Anaerobic digestion of organic matter

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<ul><li><p>This article was downloaded by: [Cornell University Library]On: 13 November 2014, At: 22:20Publisher: Taylor &amp; FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK</p><p>C R C Critical Reviews inEnvironmental ControlPublication details, including instructions for authors andsubscription information:</p><p>Anaerobic digestion of organicmatterP. N. Hobson a , S. Bousfield a , R. Summers a &amp; E. J. Kirsch ba Rowett Research Institute , Bucksburn, Aberdeen, Scotlandb Purdue University , West Lafayette, IndianaPublished online: 09 Jan 2009.</p><p>To cite this article: P. N. Hobson , S. Bousfield , R. Summers &amp; E. J. Kirsch (1974) Anaerobicdigestion of organic matter, C R C Critical Reviews in Environmental Control, 4:1-4, 131-191,DOI: 10.1080/10643387409381614</p><p>To link to this article:</p><p>PLEASE SCROLL DOWN FOR ARTICLE</p><p>Taylor &amp; Francis makes every effort to ensure the accuracy of all the information(the Content) contained in the publications on our platform. However, Taylor&amp; Francis, our agents, and our licensors make no representations or warrantieswhatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions andviews of the authors, and are not the views of or endorsed by Taylor &amp; Francis. Theaccuracy of the Content should not be relied upon and should be independentlyverified with primary sources of information. Taylor and Francis shall not be liablefor any losses, actions, claims, proceedings, demands, costs, expenses, damages,and other liabilities whatsoever or howsoever caused arising directly or indirectly inconnection with, in relation to or arising out of the use of the Content.</p><p>This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden.Terms &amp; Conditions of access and use can be found at</p><p></p></li><li><p>ANAEROBIC DIGESTION OF ORGANIC MATTER</p><p>Authors: P. N. HobsonS. BousfieldR. SummersRowett Research InstituteBucksburn, AberdeenScotland</p><p>Referee: E. J. KirschPurdue UniversityWest Lafayette, Indiana</p><p>I. INTRODUCTION</p><p>The treatment of waste products so as toreduce them in bulk and render them less obnox-ious is becoming more and more necessary inpresent-day society. Some wastes pose almostinsurmountable problems of disposal; radioactivewastes may remain dangerous for thousands ofyears. Other dry wastes, such as power station ash,may be relatively easily disposed of as land-fill orbuilding materials. Still other dry wastes may bereconstituted, as are paper, rags, and glass, intofeedstock for producing more of the same articles.Chemical wastes may be reacted to render theminnocuous, or burnt along with other wastes. Insome cases proper design of furnaces has led to areturn from burning waste in the form of heatwhich can be usefully employed. But there stillremains probably the largest fraction of the totalwaste output which is difficult or impossible totreat by physical methods. This is organic materialof animal or vegetable origin. The high watercontent of most of this material makes incinera-tion impossible or uneconomic; separation of theliquid and solid fractions can be difficult and thesefractions in themselves are still polluting. The</p><p>water content makes transport uneconomical, andthis, and the fact that the waste is subject toalmost immediate attack by microorganisms, makeon-site treatment necessary. Since the pollutionalproperties of these wastes are due in large measureto their being good substrates for microbialgrowth, they can be treated by controlled actionof microorganisms.</p><p>The growth of microorganisms can take placeeither aerobically or anaerobically. A larger rangeof substrates may be degraded aerobically thananaerobically and even many chemical wasteswhich in concentrated form are corrosive orbactericidal may, when in dilute solution, betreated microbiologically by aerobic methods.Complete action of microorganisms in the pres-ence of excess oxygen, and on a substrate balancedin carbon, nitrogen, and minerals theoreticallyresults in carbon dioxide, water, and microbialcells. Because of imbalances in the waste substratessuch complete conversion is seldom attained, butin a properly designed plant the pollutional load ofwaste waters may be reduced to acceptable levels.However, of more importance than imbalance ofsubstrates is the question of excess oxygen. Thedifficulties of supply of oxygen to the microbes</p><p>June 1974 131</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Cor</p><p>nell </p><p>Uni</p><p>vers</p><p>ity L</p><p>ibra</p><p>ry] </p><p>at 2</p><p>2:20</p><p> 13 </p><p>Nov</p><p>embe</p><p>r 20</p><p>14 </p></li><li><p>make aerobic waste-water treatment systems ofuse only for the treatment of dilute, largelysolubilized, materials. Some solids may be present,but these should be in the form of fine, suspendedparticles and their removal from the waste-water iscaused as much by entrapment and sedimentationin zoogleal floes of microorganisms as by actualdegradation. Aerobic microorganisms can, ofcourse, break down solid organic materials giventime and, again, a sufficient supply of air. Thisability is made use of in composting systems whererelatively dry, solid, organic wastes are degraded.But this process depends on the dryness of thesolids to provide a loose matrix through which aircan diffuse. If the solids are wet and compact,oxygen becomes limiting and anaerobic metabo-lism results. The other difficulty with aerobictreatment is that as respiration gives a high energyyield a large proportion of the substrate carbonand nitrogen is converted to microbial cells. In thecase of composting of solid wastes the productionof microbial cells does not matter. Indeed it is adesirable function of the process as the endproduct is generally used as fertilizer. But in thecase of treatments designed to produce a clearwater the mass of microbial cells is an embarrass-ment. These cells have to be removed from thetreated liquid. They can only be removed as a verywet sludge and, as they themselves can be subjectto microbial attack, and so are potentially pollu-tional, their disposal presents a further problem.</p><p>Anaerobic metabolism on the other hand pres-ents no problems of oxygen supply and is suitablefor breakdown of concentrated and insoluble, wet,organic wastes. This is exemplified by the fact thatfor millions of years anaerobic microbial metabo-lism has been used in the alimentary tracts ofherbivorous animals for digesting vegetable food-stuffs. "Organic" is used here in the sense ofanimal or microbial bodies or products of theirmetabolism, native vegetable materials or manu-factured products such as paper, but it does notinclude many synthetic organic chemicals whichare degraded only slowly, if at all by anaerobicprocesses. Its disadvantage from the point of viewof waste treatment is that although a mixedanaerobic flora will under the correct conditionsconvert a large amount of the carbon in an organicwaste into methane and carbon dioxide, which arenonpolluting, the process is never entirely com-plete, and some of the acidic intermediary pro-ducts remain to pollute the residual liquid, and,</p><p>except in the largely theoretical case where nitro-gen is growth-limiting, ammonia and possiblyother nitrogenous compounds may also remain.Thus in most cases the liquid from an anaerobictreatment system may not be suitable for uncon-trolled discharge. On the other hand anaerobicmetabolism is of low energy yield and the conver-sion of substrate to microbial cells is small, so thevolume of residual microbes to be disposed of isless than in an aerobic system. The residualmaterial from an anaerobic system is also stableand odorless and so presents less problems per seof disposal.</p><p>Anaerobic metabolism can then, by itself, notoften be a complete waste disposal system inreducing a liquid organic waste to present-day riverboard standards. It can be a system for reducingthe volume of, and stabilizing the solids in a waste,and for greatly reducing the pollutional load of thewaste liquid. This, in itself, may be sufficient, butgenerally some further aerobic or other treatmentof the liquid will be desirable before its finaldischarge.</p><p>Anaerobic microbial metabolism may takeplace whenever the ingress of oxygen is stoppedcompletely or limited to such an extent thataerobic microbial metabolism will quickly removethe oxygen. Thus it will take place beneath thesurface of still waters where rate of diffusion ofoxygen from the air is slow and the oxygen can beused up by microbial metabolism in the surfacelayers. This then is the basis for the simplest typeof anaerobic, waste-treatment system; the lagoon.A lagoon consists of a large tank, of a few feetminimum depth, open to the atmosphere. Wastematerial, such as farm animal excreta, is slowly runinto one end and partially purified liquid runs outat the other. Because there is little turbulence inthe lagoon the waste solids settle out. An aerobicmetabolism takes place in the top layer of theliquid, but beneath this, and in the settled solids,anaerobic metabolism slowly breaks down andstabilizes the organic matter. The principal dis-advantage of a lagoon is its size. Since themicrobial action takes place at an ambient temper-ature it is slow, and to obtain sufficient break-down of the waste its detention time must be long,a matter of months or a year or so. Thus if theoutput of waste to be treated is large, a very largelagoon is needed. This in turn brings otherdisadvantages. Unless the waste is very liquid thelagoon may have to be filled with water initially to</p><p>132 CRC Critical Reviews in Environmental Control</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Cor</p><p>nell </p><p>Uni</p><p>vers</p><p>ity L</p><p>ibra</p><p>ry] </p><p>at 2</p><p>2:20</p><p> 13 </p><p>Nov</p><p>embe</p><p>r 20</p><p>14 </p></li><li><p>obtain dispersion and settling of the solids. Thelarge surface area makes evaporation in dry clim-atic conditions and flooding in wet ones a prob-lem. While the large surface allows release ofcarbon dioxide and methane from the anaerobicmetabolism, it also allows release of sulphurcompounds and other microbial metabolic pro-ducts which can create a nuisance.</p><p>Under some conditions a lagoon can alsobecome a breeding ground for flies and otherpests. The lagoon has applications on certain sizesof farms under certain conditions, or for sometypes of factory waste, but its use is limited.</p><p>The "septic tank" in its various forms is anenclosed anaerobic system where ii) a speciallydesigned chamber or chambers a settlement anddigestion of solid waste takes place and a partiallypurified liquid is run off while gaseous endproducts are vented to the atmosphere. Feed is bygravity and is intermittent and controlled only bythe rate of flow in the sewers feeding the tank.The septic tank principle is largely used fordomestic wastes and is now generally limited tosingle house use, although some undergroundstorage pits for farm wastes may, by accident ordesign, obtain a similar process. Since, like thelagoon, a septic tank is at ambient temperature itis again a slow process and the tank must bedesigned for its particular purpose. Besides havingadequate design, both the lagoon and the septictank must be run under controlled conditions orbreakdown of the system and creation of a pooreffluent and a public nuisance may result. Also inboth cases there is a gradual accumulation of solidsand these have to be removed at intervals anddisposed of.</p><p>The third system of anaerobic waste treatmentin general use is that referred to as "anaerobicdigestion." This is anaerobic metabolism takingplace at a temperature usually above ambient andin a closed system which can be of simple orcomplex design. When it is run at elevated temp-erature the microbial metabolism is quicker thanin the lagbon or septic tank and the detention timeof the waste is less. Also the rate of charge (anddischarge) is subject to greater control than theseptic tank and the gaseous end products can becollected to do useful work.</p><p>The most common use of the anaerobic digesteris as a treatment for the solids settled off fromdomestic sewage before the aerobic treatment,combined with treatment of the microbial sludge</p><p>from the aerobic plant. This utilizes the previouslymentioned ability of the anaerobic microorganismsto degrade solid organic material in very thicksuspension. This property also makes the process apossible one for primary treatment of thickslurries such as those from farms, and of the solidresidues from, for instance, vegetable or meatprocessing factories, slaughter houses, or the fer-mentation industries. Since, of course, anaerobicmetabolism of completely dissolved compounds isalso possible, anaerobic digestion can also be usedfor soluble wastes. However, a complex flora, thecomponents of which have different growth rates,is necessary in practice, if not in theory, foranaerobic digestion and anaerobic bacteria aregenerally of lower maximum growth rate thanaerobes, so to stabilize this flora for treatment ofsoluble wastes, especially of high flow rate, asystem different from that used for high solidswastes is needed. There is also some indicationthat certain components of the anaerobic florarequire a solid surface for optimum growth andsolid components in the waste can supply this.</p><p>Since anaerobic digestion at higher than ambi-ent temperature seems the process which is con-tributing, and is most likely to contribute most tosystems of waste treatment, and is the mostamenable to control, some theoretical and prac-tical considerations of the process will be thepurpose of this review, although the materials andmethods of construction of digesters are outsidethe scope of this article. Lagoons and septic tankswill not further be considered. Also, in the presentclimate of decrease in conventional energy suppliesmicrobial production of useable gas would seem,at least on a limited scale, to offer possibilities.</p><p>Since anaerobic digesters have been part ofdomestic sewage plants for at least 60 years manypapers and reports, sometimes conflicting, on theoperation of digesters have been published. Butdetailed investigation of the microbiology andbiochemistry is comparatively recent, as it hasdepended on methods developed for use in otherfields. There is also difficulty in comparing reportson digester operation because the substrates fordigestion are heterogeneous and the methods ofanalysis commonly used are not sufficiently exactto show up differences in detailed composition ofthe wastes which may affect digestion. In effortsto correlate data on anaerobic digestion,...</p></li></ul>


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