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,
~) Oxford University Press 1986
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
352 A. Bhadra , J. M. Scharer & M. Moo-Young
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Digestion of cellulosic wastes 353
lignocellulosic substances are digested, the acid yield is considerably less because of the reduced hydrolytic activity. The optimal pH for aci