Monitoring of anaerobic digestion processes: A review perspective

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  • Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155

    Contents lists available at ScienceDirect

    Renewable and Sustainable Energy Reviews

    j ourna l h o mepage: www.elsev ier .com/ locate / rser

    Monito

    Michael Aalborg Univer

    a r t i c l

    Article history:Received 21 DAccepted 11 A

    Keywords:Anaerobic digeProcess monitProcess Analytical Technologies (PAT)Process samplingTheory of Sampling (TOS)

    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 formu-lated with respect to assessing the state of the process from chemical measurements. Conservative reactordesigns have dampened the motivation for employing new technologies, which also constitutes one of the

    Contents

    1. Introd1.1. 1.2. 1.3.

    2. Anaer2.1. 2.2. 2.3. 2.4. 2.5. 2.6. 2.7. 2.8. 2.9.

    CorresponE-mail add

    1364-0321/$ doi:10.1016/j.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. Incombination with chemometric multivariate data analysis these emerging process monitoring modalitiescarry the potential to bring AD process monitoring and control to a new level of reliability and effective-ness. 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 eld of AD monitoring and the powerfulPAT/TOS/chemometrics application potential is highlighted. The Danish co-digestion concept, which inte-grates utilisation of agricultural manure, biomass and industrial organic waste, is used as a case study.We present a rst foray for the next research and development perspectives and directions for the ADbioconversion sector.

    2011 Elsevier Ltd. All rights reserved.

    uction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3142Major applications of anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3142Bio-economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3142Resources for bio-renery concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3143

    obic digestion in Denmark: a case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3143Mobilisation of grassroot movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3143Current share of energy production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3143The farm-scale biogas plant concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3143The centralised biogas plant concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3143Main products of centralised AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3143Veterinarian benets of AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3144Fertilising benets of AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3144Socio-economic benets and externalities of AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3144Issues yet to be resolved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3144

    ding author. Tel.: +45 9940 9940; fax: +45 9940 7710.resses: mima@bio.aau.dk, michael.madsen@maskau.dk (M. Madsen).

    see front matter 2011 Elsevier Ltd. All rights reserved.rser.2011.04.026ring of anaerobic digestion processes: A review perspective

    Madsen , Jens Bo Holm-Nielsen, Kim H. Esbensensity, ACABS Research Group, Niels Bohrs Vej 8, DK-6700 Esbjerg, Denmark

    e i n f o

    ecember 2010pril 2011

    stion (AD)oring

    a b s t r a c t

    The versatility of anaerobic digestion (AD) as an effective technology for solving central challenges metin applied biotechnological industry and society has been documented in numerous publications overthe past many decades. Reduction of sludge volume generated from wastewater treatment processes,sanitation of industrial organic waste, and benets from degassing of manure are a few of the mostimportant applications. Especially, renewable energy production, integrated biorening concepts, andadvanced waste handling are delineated as the major market players for AD that likely will expand

  • 3142 M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155

    3. AD process theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31443.1. General concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31443.2. VFA as process indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31453.3. VFA quantication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3145

    4. Conclusions and recommendations from previous reviews . . . . . . . . . . . . . . . . . . . . . . . . . . . .5. Review of the modern AD process monitoring potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    5.1. Monitoring of bioprocesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.2. Principles of operation for reviewed monitoring techniques. . . . . . . . . . . . . . . . . . . 5.3. Fluorescence spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.4. Infrared spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.5. Near infrared spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6. Ultraviolet and visual spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7. Electronic tongues and noses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.8. Gas chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.9. Titration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.10. Microwaves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.11. Acoustic chemometrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.1. Quantication of relevant process parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.2. Emerging PAT modality: Raman spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Sensor interfacing issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.4. Sampling issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Uni-variate versus multivariate data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.6. Acoustic chemometrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7. Handheld and mobile process analyserspart of the new paradigm? . . . . . . . . . .

    7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8. Future research and development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    Nomenclature

    AD anaerobic digestionCOD chemical oxygen demandCSTR Continuously Stirred Tank ReactorFIA ow injection analysisFID ame ionisation detectorFT Fourier transformGC gas chromatographyHPLC high-pressure liquid chromatographyHS-GC head space gas chromatographyIR infraredLCVFA long-chained volatile fatty acidsMS mass spectrometryNIR near infraredOEM original equipment manufacturerPLS partial least squaresPA partial alkalinityPAT process analytical technologiesPCA principal component analysisPC principal componentRMSEP root-mean-square-error of predictionSBR Sequential Batch ReactorSCVFA short-chained volatile fatty acidsTA total alkalinityTKN total Kjeldahl nitrogenTOC total organic carbonTOS Theory of SamplingUV ultravioletVFA volatile fatty acidsVS volatile solids SCVFA sum of short-chained volatile fatty acids

    1. Introdu

    1.1. Major

    Anaerobbiogas prodmany purpter treatmeexpensive treduced byof biogas isorganic parwastewaterenergy proencourage being a stroagriculturaof nutrientsthe organicindustrial w

    1.2. Bio-eco

    Productseveral waynuclear fusisil fuels. Anvia the biortechnologyessential chfossil oil. Apart, but heand organicadvantage to supply bvalue produ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3145. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3147

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3147. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3147

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3147 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3147. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3148. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3148

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3149 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3149

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3149 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3150

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3150 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3150

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3150. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3150

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3150. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3150

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3152. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3152

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3153 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3153

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3153. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3153

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3153

    ction

    applications of anaerobic digestion

    ic digestion (AD), also referred to as biogasication anduction, is a versatile technology platform that can serveoses in industry and society. For example, wastewa-nt processes generate vast amounts of sludge, which iso deposit or incinerate. The sludge volume is effectively

    means of AD. Moreover, renewable energy in the form

    recovered during the microbial decomposition of thet of sludge, signicantly reducing the costs for treating. Another widespread application for AD is dedicatedduction. Recent European political incentives seek toapplication of AD in agriculture, the main motivatorsng desire for reducing the environmental impact froml activities and at the same time enhance the utilisation

    applied for crop-cultivation. Finally, also treatment of fraction of municipal solid waste (OFMSW) and organicaste is effectively done by means of AD [14].

    nomy

    ion of renewable, clean energy can be accomplished ins, e.g. by solar energy, wind energy, hydropower andon, which are all documented viable alternatives to fos-other route for production of clean, renewable energy isenery. A petrochemical renery is a well-established

    platform that has been providing the industry withemical building blocks for decades, all derived from

    biorenery is the precise biotechnological counter-re the raw materials are cultivated crops, energy crops,

    industrial residues and wastes. The biorenery has theover the petrochemical renery that it is able not onlyulk-commodities, but especially also specialised, high-cts such as enzymes, avour compounds and additives

  • M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155 3143

    for the food industry, and important pre-cursors for pharmaceuticalproduction enabling the biorening concept to be a versatile ingre-dient and raw material supplier for many industrial and societalsectors [5].

    1.3. Resour

    Conventproduction energy prowrongly cabeing utilisabundant ampose of highform the bresources arthe caterinfractions froagriculturalthese resouof a specicthere are alsand transpotodays techand/or polibiorening ing sugar plpetro-chemglucose is thbiotechnolorange of imsugar yieldsugars, are ocrops are avregions of t

    2. Anaerob

    Politicalrenewable resources (mthe public gof these balarge-scale Denmark. Tworth analyther dissemintegration

    In the fotion is descof the Danis

    Agricultcentralised The potenti4% of the avaddition, vaorganic indergy potentsuch as wheier digestibEuropean c

    For comdescriptionreferences [

    2.1. Mobilisation of grassroot movements

    Immediately following the rst energy crisis in 1973, stronggrassroot movements pursued the then novel idea of recovering

    from to fooursseverd be

    the callyecomliticae str

    techal ensoure bas

    rren

    estid 86tionpoolt veimatentraeatmrial famain4,15

    e far

    orig 197s aning cal ponomce,

    edge imp

    Throceptg una

    e cen

    ed on econf cenrodu

    lisedand er thnicd adiniti

    c wa

    ain p

    st cend poces for bio-renery concepts

    ional crops grown for human consumption and forof animal feed are politically not recommended for

    duction. Meanwhile, agricultural residues (sometimestegorised as wastes) such as straw are currently noted many places and are therefore readily available inounts. New crops and varieties engineered for the pur-

    yield (and not intended for human consumption) canasis of future biorening concepts. Large unexploitede available. These resources include organic waste fromg and food processing industries, organic by-productm chemical industries, and residues and wastes from

    activities. It is not necessarily seamless to integraterces into a biorening concept, as the origin and quality

    organic resource or waste can restrict its further use;o vast logistical issues in the collection, sorting, storage,rtation of these potential raw materials, but nothingnologies cannot solve easily as soon as the economic

    tical drivers are manifest. Crops grown specically forcan easily become an integrated part of the interest-atform, which can substitute many of the conventionalically derived compounds. The basic sugar monomere raw material for a large number of proven, industrialgical processes that are capable of delivering a wideportant industrial chemicals. Hence, crops with high, either in the form cellulose bre bundles or simplef great interest for biorening concepts and alternativeailable supporting the cultivation conditions in many

    he world [46].

    ic digestion in Denmark: a case study

    and practical hurdles in relation to integration ofenergy in the form of biogas derived from agriculturalanure, energy crops) and industrial organic waste into

    rid are numerous. A country that has overcome manyrriers and created a somewhat viable system allowingcentralised AD plants to be integrated into society ishe political as well as the technological development issing in detail as it can provide valuable input for fur-ination of AD technologies and allow their successful

    into other countries political schemes.llowing the background for this successful implementa-ribed and the remaining barriers for further expansionh AD sector are discussed.ural application of AD has led to 60 farm-scale and 22large-scale biogas plants currently being in operation.al of AD in agriculture is far from being exploited. Merelyailable manure is circulated through biogas plants. Inst amounts of agricultural residues and, to some extent,ustrial waste are still available. The unexploited bioen-ials are present in complex lignocellulosic substratesat straw, which is an abundant resource, as well as eas-le grasses and silages. This is a common feature of manyountries [4,6,7].prehensive historical details as well as technical

    of the prevailing biogas plant concept refer to the key812].

    biogassourceendeavtiated reachereduce(specifrom bThe pohensivbiogasnationnative reliabl

    2.2. Cu

    Domreacheproducplants the pasapproxlarge cater trindustThe resites [1

    2.3. Th

    Theing thefarmerpertainbiologithe ecsequenknowling andsector.nal concausin

    2.4. Th

    Basples ofidea ostock pcentraof Jutlwhethtion sigallowewhich organi

    2.5. M

    Moheat an manure in order to develop an alternative energyssil fuels based on unexploited, natural resources. These

    were supported by the Government in ofce, which ini-al grant and support schemes. The political ambitionsyond renewable energy production and also sought toenvironmental impact related to agricultural activities

    eliminating the risk of sensitive aquatic environmentsing polluted with excess nutrients applied to farmland).l will was later translated into visionary and compre-ategies for developing and documenting all aspects ofnology, economy and effects on society. Furthermore,ergy resources and future potentials from other alter-ces were thoroughly evaluated providing a sound andis for political decision-making [8,9,12,13].

    t share of energy production

    c energy consumption in Denmark (climate adjusted)4 PJ in 2009, whereof 3.9 PJ originated from biogas

    (wastewater treatment plants and agricultural biogased). The biogas production has remained constant over

    years. In 2007, agricultural biogas plants accounted forely 60% of the biogas production distributed among 21lised biogas plants and 60 farm-scale plants. Wastew-ent-related AD processes at 59 municipal sites and 4cilities contributed with 22% of the biogas production.der share came from methane recovery from landll].

    m-scale biogas plant concept

    inal farm-scale biogas plant concepts developed dur-0s were simple constructions commissioned by locald developed by local black smiths. Lack of knowledgeto optimal adjustment and control of the sensitiverocess often led to diminishing yields, if any. Hence,y in these early designs was non-existing. As a con-

    the Ministry of Trade facilitated the creation of a dissemination group with the purpose of document-roving the performance of the emerging Danish biogasughout the 1970s and the 1980s many of the origi-s were abandoned due to severe operational problemscceptable downtime [8,12].

    tralised biogas plant concept

    the experience from farm-scale operation, the princi-omy-of-scale soon became a much debated topic. Thetralising the AD plant and having manure from live-ction transported back and forth resulted in the rst

    mesophilic AD plant being build in the Northern partin 1984. Numerous operational problems questionedis design was indeed feasible, but a later reconstruc-antly increased the yield. Furthermore, the new designdition of industrial organic waste to the AD process,ated the era of co-digesting manure and industrialste [8,12].

    roducts of centralised AD

    tralised AD plants in Denmark have an onsite combinedwer (CHP) generation facility or they export the biogas

  • 3144 M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155

    to a nearby power plant. Biogas is combusted directly in suitable gasengines or turbines. The produced electricity is sold to the publicgrid and the generated heat is used either for process heat or soldto the public district heat grid. The latter option requires that theAD plant is purchase thmuch depenegotiate wand industr

    2.6. Veterin

    From a offers pathoeffective sation at 70

    benchmarkplished in reactor(s). Htures and horeduction. temperaturHence, pracand other ring manurefrom their transmittedcles, equipmmanure is aall Danish a

    2.7. Fertilis

    The digehas a fairlyents (N, P, as fertiliseressential niaccessible bthe digestanicant losorganic ma

    2.8. Socio-e

    From a offer solutiorural districmanure. Thtions betweand their cieasy to putimportant wplants. FewAD plant asPR associatpersonnel tthe AD planoff activitieactivity levelocal commthe possibisufcient wmany polititural resourpower can b

    production scheme in rural districts in developed as well as indeveloping regions [8,10,12,13].

    sues y

    k of esseen thtor aeceivtiatetify f

    Pre-ans ss renocen ordtly, as andconove yeure Ahe cocons

    indtive bblic seekal e

    rs w pro

    onlyoug, theom nts

    givention, andateriin lice o

    proceendouildalwad is h

    proc

    nera

    gented pnd od for

    the a notns arity, oks,

    simtial

    indivs, anlocated adjacent to infrastructure or industry willing toe heat energy. The economy of a given AD plant is veryndent on the contracts the board of directors is able toith energy buyers (municipalities, energy companies,y) [8,10,12].

    arian benets of AD

    veterinarian point-of-view, the centralised AD plantgen and weed reduction. For thermophilic AD plantsnitation of the manure is achieved easily. Pasteurisa-C for duration of 1 h was originally proposed as the

    for effective sanitation of manure. This could be accom-a separate batch reactor following the main biogasowever, a number of alternative operating tempera-lding times proved to have the same effect on pathogen

    For instance, digesting fresh manure at thermophilice (53.5 C) for a period of 8 h is considered adequate.tical restrictions exist on the dosage scheme for manureaw materials requiring pasteurisation. Farmers supply-

    to the AD plant can be condent that possible deceaseslivestock production are not spread on arable land or

    to other livestock farms. Thorough cleaning of vehi-ent, and personnel coming into direct contact with raw

    necessary pre-requisite and has been implemented atgricultural AD plants [10,12,16].

    ing benets of AD

    state (co-digested manure and industrial organic waste) homogeneous content with respect to major nutri-and K), which makes it easier to apply to farmland, since the fertilising value is well-declared. Moreover,trogen is present on a form (ammonium) that is easilyy plants. Using tractor and dragging-hose equipment

    te is effectively incorporated into the soil without sig-s of fertilising value. Moreover, the digestate is stabletter, which acts as a valuable soil enricher [1,10,12,16].

    conomic benets and externalities of AD

    socio-economic point-of-view centralised AD plantsns to many challenges encountered in agriculture andts. Digestate has a less offensive odour compared to rawis is important in the context of ensuring good rela-en the farmers (crop-cultivators, appliers of manure)vilian neighbours in the countryside. Although it is not

    a monetary value on this adverse effect, it is criticallyhen considering possible locations for future biogas

    people are interested in having an offensive smelling a neighbour, mostly because of rumours and negativeed with ill-operated AD plants. The need for qualiedo take care of manure handling, technical service ont itself and related instrumentation as well as the spin-s in local communities is also an important factor. Thel is generally high and the impact an AD plant has on theunity (service, trade) is signicant. On the other hand,lity for local or regional communities to become self-ith renewable, clean energy is a strong motivator forcians in this day and age. Biogas derived from agricul-ces combined with for instance wind power and solarecome important key components in the future energy

    2.9. Is

    Lachas strbetweAD seckeen rhas inito idensector.cal mebiomaplex ligsteps iCurrenratoriesocio-eties ha

    Futfrom tdirect qualitylegislathe puin AD chemicreactoshownwhere

    Althhistoryseen frAD plawhichUnintestratesraw mthe maformanof the a tremtence balbeit deman

    3. AD

    3.1. Ge

    Themediamers areduceide) in

    It isreactiofor clatextbo

    Forsequenof the genesiet to be resolved

    industrial organic waste of sufciently high qualityd the market severely leading to strong competitione AD plants themselves and between the agriculturalnd the wastewater treatment sector, which is also aer of suitable wastes for increasing biogas yields. Thisd many new research projects and programmes tryinguture raw materials that can be introduced into the ADtreating manure by various low and high technologi-is one option, but as stated above, many unexploitedsources are still readily available. However, their com-llulosic structure does require advanced pre-treatmenter to guarantee a feasible yield from the AD process.

    number of technologies are being investigated in labo- at pilot-plant installations, but reliable economic andmic assessments of these new technological possibili-t to be disseminated.D plant concepts will most likely be radically differentncepts that are prevailing today. This can be seen as aequence of the limited availability of easily digestibleustrial organic waste and the sharp competition. Also,arriers currently prevent AD plants supplying heat to

    grid from accumulating prot. Recent research efforts inspiration in well-known principles from classicalngineering disciplines. Serial digestion, where severalith varying operating conditions are connected, hasmising results compared to the conventional design,

    one main reactor is considered.h the Danish AD sector has a long and well-documented

    development in process monitoring and control asa technological point-of-view is not impressive. Manyare relying on a few simple-to-measure parameters,

    a poor indication of the state of the biological process.al organic overloading, accidental addition of toxic sub-

    other process interruptions occur frequently. Lack ofal quality control and analysis is believed to be one ofmitations for effective raw material handling. The per-f an AD plant by and large depends on the qualicationsss technicians and on the manager [10,1721]. Here isus still largely untapped potential for high-skill compe-ing and for modern process technological involvement,ys with a keen eye for capital costs; what is in heavyigh-tech low-cost technologies.

    ess theory

    l concept

    erally accepted denition of AD is a microbiologicallyrocess during which organic carbon present in biopoly-ther degradable compounds is converted to its mostm (methane) and its most oxidised form (carbon diox-bsence of oxygen (anaerobic conditions).

    the aim of this work to recite the many interestingnd phenomena occurring in the AD process. However,an overview of the AD process, as seen in many recentis given in Fig. 1.plicity, the AD process can be characterised by fourprocesses as depicted in Fig. 1. The characteristic timesidual AD process steps, hydrolysis, acidogenesis, aceto-d methanogenesis, differ signicantly. In a healthy AD

  • M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155 3145

    S

    Proteins

    Suspended or ganic mat erial (biopolymers )

    Carbohydr atesLipids

    LC

    Ste

    p 1

    Hyd

    roly

    sis

    Ste

    p 2

    Acid

    og

    en

    esis

    Ste

    p 3

    Ace

    tog

    en

    esis

    Ste

    p 4

    Me

    tha

    no

    ge

    ne

    sis

    Fig. 1. Breakdreaction stepsAD process.

    process, thedepending while the laentering theto the assufairly low in

    3.2. VFA as

    Feasiblehave been volatile fattindicative oVFA accumload or inhito the inutaken to avounclear. Hobeen the fav

    A compplants, bothreported byplants had Furthermorfor AD planwastewaterVFA concennium conce

    3.3. VFA qu

    Many ancation of VShort-chainsuch as mantication mThe most imlowing.

    Several methods fomethodologinterested iindividual VMethods fo

    ing countries have been developed, implemented and validated[2931].

    However, as indicated by the similarities between the pKa valuesof the individual VFA acids listed in Table 1, it is not possible to

    uishomah labnd patog

    and Tabible d

    [32]cal acids utineertiseatogrscalerom matoconv

    cluss

    ta-l in h(OFMAD s

    andh acs und subocesse of of paproc

    haveve bes full

    by athorrpris

    specages

    AD Organic int ermediatesCVFA, alcoho ls, lactic ac id etc.

    Inorg anic int erme diates

    CO2, H2, NH4+, H2S etc .

    Acetotrophic metha noge nesis (70 % of the CH ): CH COO H CH + CO

    Hydrotrophic metha noge nesis (30 % of the CH ): CO + H CH + H O

    Acetic ac id Carbon dioxide & h ydr ogen

    Polypepti des

    Pepti desMono- &

    disacc har idesVFA & g lycerine

    own of the anaerobic digestion process. Overview of the four principle, hydrolysis, acidogenesis, acetogenesis, and methanogenesis, in the

    duration of the former is usually from days to hourson substrate complexity and chemical environment,tter proceeds in seconds to minutes. Hence, substrate

    methanogenesis is converted instantaneously leadingmption that volatile fatty acid (VFA) levels should be

    a well-balanced system [22].

    process indicators

    and relevant indicators of the AD process conditionargued for decades. Many authors have suggestedy acids (VFA) as control parameters as these acids aref the activity of the methanogenic consortia. In case ofulation, this can be interpreted as either organic over-bition of the methanogenic microbial communities dueence of other factors. In either case, action should beid reactor failure. Relevance of specic VFA acids is stillwever, simple short-chained VFA acids (SCVFA) haveourites of many researchers [2327].

    rehensive survey of the condition of 15 Danish AD wastewater-related AD and agricultural AD plants, was

    Karakashev et al. [28]. It was shown that mesophilicgreater microbial diversity compared to thermophilic.e, VFA and ammonium levels were signicantly higherts treating manure than plants treating sludge from

    treatment. The 9 agricultural AD plants studied had

    distingChr

    researcacids aChrommobilering tois feasBrondznologifatty aare roof expchromat full-ences fof chrothe bio

    4. Conreview

    Mamentswaste of the stratesresearcprocesdenepre-prpurpossation of AD papersand hareacheplishedThe auwas sutreat aadvanttion oftration levels in the range of 3 g L1 or less, while ammo-ntrations varied from approximately 2 g L1 to 6 g L1.

    antication

    alytical methods have been developed for quanti-FA acids of relevance for AD process monitoring.ed VFA acids commonly present in sample matricesure and wastewater sludge are listed in Table 1. Quan-ethods vary in complexity and analytical performance.portant and widespread ones are discussed in the fol-

    authors have suggested different variants of titrationr efcient and cheap AD process monitoring. Thisy is certainly worth pursuing for AD plants that are notn detailed information about the relative abundance ofFA acids, but merely seek a measure for total acidity.r use in academia and industry as well as in develop-

    outstandingPind et

    itoring andinstrumentThe reviewing the musystems, thas well. Stirtion times for this is lprevent wawith small guration rfor historicauthors eminuence onables (for intransient re between the VFA acids using titration methods.tographic methods commonly found in industry andoratories are capable of separating the individual VFArovide quantitative measures of their concentrations.raphic separation is based on relative afnity between a

    a stationary phase in a separation column. Again, refer-le 1, gas chromatographic separation of the VFA acidsue to the relative large differences in boiling points.

    presented a comprehensive review outlining the tech-nd methodological developments for quantication ofby liquid and gas chromatography. These techniquesly applied by professionals and require a great deal

    and sample pre-processing. Nevertheless, automatedaphic systems have been developed and put into work

    AD plants [3336]. There are some positive experi-broad-based general process technological applicationsgraphic techniques, some of which may be relevant forersion eld [37,38].

    ions and recommendations from previous

    varez et al. [1] outlined the technological achieve-andling of the organic fraction of municipal solidSW) using AD. Their work illustrated the complexityector, which deals with many different types of sub-

    relies on many different processing techniques. Thehievements until year 2000 had mainly focused onderstanding in laboratory- and pilot-scale using well-strates and cultures. Reported studies dealing withing techniques applied to difcult substrates with theincreasing the digestibility (for instance due to solubili-rticulate organic matter) were numerous. Also, start-upesses has always been a much debated topic. Many

    suggested procedures for efcient process inoculationen focusing on reduction of the time until the process

    production capacity. Signicant savings can be accom-pplication of a proper and effective start-up procedure.s noted that the industrial interest in AD co-digestioningly low. Most industrial AD systems were designed toic type of waste only thereby not benetting from the

    of co-digestion systems. The widespread implementa-co-digestion plants in Denmark was highlighted as an

    development.al. [22] presented a comprehensive review on mon-

    control of anaerobic reactors, focusing on generalation and methods for process knowledge acquisition.

    included both low rate and high rate systems. Concern-ch applied CSTR (Continuously Stirred Tank Reactor)e authors made several remarks worth mentioning herered tank reactors commonly have long hydraulic reten-(usually in the order of 15 days or more). The reasonack of biomass immobilisation inside the reactor. Toshout of the microbial consortia, reactors are loadedamounts of feed periodically. Hence, the reactor con-esembles a Sequential Batch Reactor (SBR). However,al reasons, the term CSTR is used in AD literature. Thephasized that the feeding strategy can have signicant

    the chemical environment in the reactor. Process vari-stance pH, alkalinity, VFA, and gas yield) often displaysponses to the feeding prole. Moreover, the substrates

  • 3146 M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155

    Table 1SCVFA, data and structures.

    IUPAC nomenclature Formula CAS # MW (Da) bp (C) pKa Structure

    Ethanoic aci

    Propanoic ac

    n-Butanoic a

    2-Methylpro

    n-Pentanoic .1

    2-Methylbu .1

    3-Methylbu .1

    IUPAC: Interna : mofrom CRC Han

    fed to these(sand, breAD reactor sstrate feed sampling erecognised)Direct sensmicrobial gpretation, aof biolm. Aoff-line anarecorded onas pH, redointuition anrithms. Theshould be aor intervenmonitoringsystems be

    Vanrollemonitoringthat most way that thity disposedwithout thever, the dlarger reactsociety anding effort. Fdesigned tohensive insunintentioncareful appmodelling. covering si

    pproised enerantenere oia fo

    andnjersd CH3COOH 64-19-7 60.1

    id CH3CH2COOH 79-09-4 74.1

    cid CH3CH2CH2COOH 107-92-6 88.1

    panoic acid CH3CHCH3COOH 79-31-2 88.1

    acid CH3(CH2)3COOH 109-52-4 102

    tanoic acid CH3CH2CHCH3COOH 116-53-0 102

    tanoic acid CH3CHCH3CH2COOH 503-74-2 102

    tional Union of Pure and Applied Chemistry, CAS: Chemical Abstracts Service, MWdbook of Chemistry and Physics, 86th ed., ISBN: 0849304865.

    systems contain relatively large proportions of solidss, and particulate organic matter) compared to otherystems. Drawing representative samples from the sub-

    line and the reactor is complicated and subject to manyrrors in general not sufciently counteracted (or even

    due to the highly heterogeneous material involved.or interfaces (on-line, in-line, at-line) are subject torowth (fouling), which further complicates data inter-

    were acomprclass gof maisors wacademitoring

    Spa

    s the sensory signals can be corrupted by the presenceD plants are commonly regulated based on at-line orlytical results. Few process parameters are routinely-line and they are limited to easily accessible data suchx potential, gas production rate, and ow rates. Humand operator experience rule over dedicated control algo-

    conclusions were that process parameters preferablyvailable on-line and not rely on either human intuitiontion. Only through application of automated process

    and data interpretation could effective control of ADachieved.ghem and Lee [3] reviewed instrumentation for on-line

    of wastewater treatment plants. The authors statedwastewater treatment plants were designed in suche legislative requirements concerning the efuent qual-

    off into recipient water bodies could be guaranteede need for comprehensive process monitoring. How-esire for expansion of the treatment capacity (eitherors or shorter retention times) due to general growth in

    industry seeded the idea of intensifying the monitor-uture wastewater treatment plants will presumably be

    allow greater exibility, which necessitates compre-ight into the core of the AD process in order to avoidal disturbances. This criterion can only be met throughlication of appropriate sensor technology and processSensor technologies were divided into two classes; onemple, low-maintenance, and reliable sensors, which

    tion for ADare associavative desigguarantee ptraditional plant operanology intoto their effedue to consteristics unature. Wacompositioof society ainuxes of ment plantnot countetoxic comptrant compoitself. Withunpredictabas such alwdetected us

    Ward etoptimising resources. parametersmany AD p117.9 4.76 O

    OH

    141.2 4.87O

    HO

    163.8 4.83 O

    OH

    154.5 4.84

    OHO

    186.1 4.83

    OHO

    177 4.80

    OHO

    176.5 4.77

    OHO

    lecular weight, bp: boiling point, pKa: dissociation constant. All data

    priate for automatic control systems. The other classadvanced sensors. The authors stated that the secondlly requires signicant human intervention in the formance and calibration efforts. Typically, advanced sen-nly applied by specialists (consultants) for audits or inr research and not for dedicated on-line process mon-

    control of AD plants. and van Lier [39] analysed implemented instrumenta- process monitoring. Wastewater treatment AD plantsted with high costs. This is mainly due to the conser-n of the reactors, which are very much over-sized torocess robustness. Limited monitoring is included in

    designs putting the fate of the process in the hands of thetor. Introducing reliable monitoring and control tech-

    the sector would allow AD plants to be operated closerctive capacity limit instead of wasting reactor volumeervative design rules. The inuent (substrate) charac-ctuate over time and are partly highly heterogeneous instewater treatment plants are not able to predict futurens of the received wastewater, because the dynamicsnd industry constantly results in transients. Suddenconcentrated organic material to a wastewater treat-

    can lead to process upsets and severe disturbances, ifracted in due time. Equally serious is the presence ofounds (heavy metals, industrial chemicals, and recalci-unds) that pose a severe threat to the biological processout reliable monitoring systems that respond to suchle transients, the wastewater treatment AD process isays at risk. Many of these potential threats can only being proper on-line techniques.

    al. [4] comprehensively reviewed the possibilities forthe performance of AD reactors based on agriculturalThe authors discussed reactor congurations, process

    as well as feeding strategies in detail. It was noted thatrocesses are operated at suboptimal conditions due to

  • M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155 3147

    Spectroscopic Electro-chemical Chromatographic Other

    Fluorescence C

    dspa

    LC

    Acoustic chemometrics

    Fig. 2. Review n clasinterest, since

    lack of propcultural ADthe processgas potentiain the AD inview of thetechnologiestudies havpilot-scale.

    5. Review

    5.1. Monito

    The techitoring in tanalyse as ilatter has silabour, statof componeof advancedparadigm. Festing advaAnalytical Ttoday, we re

    5.2. Princip

    Focusingcation of nuparametersviding a cleareviewed mdepicted inthe core ofa review ofbrought in t

    For in-dciples consimplementeing, while Bmultivariat

    uores

    k andonitow inuantrobe

    ed foe uoabolue ts varg a ps distherthancatietatis starel en whany cencngthms. B

    thatFLU

    InfraredIR

    Near infraredNIR

    Raman

    VisualVIS

    UltravioletUV

    pH

    Redox potential

    Electronic tongueET

    Electronic noseEN

    G

    GC hea

    HP

    ed analytical modalities. Clusterisation of the reviewed modalities into four mai these two classes are developing signicantly at present.

    er on-line data. The complexity of the feed in many agri- plants tends to encourage plant operators to regulate

    conservatively rather than trying to exploit the full bio-l. Moreover, the monitoring systems currently applieddustry were believed to give a poor and inconsistent

    actual process state. The use of multivariate sensors and electrochemical arrays was encouraged as manye shown promising results in both laboratory-scale and

    of the modern AD process monitoring potential

    ring of bioprocesses

    nological development within the eld of process mon-he established fermentation industry is attractive tot has many similarities with the AD sector. Although thegnicantly less capital available for instrumentation ande-of-the-art technology development, miniaturisationnts along with economy-of-numbers (mass production)

    process sensors are about to cause a change in thisor a comprehensive overview of the many current inter-nced sensor alternatives, collectively known as Processechnologies (PAT), that are available on the marketfer to the recent, authoritative text edited Bakeev [38].

    5.3. Fl

    Pecwas mgas owere qcence pwas usthat thto metance dprocesas beinproces

    Anofor mequantiinterprproces

    Moloop, itain muoreswaveleproblestratedles of operation for reviewed monitoring techniques

    on the key metabolites depicted in Fig. 1, the appli-merous monitoring techniques for quantifying these

    have been reported in the literature. For the sake of pro-r and comprehensive overview of available techniques,odalities have been organised in four main groups as

    Fig. 2. The general principles of operation constituting these techniques are described in [3]. Below follows

    reported studies, where these techniques have beeno the context of monitoring AD processes.epth details about the individual measurement prin-ult Harris [40]. Vanrolleghem and Lee [3] surveyedd and emerging technologies for AD process monitor-akeev [38] presents a comprehensive work covering

    e process analysers in all industries and sciences.

    chemical ox

    5.4. Infrare

    Steyer efor determiity in an into provide instrumentoptics soonusing bre industry. Tha real procchemical spspectrum dspectra fromextensive cauthors follce

    a.c.

    Mass spectrometryMS

    Microwaves

    Titration

    ses. The spectroscopic and electro-chemical methods are of special

    cence spectroscopy

    Chynoweth [41] reported a study, where uorescencered on-line along with pH, redox potential and bio-

    a mixed culture fermentation AD process. VFA acidsied off-line using gas chromatography. The uores-

    was immersed in the culture and a single wavelengthr excitation and emission respectively. It was concludedrescence signal from NAD(P)H, which is directly related

    ic activity, gave the earliest warning of process imbal-o organic overload compared to the other measurediables. The authors suggested uorescence monitoringromising alternative technique for early indications ofturbances in AD processes.

    uorophore is the coenzyme F420, which is specicogenic bacteria. Although probes are available foron of this intracellular compound, the questions ofon and the signicance of the signal in relation to ADbility still remain open [3,41].t al. [42] equipped an AD reactor with a recirculationich a bre optic probe was placed. AD substrates con-uorophores that can interfere with for instance thee signal from NADH. Several excitation and emissions were used in order to overcome reported interferencey application of multivariate methods it was demon-

    uorescence spectroscopy could predict VFA as well as

    ygen demand with reasonable accuracy.

    d spectroscopy

    t al. [43] demonstrated the use of infrared spectroscopynation of VFA, COD, TOC, and total and partial alkalin-dustrial AD reactor. An ultra-ltration unit was useda clear liquid free of particles to be passed on to the. The authors argued that the emerging market for bre

    would allow for in-line IR analysis of AD processesoptics, which would greatly expand the uses of IR ine IR spectrometer was calibrated using samples from

    ess based on the argument that interactions betweenecies in complex matrices lead to that the resultingiffering from a simplistic linear combination of the

    the individual chemical species involved. Hence, analibration with synthetic samples is not feasible. Theowed the AD process for several months and were able

  • 3148 M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155

    to document process upsets and failures based on analysis of the IRspectra.

    Spanjers et al. [44] also followed an industrial AD process forseveral months with an IR spectrometer equipped with a CaF2transmissioa pre-treatmwithdrawnby the measwere associof the analyammoniumIt was concperform opwas a cheap

    5.5. Near in

    Nordbertroscopy fowith syntheDisturbanceimmediate Data were athe authorsand propionsurements,the referen

    Hanssonitoring of afraction of mmade to deminor distuprocess stabloading, prpH units. MBased on mpionate moThe reason

    Holm-Nwhere samwere analyto a standPrimary sato the labstep, wherreduced prmade for tTKN, and allowed foThe authorformance tpilot-scale (acknowledgyet.

    Holm-Nratory AD reAD reactors5 L reactorswith re-circing and rep(using the Twas succesgas producseen from ble AD proby adding

    by use of multivariate models it was possible to predict bothVFA acids (acetate and propionate) as well as glycerol fromthe near infrared spectra with acceptable accuracy and preci-sion.

    ragucfrare

    wase pro

    to appliequesnvesancere thThis m: la

    botng eion or feds weodells typely hltivatted borgscopscaleergy

    integiate NIRS47,48ded tte dathe Aade

    dry bi etnitor

    It wltura

    200ent

    s. In as in fo

    s weconcnce cal imme f

    ltravi

    ondo, whnd hevanral. -moe an

    tem .n measurement cell. The instrument was modied andent unit was added allowing particles to settle in the

    sample, before being sent to a 150 m lter followedurement cell. It was argued that the calibration modelsated with high uncertainties due to low concentrationstes (COD, VFA, bicarbonate, phosphate, nitrate, nitrite,, and TKN) and hence low absorbances in the IR spectra.luded that the pre-sedimentation and ltration did nottimally, as occasionally clogging occurred. The solution

    alternative to commercial ultra-ltration units.

    frared spectroscopy

    g et al. [45] reported the use of near infrared spec-r monitoring of an AD reactor in laboratory scale fedtic substrate. The probe was immersed in the reactor.s were introduced to the system and it was noted thatchanges were observed in the near infrared spectra.nalysed using chemometric multivariate methods and

    concluded that the calibration models for VFA (acetateate) were acceptable for the purpose of on-line mea-

    although they did not fully match the performance ofce methods (chromatography).

    et al. [46] applied near infrared spectroscopy for mon-n AD laboratory-scale process fed with the organicunicipal solid waste (OFMSW). Several attempts were

    liberately stress the AD process. It was noted that forrbances in the form of organic overloading, the ADilised itself after 12 h. However, for an aggressive over-

    opionate accumulated leading to a drop in pH of 0.3oreover, the biogas composition signicantly changed.ultivariate analysis of the data it was concluded that pro-dels were better and more robust than acetate models.for this was unclear.ielsen et al. [47] reported an experimental work,ples from four Austrian agricultural AD reactorssed off-line using an innovative ow cell coupledard near infrared spectrometer, the TENIRS system.mples from the full-scale reactors were broughtoratory. The ow cell incorporated a maceratione particles, straw in particular, could be effectivelyior to scanning. Multivariate calibration models werehe parameters individual VFA acids, ammonium-N,VS. It was shown that the measurement principler adequate modelling of the parameters of interest.s indicated that due to the promising model per-he system would be applied immediately on both150 L) and full-scale (1800 m3) installations. This studyed primary sampling issues, which were not optimised

    ielsen et al. [48] investigated the responses from labo-actors by adding easily digestible glycerol to laboratory

    operated on manure and observing the effects. Three operated at thermophilic temperature were equippedulating loops allowing on-line near infrared monitor-resentative sampling of the heterogeneous bioslurryENIRS system mentioned above). Organic overloadingsfully induced, which completely stopped the bio-tion. Rapid accumulation of glycerol and VFA wasoff-line analysis. The authors concluded that a sta-cess with increased biogas yield could be achieved35 g L1 glycerol. Furthermore, it was shown that

    Sarnear inlengthand thoughlywere atechnieters iabsorbbly westudy. probleseen in

    ZhaterisatreactomodelThe mvariourelativthe muover-

    Lomspectroratory that enbeing immedThe TEet al. [conclutivariaabout were mwell as

    Jacofor mosilage.agricuof yearinstrumproceshead wduratiomodelIt was ment oTechnithe the

    5.6. U

    Redsystembroth aare relin genesamplethe samthe sysitoring a et al. [49] monitored an activated sludge reactor withd spectroscopy. An immersion probe with a 10 mm path

    mounted in the reactor. Spectra were acquired dailybe was withdrawn from the reactor and cleaned thor-void interference from fouling. Multivariate methodsd for data analysis along with several variable selection. The authors noted that the limited range of the param-tigated (COD and nitrate) combined with the strong

    of water in the near infrared spectral region, proba-e main reasons for the poor correlations obtained in thestudy suffered from an experimental design evergreenck of proper analyte span; this is an error very often

    h laboratory as well as full-scale studies.t al. [50,51] used near infrared spectroscopy for charac-f the efuent from an anaerobic hydrogen-producing

    with synthetic wastewater. Multivariate calibrationre made for ethanol, acetate, propionate, and butyrate.ing performance was optimised through application ofes of spectral pre-processing methods. Meanwhile, theigh number of reported PLS-components applied forriate regression models indicate that there is a risk ofdata.

    et al. [52] surveyed the performance of near infraredy for monitoring of agricultural AD reactors in labo-

    co-digesting manure and maize silage. It was argued crops such as different types of silages already arerated in AD processes worldwide and that there is anneed for proper on-line monitoring of such installations.

    ow cell system earlier described by Holm-Nielsen] was also applied for these experiments. The authorshat near infrared spectroscopy in conjunction with mul-ta analysis is capable of providing real-time informationD process. Specically, multivariate calibration models

    allowing prediction of all the SCVFA acids of interest asmatter.

    al. [53] reported the use of near infrared spectroscopying of a full-scale agricultural AD plant processing maizeas stated that according to German authorities, 4000l AD plants were in operation in Germany by the end8. The majority of these plants were not equipped withation allowing the operators to fully optimise the ADthe reported study, a specially designed measurementmplemented at the AD plant. A campaign of 500 daysrmed the basis for the results. Multivariate calibrationre made for the two VFA acids acetate and propionate.luded that the applied routine of referencing the instru-a week probably gave rise to errors in the spectral data.

    provements, including automatic referencing, will beor further work.

    olet and visual spectroscopy

    et al. [54] designed a ow injection analysis (FIA)ich allowed quantication of sulphide in fermentationydrogen sulphide in the gas phase. Such measurementst in relation to biogas cleansing as well as AD processesThe system consisted of one detection module and twodules, which allowed quantication of said species onalyser. The results indicated excellent performance ofwith low detection limits suitable for AD process mon-

  • M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155 3149

    5.7. Electronic tongues and noses

    Rudnitskaya and Legin [55] reviewed the possibilities for mon-itoring biotechnological processes using electronic tongues andnoses. The acal sensors reported baimportant multivariatperform maThe low prialternativesing of comfouling, degsampling istype of senchemical seencountere

    NordberSemicondumultivariatconcentratifor both mhydrogen gtechnologywould sevefuture to co

    Buczkowminituariselaboratory-ular set-up multivariatstructed forconditions, performancpH value (hstatistics fodesign of th

    5.8. Gas chr

    Pind et aon in situ nected to tha frequencythe SCVFA had previouparameterstion unit cthe need foof 200 saminterventioapplication

    Boe et almatographyadjusting tesample, thecally. The gaand the consolubility rerior advantaprone to fou

    Diamantlary gas chresearch. Aby a ltrati

    performed automatically. The authors showed that the systemresponded to and veried sequences of organic overloading of thereactor.

    Boe et al. [36] further rened the principle they described ear-r usefor uation

    sys comnts. inat

    mact threac

    by voluer, tcountc. i

    atogr

    tratio

    kenh quanon aater

    neeploy

    lled bocedness g wat signformcouldtsch ring

    ent sle an

    r was throThe renceressethoance

    in aav ancessne orior plicitnts dologthorsable

    proctive ointuche titrf the

    are inlina tic syuthors emphasized that recent applications of chemi-coupled in arrays have overcome many of the earlierrriers for their successful implementation. The mostof these hurdles is low selectivity. By proper use ofe methods, the sensor arrays can be optimised and out-ny techniques found in a typical analytical laboratory.ce and robustness of these sensors make them viable

    to modern complex analytical hardware for monitor-plex processes. However, common problems such asradation due to the harsh chemical environment andsues are also themes that have to be dealt with for thissors. Meanwhile, gas phase measurements by use ofnsors has a great potential, as many of the problemsd in the broth are absent in the gas phase.g et al. [45] implemented chemical sensors (Metal Oxidectors) in the headspace of an AD reactor. By use ofe methods, the sensor data were correlated with theon of volatile compounds. Good accuracy was obtainedethane and SCVFA (acetate and propionate), whereasave suboptimal results. The authors argued that the

    was still maturing and that technological developmentrely decrease costs and strengthen the potential in theme.ska et al. [56] reported the construction of ad array of chemical sensors for monitoring of ascale AD process. The sensors were arranged in a mod-and could be integrated in a recirculation line. Applyinge methods, acceptable calibration models were con-

    VFA, COD, and pH. It was argued that changes in processin particular pH, apparently had an inuence on modele. Dividing the data set into two classes based on theirigh and low, respectively) gave improved modelling

    r acetate. A patent application was led for the modulare ow cell.

    omatography

    l. [33] presented a novel measurement principle basedmembrane ltration. A gas chromatograph was con-e system allowing automatic VFA quantication with

    of 15 min in the range from 0 to 3000 mg L1 of allacids. The authors argued that no ltration systemsly been validated in the literature. By optimising the

    for the ltration, the authors showed that the ltra-ould be operated for a period of 100 days withoutr replacing components in full-scale. An equivalentples could be analysed automatically without any

    n from technicians. This work resulted in a patent.. [34] developed a method based on headspace gas chro-. A special sample pre-treatment cell was designed. Bymperature, pH and increasing the ionic strength in the

    solubility of volatile compounds was lowered drasti-s phase was sampled, analysed by gas chromatography,centrations of individual SCVFA were determined usinglations. The authors stated that the method had a supe-ge, since it did not involve the use of any ltration unitsling and clogging.is et al. [35] described a method based on capil-romatography suitable for process monitoring andn automatic sampling module was applied followedon unit. Injections into the gas chromatograph were

    lier fodriver eliminruggedknownAD pladeterming cowto affeof the causedsampleHowevinto acsizes, echrom

    5.9. Ti

    FeitSCVFAtested wastewout thewas emcontrotion prrobustvaryinwas nothe perthat it

    Janmonitosuremprincipreactosystem8 min. a referthe regtwo mattendapplied

    LahAD protries. Ois supeto simAD plamethoThe auare capan ADalternaeight-psion, msuitablation olimits

    Motitrime on a laboratory-scale AD system. Again, the mainsing headspace gas chromatography was the effective

    of fouling problems associated with ltration units. Atem was designed based on previous experience andplications, when analysing biomass from agricultural

    For a period of 6 months, the system delivered on-lineions of SCVFA from a laboratory-scale AD reactor treat-nure subjected to various feed pulses in the attempte SCVFA levels. It was discussed that effective mixingtion cell had to be considered to eliminate problemsmainly foam formation due to addition of acid. Smallmes were preferred for several reasons in this design.

    he authors argued that future prototypes would taket the benets of larger sample volumes, larger loopn the effort of optimising the response from the gasaph.

    n

    auer et al. [29] presented an economical alternative fortication based on titration. The titration system was

    laboratory-scale AD process treating synthetic textile. Their solution facilitated analysis of samples with-d for ltration. A specially designed measurement celled in which titration was performed. The system wasy a computer, which automatically adjusted the titra-ure based on the expected SCVFA concentrations. Theof the method was tested by adding salt simulatingstewater quality from industrial processes. The methodicantly affected by these changing salt levels. Based onance and the simplicity of the method it was concluded

    be readily applied for analysis of any AD process.and Mattiasson [57] described a titrimetic method for

    of partial and total alkalinity in AD processes. The mea-ystem was based on the ow injection analysis (FIA)d was completely automated. A laboratory-scale AD

    equipped with a recirculation line connected to the FIAugh a lter, which allowed analysis of a sample everyeactor was monitored for a period of several months. As

    to the FIA method, titration was applied. The results ofion analyses showed excellent agreement between theds. The authors noted that the system needed plenty of, which must be optimised, before the concept can ben automated process environment.d Morgan [30] reviewed simple titration methods for

    monitoring with a special focus on developing coun-f the opening remarks in the paper was that titrationto any other routine monitoring method with respecty and cost. This still holds true, maybe also at manyin developed countries. The many proposed titrationies published over the years were discussed critically.

    noted that these methods, although simple in nature, of providing accurate results concerning the state ofess. Operators are free to choose from a number ofmethods, ranging from simple two-point titrations to

    titrations involving advanced calculations. In conclu- dependent on the AD reactor and system behavior, aation technique can always be used for on-site evalu-

    process state, although the resolution and detectionferior to expensive analytical hardware.et al. [31] reported the installation of an automatedstem, AnaSense, for pilot-scale monitoring of an indus-

  • 3150 M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155

    trial AD process treating wastewater. The system was based ontwo-point titration. It was argued that multi-point titration meth-ods were less applicable at industrial scale. The pilot-scale reactorwas fed with synthetic wastewater. Throughout the measurementcampaign, bon were pcalibrate thcarbon wasthe AnaSenconcluded with resoluof the systetions, howeis sufcient

    5.10. Micro

    Nacke ematter in biwas from 2based on thof water (apmicrowaveplastic procthe signal. tate from athe reactor ate modelsthe authorsapproach shcould be inuni-variatesensor wastered for spthe cell. The

    5.11. Acous

    Lomborgacoustic meof dry matteThree 5 L rethe effort toabove was into the loo4.8% to 11.4tion providprediction othe type ofpotential av

    6. Discussi

    6.1. Quanti

    Whethetrial wasteplants or ca number oare in comto-measurepotential. Ton-line usioff-line deing of the parameters

    biochemical network of coupled reactions that occur in an ADprocess.

    Comprehensive insight into the dynamics of the microbiologicalprocess can be obtained, if the monitoring program is augmented

    inclu comequi

    skillte) pnance sention multlidatalysethe s

    pres3 as tics in

    ergi

    he kscop

    an ry toant ced in. Mot in tous se tecy ha

    ars. Iethodof thees) cngth

    nsor

    AD pcompa-ltain alectrs, anl merly, oon a

    part win

    mpli

    ery e ho

    it is howutinative

    calibstly ue to

    as ron-line determinations of total dissolved organic car-rovided by a TOC-analyser. These values were used toe titration system under the assumption that dissolved

    due to SCVFA. Periodically, the agreement betweense and a gas chromatograph was checked. The authorsthat the system was capable of providing an estimatetion of between 5 min and 10 min. The performancem at low SCVFA concentrations gave unstable predic-ver, a clear trend could be observed in the data, which

    for process monitoring and control.

    waves

    t al. [58] reported a system for quantication of dryoslurry based on microwaves. The measurement range% to 14% dry matter. The principle of operation wase pronounced difference between dielectric constantsproximately 80) and solids (approximately 210). The

    radiation could be sent through plastic pipes or otheress instrumentation without signicant damping ofA laboratory-scale reactor was inoculated with diges-n agricultural biogas plant. The dry matter content inwas varied from 3% to 10%. Uni-variate and multivari-

    were built and showed good performance. However, argued that for real-world applications a multivariateould be considered as the dielectric variables possibleuenced by other constituents in the bioslurry rendering

    models useless. It was pointed out that the microwave not prone to suffer from common problems encoun-ectroscopic techniques such as fouling and clogging of

    principle was truly non-invasive.

    tic chemometrics

    et al. [52] investigated the feasibility of using passiveasurements and chemometrics to model the dynamicsr concentration changes during AD in laboratory-scale.actors digesting manure were fed with maize silage in

    increase the dry matter level. The TENIRS unit describedmodied to also include an accelerometer integratedp (non-invasive). The dry matter was increased from% resulting in 14 different levels. Multivariate calibra-ed models with acceptable accuracy and precision forf the dry matter content. This study is a role model for

    remote sensing acoustic chemometrics applicationailable today; see also [59,60] and Section 6 (Table 2).

    on

    cation of relevant process parameters

    r an AD process is designed to treat high-strength indus-water, sludge from municipal wastewater treatmento-digest agricultural resources and organic residuesf relevant process parameters and monitoring issuesmon. Classical process parameters include simple-

    parameters such as pH, temperature, and redoxhese parameters are all relatively easy to quantifyng commercially available technology. For at-line orterminations, there is no need for extensive train-personnel performing the analysis. However, these

    provide only very limited insight into the complex

    to alsooff-gaseters rhighlytivariamaintesors, thcalibramost (and vacess anabout

    Thein Fig. dynam

    6.2. Em

    To tspectrofar.

    Rammentadominnouncspectrapresennumerthat thtroscopfew yekey mmany rophorwavele

    6.3. Se

    An easily of ultrto obtysed. Etonguethe celSimilaattentito neoptical

    6.4. Sa

    A vassumelling;realityconstitresentsensorbut mocontinqualifyde volatile fatty acids (VFA), ammonia/ammonium andposition. Conventional determination of these param-res technically more complex analytical hardware anded personnel. Recent advances in development of (mul-rocess sensors have made it possible to substitute thesee-heavy methods with rugged and reliable process sen-sor-analyte quantication being based on multivariate[61]. There is still a need for specialist intervention, asivariate) process analysers require extensive calibrationion, chemometrics. However, once validated the pro-rs provide the plant operator with detailed information

    tate of the process.ent authors suggest the monitoring principle outlinedhe optimal solution for detailed deciphering of process

    practically any chemical or biochemical reactor.

    ng PAT modality: Raman spectroscopy

    nowledge of the authors, no studies concerning Ramanic monitoring of AD processes have been reported so

    spectroscopy is widely recognised as being comple- infrared spectroscopy. Moreover, the signal from theompound in AD processes, water, is much less pro-

    Raman spectra than in for instance near infraredst importantly, the superior selectivity for compoundsrace amounts in complex matrices has been proven intudies. From the literature [38,62,63] it becomes clearhnological development within the eld of Raman spec-s resulted in several interesting solutions over the pastt is possible that Raman spectroscopy will be one of thes for bioprocess monitoring in the near(er) future, as

    reported hurdles (for instance interference from uo-an be overcome by proper application of laser excitations and data processing.

    interfacing issues

    rocess is a harsh chemical environment, which mayromise process sensors. Many studies report the use

    ration or similar measures as a necessary prerequisite homogeneous, particle-free liquid that can be anal-ochemical sensors (ion-selective electrodes, electronicd electronic noses) can degrade due to poisoning ofmbrane and are subject to growth of biolm (fouling).ptical interfaces for spectroscopic techniques requires these can be subjected to fouling too and erosion dueicles (for instance sand) in the bioslurry owing by thedow.

    ng issues

    common assumption in chemical engineering is tomogenous mixtures allowing for easier process mod-easy to appreciate, why this assumption is popular. Inever, biotechnological processes such as AD are far fromg homogenous systems. The problem of acquiring rep-

    sensor signals and representative process samples forration and validation is well recognised in the literature,just as lip service. Many studies and technology reports

    describe, report and implement procedures that do notepresentative. Since most of the parameters of interest

  • M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155 3151

    Table 2Reported modelling performance of modern modalities for AD monitoring.

    Parameter Range Modality Scale Mode Reference

    SCVFA 40105 mmol L1 Titration Lab ES Feitkenhauer et al. [29] SCVFA 134900 mg L1 Titration Pilot ES Molina et al. (2008) SCVFA Classication ET Lab IS Buczkowska et al. [56] SCVFA 200800 mg L1 IR Full IS Spanjers et al. (2008) SCVFA 0.3110.20 g L1 NIR Full IS Jacobi et al. [53]H2S(g), S2(aq) 010,000 ppm FIA Lab ES Redondo et al. [54]HAc 018 mmol L1 GC Pilot ES Diamantes et al. [35]HAc 550 mmol L1 Titration Lab ES Salonen et al. (2009)HAc 250600 mg L1 IR Full IS Spanjers et al. (2008)HAc 5001700 mg L1 IR Lab ES Steyer et al. [43]HAc 0.073.08 g L1 NIR Full IS Jacobi et al. [53]HAc 0.141.72 g L1 NIR Lab IS Nordberg et al. [45]HAc 113 g L1 NIR Lab IS Lomborg et al. [52]HAc 06 g L1 NIR Lab IS Zhang et al. [50]HPr 018 mmol L1 GC Pilot ES Diamantis et al. [35]HPr 0.017.47 g L1 NIR Full IS Jacobi et al. [53]HPr 0.22.8 g L1 NIR Lab IS Hansson et al. [46]HPr 0.061.78 g L1 NIR Lab IS Nordberg et al. [45]HPr 06.2 g L1 NIR Lab IS Lomborg et al. [52]HPr 0800 mg L1 NIR Lab IS Zhang et al. [50]ind. SCVFA 0.150 mmol L1 GC + ltration Lab/full ES Pind et al. [22]ind. SCVFA 150, 40, 20, 10 mmol L1 HS-GC-FID Lab ES Boe et al. [36]PA, TA 23 g CaCO3 L1 FIA, titration Lab ES Jantsch and Mattiasson [57]TS 110 wt% Microwave Lab IS Nacke et al. [58]

    ind.: individual, lab: laboratory-scale, pilot: pilot-scale, full: full-scale, IS: in situ, ES: ex-situ, SCVFA: sum of short-chained volatile fatty acids.

    are dissolved in the liquid phase, ltration is often applied to getrid of the heterogeneous, interfering material such as particulateorganic matter, straw, and sand particles, which unfortunately isnot always the case, and therefore cannot serve the role as a uni-versal feature that circumvents the need for documentable, reliablerepresentatthis issue sdesign phas(TOS).

    Coppellabenets andrepresentatThe princip

    of mixed cultures. Often, for agricultural systems, through regu-lar supply of fresh manure it is assured that microbes that arewashed out are being replaced with fresh biomass securing micro-bial diversity and stability. The high solid content for AD reactorsfed with manure (usually in excess of 50 g L1) plus the presence

    w anremeightd in

    apprh preeve

    mus

    Fig. 3. Analytiboth the liquidthe knowledgeive samples and/or sensor signals. It is necessary to facequarely in the future and to incorporate already frome the principles promulgated by the Theory of Sampling

    [64] as well as Holm-Nielsen et al. [65] discussed the drawbacks of recycling lines and methods for assuringive samples from complex geometries such as reactors.le of operation for most AD reactors involves the use

    of strameasuis the nteractewhereis muc

    Howplants

    Destructive MIMSGCin-situ

    Non-destructive SpectroscopyEN

    gas

    Non-destructive SpectroscopyETa.c.

    in-situbroth

    ex-situbroth

    cal modalities put into context. A general PAT monitoring scheme applicable for any chem and the gas phase can be accomplished with commercial hardware. Adding a battery of (d

    level pertaining to virtually any process.d other particles, complicates spectroscopic (optical)nts. Moreover, formation of gas bubbles in the brothmare of spectroscopists. Usually these issues are coun-

    industry using so-called de-bubblers and macerators,opriate. A clear, particle-free and gas bubble-free liquidferred in conventional spectroscopy.r, the forced technical simplicity of agricultural ADt be taken into account. Sophisticated hardware solu-Destructive MSGCHPLCFIATitrationNon-destructiveSpectroscopyETENa.c.

    ical, environmental or biochemical process. The in situ interfaces toestructive) reference modalities for at-line analysis further increases

  • 3152 M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155

    tions have to be avoided as documented by the many failedattempts to introduce even only moderately advanced systems inpractical, full-scale agricultural AD plants. From a spectroscopicpoint-of-view this implies that transmission cells prone to cloggingdue to presand other cothe reectiever feasibl

    Holm-Non monitorconversion solve to comboth represpower of chdirection wfull power using propecritically nesystems. Prsine qua nonreader is re3 in Bakeevpling issuessystems.

    6.5. Uni-va

    Relativecesses handuni-variatefor analysininterferencUni-variaterelevant invided that structure (fconcentratiuni-variatemodel.

    Many estivariate remany respoThe selectivconcentratitern hiddenwhich holdappropriateden inform

    Multivarcal chemistis howeverand versation data orichemistry amultivariat

    The widhas accelerinterpretatione or mopreter. It iinformationbut also pronot relevanas principastructure (Xone that ca

    problem and a residual matrix (errors). By performing this lin-ear algebraic manoeuvre, the effective dimension of the datasetcan be reduced signicantly, since only information relevant fordescribing the analyte variation in the spectra is kept in the data

    . Forsingsiona

    a diincip

    corrndepg valhichal s

    is. Sus bering

    nex for

    singds suany , wh

    erim the s of and

    idatieredes ov

    of a al vaen a

    andchno

    deta] anrkho

    sion. metrgs.

    oust

    ong ults een eous i

    yearringble wle nomp.g. atratich isustic

    infoocesrodyy detce m

    sts smetr

    relehe aence of particles, complicated auto-calibration systemsnvenient measures probably not are applicable. Instead

    on principle (180 backscatter) should be used, wher-e.ielsen [60] presented a rst foray of a new holistic viewing the complex AD process in particular and of bio-processes in general. This study focused on (but did notpletion) the critical role of always being able to acquireentative samples as well as sensor signals, before theemometric calibrations is applied. A next step in thisas taken by Esbensen and Julius [66], who laid out theof representative sampling, chemometrics as well asr validation approaches (test set validation), that arecessary in order to prove reliable process monitoringoper representative sampling is a critical success factor

    for all of the issues included in the present review. Theferred to Esbensen and Paasch-Mortensen [67], chapter

    [38] for in-depth treatment of all relevant process sam- whether pertaining to sampling systems or to sensor

    riate versus multivariate data analysis

    ly few studies on monitoring and control of AD pro-le data using multivariate methods. Instead, classical

    techniques are put into play and used extensivelyg responses from sensors. Matrix effects (chemical

    es, etc.) are hardly detectable using these techniques. methods are inadequate, when it comes to extractingformation from often large, complex data sets. Pro-the specic information is not apparent from the dataor instance linear correlation between absorbance andon of analyte as determined at a single wavelength)

    methods will never give rise to a robust and valid

    tablished and emerging sensor techniques output mul-sponses meaning that a single sensor or array deliversnse variables for instance spectra or a series of voltages.ity of one of these response variables with respect to theon of a single analyte is often poor. However, the pat-

    in the response variables provide a sort of ngerprint,s vast amounts of meaningful information provided that

    data analytical methods are applied to extract this hid-ation.iate data analysis has its root in analytical and physi-ry. Hence, this discipline was named chemometrics. It

    important to underline that the functionality, power,lity of chemometric methods allow them to be appliedginating from almost any process, not only related tond a more proper term for this discipline would bee data analysis.espread implementation of spectroscopic techniquesated the development of capable data handling andon algorithms. Frequently, full spectra obtained fromre process analysers are delivered to the data inter-s obvious that such large datasets not only contain

    relevant for quantication of one or more analytes,vides vast amounts of auxiliary information, which ist for the problem at hand. Multivariate methods, suchl component analysis (PCA), allows the original data, the spectra) to be decomposed into two matrices;rries relevant spectral information pertaining to the

    matrixcompodimenthis tothe prdo notthem iloadinples, wabnormanalysand hamonito

    Thedictingfrom amethoAs for appliedof exption insystemon demnal valconsidprovidmanceexternEsbensmisuseand te

    Forson [70the woregreschemostandin

    6.6. Ac

    Aming resat all sNumerlast 15monitoassessadesirabof its ceters econcenapproaan acorevealstem/prin hydtativelreferenrics rechemotion ofup by t a case, where near infrared spectra have been acquired of say 1000 different wavelengths (equalling a 1000-l variable space), the PCA will typically be able to reducemension of less than 10 principal components. Sinceal components are constructed in such way that theyelate (they are orthogonal), it is possible to interpretendently. Using proper plots of calculated score andues, it is possible to detect clusters/classes of sam-

    are similar based on their spectral information. Also,amples (outliers) reveal themselves easily during PCAch use of PCA analysis is often applied in data analysisen reported in many studies pertaining to AD process.t step, to make actual regression models capable of pre-instance SCVFA, COD, and dry matter simultaneouslyle spectrum, involves use of multivariate regressionch as partial least squares regression (PLS regression).modelling exercise, proper validation must always been implementing multivariate models. Careful planningental details often helps creating the necessary varia-calibration data. However, AD processes are sensitivemicroorganisms and variation can be difcult to induce, if the process is stable. The widespread use of inter-on (cross-validation, re-sampling) is by many authors

    adequate. Unfortunately, this kind of validation usuallyer-optimistic modelling performance. The true perfor-prediction model can only be estimated by use of properlidation (independent test set). The reader is referred tond Geladi [68] for an in-depth critique of the serious

    mal-practice of cross-validation within applied sciencelogy.ils on PCA and PLS refer to Wold et al. [69]. Hskulds-d Geladi and Kowalski [71] describe the theory behindrse in multivariate data analysis, partial least squaresEsbensen [72] gives an introduction to these basicic methods based on geometrical projection under-

    ic chemometrics

    the emerging PAT modalities that have shown promis-for non-invasive process analysis, but which has notnough exploration, acoustic chemometrics range high.nnovative applications have been documented over thes showing that this technique is capable of addressing

    problems in science and industry that are not easilyith other demonstrated modalities, both because of its

    on-invasive features (clamp-on sensor) and becauselementary nature, mainly addressing physical param-verage particle size, particle size distribution, bubbleon and others. The rationale behind the success of this

    that practically all processes and unit operations emit pattern, which reects the state, and very often alsormation about composition and phases, of the sys-s. It has been documented that even minute changesnamic behavior, for example pipe line ow, are quali-ectable and very often also quantiable, using properethods to calibrate the system. Acoustic chemomet-

    quarely on the foundation of multivariate calibration;ic data analysis is a critical success factor for extrac-vant features in the complex acoustic pattern pickedpplied sensors. Applications for acoustic chemometrics

  • M. Madsen et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3141 3155 3153

    in AD process monitoring could include monitoring of dry matterdynamics, hydrolysis rate determination for particulate matter, andassessment of wear and tear of machinery and process instrumen-tation. Halstensen and Esbensen [73] give recent introduction toacoustic cherature); anis [59].

    6.7. Handhparadigm?

    Recent resulted in sRaman analeven handhrange of poploited, butsamples to tinstrumentably handhxed, dedicdevices arehence, theya minimumeliminate ttainly appeastudies [74,instrumentcalibration another [76

    It is therAD plants lart in-line/osuch handhAD reactorsscenarios caresults will

    7. Conclus

    As docuAD process and reliabilator has tooffered techagriculturalmethods ansort of coning a monoexperience

    Larger, ccept or simindustrial orequires a measures foreveal the tfeedstocks. materials isintegrated p

    The prestion procesAD reactor ciently larefuent qumodern sen

    cess can be kept within specications even at signicantly higherloads than seen today.

    For agricultural AD plants, robustness and simplicity must nec-essarily always be part of the process analyser design. The users

    rs) w audivativ, andial, rin th

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    possed mts ar hug prmen

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    basering

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    hopd adablisod sying n robhere

    from chaors, d, anh proptio

    meas ope

    sect

    wled

    haelh Ph

    nces

    ta-lvoverv0;74(iland nts. Birollegnt proemometrics (and refer to the background technical lit- easily accessible foundation for acoustic chemometrics

    eld and mobile process analyserspart of the new

    advances in the eld of mobile spectroscopy haveeveral innovative products. Infrared, near infrared, andysers are now available in battery-operated mobile andeld congurations. These units provide a whole newssibilities for process monitoring, largely still unex-

    the central issue is very clear: instead of bringing thehe instrument/laboratorywhy not bring the analytical

    to the process? And why not use a mobile, prefer-eld, analytical instrument, instead of implementing aated PAT system for full versatility and exibility? These

    specically designed with user-friendliness in mind; can be operated by personnel having received only

    of training. However, this desirable feature does nothe need for competent supervision. First forays cer-r sufciently positive to warrant continued application75]. Moreover, the structural similarities between theses and their laboratory/bench top analogues allow formodels to be transferred seamlessly from one type to].efore possible that the future of process monitoring foracking sufcient capital for upgrading to state-of-the-n-line analysers to some extent can be substituted byeld, mobile instruments, e.g. shared between several, AD plants, and/or farmers. The potential applicationn only be glimpsed at this stage: exciting studies and

    abound.

    ions

    mented by this review, there is not a clear winner formonitoring. Robustness, simplicity, accuracy, precision,ity are some of the key parameters any AD plant oper-

    consider, when focusing on purchase of any of thenologies at the market. Many small, local or regional

    AD plants can probably survive using simple titrationd occasionally having their process veried by somesultant service. This could hold for AD plants digest--substrate (for instance maize silage), where operationis well-documented.entralised, more complex AD plants, the Danish con-ilar, where co-digestion of manure and various sorts ofrganic wastes happens at thermophilic temperatures,more reliable and comprehensive approach. Simpler acidity and other key parameters will probably notrue process state for all relevant combinations of theMoreover, the lack of quality control of incoming raw

    a serious drawback. Raw material control should be anart of the process analysis hardware package.ent authors perceive that the future of anaerobic diges-s monitoring is facing a paradigm shift. Conventionaldesign for wastewater treatment plants, where a suf-ge volume efciently acts as a buffer and guaranteesality, is no longer attractive. By proper application ofsor technology and multivariate data analysis the pro-

    (farmeexpertas deripliedpotenteated simpleagriculdegreeputs e.is no li

    Thedesignnutrienanotheongoindeploy

    8. Fut

    It isresentto formeffortsmonitorevealsare bainevitasuboptThere moderproces

    It istive anthe estand foidentifmoderlevel, wbenetcult tooperatplanneresearcviable logicalprocesthe AD

    Ackno

    Micthroug

    Refere

    [1] MaAn 200

    [2] Weme

    [3] Vanmeill never adapt sophisticated systems that requiret and frequent consultancy by techniciansbut as soone generations of such systems have been suitable sim-

    scaled down economically without compromising theevolutionary information quality and quantity delin-is review, the market for such robust and technicallyems is enormous, e.g. as evidenced by more than 5000l AD plants currently in operation in Germany alone. Thebustness and affordability that can be achieved todayal Chinese or Indian markets within reach as wellthereor the commercial OEM sector.ibility for expanding the use of closely related speciallyanure analysers to also allow on-eld quantication ofpplied to farmland during fertilising seasons presentsge unexploited market worldwide. Many interestingojects are looking into the possibilities for this kind oft.

    esearch and development

    mmended that the AD community together with rep-s from industry, agriculture, OEM, and academia seeke a common strategy for research and developmentd on the modern approaches for achieving advanced

    and control. A perusal of public research databases it is far from the rule that current well-funded projectsn the state-of-the-art levels described above, which

    eads to redundant research being performed as well as utilisation of grants and schemes for applied research.ast, still only barely tapped potential in involving theoven approaches of PAT and TOS for more reliable ADnitoring and control.ed that based on the vast experiences relating to effec-vanced process monitoring and control gained fromhed fermentation industry mainly rooted in pharmacycience, the AD community will be able to benet byeasily transferable know-how. Cost-of-ownership forust, inexpensive process analysers has decreased to a

    also minor companies (and soon individual farms) can their implementation. Although it presumably is dif-

    nge the culture and mentality of all current AD plantimmediate proof-of-concept studies can be designed,d executed within the framework of already establishedogrammes. Conservative AD plant designs are no longerns in the modern economy. Capable modern techno-ns must be adapted in order to assure the best possibleration conditions and hence maximise the efciency ofor as a whole.

    gements

    Madsen acknowledges Aalborg University for funding.D. scholarship no. 562/06-7-28027.

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    Monitoring of anaerobic digestion processes: A review perspective1 Introduction1.1 Major applications of anaerobic digestion1.2 Bio-economy1.3 Resources for bio-refinery concepts

    2 Anaerobic digestion in Denmark: a case study2.1 Mobilisation of grassroot movements2.2 Current share of energy production2.3 The farm-scale biogas plant concept2.4 The centralised biogas plant concept2.5 Main products of centralised AD2.6 Veterinarian benefits of AD2.7 Fertilising benefits of AD2.8 Socio-economic benefits and externalities of AD2.9 Issues yet to be resolved

    3 AD process theory3.1 General concept3.2 VFA as process indicators3.3 VFA quantification

    4 Conclusions and recommendations from previous reviews5 Review of the modern AD process monitoring potential5.1 Monitoring of bioprocesses5.2 Principles of operation for reviewed monitoring techniques5.3 Fluorescence spectroscopy5.4 Infrared spectroscopy5.5 Near infrared spectroscopy5.6 Ultraviolet and visual spectroscopy5.7 Electronic tongues and noses5.8 Gas chromatography5.9 Titration5.10 Microwaves5.11 Acoustic chemometrics

    6 Discussion6.1 Quantification of relevant process parameters6.2 Emerging PAT modality: Raman spectroscopy6.3 Sensor interfacing issues6.4 Sampling issues6.5 Uni-variate versus multivariate data analysis6.6 Acoustic chemometrics6.7 Handheld and mobile process analyserspart of the new paradigm?

    7 Conclusions8 Future research and developmentAcknowledgementsReferences

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