Effects of mixing on the result of anaerobic digestion: Review

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<ul><li><p>Effects of mixing on the result of anaerobic digestion: Review</p><p>Johan Lindmark a,n, Eva Thorin a, Rebei Bel Fdhila a,b, Erik Dahlquist a</p><p>a Mlardalen University, School of Business, Society &amp; Engineering, PO Box 883, SE-721 23 Vsters, Swedenb ABB AB, Corporate Research, SE-721 78 Vsters, Sweden</p><p>a r t i c l e i n f o</p><p>Article history:Received 12 August 2013Received in revised form17 June 2014Accepted 17 July 2014</p><p>Keywords:Anaerobic digestionMixingContinuously stirred tank reactorIntermittently mixedTracerCFD modeling</p><p>a b s t r a c t</p><p>Mixing in an anaerobic digester keeps the solids in suspension and homogenizes the incoming feed withthe active microbial community of the digester content. Experimental investigations have shown thatthe mixing mode and mixing intensity have direct effects on the biogas yield even though there areconflicting views on mixing design. This review analyzes and presents different methods to evaluate themixing in a digester (chemical and radioactive tracers and laboratory analysis), tools for digester design(computational fluid dynamics and kinetic modeling) and current research on the effects of mixing onthe anaerobic digestion process. Empirical data on experiments comparing different mixing regimeshave been reviewed from both a technical and microbial standpoint with a focus both on full scaledigesters and in lab-scale evaluations. Lower mixing intensity or uneven mixing in the anaerobicdigestion process can be beneficial during the startup phase to allow for methanogenic biomass growthand alleviate process instability problems. Intermittent mixing has been shown to be able to yield asimilar gas production as continuous mixing but with the possibility to reduce the maintenance andenergy demands of the process. Problems often experienced with experimental design include the effectof mixing on the solids retention time, and measurement of steady state gas production because ofstartup instabilities. Further research should be aimed at studying the effects of mixing on a chemicaland microbial level and on the different stages of anaerobic digestion (hydrolysis, acidogenesis,acetogenesis and methanogenesis). The focus should be on the effects of mixing on a multiple stagedigestion process and also finding new methods to evaluate the effects of mixing in the one stagedigestion process rather than evaluating a wider range of mixing modes, intensities and substrates.</p><p>&amp; 2014 Elsevier Ltd. All rights reserved.</p><p>Contents</p><p>1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10312. Modes of mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1031</p><p>2.1. Mixing and the microbial community . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10323. Evaluating the mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1032</p><p>3.1. Tracer methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10323.1.1. Chemical tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10323.1.2. Radioactive particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1033</p><p>3.2. Laboratory analysis of mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10334. Effects of mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1033</p><p>4.1. Control parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10334.1.1. Retention time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10334.1.2. Organic loading rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1033</p><p>Contents lists available at ScienceDirect</p><p>journal homepage: www.elsevier.com/locate/rser</p><p>Renewable and Sustainable Energy Reviews</p><p>http://dx.doi.org/10.1016/j.rser.2014.07.1821364-0321/&amp; 2014 Elsevier Ltd. All rights reserved.</p><p>Abbreviations: AD, anaerobic digestion; CARPET, computer automated radioactive particle tracking; CFD, computational fluid dynamics; COD, chemical oxygen demand;HRT, hydraulic retention time; OLR, organic loading rate; SRT, solid retention time; TS, total solids; VFA, volatile fatty acids</p><p>n Corresponding author. Tel.: 46 21 10 70 44.E-mail address: Johan.Lindmark@mdh.se (J. Lindmark).</p><p>Renewable and Sustainable Energy Reviews 40 (2014) 10301047</p><p>www.sciencedirect.com/science/journal/13640321www.elsevier.com/locate/rserhttp://dx.doi.org/10.1016/j.rser.2014.07.182http://dx.doi.org/10.1016/j.rser.2014.07.182http://dx.doi.org/10.1016/j.rser.2014.07.182http://crossmark.crossref.org/dialog/?doi=10.1016/j.rser.2014.07.182&amp;domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.rser.2014.07.182&amp;domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.rser.2014.07.182&amp;domain=pdfmailto:Johan.Lindmark@mdh.sehttp://dx.doi.org/10.1016/j.rser.2014.07.182</p></li><li><p>4.2. Empirical results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10345. Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035</p><p>5.1. CFD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10355.1.1. Implementation of CFD simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1035</p><p>5.2. Mass balance and kinetic models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10376. Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10377. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1038Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1039References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1046</p><p>1. Introduction</p><p>The anaerobic digestion (AD) process has been studied inten-sively over the last few decades for its application on biomass andsolid waste digestion and for its use in industries such as wastewater treatment and agriculture [26,57,94,3,58,16]. Understandingand control of the digester and the biological processes within itare of great importance in order to improve the AD process andincrease biogas production in industrial biogas plants.</p><p>Considerable effort has focused on selection of the right substrateor mixture of substrates, pretreatment to remove contaminants,reducing substrate size and hygienization in the AD industry, but verylittle effort has been put into controlling the digestion itself. Thecontinuously stirred tank reactor (CSTR) is a very common digesterdesignwhere the content is mixed continuously to maintain the solidsin suspension and to form a homogenous mixture. Mixing mode andintensity are important control measures for the CSTR and manyinvestigations have shown that they have direct effects on the biogasyield, even though there are conflicting views on how the mixingshould be designed. Positive effects on the biogas yield have beenachieved both by increasing [35] and decreasing [80,89] mixing in theAD process.</p><p>The AD process is dependent on mixing for distribution ofmicroorganisms and nutrition, inoculation of fresh feed, homogeniza-tion of the material, removal of end products of metabolism and forevening out the temperature inside the digester [22]. Microorganismsare sensitive to mixing intensity and may not survive excessivelyforceful mixing [22]. In this reviewwe analyze and evaluate the resultsof a number of studies with the aim of formulating general rules andempirical relations between mixing and gas production in the ADprocess.</p><p>2. Modes of mixing</p><p>There are a number of different types of mixing equipmentsused in the AD industry. These include mechanical mixing,hydraulic mixing and pneumatic mixing [22]. Mechanical mixingis the most common mixing type being used in Europe today, anduses different types of propellers and agitators to homogenize thedigester content. Mechanical mixing has also been reported ashaving the highest power efficiency per volume unit mixed [9].Hydraulic mixing is performed via pumps located outside thedigester, which recirculate the AD sludge, and pneumatic mixingutilizes the gas, which is pumped back down into the digester andreleased to create a horizontal mixing action as the bubble columnrises to the surface. The quality and results of mixing can varydepending on the equipment used and the geometry of thedigester.</p><p>Aside from the mixing equipment used, mixing intensity andalternative mixing modes can be used to further influence the process.The mixing intensity can be defined as the power used per unitvolume, but is often presented as rotational speed of a mixer, or as the</p><p>flow output of a pump (gas or liquid). Regardless of how the mixingintensity is presented, a lower mixing intensity can save energy andimprove the energy balance of the system. The United States Environ-mental Protection Agency (EPA) gave a general recommendation of58W/m3 of digester volume in 1979 (US EPA), but there is no realconsensus on how much mixing is actually needed.</p><p>The digester can either be mixed continuously, using anintermittent mixing mode, or not be mixed at all. Intermittentmixing means that mixing is turned on and off according to apreset time interval that can range from a few seconds of mixingper day to an almost continuous mixing mode. The power demandof the mixer motor increases during startup of mixing, and thisincrease needs to be considered when considering energy optimi-zation of the process. Kowalczyk et al. [43] reported a 2.5% averageincrease in power consumption by the mixer motor for the first13 min following startup in their laboratory-scale setup. Theenergy demand for mixing in a full-scale digester is substantialand can vary from 14% to 54% of the total energy demand of theplant [70,43]. Kowalczyk et al. [44] reported that the energydemand of mixing could be reduced by 1229% if intermittentmixing was used instead of continuous mixing. By applying anintermittent regime, mixing for 2 h and pausing mixing for 1 h, theenergy demand was reduced by 29% compared to a continuousmixing mode, and when mixing for 7 h and pausing mixing for1 h, the energy demand was reduced by 12%.</p><p>Gas release from the liquid digestate in intermittently mixeddigesters has been shown to increase by up to 70% during mixingperiods [82,65,61]. This implies that gas release is impeded in theunmixed condition and that mixing increases the mass transfer ofthe gas from the liquid phase to the gas phase. Stafford [79]analyzed the gas transfer from liquid to gas phase during differentmixing regimes (1401000 rpm intermittent mixing) and demon-strated gradually increasing gas release with increasing mixingintensity during the first minute of mixing.</p><p>Depending on the type of substrate treated in the AD processthe feed can have different properties. Light materials such asfeathers and straw have a tendency to float on the surface [15],while heavier materials like egg shell and other heavy particlessink to the bottom. Stratification processes such as sedimentationand floatation are often seen as a problem in the digestion process,which can lead to reduced active digester volume, mechanicalproblems with pumps and stirrers, and obstruction of the releaseof biogas from the liquid phase. Keeping the digester content insuspension by mixing can solve many of these problems as well asevening out the feed concentration and temperature gradient inthe digester. Foaming on the surface is also a problem that canoccur in the absence of mixing [80] or during mixing breaks [43],but no negative effect on the gas yield has been reported. Indeed,Stroot et al. [80] describe an increase in active volume. The use of ahopper bottom digester and local agitation of the surface to reducesedimentation, flotation and foaming is a possible solution, but anevaluation of each individual substrate is needed to identifyproblems that may occur. It is important to remember that these</p><p>J. Lindmark et al. / Renewable and Sustainable Energy Reviews 40 (2014) 10301047 1031</p></li><li><p>operational problems also occur in a CSTR and are not exclusive tothe unmixed and intermittently mixed digester.</p><p>2.1. Mixing and the microbial community</p><p>High mixing intensity and the resultant shear stress has anegative effect on flock formation and can inhibit gas production[93,59,80,41]. Close proximity between some microorganisms inflocs, often referred to as juxtapositioning, is important forsyntrophic interaction. Interspecies hydrogen transfer is oneimportant syntrophic interaction, where hydrogen is transferreddirectly between acetogenic and methanogenic (producers andconsum...</p></li></ul>