Aalborg Universitet
Monitoring of anaerobic digestion processes: A review perspective
Madsen, Michael; Holm-Nielsen, Jens Bo; Esbensen, Kim
Published in:
Renewable & Sustainable Energy Reviews
DOI (link to publication from Publisher):
10.1016/j.rser.2011.04.026
Publication date:
2011
Document Version
Accepted author manuscript, peer reviewed version
Link to publication from Aalborg University
Citation for published version (APA):
Madsen, M., Holm-Nielsen, J. B., & Esbensen, K. (2011). Monitoring of anaerobic digestion processes: A review
perspective. Renewable & Sustainable Energy Reviews, 15(6), 3141-3155.
https://doi.org/10.1016/j.rser.2011.04.026
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Please cite this article as:
Madsen M; Holm-Nielsen JB; Esbensen KH. Monitoring of anaerobic digestion processes: A
review perspective. Renewable Sustainable Energy. Rev. 2011, 15, 3141–3155
Enquiries about this article should be sent to:
michael.madsen@maskau.dk
Michael Madsen
October 2011
Monitoring of anaerobic digestion processes: a review perspective
Michael Madsen*, Jens Bo Holm-Nielsen, Kim H. Esbensen
Aalborg University, Section of Chemical Engineering, ACABS Research Group
Niels Bohrs Vej 8, DK-6700 Esbjerg, Denmark
Tel.: +45 9940 9940, fax: +45 9940 7710, e-mail*: mima@bio.aau.dk
Abstract
The versatility of anaerobic digestion (AD) as an effective technology for solving central
challenges met in applied biotechnological industry and society has been documented in
numerous publications over the past many decades. Reduction of sludge volume generated from
wastewater treatment processes, sanitation of industrial organic waste, and benefits from
degassing of manure are a few of the most important applications. Especially, renewable energy
production, integrated biorefining concepts, and advanced waste handling are delineated as the
major market players for AD that likely will expand rapidly in the near future.
The complex, biologically mediated AD events are far from being understood in detail however.
Despite decade-long serious academic and industrial research efforts, only a few general rules
have been formulated with respect to assessing the state of the process from chemical
measurements. Conservative reactor designs have dampened the motivation for employing new
technologies, which also constitutes one of the main barriers for successful upgrade of the AD
sector with modern process monitoring instrumentation.
Recent advances in Process Analytical Technologies (PAT) allow complex bioconversion
processes to be monitored and deciphered using e.g. spectroscopic and electrochemical
measurement principles. In combination with chemometric multivariate data analysis these
emerging process monitoring modalities carry the potential to bring AD process monitoring and
control to a new level of reliability and effectiveness. It is shown, how proper involvement of
process sampling understanding, Theory of Sampling (TOS), constitutes a critical success factor.
We survey the more recent trends within the field of AD monitoring and the powerful
PAT/TOS/chemometrics application potential is highlighted. The Danish co-digestion concept,
which integrates utilisation of agricultural manure, biomass and industrial organic waste, is used
as a case study. We present a first foray for the next research and development perspectives and
directions for the AD bioconversion sector.
Keywords
Anaerobic digestion (AD), process monitoring, Process Analytical Technologies (PAT), process
sampling, Theory of Sampling (TOS)
1. Introduction
1.1 Major applications of anaerobic digestion
Anaerobic digestion (AD), also referred to as biogasification and biogas production, is a versatile
technology platform that can serve many purposes in industry and society. For example,
wastewater treatment processes generate vast amounts of sludge, which is expensive to deposit
or incinerate. The sludge volume is effectively reduced by means of AD. Moreover, renewable
energy in the form of biogas is recovered during the microbial decomposition of the organic part
of sludge, significantly reducing the costs for treating wastewater. Another widespread
application for AD is dedicated energy production. Recent European political incentives seek to
encourage application of AD in agriculture, the main motivators being a strong desire for
reducing the environmental impact from agricultural activities and at the same time enhance the
utilisation of nutrients applied for crop-cultivation. Finally, also treatment of the organic fraction
of municipal solid waste (OFMSW) and organic industrial waste is effectively done by means of
AD. [1], [2], [3], [4]
1.2 Bio-economy
Production of renewable, clean energy can be accomplished in several ways, e.g. by solar
energy, wind energy, hydropower and nuclear fusion, which are all documented viable
alternatives to fossil fuels. Another route for production of clean, renewable energy is via the
biorefinery. A petrochemical refinery is a well-established technology platform that has been
providing the industry with essential chemical building blocks for decades, all derived from
fossil oil. A biorefinery is the precise biotechnological counterpart, but here the raw materials are
cultivated crops, energy crops, and organic industrial residues and wastes. The biorefinery has
the advantage over the petrochemical refinery that it is able not only to supply bulk-
commodities, but especially also specialised, high-value products such as enzymes, flavour
compounds and additives for the food industry, and important pre-cursors for pharmaceutical
production enabling the biorefining concept to be a versatile ingredient and raw material supplier
for many industrial and societal sectors. [5]
1.3 Resources for bio-refinery concepts
Conventional crops grown for human consumption and for production of animal feed are
politically not recommended for energy production. Meanwhile, agricultural residues (sometimes
wrongly categorised as wastes) such as straw are currently not being utilised many places and are
therefore readily available in abundant amounts. New crops and varieties engineered for the
purpose of high yield (and not intended for human consumption) can form the basis of future
biorefining concepts. Large unexploited resources are available. These resources include organic
waste from the catering and food processing industries, organic by-product fractions from
chemical industries, and residues and wastes from agricultural activities. It is not necessarily
seamless to integrate these resources into a biorefining concept, as the origin and quality of a
specific organic resource or waste can restrict its further use; there are also vast logistical issues
in the collection, sorting, storage, and transportation of these potential raw materials, but nothing
today’s technologies cannot solve easily as soon as the economic and/or political drivers are
manifest. Crops grown specifically for biorefining can easily become an integrated part of the
interesting sugar platform, which can substitute many of the conventional petro-chemically
derived compounds. The basic sugar monomer glucose is the raw material for a large number of
proven, industrial biotechnological processes that are capable of delivering a wide range of
important industrial chemicals. Hence, crops with high sugar yield, either in the form cellulose
fibre bundles or simple sugars, are of great interest for biorefining concepts and alternative crops
are available supporting the cultivation conditions in many regions of the world. [5], [4], [6]
2. Anaerobic digestion in Denmark: a case study
Political and practical hurdles in relation to integration of renewable energy in the form of biogas
derived from agricultural resources (manure, energy crops) and industrial organic waste into the
public grid are numerous. A country that has overcome many of these barriers and created a
somewhat viable system allowing large-scale centralised AD plants to be integrated into society
is Denmark. The political as well as the technological development is worth analysing in detail
as it can provide valuable input for further dissemination of AD technologies and allow their
successful integration into other countries‘ political schemes.
In the following the background for this successful implementation is described and the
remaining barriers for further expansion of the Danish AD sector are discussed.
Agricultural application of AD has led to 60 farm-scale and 22 centralised large-scale biogas
plants currently being in operation. The potential of AD in agriculture is far from being
exploited. Merely 4 % of the available manure is circulated through biogas plants. In addition,
vast amounts of agricultural residues and, to some extent, organic industrial waste are still
available. The unexploited bioenergy potentials are present in complex lignocellulosic substrates
such as wheat straw, which is an abundant resource, as well as easier digestible grasses and
silages. This is a common feature of many European countries. [4], [6], [7]
For comprehensive historical details as well as technical description of the prevailing biogas
plant concept please refer to the key references [8], [9], [10], [11], and [12].
2.1 Mobilisation of grassroot movements
Immediately following the first energy crisis in 1973, strong grassroot movements pursued the
then novel idea of recovering biogas from manure in order to develop an alternative energy
source to fossil fuels based on unexploited, natural resources. These endeavours were supported
by the Government in office, which initiated several grant and support schemes. The political
ambitions reached beyond renewable energy production and also sought to reduce the
environmental impact related to agricultural activities (specifically eliminating the risk of
sensitive aquatic environments from becoming polluted with excess nutrients applied to
farmland). The political will was later translated into visionary and comprehensive strategies for
developing and documenting all aspects of biogas technology, economy and effects on society.
Furthermore, national energy resources and future potentials from other alternative sources were