Fractionation of organic substances during anaerobic digestion of farm wastes for biogas generation

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<ul><li><p>M1RCEN Journal, 1989, 5, 27-42 </p><p>Fractionation of organic substances during anaerobic digestion of farm wastes for biogas generation </p><p>M,M. EI-Shinnawi*, B.S. EI-Tahawy, S.A. El-Shimi &amp; Soheir S. Fahmy Department of Soil Science, Menufiya University, Shibin Elkom and Soils and Water Research Institute, Giza, Egypt </p><p>Received as revised 6 July 1988; accepted 28 July 1988 </p><p>Introduction </p><p>Present day anaerobic digestion processes yielding methane have been established by allowing events which occur in nature, e.g. in sediments, the rumen, and other anaerobic environments, to take place within designed containers. This is to make efficient use of combustible gas evolved as an accessible and cheap source of energy, especially in rural areas. By fermentation of organic matter, the farmer can simultaneously satisfy his needs for fuel and good quality manure from one source, i.e. the farm wastes, besides controlling environmental pollution. </p><p>The main digestible components of solid wastes are carbohydrates (cellulose and hemicellulose), proteins and fats. Although these components in themselves are easily digested, they can be present in wastes in such a structural form that makes their availability for biodegradation difficult. This is the case for coagulated and fibrous protein, cellose, and hemicellulose incorporated in lignic complex (Hobson et al. 1974). Rate of methanogenesis depends on the status, type and constituents of the organic matter undergoing anaerobic digestion (Sathianathan 1975; Hobson et al. 1981). </p><p>Fermentation of mixed waste materials has proved to be more effective in biogas production than that of a particular material. For instance, Park (1979) showed a synergistic effect on the gas yield when different materials were mixed with each other, e.g. sewage waste on its own gave 0.265 m 3 biogas/kg volatile solids added and </p><p>* To whom correspondence should be addressed. </p><p>9 Oxford University Press 1989 </p></li><li><p>28 Biogas from farm wastes </p><p>weeds alone produced 0.277m3/kg volatile solids fed, but their mixture (50:50) produced 0.387 m3/kg volatile solids added, with an increase of 39%. </p><p>Our work here aims to study the biochemical changes of major constituents brought about in mixtures of cow dung with crop residues during the fermentation process for biogas production. </p><p>Methods and Materials Materials </p><p>Cow dung, rice straw, maize stalks and cotton stalks were employed as substrates. The crop residues were air dried and pulverized, but cow dung was used in its fresh wet status (78% moisture, on the basis of drying at 70~ for 24 h). </p><p>Laboratory techniques Treatments including fresh cow dung alone and in combinations with each of the air- dried crop residues were performed. In the animal/plant waste combinations, the ratio 1:1, on the 70~ dry weight basis, was applied. Initial analysis of those mixtures is shown in Table 1. CaCO3 (20 g) was added to each treatment (to serve as a buffer). The ingredients were thoroughly mixed and introduced into 4-1itre laboratory biogas fermenters (Fig. 1). Tap water was added to bring both the total solids concentration to about 8% (Sathianathan 1975) and the working volume to 2 litres. The C/N ratio of the feedstocks was left unmodified. All treatments were run in duplicates; thus 40 fermenters were allocated to satisfy five-interval estimations. The fermenters were then sealed and incubated at 35 ~ </p><p>For gas sampling---l]- </p><p>Polyethylene tube I I </p><p>Biogas t~iogas </p><p>Digesting material </p><p>Polyethylene tube </p><p>1 2 3 Fermenter Water trap for Receiving excess </p><p>gas collection water </p><p>Fig. 1 Schematic diagram of the laboratory biogas digestion Unit. </p><p>Assays. Measurement of biogas yield and its content of methane was carried out every 2 days as follows: </p></li><li><p>Table</p><p> 1 A</p><p>na</p><p>lysi</p><p>s of t</p><p>he</p><p> fee</p><p>dst</p><p>ock</p><p>s use</p><p>d </p><p>Fe</p><p>ed</p><p>sto</p><p>cks </p><p>TS</p><p>* V</p><p>S* </p><p>OC</p><p>* T</p><p>N* </p><p>C/N</p><p> W</p><p>ate</p><p>r-so</p><p>lub</p><p>le P</p><p>rote</p><p>in </p><p>Fa</p><p>ts </p><p>He</p><p>mic</p><p>ell</p><p>ulo</p><p>se C</p><p>ell</p><p>ulo</p><p>se </p><p>Lig</p><p>nin</p><p> C</p><p>ell</p><p>ulo</p><p>se/ </p><p>(%) </p><p>(%) </p><p>(%) </p><p>(%) </p><p>sub</p><p>sta</p><p>nce</p><p>s (%</p><p>) (%</p><p>) (%</p><p>) (%</p><p>) (%</p><p>) Li</p><p>gn</p><p>in </p><p>(%) </p><p>(%) </p><p>r~ </p><p>,'r-, </p><p>Co</p><p>w d</p><p>un</p><p>g </p><p>(CD</p><p>) 8</p><p>.1 </p><p>73</p><p>.4 </p><p>42</p><p>.6 </p><p>1.7</p><p> 2</p><p>5.1</p><p> 13.1</p><p> C</p><p>ow</p><p> du</p><p>ng</p><p> + r</p><p>ice</p><p> stra</p><p>w </p><p>(CD</p><p>+R</p><p>) 8</p><p>.0 </p><p>71</p><p>.6 </p><p>41</p><p>.5 </p><p>0.8</p><p> 5</p><p>1.9</p><p> 1</p><p>3.3</p><p> C</p><p>ow</p><p> du</p><p>ng</p><p> + m</p><p>aiz</p><p>e st</p><p>alk</p><p>s (C</p><p>D+</p><p>M) </p><p>7.9</p><p> 7</p><p>7.8</p><p> 4</p><p>5.1</p><p> 0</p><p>.9 </p><p>50</p><p>.1 </p><p>14</p><p>.9 </p><p>Co</p><p>w d</p><p>un</p><p>g +</p><p> co</p><p>tto</p><p>n st</p><p>alk</p><p>s (C</p><p>D+</p><p>C) </p><p>8.0</p><p> 7</p><p>8.5</p><p> 4</p><p>5.5</p><p> 1</p><p>.1 </p><p>41</p><p>.4 </p><p>11</p><p>.5 </p><p>10</p><p>.8 </p><p>7.0</p><p> 3</p><p>2.4</p><p> 2</p><p>1.5</p><p> 1</p><p>7.7</p><p> 1</p><p>.2 </p><p>5.2</p><p> 4</p><p>.2 </p><p>28</p><p>.0 </p><p>25</p><p>.4 </p><p>18</p><p>.2 </p><p>1.4</p><p> 5</p><p>.8 </p><p>4.1</p><p> 2</p><p>3.8</p><p> 2</p><p>8.1</p><p> 2</p><p>0.4</p><p> 1</p><p>.4 </p><p>6.5</p><p> 4</p><p>.0 </p><p>4.0</p><p> 2</p><p>5.1</p><p> 2</p><p>2.4</p><p> 1.1</p><p>*TS</p><p> = T</p><p>ota</p><p>l so</p><p>lid</p><p>s; VS</p><p> = v</p><p>ola</p><p>tile</p><p> soli</p><p>ds (</p><p>pe</p><p>rce</p><p>nt o</p><p>f TS</p><p>); O</p><p>C =</p><p> org</p><p>an</p><p>ic ca</p><p>rbo</p><p>n; T</p><p>N =</p><p> to</p><p>tal n</p><p>itro</p><p>ge</p><p>n. </p><p>t~ </p><p>tQ </p></li><li><p>30 Biogas from farm wastes </p><p>1. biogas, by means of displacement technique, using acidified water (2% H2SO 4) to prevent the solubilization of CO2 (Maramba 1978); </p><p>2. methane, by gas-liquid chromatography (Wujick &amp; Jewell 1980), using a flame ionization detector. Gas samples were withdrawn by a 50-ml gas-tight syringe and 0.5 ml injected into a gas chromatograph fitted with stainless-steel column (120 cm 0.2m) packed with 5% (w/w) OV-101 on Chrom-PAW80-100 mesh. Nitrogen, as a carrier gas, was at 28 ml/min. The column oven was at 75~ the injection port at 100~ and the detector at 150~ Results were calculated referring to periodically-made calibration curves of pure methane. </p><p>Chemical analysis. 1. Total solids (TS) in the slurry samples dried for 24h at 70~ according to the </p><p>recommendations of the American Public Health Association (APHA 1976). 2. Volatile solids (VS) by burning the 70~ samples at 650~ to constant weight </p><p>(APHA 1976). 3. Total volatile fatty acids (VFAs), by steam-distillation of the slurry effluent </p><p>acidified with HzSO 4 to pH 1, then back-titration with NaOH (Neish 1952). 4. Individual VFAs by gas-liquid chromatography (Wujick &amp; Jewell 1980). Effluent </p><p>of liquid slurry was acidified with H3PO 4 and left to stand for 60 min at 4~ then centrifuged at 1000 rev/min for 10 min. The supernatant was acidified to pH 1 and centrifuged at the same speed for 5 min. A sample (5 ~xl) was injected into the gas chromatograph (specifications are described above) running at 160~ for column oven and 210~ for each of injection port and detector. Results were calculated referring to periodically-made calibration curves of pure major fatty acids. </p><p>5. Analyses carried out according to the methods given by Chapman &amp; Pratt (1961): In the liquid slurry: NH4~-N, by micro-steam distillation in alkaline medium. </p><p>In the slurry dried at 70~ for 24h: organic carbon by the method of Walkely &amp; Black using H2SO 4 + K2Cr207 and total N by the Kjeldahl method. </p><p>6. Analyses for the major organic constituents, performed following the methods recommended by Kononova (1966): Water-soluble substances by loss of weight of the 70~ samples subjected to addition of distilled water and filtration three times. </p><p>Crude protein by multiplying the difference between the total N and NH~--N contents by 6.25. Fats, by extracting the 70~ samples using ethanol-benzene mixture (1:1 v/v) for 12-20 h in a Soxhlet apparatus. Carbohydrate fractions: Hemicelluloses, by hydrolyzing the residue remaining after removal of water-soluble substances using 0.3M HC1 for 3h. Reducing sugars were then determined in the hydrolysate by phenol-sulphuric acid technique. Data resulting were multiplied by 0.9 to give the hemicellulose content. Cellulose, by hydrolyzing the reesidue as before using 14 ra HzSO 4 for 2.5 h. The mixture was then diluted with distilled water and left to stand for 5 h. Reducing sugars were measured in the hydrolysate and their data were multiplied by 0.9 to give the cellulose content. Lignin, by drying the residue, as for cellulose to a constant weight at 105~ then ashed in a muffle furnace at 550~ Loss of weight represented the lignin content. </p></li><li><p>M. M. El-Shinnawi, B. S. El-Tahawy, S. A. El-Shimi &amp; Soheir S. Fahmy 31 </p><p>Results and discussion </p><p>Biogas and its methane component Production of biogas and its methane component measured every 2 days during the biomethanation of cow dung and its mixtures with crop residues is illustrated in Fig. 2. Evolution of gas showed fluctuating levels during the experimental duration. Appreciable amounts of gas were produced from days 7 to 35 of incubation. The highest biogas peak was detected for cow dung + maize stalks (CD+M) (1.61itres/ litre over 2 days) and followed in descending order by cow dung + rice straw (CD+R), cow dung + cotton stalks (CD+C) and cow dung (CD) alone. However, peaks of methane content mostly did not follow those of biogas. Flammable gas started after 6-9 days of incubation, depending on the type of biomass. </p><p>Cumulative production of each of biogas and methane through the fermentation course of the various feedstocks is shown in Fig. 3. Top total volumes of both gases were gained by CD+M which produced 171itres biogas and 81itres methane/litre fermented matter, whereas CD+R, CD+C, and CD alone followed, respectively. </p><p>"E </p><p>E </p><p>E </p><p>1.6 </p><p>1.4 </p><p>1.2 </p><p>1.0 </p><p>0.8 </p><p>0.6 </p><p>0.4 </p><p>0.2 </p><p>0 1.6 (c) </p><p>1.4 </p><p>1.2 </p><p>1.0 J 0.8 </p><p>0.6 </p><p>0.4 </p><p>0.2 </p><p>27" </p><p>(a) I- (b) </p><p>i I ." </p><p>~ii\.%.; 9 "... </p><p>.! I I I </p><p>(d) </p><p>I I [ " I [ 15 30 45 60 75 0 15 30 45 60 75 </p><p>Fermentation time (days) </p><p>Fig. 2 Production of biogas and its methane component during fermentation of cow dung and its mixtures with crop residues. (a) cow dung, (b), cow dung + rice straw, (c) cow dung + maize stalks, (d) cow dung + cotton stalks. - - Biogas, -... methane. </p></li><li><p>32 Biogas from farm wastes </p><p>18, </p><p>16 </p><p>14 </p><p>(9 </p><p>== t - </p><p>(9 D </p><p>8 o </p><p>"O o o. 6 </p><p>o~ </p><p>(5 4 </p><p>0 ~..-~... v - 15 </p><p>9 / . . j , . . .~ . . . .A . - . .~- . .A . . .~- .~ i </p><p>y .~,...~'"':.., ~...X....X...~..-~""X'"'X / - . " x . .~ .... </p><p>/ "" "' 0 " 'O ' " 'O ' "O ' "O ' "O 0 . . ,0 . . . . / ...'.x" o .... 9 9 .,r .~.-'" o ' " . ~. . .e . . . . . . . . . . o . . .~ . .~. .4 /'..::(.x ..~:-.'..o .... "'" </p><p>.,,~" .~::" +0" / ' </p><p>.~ . . " </p><p>I I I I 30 45 60 75 </p><p>Fermentation time (days) </p><p>Fig. 3 Cumulative biogas and methane production during fermentation of cow dung and its mixtures with crop residues. Biogas, .-.. methane. O, cow dung; x, cow dung + rice straw; A, cow dung + maize stalks, 9 cow dung + cotton stalks. </p><p>The above results are attributed to the type of raw biomass. For instance, molecular composition of the constituents of the plant materials undergoing digestion is of significance, e.g. the high lignin content of cotton stalks (Table 1) retards its biodegradation. The higher content of water-soluble substances and the lower C/N ratio of maize stalks makes them superior to rice straw. Moreover, the presence of growth-promoting substances, most probably in plant materials, enhances the microbial population - - thus mixing the cow dung with crop residues is beneficial for methanogenesis (Chengdu 1979; Hobson et al. 1981; Han 1982). </p><p>Total and volatile solids Contents of both TS and VS of the digesting materials decreased with time (Fig. 4). Such decreases are due to the bioconversion of the organic substances into gases (CH4 and CO2 ) and water. The extent of disappearance for both solids followed the order: CD+M&gt;CD+R&gt;CD+C&gt;CD. Sharp depressions occurred within the first month </p></li><li><p>M. M. El-Shinnawi, B. S. EI-Tahawy, S. A. El-Shimi &amp; Soheir S. Fahmy 33 </p><p>851 </p><p>80 </p><p>75 </p><p>70 </p><p>~ 65 g g ~ 6O </p><p>~ 55 P, o </p><p>~ 5o </p><p>45 </p><p>40 </p><p>-X ,,A </p><p>- \ \ </p><p>\ "~ .~..o-. . . . . . </p><p>~k,~ '~'x~ - ~ ~' ,o -x </p><p>-A </p><p>I I I I I 15 30 45 60 75 </p><p>Fermentation time (days) </p><p>Fig. 4 Changes of total and volatile solids during fermentation of cow dung and its mixtures with crop residues. Total solids, ---- volatile solids. e, cow dung; x, cow dung + rice straw; A, cow dung + maize stalks, 9 cow dung + cotton stalks. </p><p>and thereafter the rate of solids diminution slowed down. This correlated with the changes in the bacterial growth dynamics, which bloomed at the early stages but then steadily tended to decline by advancing the process duration as a result of nutritional exhaustion and/or accumulation of by-products. </p><p>Production rates of biogas and its methane component, calculated at the end of the digestion course of the various feedstock materials, are listed in Table 2. Figures of biogas indicated that CD+R gave the highest rate for VS added (2891itres/kg) and followed by CD+M, CD+C, and CD, respectively, whereas CD+C attained the maximum level for VS consumed (l1791itres/kg); CD+R, CD+M and CD were successively lower. CD+R was the highest producer of methane for VS added (1371itres/kg); CD+M, CD and CD+C followed descendingly, whilst CD surpassed all of the tested biomass in relation to the VS consumed and gave 636 litres/kg and </p></li><li><p>Ta</p><p>ble</p><p> 2 P</p><p>rod</p><p>uc</p><p>tio</p><p>n </p><p>rate</p><p>s of </p><p>bio</p><p>ga</p><p>s an</p><p>d it</p><p>s m</p><p>eth</p><p>an</p><p>e </p><p>aft</p><p>er 7</p><p>5 d</p><p>ay</p><p>s of fe</p><p>rme</p><p>nta</p><p>tio</p><p>n </p><p>of th</p><p>e d</p><p>iffe</p><p>ren</p><p>t fee</p><p>dsto</p><p>ck</p><p>s </p><p>in r</p><p>ela</p><p>tio</p><p>n to</p><p> to</p><p>tal a</p><p>nd</p><p> vo</p><p>lati</p><p>le </p><p>so</p><p>lid</p><p>s a</p><p>mo</p><p>un</p><p>t ad</p><p>de</p><p>d an</p><p>d c</p><p>on</p><p>su</p><p>me</p><p>d </p><p>4~</p><p>r~ </p><p>Fe</p><p>ed</p><p>sto</p><p>cks </p><p>Bio</p><p>gas </p><p>Me</p><p>tha</p><p>ne</p><p>To</p><p>tal v</p><p>olu</p><p>me</p><p> R</p><p>ate</p><p> of p</p><p>rod</p><p>uct</p><p>ion</p><p>, Vkg</p><p> of </p><p>To</p><p>tal v</p><p>olu</p><p>me</p><p> (U</p><p>dig</p><p>est</p><p>er)</p><p> (a</p><p>t (g</p><p>dig</p><p>est</p><p>er)</p><p> (a</p><p>t 75 d</p><p>ays)</p><p> TS* </p><p>VS* </p><p>VS* </p><p>75 d</p><p>ays)</p><p> a</p><p>dd</p><p>ed</p><p> a</p><p>dd</p><p>ed</p><p> co</p><p>nsu</p><p>me</p><p>d </p><p>Ra</p><p>te of p</p><p>rod</p><p>ucti</p><p>on</p><p> (l</p><p>/kg) o</p><p>f </p><p>TS</p><p> V</p><p>S </p><p>VS </p><p>ad</p><p>de</p><p>d </p><p>ad</p><p>de</p><p>d </p><p>co</p><p>nsu</p><p>me</p><p>d </p><p>Cow</p><p> du</p><p>ng</p><p> (C</p><p>D) </p><p>25 </p><p>Cow</p><p> du</p><p>ng</p><p> + ric</p><p>e st</p><p>raw</p><p> (C</p><p>D+</p><p>R) </p><p>33 </p><p>Cow</p><p> du</p><p>ng</p><p> + m</p><p>aiz</p><p>e sta</p><p>lks (</p><p>CD</p><p>+M</p><p>) 34 </p><p>Cow</p><p> du</p><p>ng</p><p> + c</p><p>ott</p><p>on</p><p> sta</p><p>lks (</p><p>CD</p><p>+C</p><p>) 2</p><p>8 </p><p>154 </p><p>20</p><p>7 </p><p>1101 </p><p>14 </p><p>207 </p><p>289 </p><p>1177 </p><p>16 </p><p>217 </p><p>278 </p><p>1176 </p><p>17 </p><p>176 </p><p>22</p><p>4 </p><p>1180 </p><p>11 </p><p>89 </p><p>120 </p><p>636 </p><p>98 </p><p>137 </p><p>560 </p><p>10 </p><p>134 </p><p>566 </p><p>70</p><p> 8</p><p>9 </p><p>468 </p><p>*TS =</p><p> To</p><p>tal s</p><p>oli</p><p>ds;</p><p> VS</p><p> = v</p><p>ola</p><p>tile</p><p> soli</p><p>ds </p></li><li><p>M. M. El-Shinnawi, B. S. El-Tahawy, S. A. El-Shimi &amp; Soheir S. Fahmy 35 </p><p>followed by CD+M, CD+R and CD+C, respectively. Consequently, mixing the cow dung with the crop residues increased the amounts of biogas produced but decreased the methane content on the basis of VS consumed. This is attributed to the high cellulose content of the plant materials (National Academy of Sciences, 1981). The overall loss of VS ranged between 18 and 25%. </p><p>Again, the nature of feedstocks governed the rate of disappearance of VS in the digesters. The C/N ratio, contents of both water-soluble substances and lignin, and initial VS content appeared to determine the extent of breakdown of the organic matter, as mentioned above. The lower C/N ratio of cow dung might contribute to the high rate of methane generated (for VS consumed), according to Hill (1979) who noted that the percentage of methane gas increased with decreasing C/N ratio of the feedstocks. </p><p>Volatile fatty acids Total VFAs. The contents of total VFAs was increased within the first 15 days of incubation, and thereafter was severely diminished (Fig. 5). CD exhibited the greatest VFA formation throughout, while CD + R, CD + C and CD +M followed, respectively. Amounts of VFAs formed by CD during the first 2 weeks, reached 93 mEq/litre). </p><p>100 </p><p>90 </p><p>~" 80 </p><p>t~ E 7O tD3 C </p><p>t - </p><p>60 </p><p>~ 50 W </p><p>E ~ 40 i f ) &lt; l.J_ </p><p>- - ao </p><p>i.-- 20 </p><p>10 </p><p>x </p><p>I I I I I o 15 30 45 60 75 </p><p>Fermentation time, days </p><p>F ig . 5 Changes of total volatile fatty acids during fermentation of cow dung and its mixtures with crop residues, e, cow dung; x...</p></li></ul>