Anaerobic digestion of native cellulosic wastes
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MIRCEN Journal, 1986, 2, 349-358
Anaerobic digestion cellulosic wastes
A. Bhadra, J. M. Scharer & M. Moo-Young Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Received 16 April 1985; revised and accepted 18 March 1986
The world-wide significance of cellulose as a potential energy source is appreciated by recognizing that photosynthesis produces 2 106 tonnes of dry biomass containing 50% cellulose each year (Reese etal. 1972). The energy equivalent of this cellulose is approximately 6 times the current global energy consumption. Most of this cellulose, however, occurs as a cellulose-lignin matrix in lignocellulosic materials. The inherent recalcitrant nature of lignocellulosic materials imposes formidable problems with respect to economical bioconversion of this resource to gaseous and liquid fuels. To date, the bioconversion of native lignocellulosic materials to methane (a gaseous fuel) has been the most popular route on a global scale. The process, known as anaerobic digestion, offers several advantages over alternative bioconversion strategies. These advantages reflect the technological simplicity of the system. Most anaerobic digestions can be performed with elective microbial cultures with no particular attempt to exclude contaminants. Thus, no pro-sterilization or aseptic conditions are necessary. Besides cellulose, the mixed microbial culture can use hemicelluloses, starches, proteins, and lipids of raw materials as substrates. The product gas containing 50-80% methane can be used directly as fuel with minimal post- fermentation treatment. Usually, low levels of hydrogen sulphide, an undesirable constituent, can be readily removed with iron shavings. During the process, energy- intensive unit operations such as continuous mixing and aeration are not required. The liquid effluent is characteristically low in both animal and plant pathogens. This effluent can be used as a soil conditioner and fertilizer.
From a biochemical point of view, the bioconversion of native cellulosic materials consists of three stages. Cellulose is first hydrolyzed to soluble sugars by a cellulase enzyme complex. These sugars, in turn, are degraded by two major groups of micro- organisms to a variety of small, dissolved molecules of which volatile fatty acids comprise the dominant fraction. These groups of micro-organisms are referred to as acidogenic (acid forming) and acetogenic (acetic acid-forming). In the third stage,
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350 A. Bhadra, J. M. Scharer & M. Moo-Young
acetic acid, hydrogen, and carbon dioxide are converted to methane by methanogenic (methane forming) bacteria.
The acid-forming bacterial population consists of cellulolytic and non-cellulolytic organisms. The interaction between these two groups of organisms in acidogenesis is very complex. Biosynthesis and extracellular cellulase activities are constrained by the presence of non-cellulolytic bacteria which compete with the cellulolytic bacteria for the soluble products of hydrolysis. According to Bryant (1973), non-cellulolytic organisms may provide some essential nutrients such as vitamins, growth factors and branched chain fatty acids for the cellulolytic species. Cellulose and glucose inhibit enzyme activity but their effect on cellulase biosynthesis is not well understood (Scharer & Moo-Young 1979). The combined action of cellulolytic and non- cellulolytic flora results in hydrolysis of cellulose to solubilized saccharides and their conversion to volatile fatty acids, carbon dioxide and hydrogen.
The microbiology of methanogenic bacteria has been systematically studied only in the past two decades. Altogether, about a dozen species of methanogenic bacteria have been isolated and maintained in pure cultures. They include short rods and curved rods (Methanobacterium), cocci (Methanococcus), spiral organisms (Methano- spirillum), and sarcinas (Methanosarcina). The biochemical mechanism of methane formation from either organic acids or carbon dioxide and hydrogen is not well known. The electron transport mechanism or the intermediates of the postulated sequential steps are yet to be proven experimentally (Stadtman 1967; Tzeng et al. 1975a). The presence of coenzyme M (2-mercapto ethane sulphonic acid), which affects methyl group transfer in methanogens has been established by many workers. Recently, several species of Methanobacterium have been found to possess a low molecular weight, fluorescent cofactor, co enzyme F420, which is believed to assist in low potential electron transport in a NADP-linked reversible oxidoreductase system (Tzeng et al. 1975a, b; Cheeseman et al. 1972).
Anaerobic digestion process
It is possible to separate the acid forming and methane forming stages and to culture each group of bacteria in isolated environments. The majority of conventional anaerobic bioreactors, however, consist of a single stage. Although optimum growth conditions for each baterial group are rarely achieved in a one-stage system, a balanced population can be maintained by making use of the syntrophic relationships amongst the various organisms. Methanogens growing in the same vessel as the acid- forming bacteria help in controlling the acid level as well as the pH.
Single-stage anaerobic digesters have been used for methane generation from industrial effluents, sewage sludge and agricultural wastes. Animal wastes are particularly suitable as feedstock. Animal manure production and its characteristics are summarized in Table 1. These manures can be digested directly, but the digestible carbon to nitrogen (C : N) ratio in most manures is sub-optimal for maximum biogas productivity. Animal manures contain excess nitrogen. Optimum C:N ratios of 21 :1 to 35:1 can be achieved by supplementing the digestion feed with lignocellulosic wastes such as cornstover, straw, rice husk, etc. For example, the
Digestion of cellulosic wastes 351
Table 1 Manure production and characteristics per 1000 kg live weight*
Item Units Dairy Beef Swine Poultry
Cow Heifer Yearling Feeder Feeder Breeder Layer Broiler
Raw waste (RW) kg/day 81.9 74.0 89.9 59.9 65.0 50.0 52.9 70.9 Faeces/urine ratio kg/kg 2.2 1.2 1.8 2.4 1.2 1.2 -- -- Density kg/m 3 1005 1003 1010 1010 1010 1010 1050 1050 Total solids (TS) kg/day 10.4 9.3 11.5 6,8 6.0 4.9 13.4 17.0
% RW 12.7 10.8 12.6 ll.6 9.2 8,6 25.2 25.2 Volatile solids kg/day 8.4 -- -- 6.0 4.9 3.1 9.5 11.9
% TS 82.5 -- -- 85.0 80.0 75.0 70.0 70.0 BOD5 % TS 16.5 -- -- 23.0 33.0 30.0 27.0 --
*Compiled from publication ASAE D384, ASAE Agricultural Sanitation and Waste Management Committee (1976).
addition of cornstover to swine manure (1 : 3 mass ratio) enhanced gas productivities by 63% in the case of thermophilic operation (55~ and 65% in the case of mesophitic operation (39~ (Fujita et al. 1980).
The operating characteristics of single-stage anaerobic digesters with animal manure as feedstock are summarized in Table 2. In general, lower operating temperatures require longer retention times of the solids. At loading rates of up to c. 5 kg volatile solids (VS)/m3/d at mesophilic conditions and 15 kg VS/m3/d at thermophilic conditions, the biogas productivity of (m 3 of gas/m 3 volume/day) increases linearly with loading. In general, biogas productivities range from 0.5-1.5 m3/m3/day at mesophilic conditions (20-40~ and from 1.0-2.5 m 3 biogas/m3/day at thermophilic conditions (40-60~ For most operating conditions, 30-50% of the volatile solids fed to the digester is converted to gas. The composition of the gaseous products does not depend on the operating temperature. For most feedstocks, the liquid effluent can be sprayed on farm land as a source of fertilizer or recycled as a diluent of the raw waste.
Recent advances in anaerobic bioconversion of native cellulosic wastes involve two- stage operation. In these systems, cellulolytic and acidogenic bacteria are physically separated from acetogenic and methanogenic species. By separating these microbial functions, each stage can be engineered to maintain optimum conditions. In this way, the methanogenic bacteria, which are the most sensitive to unfavourable environmental conditions, are protected against shock loads and pulses of inhibitory compounds.
Research by us and other workers (Khan et aL 1983; Baccay & Hashimoto 1984; Koster 1984; Bhadra et al. 1985) has shown that the anaerobic hydrolysis of cellulose can be improved upon by the introduction of two-stage processes. The hydrolysis of cellulose is considerd the rate-limiting step in the first stage, since the dissolved sugars are rapidly converted to acids. Volatile acids production from cellulose and lignocellulosic materials are shown in Table 3. Volatile fatty acid yields have been reported to be 0.7-0.8 g/g cellulose with pure cellulosic preparations. However, when
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lignocellulosic substances are digested, the acid yield is considerably less because of the reduced hydrolytic activity. The optimal pH for acid production from lignocellu- ~osics is 5.5 to 6.0 (Zoetmeyer etal. 1982). This compares with observed optimal pH range of 7.0-7.5 for methane generation from fatty acids.
In the two-stage concept, methane generation from dissolved organic acids is confined to the second stage of the process. Recent trends in process design involve the separation of the residual solids after the first stage and using the liquid fraction, containing volatile fatty acids and other dissolved substances, as feedstock in the second stage. Several types of high-rate bioreactors, with enriched biomass, have been employed to increase the conversion efficiency and reduce the retention time. The most promising bioreactor configurations are briefly described below on the basis of the techno-economics of the conversion.
The upflow anaerobic sludge blanket (UASB) process was developed by Lettinga et al. (1980, 1983). In this bioreactor, the feed solution flows through a thick layer of pelletized biomass. Methane generation occurs primarily on the surface of the pellets. Due to adhering gas bubbles, the pellets rise to the liquid surface, where the gas is collected in a funnel-like device. The pellets, stripped of gas, sink back into the sludge blanket. This process requires no external sludge recycle, tolerates some suspended solids loading, and operates at high organic loading. The process efficiency can be as high as 97?/0, One disadvantage of the UASB process is the relatively long start-up time of several months, required to form the bacterial pellets.
Fluidized bed bioreactors on the other hand, usually contain c. 30% by volume of sand or other fine support materials. In this type of digester, methanogenic and acetogenic bacteria are usually attached to the surface of sand particles. Continuous recirculation of the liquid in the digester results in the suspension of the sand particles. Loading rates as high as 15 kg VS/m3/d can be used under mesophilic conditions (Boening & Larsen 1982). This is approximately three times the loading rate in conventional anaerobic digesters.
In fixed film bioreactors, porous support materials are used for bacterial attachment. The downflow fixed film reactor was developed by van den Berg & Kennedy (1981, 1982). Suitable support materials include red draintile, potter's clay, Raschig rings, charcoal, activated carbon and variously pre-treated plastic supports. At optimum surface area-to-volume ratios (c. 240 m2/m3), a six-fold increase in biogas productivity in comparison to conventional systems has been observed. The efficiency of acids conversion is 75-95%. Although fixed film bioreactors can be operated in either a downflow or an upflow mode, the downflow mode of operation is generally preferred since channelling and plugging by suspended solids are overcome.
Currently, two-stage systems, particularly those involving the use of high-rate bioreactors in the second stage of methane production, are mainly at the laboratory and pilot-plant level of development. A few full-scale units have been constructed, but operating data are not yet available. The techno-economic feasibility of two-stage systems will depend on the extent of realization of the improved performance observed under ideal conditions in laboratory and pilot-plant projects.
At present, the vast resources of native cellulosic wastes remain under-utilized as feedstock for bioconversion to useful products. Considering its technological
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simplicity, anaerobic digestion to gaseous fuels should be considered as an integral part of the overall biomass-use strategy. Although the technical feasibility of anaerobic bioconversion of native cellulose has been demonstrated and significant technological advances have been accomplished, problems relating to low rates of hydrolysis of lignocellulosics, process efficiency, stability and control remain unsolved. Improvements in microbial activity may arise through genetic engineering in the future. More immediate goals would be the commercial development of starter cultures, microbial pellets, and microbial films for use in appropriate high-rise bioreactors. Improvements brought about by process engineering are likely to give higher conversion efficiency at lower hydraulic retention times. At present, physical processes such as contact between bacteria and solids and mass transfer rates have not been optimized. Thus, the full potential of the anaerobic process is yet to be realized.
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358 A. Bhadra, J. M. Scharer & M. Moo-Young
Anaerobic digestion is an extensively used bioconversion process to produce gaseous fuel from native lignocellulosic materials. It consists essentially of two steps; acidogenesis and methanogenesis. Most conventional anaerobic digesters are single-stage systems. Animal waste, agricultural residues, sewage sludge and industrial effluents are suitable as feedstock. Biogas productivity in single stage digestion ranges from 0.5-1.5 m3/m3/day at mesophilic (20-40~ and 1.0-2.5 m3/m3/day at thermophilic (40-60~ temperatures, and about 30-50% of the volatile solids are converted to biogas. In two-stage systems, acidogenesis is separated from methanogenesis, which improves cellulose hydrolysis and process efficiency. Recent advances in digester design include the introduction of the upflow anaerobic sludge blanket, and fluidized-bed and fixed-film bioreactors, which are operated at much higher loading. Process efficiency can be as high as 97%. An overview of these technologies is presented.
Fermentation anadrobie de ddchets cellulosiques naturels La fermentation ana6robie est un proc6d6 de bio-conversion tr6s utilis6 pour produire des carburants gazeux ~ partir de ligno-cellulose. EUe comporte essentiellement deux 6tapes: acidogen6se et m6thanogen6se. La plupart des digesteurs usuels sont des syst6mes ~ un seul 6tage. Les d6chets animaux, les r6sidus agricoles, les boues d'6pandage et certains effluents industriels sont des substrats appropri6s. Dans les fermentations ~ un seul 6tage, la productivit6 en bio-gaz varie entre 0,5 et 1,5 m 3 par m 3 et par jour pour les temp6ratures m6sophiles (20-40~ et de 1,0 ~t 2,5 pour les temp6ratures thermophiles (40-50~ d'autre part environ 30 ~t 50% des solides volatils sont convertis en bio-gaz. Dans les syst~mes h deux 6tages, l'acidogenbse est s6par6e de la m6thanogen6se, ce qui am61iore l'hydrolyse de la cellulose et l'efficacit6 du proc6d6. Les progr6s r6cents concernant la structure des fermenteurs comprennent la circulation du d6p6t ana6robie et la r6alisation de bior6acteurs h lit fluidis6 et film-fix6, ce qui permet d'op6rer avec des charges tr6s sup6rieures. L'efficacit6 peut atteindre 97%. Un aper~u de ces nouvelles technologies est pr6sent6 dans l'article.
Digesti6n anaerrbica de desechos celulrsicos La digesti6n anaer6bica es un proceso de bioconversi6n muy utilizado para producir gas combustible a partir de materiales de tipo lignocelulrsico. El proceso consta esencialmente de dos etapas acidogrnesis y metaogrhesis. La mayor~a de los digestores de tipo convencional son de una sola etapa. Como material de partida pueden utilizarse residuos d6 ganado, agricolas, lodos de depuradoras y efluentes industriales. La producci6n de biog~s en un digestor de una sola etapa va desde 0.5-1.5 m3/m 3 dia cuando el proceso transcurre a temperaturas mes6filas hasta 1.0-2.5 m3/m 3 dia cuando lo hace a temperaturas term6filas, transform~indose entre un 30 y un 50% de los s61idos vohitiles en biog~s. En los sistemas con dos etapas la acidogrnesis se separa de la metanogrnesis con lo cual se mejora la hidr61isis de la celulosa y por lo tanto la eficiencia del proceso. Entre los recientes adelantos en el disefio de digestores cabe incluir la introducci6n de una manta de flujo ascendente para fangos anaer6bicos y bioreactores con camas fluidas y de l~mina fija que pueden usarse con cargas mayores. La eficiencia del proceso puede llegar entonces hasta 97%. En este trabajo se pasa revista a todas estas nuevas tecnologias.