown in Fig. 9.
Typical dimensioning practice (in Denmark) leads to heat exchanger instal-
lations with a resulting temperature difference of 10–15 °C. The remaining heat-
16 I. Angelidaki et al.
Fig. 9. Different heat exchanger configurations
ing up to process temperature, and for heat loss compensation, is by hot water
coils in the reactors or with a final biomass/water heat exchanger plusfewer coils
for heat loss compensation. In this connection, it is important to recognize and
include the heat loss due to evaporation of the water contained in the humid bio-
gas leaving the reactor. A typical heat exchanger used in full scale biogas plants
is shown in Fig. 10.
On the input side of heat exchangers (and pumping systems) the risk of clog-
ging is manly due to the possible presence of foreign objects. On the digestate
side of heat exchangers, one must reckon with the formation of struvite
(MgNH4PO4), which forms due to over-saturation when the biomass is cooled.
The scaling occurs mainly in the cold sections and will gradually close flow chan-
nels. The struvite needs to be dissolved by flushing with a circulating weak acid
at regular intervals. Residual scaling also occurs in cold pipes down stream of
heat exchangers, although at a much slower rate.
3.6.4
Biogas Cleaning
Dry biogas from manure generally consist of approx. 60–70% v/v methane
(CH4), 30–40% v/v carbon dioxide (CO2), 1–2% v/v nitrogen (N2), 1000–
3000 ppm hydrogen-sulfide (H2S) and 10–30 ppm NH3 . The concentration of
hydrogen-sulfide stated is typical when operating on manure alone. Other 
Applications of the Anaerobic Digestion Process 17
Fig. 10. Typical exchanger used in full scale biogas plants
substrates, even in relatively small amounts, can result in quite different levels,
both lower and higher.
Raw biogas is a humid gas, which needs to be cooled and carefully drained be-
fore utilization. Early attempts to dry biogas using regenerative absorption
equipment designed for pressurized air proved difficult due to accelerated ab-
sorbent decomposition, and the preferred solution has instead become to sim-
ply cool the biogas and to design the gas system with care to avoid undrained low
points. In the case where the biogas is to be utilized in a boiler and emission re-
quirements for sulfur-dioxide are not too strict, no further cleaning is necessary.
In many cases, however, the biogas is utilized in gas engines for Combined Heat
and Power (CHP) generation. In this case the hydrogen-sulfide content needs to
be kept below approx. 700 ppm for conventional gas engines in order to avoid ex-
cessive corrosion and too rapid (and costly) lubrication oil deterioration. One
method of reducing the hydrogen-sulfide content is to add a commercial ferrous
solution to the reactor feed. Ferrous compounds bind sulfur as insoluble prod-
ucts in the liquid phase, which reduces the evolution of gaseous hydrogen-sulfide.
The method has been utilized in a number of plants, but is rather costly if applied
continuously, since the consumption of ferrous material on a stoichiometric ba-
sis has proven to be 2–3 times the desired reduction in gaseous hydrogen-sulfide.
In some cases, however, ferrous containing waste products can be obtained for
codigestion most of the year, with commercial ferrous addition acting as a back
up possibility only.
Often secondary H2S biogas cleaning is necessary, and in this case a biologi-
cal oxidation process has become the dominating solution. By injection of a small
amount of air (2–8 % v/v) into the raw biogas, the hydrogen-sulfide content can
be biologically oxidized either to free (solid) sulfur or (aqueous) sulfurous acid
according to following reactions:
2 H2S + O2 Æ H2O + 2 S (1)
2 H2S + 3 O2 Æ 2 H2SO3 (2)
The reaction will occur spontaneously and can take place in the reactor 
headspace on the floating layer (if any) and reactor walls, if air is injected 
directly in the headspace. Due to the acidic nature of the products with a 
risk of corrosion and the dependency of a stable floating layer, it is often 
preferred to isolate the process in a separate reactor as shown schematically 
in Fig. 11.
The reactor is somewhat similar to a scrubber, consisting of a porous filling
(randomly packed plastic elements or similar) where microorganisms can grow,
and a sump, pump, and nozzle arrangement allowing regular showering of the
filling. Showering has the function of washing out acidic products and supplying
nutrients to the microorganisms. The sump must therefore contain a liquid with
a high alkalinity and contain essential nutrients, for which digested manure,
preferably screened, is the ideal and readily available choice.A reactor loading of
approx. 10 m3/h of biogas per m3 of reactor filling and a process temperature
around 35 °C has been the normal choice, and the process has proven very effi-
cient, provided that sufficient air is injected (slightly more than stoichiometri-
18 I. Angelidaki et al.
cally needed). Sump pH must be maintained at 6 or higher.A washing procedure,
where the filling elements are gurgled through with an air/water mixture, has to
be carried out at regular intervals in order to prevent free sulfur deposits from
closing the reactor filling.
In some cases, where biogas is stored or passing through a digestate after 
storage, the H2S reactor is omitted and only air injected. Cleaning is then 
relying on the formation of a floating layer in the after storage, on which 
the microorganisms can grow and perform the oxidation. A floating layer can
usually be maintained with the choice of a low mixing intensity, without too
many problems for operating the tank as a buffer storage. This solution is 
certainly more cost effective, but can also be more unreliable as floating 
layers can be rather unstable, i.e., sinking overnight perhaps to resurface some
days later. At least some periods with reduced cleaning efficiency can be ex-
pected.
In Fig. 12, a reactor used for biological hydrogen-sulfide reduction is 
shown.
Applications of the Anaerobic Digestion Process 19
Fig. 11. Schematic diagram of system for biological H2S oxidation
3.6.5
Biogas Transmission
Biogas Plants are usually situated at some distance from the nearest town, while
biogas utilization in an engine CHP installation often takes place inside or in the
vicinity of the town in order to connect to a district heating system. As a conse-
quence biogas often has to be transported up to several km between the plants.
The first plants utilized high pressure (2–4 bar) dry transmission with piston
compressors and absorption drying equipment.As mentioned earlier, absorption
drying presented problems, and as piston compressors also proved vulnerable
and operationally expensive, which has caused the low pressure wet transmission
to be gradually taken over. This solution usually involves a transmission pressure
in the range 400–700 mbar in order to deliver at 100–200 mbar, utilizing Roots
blowers and a carefully planned buried transmission line, laid with a minimum
3–5‰ slope (preferably in the flow direction) with a limited number of con-
densate wells. The bulk of condensation occurs in the first few wells within
100–500 m from the plant. Wells are equipped with valves for regular manual
emptying, and the wells are usually designed to hold up to a weeks amount of
condensate, meaning that the first wells must be quite large (typically 2 m3), while
the last only need to be small (typically 100 liters).
20 I. Angelidaki et al.
Fig. 12. Hydrogen sulfide reduction reactor, 80 m3 with 50 m3 filling material. H2S is oxidized
by a biological process to acidic products or free sulfur, with injection of a small amount of
atmospheric air upstream of the reactor
At transmission distances up to a few hundred meters, high speed centrifugal
blowers and a transmission pressure of approx. 100 mbar can be employed with
advantage.
3.6.6
Fiber Separation
Manure and other types of biomass contain fibers (from straw) and other organic
structures which are difficult for anaerobic organisms to access.A typical degra-
dation “efficiency” of 50–60% is often seen in manure based systems, meaning
that the digestate still contain 40–50% of the original organic dry matter content
primarily as fibers. Some of the early plants included separators to take out a part
of the fibers for production of commercial compost. Marketing thiscompost in
relatively small scale proved quite difficult, and compost production has ceased
in most plants. Some of the newest plants in Denmark again include separation
equipment. The intention now is to produce a fiber fraction with a dry matter
content in excess of 45%, which can be utilized as a supplementary fuel in wood
chip boilers. This way the overall energy efficiency can be raised by approx. 15%
through additional heat production. A side benefit, which in the near future
might add to the feasibility of this scheme, is the removal of surplus phosphorous
which is predominantly attached to the fibers.
Screw type separators dominate, sometimes in combination with a pre-sepa-
ration in order to increase the capacity (Fig. 13).
Applications of the Anaerobic Digestion Process 21
Fig. 13. Fiber collection wagon, with distribution screw
3.6.7
Odor Control Systems
One of the benefits from anaerobic digestion of manure is a considerable reduc-
tion in odor nuisance when spreading manure on fields. The biogas plant itself
can, however, give rise to local odor nuisance if care is not taken to limit odor
emissions.
As a first preventive measure, it is of course desirable to situate the plant (if
possible) without immediate neighbors. In addition all storage tanks should be
covered. Due to service openings and as unloading/loading and internal pump-
ing generates tank breathing, it is necessary to arrange a weak suction from the
storage tanks and other contaminated sections of the plant. This air must be
treated in order to avoid odor emission.
Air treatment is usually chosen either as a biological cleaning in a compost 
filter or as a combustion. Compost filters are relatively cheap but operational 
experience. Careful attention and maintenance are required in order to ensure 
satisfactory cleaning, as filters sometimes fail due to acidification (oxidation of
hydrogen-sulfide), might collapse and become too dense, or might run dry. Com-
bustion in a boiler in conjunction with generating process heat for the biogas plant
has been tried, but the need for combustion air is limited, enforcing a compromise
on venting flow. Combustion in engines (together with the biogas) has so far not
been attempted, due to the fear of affecting the engine negatively. In a few plants,
a system involving combustion with regenerative heat exchanging has been em-
ployed, a technique commonly utilized for the combustion of gaseous solvents in
industrial ventilation air.Although a rather expensive solution, results are positive.
3.7
Operational Experience and Results
3.7.1
Production Results
Figure 14 shows the average monthly biogas production during the first opera-
tional period of two thermophilic Biogas plants, both with a biomass treatment
capacity of 100,000 ton biomass per year. The figure shows typical start-up 
periods of 4–6 months and illustrates the relative stability of the production,
once normal operation is obtained. The sudden rise in biogas production of one
of the biogas plants after month 13, was due to the introduction of residual fish
oil as a co-substrate.
Consumption of process energy of relatively new plants typically amount to
approx. 4–5 kWh per m3 biomass treated for mixing, pumping, control etc., and
a heat consumption of 15–25 kWh per m3 of biomass treated, including con-
sumption for hot water, heating of buildings, etc. This should be compared to a
biogas yield of approx. 30 m3 per m3 biomass, corresponding to a lower calorific
value (LCV) yield of approx. 200 kWh, i.e., 10–15% of the energy produced is
consumed in the process. These figures represent the state of the art level for
large-scale manure based biogas plants in Denmark, achieved as a result of the
technical development described shortly above.
22 I. Angelidaki et al.
4
Digestion of High-Solid Wastes
A special problem arises when solid wastes are to be treated.Anaerobic treatment
of waste water and other liquid types of wastes have been used for decades. How-
ever, treatment of high-solids wastes such as municipal solid waste (MSW) is a
relatively new application of anaerobic treatment, and many technical aspects
need special consideration when inhomogeneous urban waste is the source ma-
terial.
“High-solid wastes” might be defined as organic material with a content of
solids between 10 to 40%, which is not fluid. The most important type of high
solid wastes is Municipal Solid Waste (MSW) or Household Solid Waste (HSW).
The most common way to dispose of MSW has until recently been land filling
or incineration. However, in the recent years attention has focused on recycling.
By recycling the valuable elements of waste are converted to useful products and
returned to the supply chain, minimizing the pressure on land filling and incin-
eration facilities, while also conserving primary resources.
Recycling started as a positive but minor part of waste management, but has
become a more and more dominant element in the waste policies and strategies
in many countries.
MSW is a heterogeneous waste, which can be divided into the following frac-
tions:
– an organic biodegradable fraction, consisting of food residues and sometimes
garden wastes
Applications of the Anaerobic Digestion Process 23
Fig. 14. Average monthly biogas production from thermophilic manure based biogas plants:
from Thorsoe (1994) and Blaabjerg (1996)
– a combustible fraction, consisting of an organic recalcitrant fraction, such as
wood, paper and plastic
– an inert fraction, consisting of stones, glass, metal and other inorganic parts
Waste composition varies enormously, both between countries and between lo-
cations in the same country. In urban built-up areas, MSW is less rich in organic
biodegradable material compared to rural areas and the composition also varies
with the social level of the area.
The organic biodegradable fraction of the MSW can be treated by anaerobic
digestion to produce energy and fertilizer. By anaerobic degradation, one ton of
the organic fraction of MSW can give a net energy yield of 100 to 200 kWh elec-
tricity plus heat.
Most often the technical aspects involved are:
– source separation and collection
– separation (removal of plastic bags or other packing and foreign elements)
– transfer (transporting the material through the process/plant)
– heating the substrate
– reactor mixing
– sanitation of the biomass
Digestion of high solids wastes can take place as a continuous or batch process.
Continuous processes are most desirable on a larger scale. There are many waste
treatment systems specifically designed for treatment of municipal waste (MSW).
The most treatment systems consist of the following steps:
– Pretreatment
– Biological treatment
– Post-treatment
4.1
Pretreatment of Municipal Solid Waste
Municipal solid waste typically contains less than 1/3 organic material. In order
to treat MSW biologically, a sorting of the waste is needed. The sorting can be
achieved by either source-separation, mechanical separation, or by hand sorting
techniques.
Source-separated municipal waste (SSMSW), i.e., waste sorted into frac-
tions where it is generated, is the preferred solution from a quality point of
view, since it is difficult to derive a clean organic fraction once the waste has 
been mixed.
Such systems are found only in limited areas. However, in several places in 
Europe, work is being done, in order to introduce source separation of the MSW.
Source-separated collection was originally introduced to derive certain valuable
components, due to the increased cost of raw materials and a wish to limit the use
of primary resources and energy to process new raw materials.
As a consequence, source-sorted collections were introduced mainly for paper,
valuable metals (such as aluminum and copper), and glass etc. Lately source-
separation has also included separation of an organic degradable fraction. This
24 I. Angelidaki et al.
is done by including separate waste bins and collection systems in the house-
holds.An alternative scheme, which has been implemented in some areas,
is source separation into bags of different colors, but collected with only one
truck and later sorted out into seperate fractions by an optical sorting machine.
This way the extra cost of separate collection or multi compartment collection
trucks can be avoided (by utilizing existing collection trucks), albeit at the 
expense of a sorting machine.
Most of the MSW is, however, still not source-separated. Therefore, pretreat-
ment is often needed to sort out the organic fraction of mixed MSW. The main
goal in the different pretreatment systems is to separate the organic fraction from
the inorganic material. Furthermore, effort is applied in recycling several useful
components, such as ferrous metals.
The fraction obtained by mechanical separation is usually more contaminated
then the fraction obtained by source-separation, and a fraction of the organic
material will be lost with the other fractions sorted out. It is especially the con-
tent of heavy metals and plastics that is higher in the mechanical separated MSW.
Even in case of source separation, some form of pretreatment is often needed to
guard the plant and end product against elements included in the organic frac-
tion by mistake or carelessness.
4.2
Post Treatment
Following the biological treatment, the digested material (known as digestate or
effluent) is usually dewatered to 50–55% TS with a screw press, filter press or
other types of dewatering systems. The press cakes are refined with sieve and
composted aerobically. At this point, the compost can be further cleaned by
screening, to remove unwanted material such as small pieces of glass or plastic.
The compost is often offered for sale as a soil conditioner or potting soil (nitrate
approx. N = 0.25 kg/ton, total N approx. 7–8 kg/ton). Press liquid, which may con-
tain high concentrations of volatile fatty acids, is centrifuged, recircled or sent to
wastewater treatment.
4.3
Biological Treatment
There are various classification principles of the existing anaerobic systems for
treatment of high solids wastes.
There are batch and continuous systems according to the feeding procedure
of the wastes to the reactors. Most systems are continuous systems. However,
there also exist batch systems such as the Biocel process.
The major difference between the various systems is whether a “wet” or “dry”
methanization process is used. Digestion of the MSW can take place either in the
mesophilic or the thermophilic temperature range, and the hydraulic retention
time is 10 to 30 days, depending on the process temperature, the technology used,
and the waste composition.
Applications of the Anaerobic Digestion Process 25
4.3.1
Wet Digestion Systems
With wet digestion systems the total solids content of the waste has to be re-
duced to concentrations below 20% in order to create a pumpable slurry. This 
can be done either by adding water, such as recycled process water, or by co-
digestion where MSW is mixed with more dilute streams such as sewage sludge
or manure.
Codigestion Process
Treating high solid waste requires technically complicated and expensive treat-
ment systems. Therefore, it is desirable, if possible, to avoid treating the waste as
a solid, but instead mix it with other more dilute waste.
A new concept, which has been applied successfully in several biogas plants in
Denmark (Vegger, Sinding Ørre, Studsgård and Århus) is mixing MSW with ma-
nure. Sweden's largest biogas plant in Kristianstad work with the same co-di-
gestion concept: household waste is treated together with agricultural waste and
industrial waste. The plant at Kristianstad has a capacity of 73,000 tons of bio-
mass per year, of which 15% is OFMSW, 18% is industrial organic waste, and 67%
is manure. The biogas yield is around 40 m3 biogas per ton feedstock and the pro-
duction covers the heat requirements of 600 to 800 households. In this way some
of the difficult technical problems treating high solid wastes are avoided. Also
economy of scale can be achieved and the waste enters into an established nu-
trient/fertilizer recycling system. The plants generally operate at thermophilic
temperatures.
Another example of the excellent performance of a large-scale co-digestion
process of the organic fraction of municipal solid waste is the sewage treatment
plant in Grindsted, Denmark where MSW is mixed with sewage sludge.
The Waasa Process
Another process using codigestion is the Waasa process. The process has been
tested on a number of wastes. In the Waasa plant in the city of Vaasa, Finland the
following types of wastes have been treated over the years since it started up in
1989; mechanically or source-separated MSW, sewage sludge, slaughterhouse
waste, fish waste, and animal manure. The process operates with a TS content of
approx. 10–15% and can operate both at mesophilic and thermophilic temper-
atures.At the Waasa plant both mesophilic and thermophilic treatment methods
are in operation in two parallel reactors. Today the Waasa Process is also in op-
eration in Kil, Sweden and outside Tokyo, Japan. Furthermore, a plant in Gronin-
gen, the Netherlands is under construction. One characteristic of the Waasa
Process is its main reactor, which is divided into various zones in a simple way.
The first zone is made up of a pre-chamber inside the main reactor.
The mixing in the reactor is by pneumatic stirring, where biogas is pumped
through the base of the reactor. A small part of the digestate is mixed into the
newly fed bio waste to speed up the process by inoculation.
26 I. Angelidaki et al.
4.3.2
Dry Digestion Systems
In many cases, especially in large urban areas, it is difficult to find other wastes
to codigest MSW with. Dry digestion systems can cope with solids as high 
as 35%.
The VALORGA Process
The Valorga process was developed in France and is a semi-dry process (Fig. 15).
The process is a mesophilic process and takes place in the following way: after
pretreatment the waste is mixed with recycled process water. The process water
is gained from separation of the reactor effluent by centrifugation, filtration or
other types of separation.After mixing with process water, the influent is pumped
into the reactor. The reactor is of the fully mixed reactor type. Mixing is taking
place by pneumatic stirring, i.e., the produced biogas is compressed and sent
through the contents of the reactor.
Only a small amount of water is recirculated, and the total solids content of the
waste in the reactor is still high. Other processes also use the same principle, i.e.,
recirculation of small or large amounts of process water.
The Valorga process is a relatively widely used process. There are several 
full-scale plants worldwide, such as in Amiens, France (85,000 ton/year),
Grenoble, France (16,000 ton/year), Tilburg, Netherlands (52,000 ton/year)
(Fig. 16), Papeete, Tahiti (90,000 tons/year) and Tamara in French Polynesia
(92,000 tons/year).
The DRANCO Process
The Dranco (Dry Anaerobic Composting) process is a true dry-process for treat-
ment of the organic fraction of MSW (Fig. 17). Indeed, this process requires a
high total solids content in the reactor in order to have optimal performance.
Applications of the Anaerobic Digestion Process 27
Fig. 15. Principle diagram of the Valorga process
Therefore, it is often recommended to mix non-recyclable paper or garden waste
into the MSW in order to achieve a sufficiently high TS content. The Dranco
process is a thermophilic process.
The process takes place in the following way: after the waste is pretreated and
screened it is mixed with recirculating material from the reactor. Three quarters
of the reactor content is recycled. Mixing of the waste with this large amount of
digested material ensures inoculation of the incoming material.
The biomass paste is pumped using piston pumps developed to pump con-
crete casting mixtures.
The reactor is a downward plug-flow type reactor. In the reactorno significant
mixing takes place. If the waste consists of high amounts of easily degradable 
28 I. Angelidaki et al.
Fig. 16. View of a Valorga biogas plant (Tilburg, Holland). The plant has capacity of
52,000 tonnes per year of source separated VGF (Vegetable-Garden-Fruit) waste
Fig. 17. Principle diagram of the Dranco process
material, the degradation can cause liquefaction of some of the material in the 
reactor, preventing the plug flow principle with the risk that some particles will
flow through the reactor without achieving the required retention time. There-
fore, addition of recalcitrant ligno-cellulose material is usually recommended.
The digested biomass is extracted from the bottom of the reactor by a special
patented sliding frame, pulling material evenly from the reactor cross section into
an extraction screw channel.
Today, there are several DRANCO plants in operation, Fig. 18, such as the one
in Brecht, Belgium (12,000 ton/year), Salzburg,Austria (20,000 ton/year), Bassum,
Germany (13,500 ton/year), and Kaiserslautern, Germany (20,000 ton/year).
The Kompogas Process
The Kompogas process is a dry process developed in Switzerland. The process
operates in the thermophilic range with a hydraulic retention time of approx.
15 days. The reactor is a horizontal cylinder and the flow through the reactor is
plug flow. In the reactor a stirrer provides some mixing of the waste. Recircula-
tion of a part of the effluent to the incoming substrate ensures inoculation.
4.3.3
Multi-stage Anaerobic Digestion Systems
Most of the digestion systems for anaerobic treatment of MSW are single-stage
reactor systems. However, multi-stage digestion systems are also used. These are
usually two-stage systems, although 3-stage systems have also been proposed.
The idea with the multi-stage systems is to separate the different phases of the
anaerobic digestion process, in order to be able to apply optimal conditions in
Applications of the Anaerobic Digestion Process 29
Fig. 18. View of a Dranco biogas plant
each of them. Different operating conditions, such as pH and retention time can
be kept in the different stages.
The BTA Process
The BTA process was developed in Germany (Kubler and Schetler 1994). The
process is a multi-stage process (Fig. 19). Pretreatment is centered on a hy-
dro-pulper, which receives the source-sorted waste from a screw-mill, which
opens bags and disintegrates larger agglomerated particles. In the hydro-pulper,
the waste is mixed with recirculated process water and the organic material is dis-
solved through intense agitation. Floating light elements (plastic, wood etc.) 
are skimmed from the surface by a periodically operated rake and heavy 
items (metal, glass etc.) are removed by sedimentation. The pretreatment pro-
duces a pulp of approx. 10% total solids (TS). The pulp is pumped to a buffer tank
where acidification occurs. The effluent from the acidification reactor is dewa-
tered by centrifugation. The thin liquid fraction is fed into a high flow biofilm
reactor; while the thick fraction of undissolved material is mixed with process
water and fed to a CSTR reactor where further hydrolysis and acidification takes
place. The effluent from the CSTR reactor is once again dewatered, and the 
liquid fraction fed into the biofilm reactor for methanization at mesophilic con-
ditions.
A part of the thick fraction is removed from the process to take out inert and
undegradable material with a TS content of approx. 35%. This “compost”contains
approx. 0.2–0.3 kg N per ton. Excess process water, which contains most of the
nitrogen (about 4–8 kg N/ton original solid waste; see below) and other nutri-
ents that were in the original waste, is disposed of to the sewer system or recy-
cled to the agricultural sector as a fertilizer if possible. The volatile solids (VS)
destruction is predicted to approx. 85%.
30 I. Angelidaki et al.
Fig. 19. Principle diagram of the BTA process
4.4
Summary of Processes Used for Anaerobic Treatment of Solid Wastes
There is a wide variety of configurations for anaerobic treatment of solid wastes.
One way to classify these reactor systems is depicted in Fig. 20. The batch sys-
tems can be considered as accelerated landfill systems. These systems are simple
and comparatively cheap. MSW is loaded batch wise in a closed vessel contain-
ing inoculum from a previous batch digestion. During the digestion period,
leachate is recirculated for mixing purposes of substrate, microorganisms and
moisture.When the digestion is complete, the digested material is unloaded and
a new batch digestion is initiated.
An alternative to batch digestion is the leach bed process, where the leachate
from the base of the reactor is exchanged between established and new batches
to facilitate start up, inoculation and removal of volatile acids in the active reac-
tor.When the methanogenesis is established, the reactor is connected to another
reactor containing new MSW. This concept has also been described as sequential
batch anaerobic composting (SEBAC).
The continuously operating systems can be divided into completely mixed and
plug-flow systems. The completely mixed systems can again be classified as sys-
tems based on recirculation of process water for dilution of the incoming MSW
and in systems based on the codigestion concept. Codigestion is especially well
established in Denmark. Several systems are operating on the multi-stage diges-
tion concept. However, one-stage systems are much simpler and cheaper and
therefore, considerably more widespread.
Applications of the Anaerobic Digestion Process 31
Fig. 20. Classification of anaerobic solid waste digestion systems
5
References
1. McInerney MJ, Bryant MP, Stafford DA (1980) Metabolic stages and energetics of micro-
bial anaerobic digestion. In: Stafford DA, Wheatley BI, Hudges DE (ed) Anaerobic diges-
tion. Applied Science, London
2. Gujer W, Zehnder AJB (1983) Water Sci Technol 15:127
3. Allison MJ (1978) Appl Environ Microbiol 35:872
4. Switzenbaum MS, Giraldo-Gomez E, Hickey RF (1990) Enzyme Microb Technol 12:722
5. Ahring BK, Sandberg M, Angelidaki I (1995) Appl Microbiol Biotechnol
6. Dugba PN, Zhang H (1999) Bioresour Technol 68:225
7. Whitmore TN, Lazzari M, Lloyd D (1985) Biotechnol Lett 7 :283
8. van Lier JB, Rebac S, Lettinga G (1996) In: Proceedings of the IASWQ-NVA Int. conf. on 
advanced wastewater treatment
9. Varel VH, Hashimoto AG, Chen YR (1980) Appl Environ Microbiol 40:217
10. Hashimoto AG (1982) Agriculural Wastes 4 :345
11. Ahring BK (1995) Ant van Leeuw 67:91
12. Chen YR, Day DL (1986) Agriculural Wastes 16:313
13. Fang HHT, Wai-Chung Chung D (1999) Water Sci Technol 40:77
14. Archer DB (1983) Enzyme Microb Technol 5 :162
15. Buhr HO, Andrews JF (1977) Wat Res 11:129
16. Varel VH, Isaacson HR, Bryant MP (1977) Appl Environ Microbiol 33:298
17. Hashimoto AG (1982) Biotechnol Bioeng 14:2039
18. Mackie RI, Bryant MP (1981) Appl Environ Microbiol 41:1363
19. Madamwar D, Patel A, Patel V (1990) J Ferment Bioeng 70:340
20. Casali GB, Senior E (1989) J Chem Tech Biotechnol 44:31
21. Hashimoto AG, Varel VH, Chen YR (1981) Agriculural Wastes 3 :241
22. Hashimoto AG (1983) Biotechnol Bioeng 25:185
23. Nyns EJ, Schönborn W (1986) Biomethanation processes, Berlin: Wiley-VCH Weinheim,
(8): Microbial degradations, p 207
24. Kato MT, Field JA, Kleerebezem R, Lettinga G (1994) J Ferment Bioeng 77:679
25. Rebac S, Ruskova J, Gerbens S, van Lier JB, Stams AJM, Lettinga G (1995) J Ferment Bioeng
80:499
26. Björnsson L (2000) Intensification of the biogas process by improved process monitoring
and biomass retention, Dissertation, Lund University, Lund, Sweden
27. Zinder SH (1993) Physiology and ecology of methanogens. In: Ferry JG (ed) Me-
thanogenesis. Ecology, physiology, biochemistry and genetics. Chapman and Hall,
New York
28. Moosbrugger RE, Wentzel MC, Ekama GA, Marais GR (1993) Water SA 19:11
29. Mosey FE, Fernandes XA (1989) Water Science and Technology 21:187
30. Wilcox SJ, Hawkes DL, Guwy